INF

OR

MA

TIO

N O

N S

ELE

CT

ED

CLI

MA

TE

AN

D

CLI

MA

TE

-CH

AN

GE

IS

SU

ES

By

Har

ry F

. Li

ns,

Eric

T.

Sun

dqui

st,

and

Thom

as A

. A

ger

U.S

. G

EO

LOG

ICA

L S

UR

VE

Y

Ope

n-Fi

le R

epor

t 88-

718

Res

ton,

Virg

inia

1988

DE

PA

RT

ME

NT

OF

TH

E IN

TE

RIO

R

DO

NA

LD P

AU

L MO

DE

L, Secretary

U.S

. GE

OLO

GIC

AL S

UR

VE

Y

Dallas L. P

eck, Director

For additional information

write to:

Office of the D

irector U

.S. Geological S

urvey R

eston, Virginia

22092

Copies of this report can be

purchased from:

U.S. G

eological Survey

Books and Open-File R

eports SectionB

ox 25425Federal C

enter, Bldg. 810D

enver, Colorado

80225

PR

EF

AC

E

Dur

ing

the

sprin

g an

d su

mm

er o

f 19

88,

larg

e pa

rts

of

the

Nat

ion

wer

e se

vere

ly

affe

cted

by

in

tens

e he

at

and

drou

ght.

In

man

y ar

eas

agric

ultu

ral

prod

uctiv

ity

was

si

gnifi

cant

ly

redu

ced.

Th

ese

even

ts

stim

ulat

ed

wid

espr

ead

conc

ern

not o

nly

for t

he im

med

iate

effe

cts

of

seve

re

drou

ght,

but

also

fo

r th

e co

nseq

uenc

es o

f pot

entia

l cl

imat

ic c

hang

e du

ring

the

com

ing

deca

des.

C

ongr

ess

held

hea

rings

reg

ardi

ng th

ese

issu

es,

and

vario

us

agen

cies

w

ithin

th

e E

xecu

tive

Bra

nch

of g

over

nmen

t be

gan

prep

arin

g pl

ans

for

deal

ing

with

the

dro

ught

and

po

tent

ial

clim

atic

cha

nge.

As

par

t of

the

fa

ct-f

indi

ng

proc

ess,

th

e A

ssis

tant

S

ecre

tary

of

the

Inte

rior

for

Wat

er a

nd

Sci

ence

ask

ed t

he G

eolo

gica

l S

urve

y to

pr

epar

e a

brie

fing

that

wou

ld in

clud

e ba

sic

info

rmat

ion

on c

limat

e, w

eath

er p

atte

rns,

an

d dr

ough

t; th

e gr

eenh

ouse

effe

ct a

nd

glob

al w

arm

ing;

and

clim

atic

cha

nge.

Th

e br

iefin

g w

as l

ater

upd

ated

and

pre

sent

ed

to t

he

Sec

reta

ry

of

the

Inte

rior.

The

Sec

reta

ry t

hen

requ

este

d th

e G

eolo

gica

l Su

rvey

to o

rgan

ize

the

brie

fing

mat

eria

l in

text

for

m.

The

mat

eria

l co

ntai

ned

in t

his

repo

rt re

pres

ents

the

Geo

logi

cal

Sur

vey

resp

onse

to

the

Sec

reta

ry's

req

uest

. Th

e re

port

is

divi

ded

into

th

ree

dist

inct

se

ctio

ns, c

onfo

rmin

g to

the

thre

e el

emen

ts

that

com

pose

d th

e or

igin

al

brie

fings

.

CO

NT

EN

TS

Pre

face

m

The

Wea

ther

and

Clim

ate

Sys

tem

1

by

Har

ry F

. Li

ns

The

Gen

eral

Circ

ulat

ion

of th

e A

tmos

pher

e 1

Zona

l and

Mer

idio

nal C

ircul

atio

n 2

Glo

bal A

ssoc

iatio

ns (

Tele

conn

ectio

ns)

3

Pre

cipi

tatio

n V

aria

bilit

y an

d Tr

ends

4

Dro

ught

5

The

Dro

ught

of

1988

7

The

Dro

ught

in a

Glo

bal P

ersp

ectiv

e 10

The

Gre

enho

use

Eff

ect

and

Glo

bal

War

min

g 11

by

Eric

T. S

undq

uist

Crit

ical

Que

stio

ns a

nd E

ssen

tial F

acts

11

Ene

rgy

Bal

ance

of t

he E

arth

's S

urfa

ce

and

Atm

osph

ere

12

Long

-Ter

m C

hang

es in

Atm

osph

eric

C

arbo

n D

ioxi

de

13

His

toric

al C

hang

es in

Atm

osph

eric

Car

bon

Dio

xide

14

His

toric

al C

hang

es in

Atm

osph

eric

Met

hane

15

His

toric

al G

loba

l War

min

g 16

Glo

bal W

arm

ing

in th

e 19

80's

17

Pre

dict

ing

the

Gre

enho

use

Effe

ct

18

Maj

or U

ncer

tain

ties

in G

loba

l C

limat

e M

odel

P

roje

ctio

ns

19

Maj

or U

ncer

tain

ties

in R

egio

nal

Clim

ate

Pre

dict

ions

20

The

Gre

enho

use

Effe

ct in

a G

eolo

gica

l Con

text

21

Clim

ate

Cha

nge

in t

he G

eolo

gic

Pas

t 22

by

Thom

as A

. Age

r

Per

spec

tives

on

Long

-Ter

m C

hang

es in

C

limat

e 22

Clim

ate

Cha

nge

Dur

ing

the

Pas

t 13

0 M

illio

n Y

ears

23

Mila

nkov

itch

Forc

ing

of C

limat

e C

hang

e 24

The

Last

1,0

00 Y

ears

25

Ref

eren

ces

26

TH

E

WE

AT

HE

R

AN

D

CL

IMA

TE

S

YS

TE

M

by

Har

ryF.

Lins

___ _

_______T

he

Gen

eral

C

ircu

latio

n of

the

A

tmos

pher

e

The

gl

obal

w

ind

syst

em,

mor

e sp

ecifi

cally

re

ferr

ed

to

as

the

gene

ral

circ

ulat

ion

of t

he a

tmos

pher

e, f

unct

ions

to

trans

port

war

m a

ir fro

m e

quat

oria

l re

gion

s to

war

d th

e po

les,

and

to m

aint

ain

a re

turn

flow

of

col

d ai

r fro

m p

olar

to

tropi

cal

latit

udes

. It

resu

lts f

rom

fou

r pr

imar

y co

nditi

ons:

. so

lar

radi

atio

n,

the

rota

tion

of

the

Ear

th,

the

dist

ribut

ion

of l

and

mas

ses

and

seas

acr

oss

the

Ear

th's

sur

face

, an

d tid

al f

orce

s.

Sol

ar

radi

atio

n an

d th

e E

arth

's r

otat

ion

com

bine

to

form

the

"br

ute

forc

e" d

river

of

the

syst

em.

They

ar

e,

by

far,

the

dom

inan

t fa

ctor

s.

Topo

grap

hic

and

ther

mod

ynam

ic d

iffer

ence

s ar

isin

g fro

m th

e sp

atia

l dis

tribu

tion

of la

nd a

nd

sea

are

maj

or m

odul

ator

s of

the

bru

te f

orce

co

mpo

nent

. Ti

dal f

orce

s ar

e m

inor

mod

ulat

ors

of b

rute

forc

e.Th

e ge

nera

l ci

rcul

atio

n ha

s bo

th

verti

cal

and

horiz

onta

l st

ruct

ure

(fig.

1).

The

verti

cal

stru

ctur

e, c

ompo

sed

of t

he t

ropi

cal

Had

ley,

mid

-latit

ude

Ferr

el,

and

a ve

ry w

eak

and

inte

rmitt

ent

pola

r ce

ll, m

ixes

hea

t an

d m

oist

ure

thro

ugh

the

dept

h of

th

e at

mos

pher

e.

The

horiz

onta

l st

ruct

ure,

m

anife

st a

s th

e tro

pica

l tra

de w

inds

, m

id-

latit

ude

wes

terli

es,

and

pola

r ea

ster

lies,

pe

rform

s th

e sa

me

func

tion

acro

ss th

e E

arth

's

surfa

ce.

Thes

e la

rge-

scal

e fe

atur

es p

rimar

ily

dete

rmin

e th

e br

oad

patte

rn o

f glo

bal c

limat

e.

Em

bedd

ed w

ithin

thi

s la

rge-

scal

e sy

stem

are

sm

alle

r ci

rcul

atio

ns w

hich

ten

d to

per

turb

the

gl

obal

flow

. Th

ese

smal

ler-

scal

e cy

clon

ic (

low

pr

essu

re)

and

antic

yclo

nic

(hig

h pr

essu

re)

circ

ulat

ions

are

res

pons

ible

for

the

trans

itory

, sh

ort-

term

va

riatio

ns

in

atm

osph

eric

co

nditi

ons,

whi

ch w

e re

fer t

o as

the

wea

ther

.

Man

uscr

ipt a

ppro

ved

for p

ublic

atio

n D

ecem

bers

, 19

88

Air

sink

s

Figu

re 1

. S

chem

atic

dep

ictio

n of

the

gene

ral c

ircul

atio

n of

the

Earth

's a

tmos

pher

e.

(Sou

rce:

W

ashi

ngto

n an

d P

arki

nson

, 19

86).

Zonal and

Meridional

Circulation

Large-scale m

ovements

of w

arm

air polew

ard and cold air equaterward provide a

balance betw

een energy

surpluses at

low

latitudes and energy deficits at high latitudes. W

hen the temperature difference betw

een high and low

latitudes is relatively small, as is typical in

summ

er, the circulation of the atmosphere is

weaker and m

ore zonal, or more nearly w

est to east (fig. 2).

When the tem

perature contrast is high, as in w

inter, the circulation may becom

e stronger and m

ore meridional, or north-south.

Thus, seasonal variations in weather patterns can

typically be characterized by the persistence of one or the other of these tw

o circulation regimes.

Changes

in the

amplitude

of these

planetary-scale circulation "waves" reflect the

positions and intensities of high and low pressure

systems around the globe.

The region beneath a w

ave crest is characterized by high pressure. It is typically referred to as a high pressure "ridge." Such ridges serve to m

ove relatively warm

air polew

ard and are generally associated with fair

weather conditions.

The wave depressions are

regions of low pressure, and are referred to as low

pressure "troughs."

Troughs of low pressure

within the general circulation m

ove relatively cold air equatorw

ard and are generally associated with

stormy conditions.

Occasionally the am

plitude of a low

pressure trough becomes so great that the

circulation folds back onto itself, thereby cutting the low

pressure system off from

the general circulation.

Without the m

omentum

provided by the general circulation such "cut-off" low

s tend to persist

over an

area, som

etimes

bringing storm

iness for a week or m

ore. In contrast,

protracted periods of fair weather can accom

pany a high pressure ridge cut-off from

the general circulation.

Such ridges are frequently referred to as "blocking" highs, because they tend to block the m

ovement of storm

systems into an area.

TY

PIC

AL S

UM

ME

R C

ON

DIT

ION

SZO

NA

L

ME

RID

ION

AL

CU

T-OFF

TY

PIC

AL W

INT

ER

CO

ND

ITIO

NS

Figure 2. Zonal and meridional com

ponents of the horizontal circulation of the atmosphere.

The letter L refers to low

pressure and H to high pressure.

Glo

bal

Ass

oci

atio

ns

(Tel

eco

nn

ecti

on

s)R

ecen

t re

sear

ch

indi

cate

s th

at

wea

ther

pat

tern

s in

spe

cific

reg

ions

of

the

glob

e te

nd t

o ex

hibi

t con

sist

ent

resp

onse

s to

ce

rtain

lar

ge-s

cale

atm

osph

eric

and

oce

anic

ci

rcul

atio

ns.

Thes

e as

soci

atio

ns,

frequ

ently

re

ferre

d to

as

tele

conn

ectio

ns, p

rovi

de a

bas

is

for

impr

ovin

g w

eekl

y w

eath

er p

redi

ctio

n an

d se

ason

al c

limat

e fo

reca

stin

g.

The

conc

ept

of

tele

conn

ectio

ns a

ctua

lly re

fers

to th

e st

atis

tical

cr

oss-

corr

elat

ion

betw

een

atm

osph

eric

an

omal

y pa

ttern

s (u

sual

ly

atm

osph

eric

pr

essu

re)

sepa

rate

d by

syn

optic

(1,

000-

2,50

0 km

) to

hem

isph

eric

dis

tanc

es.

The

syno

ptic

, or

m

ore

prox

imal

te

leco

nnec

tions

are

ass

ocia

ted

with

var

iatio

ns

in t

he l

ocat

ion

and

inte

nsity

of

the

pers

iste

nt

subt

ropi

cal h

igh

pres

sure

regi

ons

(suc

h as

the

Ber

mud

a H

igh)

and

sub

pola

r lo

w p

ress

ure

regi

ons

(suc

h as

the

Ale

utia

n Lo

w).

The

glob

al,

or m

ore

rem

ote

tele

conn

ectio

ns s

tem

fro

m

even

ts

such

as

El

N

ino-

Sou

ther

n O

scill

atio

n (E

NSO

). R

esea

rch

to d

ate

(198

8)

indi

cate

s th

at E

NSO

eve

nts

gene

rally

coi

ncid

e w

ith d

istin

ct s

easo

nal

prec

ipita

tion

anom

aly

patte

rns,

suc

h as

abo

ve n

orm

al p

reci

pita

tion

alon

g th

e G

ulf a

nd S

outh

east

Atla

ntic

coa

sts

of

the

Uni

ted

Sta

tes

in w

inte

r an

d in

the

Gre

at

Basi

n in

sum

mer

(fig

. 3)

. H

igh

prec

ipita

tion

amou

nts

also

are

foun

d in

sou

thea

ster

n So

uth

Am

eric

a in

sum

mer

, th

e ce

ntra

l E

quat

oria

l Pa

cific

thro

ugho

ut th

e ye

ar,

sout

hern

Ind

ia in

au

tum

n, a

nd e

ast

cent

ral

Afri

ca d

urin

g th

e w

inte

r. D

roug

ht t

ends

to

acco

mpa

ny E

NSO

ep

isod

es i

n so

uthe

aste

rn A

frica

in

sum

mer

, m

ost o

f In

dia

durin

g th

e su

mm

er,

north

east

ern

Sout

h A

mer

ica

and

adja

cent

Car

ibbe

an b

asin

in

sum

mer

, an

d in

a b

road

re

gion

of

the

wes

tern

P

acifi

c in

clud

ing

Indo

nesi

a an

d Au

stra

lia in

var

ious

sea

sons

.

It0»

LOC

AT

ION

KE

Y

A -

Indo

nesi

a, A

ustra

lia, a

nd w

este

rn P

acifi

c O

cean

B - C

entra

l Equ

ator

ial P

acifi

c O

cean

C -

Gre

at B

asin

D -

Gul

f and

Sou

thea

st A

tlant

ic C

oast

s of

U.S

.E

- Nor

thea

ster

n So

uth

Am

eric

a an

d C

arib

bean

F - S

outh

east

ern

Sout

h A

mer

ica

G -

East

Cen

tral A

frica

H

- Sou

thea

ster

n A

frica

I

- Sou

ther

n In

dia

J -

Nor

ther

n an

d C

entra

l Ind

ia

Figu

re 3

. R

egio

ns e

xhib

iting

a c

onsi

sten

t pre

cipi

tatio

n re

spon

se s

igna

l to

El N

ino-

Sou

ther

n O

scill

atio

n ep

isod

es.

(Sou

rce:

Rop

elew

ski a

nd H

alpe

rt, 1

987)

.

Precipitation

Varia

bility

and T

rendsP

recipitation is a highly variable quantity both in space and tim

e. A

nalysis of

precipitation data

for the

Northern

Hem

isphere from the m

id-19th century to the present (B

radley and others, 1987),

and of

much

longer proxy

series reconstructed from

tree rings, indicate that significant

regional variations

have occurred over decadal and longer tim

e scales.

Hem

ispherically, decadal

and longer fluctuations in precipitation are also evident, although there does not appear to be

any system

atic trend over the past

century (fig. 4A).P

atterns in specific regions have been

variable (fig.

48). E

urope, for

example, has undergone a sm

all but steady increase in precipitation since the 1860's. In the U

nited States, precipitation declined betw

een the 1880's and the 1930's. S

ince the

1930's, how

ever, there has been

a general increase w

ith the most m

arked rise occurring

since the

mid-1950's.

Precipitation over the S

oviet Union has

increased significantly since the 1880's, w

ith most of the change occurring before

1900 and after 1940. In N

orthern Africa

and the Middle East, precipitation exhibited

little trend betw

een 1900

and the late-

1940's. S

ince then, however, precipitation

has declined dramatically.

In Southeastern

Asia, a relatively w

et period in the 1920's and early 1930's separated tw

o protracted periods of relative dryness.

The first was

centered around

1900 and the second

extended from the

mid-1960's until the

present.

During the

past 30 to 40

years there

have been

some

significant differences in the tem

poral distribution of precipitation on a latitudinal basis.

In the m

iddle latitudes

(35° to 70°

N),

for exam

ple, a significant precipitation increase has

occurred since

1940. In the

sub­ tropical latitudes (5° to 35° N), how

ever, a significant decrease is evident since the m

id-1950's. In the north equatorial region

(0° to 5° N), little or no trend is evident in

the record during the past century.

IC«

0.50

<D C

L

0.40

1900 1940

Year

19M

B

0.75

0.50

0.25

0.75

0.50

0.25

0.50

United S

late*

0.75

0.50

0.25

0.75

0.50

0.25 J

SovJet Union

No

rth* A

Mca and M

iddle East

$*&

rtyr\SM

. t ii

* wii

Southeastern A

aia

1900 1940

VM

r

Figure 4. A

nnual precipitation index (m

ean of percentiles of the gamm

a distribution for Northern

Hem

isphere continental (A) and constituent regions (8). V

ertical dashed line shows year in w

hich50 percent of the grid points becam

e available for analysis. C

urved line shows the sm

ooth trendline fitted through individual values.

(Source:

Bradley and others, 1987; C

opyright 1987 by the AA

AS

).

Dro

ug

ht

In 1

975

the

Wor

ld M

eteo

rolo

gica

l O

rgan

izat

ion

issu

ed

a S

peci

al

Envi

ronm

enta

l R

epor

t on

Dro

ught

. In

tha

t re

port,

H

elm

ut L

ands

berg

, th

e fa

ther

of

Am

eric

an c

limat

olog

y, n

oted

tha

t af

ter

50

year

s of

cl

imat

olog

ical

an

d st

atis

tical

an

alys

es i

t w

as c

lear

tha

t "d

roug

ht i

s a

recu

rren

t ph

enom

enon

. Th

ere

is n

o ne

ed

to in

voke

clim

atic

cha

nge

as a

cau

se e

ach

time

drou

ght o

ccur

s.

Qui

te th

e co

ntra

ry is

ev

iden

t: dr

ough

t is

an in

tegr

al,

if irr

egul

ar,

com

pone

nt o

f cl

imat

e as

it

exis

ts n

ow"

(Lan

dsbe

rg,

1975

).Pa

rt of

the

prob

lem

ass

ocia

ted

with

un

ders

tand

ing

drou

ght

resu

lts f

rom

the

fa

ct t

hat

ther

e ar

e at

lea

st t

hree

diff

eren

t ki

nds

of

drou

ght:

met

eoro

logi

cal,

ag

ricu

ltura

l, an

d hyd

rolo

gic

al.

Met

eoro

logi

cal

drou

ght

is

gene

rally

de

fined

in

term

s of

low

er t

han

aver

age

prec

ipita

tion

for

som

e tim

e pe

riod.

A

gric

ultu

ral d

roug

ht r

efer

s to

a s

horta

ge o

f w

ater

in t

he r

oot

zone

of

crop

s su

ch t

hat

the

yiel

d of

pla

nts

is r

educ

ed c

onsi

dera

bly

(a la

ck o

f soi

l moi

stur

e).

Hyd

rolo

gic

drou

ght

is

defin

ed

in

term

s of

lo

w

leve

ls

of

stre

amflo

w,

rese

rvoi

r st

orag

e,

grou

nd

wat

er,

or s

ome

com

bina

tion

of t

hese

. A

drou

ght

epis

ode

may

con

tain

al

l th

ree

elem

ents

, tw

o of

them

, or o

nly

one.

Win

ter

drou

ghts

, fo

r ex

ampl

e,

are

ofte

n on

ly

met

eoro

logi

cal e

vent

s si

nce

it is

a t

ime

of

little

or n

o ag

ricul

tura

l dem

and

for w

ater

and

ge

nera

lly to

wer

wat

er s

uppl

y ne

eds.

Dro

ught

is a

lso

high

ly v

aria

ble

both

in

spa

ce a

nd ti

me.

In

figu

re 5

, sta

ndar

dize

d sc

ores

of

the

Pal

mer

Dro

ught

S

ever

ity

Inde

x ar

e pl

otte

d fo

r ni

ne r

egio

ns a

roun

d th

e U

nite

d S

tate

s.

Sev

eral

po

ints

are

ap

pare

nt

from

th

ese

plot

s.

Firs

t, th

e oc

curre

nce

of d

ry y

ears

var

ies

cons

ider

ably

fro

m r

egio

n to

reg

ion.

S

econ

d, t

here

is a

re

lativ

ely

high

yea

r-to

-yea

r va

riabi

lity

in

near

ly a

ll re

gion

s.

Ther

e do

es a

ppea

r, ho

wev

er,

to b

e so

me

tend

ency

for

sev

ere

drou

ght t

o pe

rsis

t ove

r sev

eral

yea

rs in

the

Wes

t Nor

th C

entra

l, So

uthw

est,

and

Sout

h re

gion

s.

Third

, al

thou

gh s

ome

regi

ons

appe

ar t

o ex

hibi

t de

cada

l-sca

le

trend

s to

war

d bo

th

wet

ness

an

d dr

ynes

s,

no

regi

on e

xhib

its a

ny s

yste

mat

ic t

rend

ove

r se

cula

r tim

e sc

ales

or

over

the

85-

year

pe

riod

of

reco

rd.

In o

ther

wor

ds,

the

frequ

ency

of

drou

ght

acro

ss t

he U

nite

d S

tate

s do

es n

ot a

ppea

r to

hav

e ch

ange

d du

ring

the

past

cen

tury

.

Dro

ug

ht

W. N

. CE

NT

RA

LE. N

. CE

NT

RA

L

NO

RT

HE

AST

SOU

TH

WE

STSO

UTH

SOU

THEA

ST

Figure 5. Annual m

aximum

, minim

um, and m

edian principal component scores of the P

almer D

rought Severity Index, 1895-1980.

The plotted scores contrast years of relative w

etness (above the zero line) with those of relative dryness (below

the zero line). (S

ource: Karl and Koscielny, 1982).

The

Dro

ught

of

1988

Dur

ing

the

sprin

g of

198

8 a

seve

re,

thou

gh

rela

tivel

y sh

ort-

term

dr

ough

t de

velo

ped

over

the

Nor

ther

n P

lain

s an

d M

idw

est

regi

ons

of th

e U

nite

d S

tate

s.

The

cond

ition

s in

the

se a

reas

ar

ose

amid

st

long

er-t

erm

dr

ough

t th

at

had

been

pl

agui

ng p

arts

of t

he W

est

sinc

e 19

87 a

nd

part

s of

th

e S

outh

east

si

nce

1984

. A

lthou

gh l

ittle

was

rep

orte

d ab

out

thes

e on

goin

g ep

isod

es,

the

even

t in

th

e N

atio

n's

hear

tland

qu

ickl

y be

cam

e a

natio

nal n

ews

item

.Th

e N

orth

ern

Pla

ins-

Mid

wes

t dr

ough

t ha

d its

beg

inni

ngs

in

Febr

uary

w

hen

prec

ipita

tion

from

the

wes

tern

Gre

at

Lake

s to

the

Sou

ther

n P

lain

s w

as b

elow

to

muc

h-be

low

nor

mal

. By

Mar

ch,

the

low

pr

ecip

itatio

n co

uple

d w

ith a

bove

nor

mal

te

mpe

ratu

res

to b

egin

wid

espr

ead

dryi

ng

of s

oils

in

the

north

ern

half

of t

he G

reat

Pla

ins.

D

urin

g A

pril

the

situ

tatio

n w

orse

ned

as

both

belo

w-n

orm

al

pre

cip

itatio

n

and

above

-norm

al

tem

pera

ture

s co

vere

d m

uch

of

the

Nor

ther

n Pl

ains

and

Mid

wes

t. Th

us,

at th

e cr

itica

l tim

e of

pla

nt g

erm

inat

ion,

whe

n so

il m

oist

ure

need

s ar

e re

lativ

ely

high

, ele

vate

d

tem

pera

ture

s an

d lo

w

prec

ipita

tion

seve

rely

de

plet

ed

soil

moi

stur

e re

serv

es

and

redu

ced

crop

de

velo

pmen

t (fi

g. 6

A

and

B).

Con

ditio

ns

cont

inue

d de

terio

ratin

g th

roug

h Ju

ne,

with

te

mpe

ratu

res

aver

agin

g 6

to 1

2 de

gree

s F

abov

e no

rmal

ove

r m

ost

of t

he N

orth

ern

Pla

ins

and

Mid

wes

t, w

ith p

reci

pita

tion

less

th

an 4

0 pe

rcen

t of

norm

al o

ver

muc

h of

the

regi

on (

fig. 6

A, B

, and

C).

In

July

an

d A

ugus

t co

nditi

ons

bega

n ch

angi

ng.

Alth

ough

tem

pera

ture

s re

mai

ned

abov

e no

rmal

acr

oss

muc

h of

the

Nat

ion

thro

ugho

ut t

he

sum

mer

, pr

ecip

­ ita

tion

in t

he N

orth

ern

Pla

ins

and

Mid

wes

t re

turn

ed

to

near

no

rmal

co

nditi

ons.

S

tream

flow

lag

ged

behi

nd p

reci

pita

tion

in

reco

verin

g as

muc

h of

the

rain

wen

t tow

ard

rech

arge

an

d,

beca

use

of

the

high

te

mpe

ratu

res,

to

incr

ease

d ev

apot

rans

- pi

ratio

n.

Still,

by

sum

mer

's e

nd th

e dr

ough

t

Figu

re 6

. S

eque

nces

of

met

eoro

logi

cal

and

hydr

otog

ical

con

ditio

ns a

cros

s th

e U

nite

d S

tate

s du

ring

the

sprin

g se

ason

an

d th

e m

onth

s of

Jun

e an

d A

ugus

t. A

tem

pera

ture

. B,

pre

cipi

tatio

n.

C,

stre

amflo

w.

D, a

tmos

pher

ic c

ircul

atio

n at

the

10,0

00-fo

ot l

evel

. (S

ourc

es:

met

eoro

logi

cal d

ata

- N

atio

nal O

cean

ic

and

Atm

osph

eric

Adm

inis

tratio

n,

Nat

iona

l Wea

ther

Ser

vice

and

Clim

ate

Ana

lysi

s C

ente

r; st

ream

flow

dat

a - U

.S.

Geo

logi

cal S

urve

y.)

TEM

PE

RA

TUR

E P

ER

CE

NTI

LES

M

arch

. Ap

ril, a

nd M

«y 1

986

TEM

PE

RA

TUR

E P

ER

CE

NTI

LES

Ju

ne.

1986

TEM

PE

RA

TUR

E P

ERC

EMTI

LES

Aug

ust.

1988

The D

rought of 1988

BP

RE

CIP

ITATIO

N P

ER

CE

NT

IES

M

arch. AprH. and May 1968

PR

EC

IPITA

TION

PERC

ENTILES

Juna. 1988

RANGES

STREA

MFLO

W D

UR

ING

APR

ILSTR

EAM

FLOW

DU

RIN

G JU

NE

STREA

MFLO

W D

UR

ING

AU

GU

ST

Normal

Below

noniuu

8

The

Dro

ught

of

1988

of 1

988

was

ess

entia

lly o

ver

in m

ost p

arts

of

th

e N

orth

ern

Pla

ins

and

Mid

wes

t. A

lthou

gh t

his

even

t was

qui

te s

ever

e in

a

shor

t-ter

m s

ense

, w

hen

cons

ider

ed in

the

cont

ext

of d

roug

ht o

ver

an e

ntire

yea

r, 19

88 m

ay o

r may

not

be

rem

embe

red

as a

gr

eat

drou

ght

year

. C

ondi

tions

dur

ing

Oct

ober

, N

ovem

ber,

and

Dec

embe

r w

ill

cont

ribut

e si

gnifi

cant

ly t

o ho

w

1988

is

reco

rded

in th

e dr

ough

t rec

ord

book

s.Pe

rhap

s of

gre

ater

sig

nific

ance

is

tr.3

caus

e of

thi

s dr

ough

t. W

hy d

id i

t oc

cur?

To

ans

wer

thi

s fir

st q

uest

ion

we

need

on

ly

look

to

th

e at

mos

pher

ic

circ

ulat

ion

durin

g th

e sp

ring

and

sum

mer

m

onth

s (fi

g. 6

D).

Dur

ing

April

, the

circ

ulat

ion

over

the

Nor

th A

mer

ican

sec

tor

at

10,0

00

feet

w

as

char

acte

rized

by

ab

norm

ally

dee

p tro

ughs

(stip

pled

are

as) o

f lo

w p

ress

ure

over

the

Ale

utia

n Is

land

s an

d N

ewfo

undl

and

with

an

abno

rmal

ly s

trong

rid

ge (

cros

s-ha

tche

d ar

ea) o

f hig

h pr

essu

re

exte

ndin

g no

rthw

ard

from

the

U.S

. ce

ntra

l R

ocki

es t

o th

e N

orth

wes

t Te

rrito

ries

of

Can

ada.

Thi

s rid

ge w

as re

spon

sibl

e fo

r the

w

arm

, dr

y co

nditi

ons

that

occ

urre

d at

that

tim

e ov

er

the

Nor

ther

n R

ocki

es

and

north

ern

Gre

at P

lain

s.

By J

une,

the

rid

ge

of h

igh

pres

sure

exp

ande

d dr

amm

atic

ally

to

enc

ompa

ss m

ore

than

hal

f the

U.S

. and

C

anad

a.

The

abno

rmal

int

ensi

ty o

f th

is

ridge

lead

to th

e un

usal

ly h

igh

tem

pera

ture

s an

d re

duce

d pr

ecip

itatio

n ac

ross

the

cent

ral p

art o

f the

cou

ntry

at t

hat

time.

In

Jul

y, h

owev

er,

and

pers

istin

g in

Au

gust

, the

circ

ulat

ion

patte

rn c

hang

ed a

s bo

th t

he m

id-c

ontin

ent

high

and

the

low

s on

bot

h co

asts

bro

ke d

own.

In

fac

t, th

e at

mos

pher

ic c

ircul

atio

n fo

r bo

th J

uly

and

Aug

ust

was

nor

mal

. Th

is,

in i

tsel

f, di

dn't

brin

g an

end

to th

e ho

t and

dry

con

dtio

ns,

sinc

e re

cove

ring

from

su

ch

cond

ition

s ta

kes

time.

It d

id, h

owev

er, s

igna

l an

end

to

the

abno

rmal

circ

ulat

ion

cond

ition

s w

hich

w

ere

dire

ctly

res

pons

ible

for

the

dro

ught

. O

f cou

rse,

the

real

que

stio

n of

wha

t cau

sed

the

drou

ght i

s th

e qu

estio

n of

wha

t cau

sed

the

circ

ulat

ion

anom

alie

s.

ATM

OS

PH

ER

IC C

IRC

ULA

TIO

N IN

APR

IL

D

ATM

OS

PH

ER

IC C

IRC

ULA

TIO

N IN

JU

NE

ATM

OS

PH

ER

IC C

IRC

ULA

TIO

N IN

AU

GU

ST

Low

pre

ssur

eH

igh

pres

sure

The D

rought in a G

lobal P

erspectiveIn recent years, a grow

ing amount of evidence

has been acquired indicating that atmospheric and

oceanic conditions in one part of the globe affect the w

eather and climate in another part of the globe.

The El

Nino/S

outhern O

scillation is

one of the

most

frequently cited

examples.

How

ever, during the

drought of 1988 there was no El Ni no occurring in the

tropical Pacific. So w

hat can we say about the relation

of this drought to regions of anomalous conditions

elsewhere

around the w

orld? In

short, nothing

definitively at present. A

lthough several scientists have hypothesized that the drought w

as linked to anom

alously warm

sea surface temperatures in the

tropical Pacific (north of the

El Nino region), this

explanation has not yet gained general acceptance. In

addition, despite the "teleconnections" or correlations betw

een pressure patterns in one part of the world

with those in another, m

ost such correlations are weak

to moderate and tend to be strongest during the w

inter season w

hen pressure gradients are strongest. W

est central C

anada, the area where the anom

alous high pressure ridge w

as centered, is not a region where

anomaly

patterns correlate

well

with

patterns elsew

here around the world in spring and sum

mer (fig.

7). M

oreover, the global meteorological anom

alies during June, 1988 do not appear to reflect a global clim

ate change. O

ne can look at charts such as those in figure 7 for any m

onth of the year, in any year, and see tem

perature and precipitation anomalies of sim

ilar or even greater spatial extent w

idely distributed around the w

orld. There do not appear to be any trends

evident in

these anom

alies that are

in any w

ay indicative of a condition differentiable from

the natural variability of the clim

ate system.

B

Shading depict* region* where tem

perature anom

aNM w

ere Mtim

ated to be within

the warm

Mt 10%

or cokleet 10% of

cNmatotogtceJ occurrence*

June, 1988

flOflM

wM

Were1 M

on

Mtttd

tO b

e W

ithm

the wetteet 10%

or driest 10% of

c»matok>gic«l occurrence*

Jun

e, 19

88

Figure 7. Global tem

perature (A) and precipitation (B) anomalies for June, 1988.

(Source: N

ational O

ceanic and Atm

ospheric Adm

inistration, Clim

ate Analysis C

enter).

10

TH

E G

RE

EN

HO

US

E

EF

FE

CT

AN

D

GL

OB

AL

WA

RM

ING

by

Er

ic T

. Su

ndqu

ist

Cri

tical

Q

uest

ions

and

E

ssen

tial

Fact

s

Que

stio

ns

Thro

ugho

ut t

he s

umm

er o

f 19

88,

the

gree

nhou

se

effe

ct

rece

ived

w

ide

atte

ntio

n in

the

pre

ss a

nd i

n a

varie

ty o

f pu

blic

dis

cuss

ions

thr

ough

out

the

wor

ld.

Thi

s at

tent

ion

focu

sed

on

seve

ral

ques

tions

th

at

are

criti

cal

to

our

unde

rsta

ndin

g of

cu

rren

t cl

imat

e an

d cl

imat

e ch

ange

, inc

ludi

ng:

o W

hat

Is t

he g

reen

hous

e ef

fect

?o

Doe

s th

e cu

rren

t gl

obal

war

min

g co

nfirm

the

gre

enho

use

effe

ct?

o W

as t

he 1

988

U.S

. dr

ough

t a

resu

lt of

the

gre

enho

use

effe

ct?

o H

ow w

ell

can

the

gree

nhou

se

effe

ct b

e pr

edic

ted?

This

sec

tion

pres

ents

bac

kgro

und

info

r­

mat

ion

that

is

usef

ul i

n th

e di

scus

sion

of

thes

e qu

estio

ns.

Fact

s

The

gree

nhou

se

effe

ct

is

the

abso

rptio

n of

long

-wav

e ra

diat

ion

by c

erta

in

gase

s in

the

Ear

th's

atm

osph

ere.

Th

is

phen

omen

on h

as b

een

docu

men

ted

and

stud

ied

for m

ore

than

a c

entu

ry.

Alth

ough

sc

ient

ists

con

tinue

to

disc

over

new

and

so

met

imes

une

xpec

ted

aspe

cts

of t

he

gree

nhou

se e

ffect

, th

e fo

llow

ing

poin

ts a

re

now

firm

ly e

stab

lishe

d:Th

e gr

eenh

ouse

ef

fect

Is

a

fact

, no

t a

theo

ry.

Its b

asic

phy

sica

l pr

inci

ples

ar

e w

ell

unde

rsto

od.

The

spec

ific

wav

elen

gths

of

radi

atio

n ab

sorb

ed

by a

tmos

pher

ic t

race

gas

es a

re p

reci

sely

kn

own.

Th

e ab

sorp

tion

of r

adia

tion

by a

ga

s ca

uses

its

tem

pera

ture

to

incr

ease

. Th

e sc

ient

ific

chal

leng

e as

soci

ated

with

the

gree

nhou

se

effe

ct

is

not

to

esta

blis

h w

heth

er it

exi

sts,

but

rat

her

to u

nder

stan

d th

e m

anife

stat

ion

of

its

basi

c ph

ysic

s th

roug

hout

the

Earth

's im

men

sely

com

plex

cl

imat

e sy

stem

.Th

e gr

eenh

ouse

ef

fect

Is

a

na

tura

l p

hen

om

en

on

. Th

e at

mos

pher

ic g

ases

whi

ch a

bsor

b th

e m

ost

long

-wav

e ra

diat

ion

are

wat

er v

apor

and

ca

rbon

dio

xide

, w

hich

hav

e be

en p

rese

nt

in t

he E

arth

's a

tmos

pher

e fo

r bi

llion

s of

ye

ars.

W

ithou

t th

e gr

eenh

ouse

effe

ct o

f th

ese

two

gase

s (a

nd w

ithou

t the

effe

ct o

f cl

ouds

for

med

by

the

co

nden

satio

n of

w

ater

vap

or),

the

earth

's a

vera

ge s

urfa

ce

tem

pera

ture

wou

ld p

roba

bly

be n

ear

-18

degr

ees

Cel

sius

(0

degr

ees

Fahr

enhe

it).

The

gree

nhou

se

effe

ct

was

va

riab

le

befo

re

hum

an

Influ

ence

. Th

ere

is

good

ev

iden

ce f

or s

igni

fican

t ch

ange

s in

the

atm

osph

eric

con

cent

ratio

ns

of c

arbo

n di

oxid

e an

d m

etha

ne (

anot

her

gree

nhou

se g

as)

durin

g th

e la

st 1

60,0

00

year

s.

The

gree

nhou

se

effe

ct

Is

pres

ently

ch

angi

ng

as

a re

sult

of

hum

an

activ

ities

. D

urin

g th

e la

st 2

00

year

s, a

nthr

opog

enic

pro

duct

ion

of c

arbo

n di

oxid

e ha

s ca

used

at

mos

pher

ic

conc

entra

tions

to i

ncre

ase

by 2

5 pe

rcen

t. A

tmos

pher

ic m

etha

ne c

once

ntra

tions

hav

e do

uble

d.

Oth

er "

gree

nhou

se"

gase

s --

ni

trou

s ox

ide,

ha

loca

rbon

s,

and

tropo

sphe

ric o

zone

~ a

re a

lso

know

n to

be

incr

easi

ng in

the

atm

osph

ere

at s

igni

fican

t ra

tes

due

to

hum

an

activ

ities

. Th

e co

mbi

ned

influ

ence

of

thes

e ch

ange

s in

at

mos

pher

ic c

ompo

sitio

n is

the

prin

cipa

l re

ason

for

con

cern

ab

out

pres

ent

and

futu

re c

hang

es in

the

gree

nhou

se e

ffect

.

11

Energy

Balance of the

Earth's S

urface and A

tmosphere

The com

plexity of

the global

climate system

, of which the greenhouse

effect is a part, is illustrated in figure 8. Pathw

ays of incoming solar (short-w

ave) radiation appear in the dark-shaded areas on the left side of the illustration; pathw

ays of long-w

ave radiation are depicted on the right.

The units shown are relative to the

total incoming solar radiation value of 100.

At the outer edge of the Earth's

atmosphere, incom

ing solar short-wave

radiation is balanced by predominantly

long-wave outgoing radiation.

The energy balance at the Earth's surface is dom

inated by long-w

ave radiation exchange with the

atmosphere.

The long-w

ave energy

exchanged at the Earth's surface is much

larger in magnitude than the am

ount of short-w

ave solar energy that reaches the Earth's surface.

This ampliation of the

Earth-surface energy balance is caused by clouds and the greenhouse effect.

An idealized

equilibrium energy

balance is depicted in the figure. The units of

incoming

solar energy

are exactly

balanced by an equal number of units of

outgoing radiation. In reality, the energy

balance is currently shifting as a result of changing concentrations of greenhouse gases. The exact nature of this shift is very difficult to understand and predict because it involves all of the intricate feedbacks that com

prise the Earth's climate system

.

OUTGOING RADIATION

OU

TER

SPAC

E

HYD

P1O

LITHO

SP

HE

ftE

KN

SIB

IE A

ND

LA

TE

NT

H

EA

T F

LU

X B

Y A

IM M

OT

ION

43

NE

T A

BS

OR

PTIO

N

BY

M?O

. CO

,AN

O

CLO

UD

S

»"V

AB

SO

RP

TION

OF

AB

SO

RP

TIO

N O

FD

IRE

CT

SO

LAR

D

IFF

US

E S

KY

AN

DR

AD

IAT

ION

C

LO

UD

RA

DIA

TIO

NN

ET

LON

G-W

AV

E

RE

RA

DIA

TIO

N

SE

NS

IBLE

AN

D L

AT

EN

T

IDO

V H

EA

T T

RA

NS

PO

RT

SE

NS

IBLE

L

AT

EN

T

NE

AT

H

EA

T

tfllrz*

TR

AM

SP

OM

T «

V

OC

f AN

CW

NM

fNT

S

Figure 8. E

nergy balance of the earth's surface and atmosphere, in percent.

(Source:

Schneider

and Mesirow

. 1976).

12

Long

-Ter

m

Cha

nges

in

A

tmos

pher

ic

Car

bon

Dio

xide

Evid

ence

for c

hang

es in

the

gree

n­

hous

e ef

fect

be

fore

hu

man

in

fluen

ce

appe

ars

in fi

gure

9.

The

plot

ted

data

are

de

rived

from

ana

lyse

s of

a 2

,000

-met

er ic

e co

re fr

om V

osto

k, A

ntar

ctic

a, b

y a

team

of

Fren

ch a

nd S

ovie

t sc

ient

ists

(Ba

rnol

a an

d ot

hers

, 19

87).

The

core

co

ntai

ns i

ce

depo

site

d as

sno

w o

ver

the

last

160

,000

ye

ars.

M

easu

rem

ents

of

deut

eriu

m (

an

isot

ope

of h

ydro

gen)

in

the

ice

prov

ide

a re

cord

of t

he te

mpe

ratu

res

at w

hich

the

ice

was

for

med

. A

naly

ses

of

air

bubb

les

trapp

ed in

the

ice

prov

ide

a co

rresp

ondi

ng

reco

rd o

f pas

t atm

osph

eric

com

posi

tions

.Th

e to

wer

cur

ve in

the

grap

h sh

ows

deut

eriu

m v

alue

s co

rres

pond

ing

to t

he

ages

ind

icat

ed o

n th

e lo

wer

axi

s.

The

reco

rd

clea

rly

show

s re

lativ

ely

high

de

uter

ium

val

ues

(cor

resp

ondi

ng to

hig

her

tem

pera

ture

s) d

urin

g th

e la

st 1

0,00

0 ye

ars,

to

wer

val

ues

repr

esen

ting

the

last

ice

age,

an

d hi

gh

valu

es

for

the

prec

edin

g in

terg

laci

al p

erio

d fro

m a

bout

120

,000

to

140,

000

year

s ag

o.

The

uppe

r cu

rve

show

s ca

rbon

di

oxid

e co

ncen

tratio

ns

corr

espo

ndin

g to

the

sam

e tim

e.

The

data

sh

ow th

at a

tmos

pher

ic c

arbo

n di

oxid

e an

d cl

imat

e ch

ange

s w

ere

clos

ely

rela

ted

thro

ugho

ut t

he l

ast

160,

000

year

s.

Ice

core

ana

lyse

s fo

r met

hane

indi

cate

a s

imila

r co

rrela

tion

with

clim

ate.

Unf

ortu

nate

ly,

it is

not

pos

sibl

e to

se

para

te

past

ch

ange

s in

cl

imat

e an

d at

mos

pher

ic c

ompo

sitio

n in

to c

ause

ver

sus

effe

ct.

Atm

osph

eric

ca

rbon

di

oxid

e co

ncen

tratio

ns

are

very

se

nsiti

ve

to

chan

ges

in o

ther

com

pone

nts

of th

e gl

obal

carb

on

cycl

e,

part

icul

arly

th

e ca

rbon

co

ntai

ned

in th

e oc

eans

, pl

ants

, an

d so

ils.

Alth

ough

it

is t

empt

ing

to a

ttrib

ute

past

cl

imat

e ch

ange

s to

th

e ch

angi

ng

gree

nhou

se

effe

ct,

man

y hy

poth

esiz

ed

expl

anat

ions

fo

r pa

st

carb

on

diox

ide

chan

ges

invo

ke c

limat

e ch

ange

as

a ca

use

of c

arbo

n-cy

cle

chan

ge.

Like

the

clim

ate

syst

em,

the

glob

al c

arbo

n cy

cle

cont

ains

man

y co

mpl

ex f

eedb

acks

whi

ch

are

all a

ffect

ed b

y a

pertu

rbat

ion

anyw

here

in

the

syst

em.

Q?

280

260

240

* 22

°CO tr CM

O

O

200

180

Car

bon

diox

ide \

160

140

120

100

80

60

40

20

0

Year

s be

fore

pre

sent

(th

ousa

nds)

Fig

ure

9.

Long

-term

var

iatio

ns in

car

bon

diox

ide

and

deut

eriu

m a

s de

term

ined

from

the

Vos

tok

ice

core

. (S

ourc

e: B

arno

la a

nd o

ther

s, 1

987)

.

13

Historical

Changes

in A

tmospheric

Carbon

Dioxide

Analyses

of ice

cores from

G

reenland and Antarctica show

that the atm

ospheric CO

2 concentration remained

near 280 parts per million for nearly all of

the 10,000 years since the end of the last ice age.

Then, in the

late eighteenth

century, atm

ospheric C

O2

began to

increase. The increase, determ

ined by m

easuring ice

core air

bubbles and,

beginning in

the early

1950's, by

con

tinu

ou

s d

irect

atm

osp

he

ric m

easurements, appears in figure 10.

The historic

rate of

increase has

been exponential,

rising to

a present-day

atmospheric C

Og concentration of about

350 parts per million.

The source of this carbon dioxide increase is predom

inantly the burning of fossil fuels, w

ith smaller contributions from

changes

in land

use (for

example,

deforestation) and

the production

of cem

ent from lim

estone. In fact, the total

amount of C

O2 released by these activities

during the last 200 years is about twice the

amount

of carbon

dioxide that

has rem

ained as added atmospheric CO

a- The rem

ainder of the anthropogenic carbon dioxide

has been

removed

from

the atm

osphere by carbon-cycle exchange, prim

arily with the oceans.

There is no doubt that the atm

ospheric CO

2 Increase show

n here

is the

result of

human

activities, but there is significant uncertainty about the response of the global carbon cycle to this perturbation in one of its key com

ponents.

o- u><ntn

Z

O

-

UJ

_

oen

O

_1720 1760

1800 1840

1880

Y E

fl

R

19201960

2000

Figure 10. Variations in the concentration of atmospheric C

O2 as determ

ined from ice cores in

Greenland and Antarctica. (Source: Siegenthaler and O

eschger, 1987).

14

His

toric

al

Cha

nges

in

A

tmos

pher

ic

Met

hane

The

hist

oric

al

incr

ease

in

at

mos

pher

ic

met

hane

, de

rived

fro

m

anal

yses

of i

ce c

ore

air b

ubbl

es a

nd, f

or th

e la

st

deca

de,

from

di

rect

at

mos

pher

ic

mea

sure

men

ts,

appe

ars

in

figur

e 11

. A

lthou

gh t

he r

elat

ive

incr

ease

in m

etha

ne

(from

800

to

1600

par

ts p

er b

illion

) ha

s be

en

grea

ter

than

th

at

for

CO

a,

the

sour

ces

of th

e m

etha

ne in

crea

se a

re m

uch

less

un

ders

tood

. Th

e tim

ing

and

mag

nitu

de o

f the

cha

nge

stro

ngly

indi

cate

hu

man

influ

ence

. Th

e m

ost

likel

y so

urce

s se

em

to

be

rice

culti

vatio

n,

ente

ric

ferm

enta

tion

in a

nim

als,

bio

mas

s bu

rnin

g,

and

foss

il fu

els.

U

nlik

e ca

rbon

dio

xide

, m

etha

ne is

act

ivel

y co

nsum

ed b

y ch

emic

al

reac

tions

in

the

atm

osph

ere.

H

owev

er,

beca

use

met

hane

is

no

t as

re

adily

ab

sorb

ed b

y pl

ants

and

the

oce

ans,

its

av

erag

e lif

etim

e in

the

atm

osph

ere

may

be

grea

ter

than

th

at

of

carb

on

diox

ide.

P

redi

ctin

g fu

ture

co

ncen

trat

ions

of

m

etha

ne is

ver

y di

fficu

lt be

caus

e th

ere

are

so

man

y un

certa

intie

s ab

out

both

its

so

urce

s an

d si

nks.

Figu

re 1

1.

Atm

osph

eric

met

hane

var

iatio

ns

over

the

past

few

cen

turie

s. (

Sour

ce:

Pear

man

and

Fra

ser,

1988

).

^ 1

,600

ri ci d. c o ^3 1

,200

* c o c o 0 ^ g 80

0

i i

i i

r I -

-

i

T

1 t* §

1

i i

^ *

*T

T16

00

1700

1800

1900

Yea

r

15

Historical

Global

Warm

ing

Histo

rical

mean

glo

bal

temperatures can be estim

ated using data from

meteorological stations throughout

the world. These stations are not uniform

ly distributed, how

ever, and spatial averaging assum

ptions must, therefore, be applied to

the stations that are available.

Other

corrections m

ust be

used for

stations w

here m

easurement

techniques or

locations have changed during the period of record.

Nevertheless, the data show

a statistically significant global w

arming trend.

A graph of norm

alized mean global

temperatures from

1880 through

1987, com

piled by Hansen and Lebedeff (1988),

appears as figure 12. The overall w

arming

apparent in this record is close to 0.8

degrees Celsius (1.4 degrees Fahrenheit).

Hansen and Lebedeff estim

ate that 0.1 to 0.2 degrees of this trend can be attributed to the heat released locally in urban areas, w

here many m

eteorological stations are located.

Their estimate of the overall

warm

ing during the last century, including uncertainties, is 0.4 to 0.8 degrees C

elsius (0.7 to

1.4 degrees Fahrenheit).

This estim

ate is supported independently by other com

pilations (Jones

and others,

1986), including analyses of sea surface tem

perature data. The average global

warm

ing over the last century is roughly consistent w

ith the magnitude of w

arming

expected from the increase in greenhouse

gases during the same period.

Oo

Annual Mean

5 Year Mean

1880*I9

00

I9

20

I9

40

DateI960

I960

Figure 12. N

ormalized m

ean global temperatures, 1880-1987.

(Source:

Hansen and Lebedeff, 1988).

16

Glo

bal

War

min

g in

the

19

80's

Glo

bal

mea

n te

mpe

ratu

res

for

the

year

s 19

80,

1981

, 19

83,

and

1987

wer

e th

e hi

ghes

t in

the

his

toric

al r

ecor

d.

The

high

glo

bal

mea

n te

mpe

ratu

res

in 1

980,

19

83,

and

1987

wer

e as

soci

ated

w

ith

pron

ounc

ed w

arm

ing

at l

ow l

atitu

des.

In

pa

rticu

lar,

the

tem

pera

ture

pea

ks o

f 19

83

and

1987

wer

e co

ntem

pora

neou

s w

ith "

El

Nin

o" c

ondi

tions

at l

ow la

titud

es (

figur

e 13

). H

isto

rical

ly,

such

eve

nts

have

ofte

n be

en

follo

wed

with

in a

few

yea

rs b

y to

w la

titud

e co

olin

g ("

La

Nin

a"

cond

ition

s)

and

a co

rres

pond

ing

drop

in

m

ean

glob

al

tem

pera

ture

s.

In

cont

rast

to

th

e ob

serv

atio

n th

at g

loba

l w

arm

ing

in t

he

1980

's h

as b

een

asso

ciat

ed p

rimar

ily w

ith

low

-latit

ude

war

min

g,

clim

ate

mod

el

sim

ulat

ions

of t

he e

ffect

s of

a g

reen

hous

e w

arm

ing

indi

cate

tha

t th

e w

arm

ing

shou

ld

be m

ost p

rono

unce

d at

hig

h la

titud

es.

The

glob

al t

empe

ratu

re d

ata

for

early

198

8 sh

ow c

ontin

ued

war

min

g.

It is

cl

ear

that

the

19

80's

ar

e th

e w

arm

est

perio

d fo

r whi

ch g

loba

l mea

n te

mpe

ratu

res

can

be

relia

bly

estim

ated

fr

om

met

eoro

logi

cal

mea

sure

men

ts (

see

figur

e 12

). D

oes

this

con

firm

the

pres

ence

of t

he

antic

ipat

ed

CO

a-in

duce

d gr

eenh

ouse

w

arm

ing?

A

lthou

gh a

few

sci

entis

ts h

ave

argu

ed y

es, m

ost b

elie

ve th

at it

is to

o ea

rly

to

attri

bute

the

glo

bal

war

min

g to

the

gr

eenh

ouse

effe

ct.

Mos

t sc

ient

ists

are

re

luct

ant t

o em

phas

ize

the

war

min

g of

the

19

80's

mor

e th

an t

he o

vera

ll ce

ntur

y-lo

ng

war

min

g tre

nd.

For e

xam

ple,

the

war

min

g of

the

late

193

0's

was

the

war

mes

t pe

riod

in th

e re

cord

to th

at ti

me,

yet

it w

as fo

llow

ed

by th

ree

deca

des

of c

oole

r tem

pera

ture

s.

Mos

t sc

ient

ists

agr

ee t

hat,

alth

ough

the

hi

stor

ical

gl

obal

w

arm

ing

is

caus

e fo

r se

rious

con

cern

, w

e ca

nnot

yet

det

ect

a

clim

atic

cha

nge

caus

ed b

y th

e gr

eenh

ouse

ef

fect

bec

ause

it m

ay b

e ob

scur

ed b

y ot

her

caus

es o

f glo

bal t

empe

ratu

re v

aria

tion.

B

Low

Latit

udes

(2

3.6'

N-2

3.6'

S)

i i

Low

Latit

udes

(2

3.6*

N-2

3.6*

S)

1.1

I9M

1970

19

75Da

te19

78

I960

1982

19

84

Dote

1986

1987

-0.2

Figu

re 1

3. G

loba

l mea

n su

rface

air

tem

pera

ture

. A,

Ann

ual v

aria

tions

, 19

58-1

987.

B,

Mon

thly

var

iatio

ns,

1978

-198

7. T

he le

tters

E a

nd V

mar

k th

e ye

ars

of m

ajor

El N

ino

and

volc

anic

eru

ptio

ns,

resp

ectiv

ely.

(S

ourc

e:

Han

sen

and

Lebe

deff,

198

8)

17

Pre

dictin

g

the G

reenhouse E

ffectP

redicting future climates is one of

the m

ost challenging

and im

portant scientific problem

s of our time.

Predicting

the climatic effects of trace gases requires

the most sophisticated clim

ate models and

the m

ost pow

erful supercom

puters. C

hanges in mean global tem

perature and precipitation, as com

puted by five of the m

ost widely

cited atm

ospheric general

circulation models (G

CM

's), are given in Table 1.

These changes were com

puted for equilibrium

climate conditions after a

doubling of

atmospheric CC>2, w

hich is expected to occur som

etime during the

next century. M

ean global temperatures

are expected to increase by about 2 to 5 degrees

Celsius,

while

mean

global precipitation is predicted to rise by about 7 to

15 percent. The global precipitation

increase is

accompanied

by a

corresponding increase in the m

odeled global

evaporation. The

effects of

increasing concentrations of trace gases other than CC>2 are expected to accelerate and add to the effects m

odeled here.The significance of these predicted

climate

effects can

be gaged

by com

parison to

the estim

ated 5-degree

Celsius m

ean global temperature change

associated with the end of the last ice age.

Unfortunately,

there is

no consistency

among

greenhouse predictions

beyond the

global equilibrium

m

ean effects

represented by the results shown in the

table. There are still too many uncertainties

in climate

models to realistically predict

regional effects, or to project changes that w

ill occur during the time of transition to a

new equilibrium

climate.

Model/S

tudyC

hange inTem

perature P

recipitation (°C

)_____(P

erce

nt)

Geophysical Fluid D

ynamics

Laboratory / Wetherald and

Manabe (1988)

Goddard Institute for S

pace S

tudies / Hansen and others

(1984)

National C

enter for Atm

ospheric R

esearch / Washington and

Meehl (1984)

Oregon S

tate University /

Schlesinger and Zhao (1988)

United K

ingdom M

eteorological O

ffice / Wilson and M

itchell (1987)

4.0

4.2

3.5

2.8

5.2

8.7

11.0

7.1

7.8

15.0

Tabto 1. Changes in the global m

ean surface air temperature (Ts

) and precipitation rate (P) simulated

by various atmospheric general circulation/m

ixed-layer ocean models for a C

O2 doubling.

(Source:

Schlesinger, 1988).

18

Maj

or

Unc

erta

intie

s in

G

loba

l C

limat

e M

odel

P

roje

ctio

ns

Man

y fu

ndam

enta

l un

certa

intie

s ar

e w

idel

y ac

know

ledg

ed t

hrou

ghou

t th

e cl

imat

e an

d tr

ace

gas

rese

arch

co

mm

uniti

es.

Thes

e in

clud

e th

e fo

llow

ing

prob

lem

s:

o P

redi

ctin

g fu

ture

gr

eenh

ouse

ga

s an

d ae

roso

l em

issi

ons

requ

ires

a va

st a

rray

of

assu

mpt

ions

abo

ut f

utur

e te

ch-n

olog

ies,

ec

onom

ic

and

soci

al

deve

lopm

ents

, in

tern

atio

nal

rela

tions

hips

, an

d re

sour

ce n

eeds

and

ava

ilabi

litie

s.

o A

th

orou

gh

know

ledg

e of

th

e so

urce

s an

d si

nks

of a

tmos

pher

ic t

race

ga

ses

requ

ires

unde

rsta

ndin

g th

e gl

obal

b

log

eoch

emic

al

cycl

es

of

carb

on,

nitro

gen,

sul

fur,

and

rela

ted

elem

ents

.

o C

limat

e w

arm

ing

will

be

dela

yed

by

ocea

n he

at

exch

ange

, w

hich

w

ill

abso

rb s

ome

of t

he h

eat

reta

ined

in

the

atm

osph

ere.

U

nder

stan

ding

thi

s pr

oces

s w

ill

requ

ire

coup

ling

non-

equi

libriu

m

mod

els

of

atm

osph

eric

an

d oc

eani

c ci

rcul

atio

n.

o M

any

clim

ate

mod

el u

ncer

tain

ties

are

attri

bute

d to

the

eff

ects

of

clou

ds,

whi

ch c

an b

oth

enha

nce

and

atte

nuat

e w

arm

ing

at t

he e

arth

sur

face

, de

pend

ing

on t

heir

inte

ract

ions

with

oth

er fe

edba

cks

in th

e cl

imat

e sy

stem

.

o Th

e h

ydro

log

lc

cycl

e is

the

m

ost

impo

rtant

an

d po

orly

un

ders

tood

co

mpo

nent

of e

nerg

y an

d w

ater

exc

hang

e at

the

land

sur

face

.

o C

limat

e m

odel

ers

are

incr

easi

ngly

em

phat

ic t

hat

thei

r w

ork

cann

ot y

et b

e us

ed t

o pr

edic

t an

ythi

ng o

n a

regi

onal

sc

ale.

19

Major

Un

certa

intie

s in

Regional

Clim

ate P

redictio

ns

The follow

ing statem

ent is

representative of the consensus among

climate m

odelers concerning predictions of regional

effects such

as droughts:

"Although

the results

of the

general circulation m

odels often agree well w

ith each other and w

ith historical surface air tem

perature data

over large

scales (global/hem

ispheric/zonal), ... [they are]

simply

not yet

ready to

be used

for quantitative

prediction at

anything approaching even a m

ulti-state region, let alone

a single

surrogate gridpoint

representing a particular state, county or city.

Over such sm

all scales, a wide range

of responses

is currently

predicted" (G

rotch, 1988).

The model results show

n in figure 14

exemplify

the differences

among

climate

model

simulations

of regional

effects. These plots show

soil moisture

simulated for three regions by tw

o different general circulation m

odels developed by the

National

Center

for A

tmospheric

Research (N

CA

R)

and the Geophysical

Fluid D

ynamics

Laboratory (G

FDL).

Although these m

odel simulations are not

strictly com

parable, they

illustrate the

inconsistencies among

regional climate

predictions. These

differences in soil

moisture projections reflect the

models'

disagreement on

regional land

surface w

ater balances

under greenhouse

conditions. The solid curves approxim

ate seasonal soil m

oisture trends for present

atmospheric

CO

2 levels, w

hereas the dashed

curves represent

equilibrium

seasonal trends for increased CC>2 levels (doubled in the N

CAR

model; quadrupled

in the GFD

L model).

For the regions shown, there are

obvious differences between the m

odels in their sim

ulations of both present-day and increased-C

C>2 conditions.

For the Great

Plains,

the G

FDL

model

predicts decreased soil m

oisture with increasing

CO2- The N

CAR

model, how

ever, predicts increased

or nearly

unchanged soil

moisture for increased C

O2 in central N

orth Am

erica. Clearly, it is prem

ature to attempt

to link regional drought to the greenhouse effect.

20

NC

AR

- Northern C

anada

lxC

02

z

JF

MA

MJJA

SO

NO

J

1xCO2, ^~

4xCO

2

GFD

L - Northern C

anada

J»

*»

«J

J»

tO

HO

j

NC

AR

- Central N

orth Am

erica

IxCO

1412

<jp

o

UJ

8

S= 4

8

NC

AR - U

kraine2xC0

2

JFMAMJJASONDJ

2 JFMAMJJASONOJ

4xCO

2

GFD

L - Great P

lains

(T "°'

2 11.0

to

GFD

L - Southern E

urope

4xC

O2

JtMAMJJASONDj

Fig

ure 14.

Annual cycle of soil m

oisture amounts (cm

) averaged over several regions, as calculated by the National C

enter for Atm

ospheric Research

(NC

AR

) and Geophysical F

luid Dynam

ics Laboratory (GF

DL) general circulation m

odels. (S

ource: M

eehl and Washington, 1988).

The

Gre

enho

use

Eff

ect

in

a G

eolo

gica

l C

onte

xtG

eolo

gist

s ha

ve l

ong

hypo

thes

ized

tha

t ch

ange

s in

the

gre

enho

use

effe

ct m

ay h

ave

cont

ribut

ed t

o cl

imat

e ch

ange

in

the

geol

ogic

pa

st.

This

pos

sibi

lity

is s

trong

ly s

uppo

rted

by

the

reco

rd o

f at

mos

pher

ic c

limat

e an

d ca

rbon

di

oxid

e an

d m

etha

ne c

hang

es f

rom

ice

cor

es.

Thus

, th

e ge

olog

ic r

ecor

d of

fers

the

pos

sibi

lity

of te

stin

g cl

imat

e pr

edic

tions

usi

ng d

ata

from

rea

l pa

st

chan

ges

in t

he g

reen

hous

e ef

fect

an

d ot

her c

limat

e pa

ram

eter

s.O

n th

e ba

sis

of t

he g

eolo

gica

l re

cord

of

clim

ate

and

atm

osph

eric

cha

nge,

it a

ppea

rs th

at

the

pres

ent-d

ay a

tmos

pher

ic C

Q2

conc

entra

tion

is h

ighe

r, as

a r

esul

t of

hum

an a

ctiv

ities

, tha

n at

an

y tim

e du

ring

the

last

se

vera

l hu

ndre

d-

thou

sand

yea

rs.

Hyp

othe

tical

pas

t an

d fu

ture

at

mos

pher

ic C

O2

leve

ls a

re c

ompa

red

in f

igur

e 15

. Th

e di

agra

m's

tim

e sc

ale

disc

ontin

uity

(lo

garit

hmic

for

the

left-

hand

sec

tion

and

linea

r fo

r th

e rig

ht-h

and

side

) em

phas

izes

th

e ex

trem

ely

rapi

d ra

te

at

whi

ch

futu

re

CO

2 co

ncen

tratio

ns m

ay r

ise.

Mos

t ge

olog

ists

be

lieve

tha

t, to

fin

d ge

olog

ic a

nalo

gs o

f po

ssib

le fu

ture

atm

osph

eric

CC

>2 l

evel

s, i

t is

nec

essa

ry t

o go

bac

k at

lea

st

seve

ral m

illio

n ye

ars

in th

e ge

olog

ic r

ecor

d.

The

evid

ence

fo

r hi

gher

at

mos

pher

ic

C02

co

ncen

tratio

ns i

n th

e di

stan

t ge

olog

ic p

ast

is

indi

rect

, re

quiri

ng in

fere

nces

from

sed

imen

t dat

a an

d ge

oche

mic

al

mod

els.

To

re

late

th

is

evid

ence

to

pres

ent

and

futu

re c

hang

es i

n th

e gr

eenh

ouse

effe

ct,

we

will

nee

d to

acq

uire

a

muc

h br

oade

r un

ders

tand

ing

of b

oth

the

long

- te

rm p

roce

sses

tha

t af

fect

ed g

eolo

gic

carb

on-

cycl

e an

d cl

imat

e ch

ange

, an

d th

e sh

ort-t

erm

pr

oces

ses

that

will

dom

inat

e tre

nds

durin

g th

e co

min

g de

cade

s an

d ce

ntur

ies.

5000

-

600

-

300

10

0

200

300

NO

W

Fig

ure

15.

Ove

rvie

w o

f the

his

tory

of a

tmos

pher

ic C

O2

from

10®

yea

rs a

go to

the

pres

ent d

ay

on a

log-

log

scal

e. F

or c

ompa

rison

, the

pos

sibl

e fu

ture

06

2 le

vels

pro

ject

ed fo

r the

nex

t fe

w c

entu

ries

are

show

n on

a li

near

tim

e sc

ale.

(So

urce

: G

amm

on a

nd o

ther

s, 1

985)

.

21

CL

IMA

TE

C

HA

NG

E

IN

TH

E

GE

OL

OG

IC

PA

ST

by

Thomas A. A

gerP

erspectives on

Long-Term

C

hanges in

Clim

ate

Much of our understanding of the

workings of the global clim

ate system is

based upon

the in

strum

en

tal

meteorological

observations that

have been recorded system

atically around the w

orld during the past century. Our concept

of w

hat constitutes

"normal"

climate,

therefore, is heavily influenced by average global clim

atic conditions covering only a short span of tim

e. By exam

ining the geologic record w

e can begin to see global clim

ates from

a

much

longer tim

e perspective.

We can look back far beyond

the past century to see how variable clim

ate has been during the past 1,000, 10,000, 100,000, or even m

illions of years.E

vidence of

past clim

atic conditions is preserved in the geologic record in m

arine and lake sediments, in

glacier ice, in peat and coal deposits, in cave

deposits, coral

reefs, and

other stratified deposits.

Innovative methods

have been

developed to

extract inform

ation about

past clim

ates from

geologic evidence and to date these past

events by application of several isotopic techniques and other dating m

ethods. It is,

therefore, possible

to reconstruct

the broad

outline of global

climate

history spanning m

any millions of years.

More

detailed climate reconstructions over tim

e scales of centuries to tens of thousands of years are now

available from m

any parts of the

world.

By exam

ining changes

in assem

blages of

climate-sensitive

organisms represented by fossil rem

ains over tim

e, it is possible to reconstruct the consequences of past clim

ate changes upon

ecosystems

in both

marine

and terrestrial environm

ents. S

tudies of air bubbles and dust preserved in glacier ice provide

insights into

the changing

composition of the atm

osphere through tim

e during the past 150,000 years. Ice

core studies also allow us to docum

ent periods of tim

e when volcanic eruptions

were m

ore frequent, when w

inds carried greater

amounts

of dust

from

the continents to distant ice sheets and ice caps.

The geologic record of past climate

change provides insights into the causes and m

echanisms of clim

ate change, the rates at w

hich some past clim

ate changes occurred,

and preserves

important

information about

the effects of

those changes on ecosystem

s, landscapes, and the oceans.

The geologic record tells us that there is no single cause of clim

ate change.

Many factors influence global

climate. The geologic record indicates that

climate varies on all tim

e scales due to natural causes.

Those causes are not all w

ell understood,

or perhaps

not all

influences on climate are yet recognized.

The causes and predictability of climate

variation on

time

scales of

greatest relevance to hum

an populations (decades to centuries) are not w

ell understood. To

illustrate how m

uch climate has varied in the

past, global climate trends on several tim

e scales w

ill be briefly examined.

22

Clim

ate

Cha

nge

Dur

ing

the

Pas

t 13

0 M

illio

n Y

ears

A r

econ

stru

cted

tem

pera

ture

cur

ve,

span

ning

the

past

130

milli

on y

ears

, app

ears

in

figur

e 16

. Th

e cu

rve

reco

nstru

cts

chan

ges

in

the

tem

pera

ture

of d

eep

ocea

n w

ater

s at

low

la

titud

es o

ver t

ime.

It

is, t

here

fore

, col

der t

han

mea

n gl

obal

air

tem

pera

ture

s. N

ever

thel

ess,

it

is u

sefu

l fo

r ill

ustra

ting

the

prob

able

glo

bal

tem

pera

ture

tre

nds

over

tha

t ve

ry l

ong

time

inte

rval

. Th

e cu

rve

show

s th

at t

he d

omin

ant

trend

ove

r the

pas

t 130

milli

on y

ears

has

bee

n to

war

d co

oler

cl

imatic

co

nditi

ons.

S

uper

impo

sed

upon

thi

s lo

ng-te

rm t

rend

are

si

gnifi

cant

tem

pera

ture

osc

illat

ions

that

last

ed

up t

o se

vera

l m

illio

n ye

ars.

Th

ese

perio

dic

war

min

g an

d co

olin

g ev

ents

had

sig

nific

ant

impa

cts

upon

glo

bal

ecos

yste

ms

both

in

the

mar

ine

and

terre

stria

l rea

lms.

Cau

ses

of th

ese

long

-term

and

inte

rmed

iate

-term

clim

ate

trend

s ar

e no

t w

ell

unde

rsto

od.

On

time

scal

es o

f te

ns

of

mill

ions

of

ye

ars,

ho

wev

er,

plat

e te

cton

ics

influ

ence

d gl

obal

clim

ate

by s

hifti

ng

the

posi

tions

of

cont

inen

ts,

caus

ing

inte

rval

s of

mou

ntai

n bu

ildin

g an

d vo

lcan

ic a

ctiv

ity, a

nd

influ

enci

ng t

he d

istri

butio

n an

d de

pths

of

ocea

ns

and

seas

. A

ll of

th

ese

fact

ors

influ

ence

glo

bal c

limat

e.

The

grad

ual s

hift

of

cont

inen

tal

land

mas

ses

into

pol

ar l

atitu

des,

an

d th

e op

enin

g of

the

Atla

ntic

Oce

an w

ere

impo

rtant

fac

tors

con

tribu

ting

to

long

-term

co

olin

g tre

nds

durin

g th

e pa

st

100

mill

ion

year

s. I

nten

se v

olca

nism

can

cau

se s

hort-

term

co

olin

g of

clim

ate

by in

ject

ing

ash

parti

cles

into

th

e st

rato

sphe

re,

whe

re

the

ash

refle

cts

inco

min

g ra

diat

ion

into

spa

ce.

Pro

long

ed

volc

anis

m m

ay c

ause

lo

nger

-term

clim

ate

chan

ge b

y al

terin

g th

e at

mos

pher

e's

carb

on

diox

ide

cont

ent.

Hig

h m

ount

ain

rang

es

influ

ence

atm

osph

eric

circ

ulat

ion

patte

rns

and

serv

e as

acc

umul

atio

n ce

nter

s fo

r ic

e an

d sn

ow.

O

30

o: UJ o 2

0U

J a.

o 1

0a.

O

1

T

I I

CO

MP

OS

ITE

TE

MP

ER

AT

UR

E

CU

RV

E F

OR

LO

W L

AT

ITU

DE

S

140

120

100

80

60

40

MIL

LIO

NS

OF

YE

AR

S A

GO

20

0

Fig

urt

16.

Dee

p se

a w

ater

tem

pera

ture

his

tory

for t

he p

ast

130

mill

ion

year

s.

(Sou

rce:

D

ougl

as a

nd W

oodr

uff,

1981

).

23

Milankovitch

Forcing

of Clim

ate Change

About 2.5 m

illion years ago the clim

ate regime of the Earth changed into an

unusual m

ode of

high am

plitude oscillations betw

een long intervals of cool or cold clim

ate interrupted by generally shorter intervals of w

armer clim

ate. The

reasons for this shift are not completely

clear. The em

ergence of the Isthmus of

Panam

a and the opening of the Bering

Strait about 3 m

illion years ago altered oceanic circulation

patterns, and these events

may

have contributed

to the

change in global climate pattens.

One of the causes of the m

ajor clim

atic oscillations during the past million

years, how

ever, has

been identified.

During the 1920's and 1930's a theory w

as developed by a Y

ugoslavian engineer and m

athematician,

Milutin

Milankovitch.

Xccording to his theory, the sequence of ice ages periodically interrupted by w

armer

interglacials and interstadials of the past m

illion years could be largely explained by celestial m

echanics. By calculating the

changes in the Earth's orbital param

eters through tim

e - axial precession, axial tilt,

and orbital eccentricity

-- M

ilankovitch show

ed that

different latitudes

would

receive differing amounts of solar energy

over time scales of tens of thousands to

hundreds of thousands of years. The

theory could not be tested adequately until the past tw

o decades or so, when detailed

studies of marine sedim

ent cores from all

around the

world

showed

strong agreem

ent between the frequencies of

past environm

ental changes

and M

ilankovitch's calculated sequence of solar insolation changes.

The chronology of

24

past ice ages and warm

interglacials fit the predictions of the theory quite w

ell. The

Milankovitch theory does not explain all the

observed variation in global climates of the

past million years, but it does show

that orbital geom

etry has been the dominant

influence on

time

scales of

tens of

thousands of years or more.

Projecting the orbital param

eters into the future indicates that if hum

an influence on the greenhouse effect w

ere not of imm

ediate concern, we

might instead be concerned about the

gradual onset of a new glacial interval.

It w

ould probably require 10,000 years to

attain full-glacial

conditions, how

ever. Even the early stages of a global cooling could

have serious

consequences for

human populations.

Oxygen

isotope variations

measured

from

fossil foram

inifera preserved in low

latitude marine sedim

ent cores during the past 750,000 years are plotted in figure 17.

The isotopic variations w

ere initially believed to reflect sea surface tem

perature changes through time.

Later w

ork indicated that changes in global ice volum

e also contribute to

variations in isotopic com

position of foraminifera.

A

temperature

scale added to the curve

(figure 17) shows that the global m

ean annual tem

perature (MAT) has varied about

1°C (1.8 degrees F) above current M

AT

TOO (G

lacial CN

mrtM

)

and as much as 5°C

(9 degrees F) below

current M

AT

during the

past 750,000

years. The peaks in the curve that intersect the dashed line indicate past intervals of w

arm interglacial clim

ates. The current

interglacial has lasted for about 10,000 years, and the record indicates that past interglacials lasted about 8,000 to 12,000 years.

Such w

arm intervals have been

relatively rare during the past 2.5 million

years and intervals warm

er than today are even rarer over that tim

e. O

nly a few

percent of that time has been characterized

by global temperatures com

parable to that of the present tim

e. Som

e climate m

odels predict

that global

mean

annual tem

peratures m

ay rise 2 to 5 degrees

Celsius (3.6 to 9.0 degrees F) above the

present mean tem

perature as a result of hum

an influence

on the

greenhouse effect.

If that prediction

proves to be

correct, such a global warm

ing would have

no precedent in the Earth's climatic history

during the past 2.5 million years or m

ore. M

any of the organisms now

living on earth evolved during the past several m

illion years of generally cooler clim

ates. It may be

difficult, therefore, to predict how these

species, and the ecosystems that they are

a part of, would react to a rapid global

warm

ing of such magnitude.

Figure 17. C

omposite oxygen

isotope curve for the past 750,000 years of Earth history. (S

ource: Em

iliani, 1978).

V*MT» A

go x 1000

The

Last

1,

000

Yea

rsEv

en d

urin

g th

e pa

st 1

0,00

0 ye

ars

of

a w

arm

in

terg

laci

al

clim

ate

(the

Hol

ocen

e),

the

time

span

dur

ing

whi

ch

hum

an c

ivili

zatio

n de

velo

ped,

the

re h

ave

been

os

cilla

tions

of

gl

obal

cl

imat

e of

su

ffici

ent

mag

nitu

de

to

sign

ifica

ntly

in

fluen

ce h

uman

his

tory

. G

loba

l m

ean

tem

pera

ture

s os

cilla

ted

abou

t 1.

0 de

gree

C

elsi

us o

r m

ore

abov

e an

d be

low

the

pr

esen

t m

ean

valu

e du

ring

the

Hol

ocen

e.

Clim

ate

chan

ge

of

that

m

agni

tude

is

su

ffici

ent

to c

ause

gla

cier

adv

ance

s an

d re

treat

s in

alp

ine

and

pola

r re

gion

s, a

nd to

si

gnifi

cant

ly i

mpa

ct h

uman

pop

ulat

ions

in

man

y pa

rts o

f the

wor

ld.

Cha

nges

in c

rop

yiel

ds,

the

type

s of

cro

ps t

hat

coul

d be

ra

ised

, an

d th

e fre

quen

cy o

f cr

op f

ailu

res

wer

e so

me

of

the

man

ifest

atio

ns

of

Hol

ocen

e cl

imat

e flu

ctua

tions

. Th

e N

orse

co

loni

zatio

n of

Gre

enla

nd o

ccur

red

durin

g a

war

m in

terv

al t

hat

perm

itted

set

tlers

to

grow

oat

s,

rye,

an

d ba

rley.

D

eclin

ing

tem

pera

ture

s be

twee

n ab

out

1250

A.D

. an

d 14

50 A

.D.

led

to th

e di

sapp

eara

nce

of

the

Gre

enla

nd

colo

nies

as

ag

ricul

ture

be

cam

e pr

ecar

ious

and

sea

ice

drif

ted

farth

er a

nd fa

rther

sou

th.,

inhi

bitin

g co

ntac

t w

ith E

urop

e.

Col

d te

mpe

ratu

res

pers

iste

d in

Eur

ope

betw

een

abou

t 14

30 A

.D.

and

1850

A.D

. Th

is c

old

inte

rval

is c

alle

d th

e "L

ittle

Ice

Age

." Th

e ha

rdsh

ips

caus

ed b

y th

is l

ong

inte

rval

of

seve

re w

inte

rs a

nd

mor

e fre

quen

t cro

p fa

ilure

s is

pre

serv

ed in

th

e ar

t, lit

erat

ure,

and

com

mer

cial

rec

ords

of

the

tim

e.

Gla

cier

s ad

vanc

ed i

n th

e m

ount

ains

of E

urop

e an

d A

lask

a du

ring

the

Littl

e Ic

e Ag

e.

Yet

the

mag

nitu

de o

f th

e gl

obal

te

mpe

ratu

re

chan

ge

(abo

ut

1 de

gree

C) w

as s

mal

l in

com

paris

on w

ith th

e

tem

pera

ture

dec

line

durin

g th

e co

ldes

t par

t of

the

maj

or g

laci

al in

terv

als

durin

g th

e pa

st

1 m

illio

n ye

ars

(abo

ut 5

deg

rees

C c

olde

r th

an th

e pr

esen

t glo

bal m

ean)

.Th

ere

is c

urre

ntly

a c

ontro

vers

y ab

out

the

sign

ifica

nce

of

an

appa

rent

in

crea

se o

f ab

out

0.6

degr

ee C

in

the

glob

al m

ean

annu

al te

mpe

ratu

re d

urin

g th

e pa

st c

entu

ry.

One

par

t of t

he c

ontro

vers

y re

late

s to

whe

ther

the

met

eoro

logi

cal d

ata

are

suffi

cien

tly r

elia

ble

to d

etec

t a

chan

ge

of th

at m

agni

tude

ave

rage

d gl

obal

ly.

The

othe

r pa

rt of

the

cont

rove

rsy

is w

heth

er o

r no

t th

e te

mpe

ratu

re i

ncre

ase,

if

it is

rea

l, re

pres

ents

nat

ural

var

iatio

n in

clim

ate

or

whe

ther

it is

an

early

man

ifest

atio

n of

the

chan

ging

gr

eenh

ouse

ef

fect

fr

om

incr

ease

s in

car

bon

diox

ide

and

othe

r tra

ce

gase

s in

the

atm

osph

ere

durin

g th

e pa

st

cent

ury

of i

nten

se i

ndus

trial

dev

elop

men

t an

d po

pula

tion

grow

th.

From

the

geol

ogic

pe

rspe

ctiv

e, th

e ap

pare

nt te

mpe

ratu

re r

ise

of th

e pa

st c

entu

ry m

ay s

impl

y re

pres

ent a

na

tura

l w

arm

ing

trend

tha

t is

par

t of

the

os

cilla

tion

tow

ard

war

mer

co

nditi

ons

follo

win

g th

e en

d of

the

Litt

le I

ce A

geen

£

*

I I

i

»» in 1

(figu

re 1

8).

The

pote

ntia

l for

hum

an in

fluen

ce

on c

limat

e ap

pear

s to

be

a va

lid c

once

rn fo

r th

e w

orld

's h

uman

pop

ulat

ions

. Th

e gl

obal

cl

imat

e sy

stem

is

co

mpl

ex

and

poor

ly

unde

rsto

od

and,

th

eref

ore,

it

is

very

di

fficu

lt to

pre

dict

its

fut

ure

resp

onse

to

hum

an in

fluen

ces

such

as

chan

ges

in th

e co

mpo

sitio

n of

th

e at

mos

pher

e.

It is

im

porta

nt t

o ke

ep i

n m

ind,

how

ever

, th

at

ther

e is

a la

rge

amou

nt o

f nat

ural

var

iabi

lity

in t

he c

limat

e sy

stem

. It

is,

ther

efor

e,

prem

atur

e to

attr

ibut

e in

divi

dual

"un

usua

l" cl

imat

e ev

ents

suc

h as

the

drou

ght o

f 198

8 in

the

Uni

ted

Sta

tes

or th

e su

cces

sion

of

"war

mer

-than

-nor

mal

" ye

ars

durin

g th

e 19

80's

to th

e an

thro

poge

nic

enha

ncem

ent

of

the

gree

nhou

se

effe

ct

at t

his

time.

H

isto

rical

dat

a pr

ovid

e a

brie

f glim

pse

of th

is

natu

ral v

aria

bilit

y, a

nd th

e ge

olog

ic r

ecor

d do

cum

ents

sig

nific

ant

natu

ral

varia

bilit

y ov

er a

bro

ad s

pect

rum

of

time

scal

es.

Ant

icip

atin

g fu

ture

cl

imat

e ch

ange

will

re

quire

und

erst

andi

ng n

ot o

nly

the

futu

re

gree

nhou

se e

ffect

, bu

t al

so t

he n

atur

ally

va

riabl

e cl

imat

e sy

stem

.

Figu

re 1

8. W

inte

r tem

pera

ture

est

imat

es f

or

Eas

tern

Eur

ope

durin

g th

e pa

st 1

,000

ye

ars,

bas

ed la

rgel

y on

his

toric

non

- in

stru

men

tal r

ecor

ds.

(Sou

rce:

Lam

b,

1977

).

1000

1300

16

00

YE

AR

A.D

.19

00

25

RE

FE

RE

NC

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26