radiation basics lecture9 18may2018final forpdf · • global dimming counter-balances increasing...

Radiation and Climate Change FS 2018 Martin Wild

Exam information

Exam, 1. 6. 2018• Takes place in RZ F21, approx. 60 - 70 Min.• Covers all material discussed in the lectures and in the computer lab• No supporting material allowed (keine Hilfsmittel), except a simple

pocket calculator with trigonometric and power functions (no calculators on smartphones, no pocket computers). We will provide a number of TI30 calculators, if you cannot bring an appropriate calculator.

• Emphasis is on understanding of concepts• You should know the simple laws and formulas, but not the more

complex ones• Please be here at 8 15, we will need some time to check your IDs

and possibly the pocket calculators, and start the exam towards 8 30.


7. Decadal changes in Earth radiation balance and climate response


Radiative forcing over past 100 years

Radiative imbalance currently

estimated at 0.85 W/m-2

(0.75 W/m-2 over 1993-2003)

From Hansen et al. (2004)

Model simulations


Radiative imbalance at TOA 1860-2100

Slingo and Webb 1997, QJRMS

Model simulations

Radiative imbalance: where does the energy go?


0.75 Wm-2

Measurements of Ocean Heat Content


Argo: • Global array of 3800 floats provides since 2003

100,000 temperature/salinity profiles and velocity measurements per year distributed over the global oceans at about 3�degree spacing.

• Floats cycle to 2000m depth every 10 days.

Measurements of Ocean Heat Content


Von Schuckmann et al. JGR 2009


Additional reading

Available on the course website:http://www.iac.ethz.ch/edu/courses/master/modules/radiation-and-climate-change.html


Response of the climate system to radiative forcings


Climate sensitivity is a measure of the equilibriumglobal surface air temperature change for a particularradiative forcing, usually given as a °C change perW/m2 forcing.

A standard experiment to determine this value in aclimate model is to look at the doubled CO2 climate,and so equivalently, the climate sensitivity is oftengiven as the warming for doubled CO2 (i.e. from 280ppm to 560 ppm)(“2xCO2 climate sensitivity”)

Climate sensitivity


Equilibrium 2 x CO2 temperature response

IPCC TAR

GCMs with mixed layer ocean calculated into equilibrium under 2 x CO2 forcing



From Murphy et al 2009

Apply step-function radiative forcing (e.g., instantaneous doubling of CO2: system responds by change in temperature and resulting change in emitted longwave radiation)

TOA Imbalance

Outgoing longwave radiation

(imbalance)

Forcing

Response


Radiative forcing and climate sensitivity (I)

TOA radiation balance: N = SW - LWWhere N = TOA Net Radiation (“imbalance”), SW = absorbed solar radiation, LW = longwave outgoing radiation

Planet in radiative equilibrium:N = SW - LW = 0

Climate change:Apply radiative forcing F, climate system responds by a change in Temperature and resulting change in longwave emitted flux (Temperature dependent)N= F – l D T (“imbalance = forcing – response”)wherel= climate feedback parameter (Wm-2/�C)


Equilibrium surface temperature response to imposed radiative forcing:At new equilibrium N=0DT= l-1 Fwhere F = Radiative forcing (Wm-2)l-1 = equilibrium climate sensitivity parameter (°C/Wm-2)DT= Equilibrium surface temperature response (°C)

Example: Climate sensitivity to doubling CO2

F = 4 Wm-2 (2 x CO2 radiative forcing)DT= 3°C=> l-1 = DT/F = 0.75°C / Wm-2

Radiative forcing and climate sensitivity (II)



IPCC TAR

GCMs with mixed layer ocean calculated into

equilibrium under 2 x CO2 forcing (ca. 4 Wm-2)

l-1= DT/F=2°C/4Wm-2

= 0.5°C/Wm-2

l-1= DT/F=5°C/4Wm-2

= 1.25°C/Wm-2


Decadal changes in surface radiation

Changes in downward longwave radiation

most directly affected by changes in atmospheric greenhouse gases

expected to undergo largest changeof all energy balance componentsin coming decades

CMIP5 models suggest increase of 6 Wm-2 since 1870

Only monitored since the initiation ofBSRN in the early 1990s

Downward longwave radiation in CMIP5 models

Greenhouse Gases

6 Wm

-2

1870-2005

Wild et al. 1997 J. Climate / Wild 2016, AIP proc.

Increasing greenhouse effectat the surface


What can we see in currently available records

of downward longwave radiation?Baseline Surface Radiation Network (BSRN)

Observed changes in downward longwave radiation

Longterm monitoring of downward longwave radiation is acentral objective of BSRN


Philipona et al. 2004

Wild et al. 2016

Widespread increase in observed downward longwave radiation

South Pole

Observed increase at all BSRN sites since 1992: +2 Wm-2/decade

Alpine sites

Greenhouse Gases

Observed changes in downward longwave radiation

25 Wm

-210 Wm

-2

RCP 8.5

RCP 4.5

CMIP5 projections 21th century

2010-2030: RCP8.5:+2.2 Wm-2/decRCP4.5:+1.7 Wm-2/dec

Observed: +2 Wm-2/dec

10 CMIP5 Models

Future changes in downward longwave radiation

Wild et al. 1997 J. Climate / Wild 2016, AIP proc.


Changes in surface solar radiation 1950s-1980s

Substantial decline in solar radiation at Earth surface:�global dimming�

Decrease in surface insolation 1950s-80s:Gilgen Wild Ohmura (1998): -9 Wm-2

Stanhill and Cohen (2001): -10 Wm-2

Liepert (2002): -7 Wm-2

=>ca. 2% decrease per decade6% over the period 1950s-1980s

Surf

ace

sola

r rad

iatio

n (W

m-2

)

Surface solar radiation at Potsdam, Germany


Global dimming: potential causes

Stanhill and Cohen (2001)

Increase in fossil fuel emissions between 1960 - 1990,In line with decrease in solar radiation at the surface


Both direct and indirect aerosol effects (cloud albedo/cloud lifetime) reduce the amount of solar radiation reaching the ground

Global dimming: potential causes

Direct and indirect aerosol effects

Direct effects Indirect effects: cloud albedo and lifetime

Less precipitation > longer lifetime


Extending the records beyond the 1980s

All studies on solar dimming used data only prior to 1990

=> Extend observational records from 1980s to present

?Surface Solar Radiation at Potsdam

Surf

ace

sola

r rad

iatio

n (W

m-2

)

Wild et al. 2005: Science 308


Extending the records beyond the 1980s

All studies on solar dimming used data only prior to 1990

=> Extend observational records from 1980s to present

Surface Solar Radiation at Potsdam

Surf

ace

sola

r rad

iatio

n (W

m-2

)

Wild et al. 2005: Science 308


Additional reading



From ISCCP (NASA/ Bill Rossow)

Anomalies in global cloud cover from satellite 1983-2002

International Satellite Cloud Climatology Project

From dimming to brightening: Causes

Wild et al, 2005, Science

Recent recovery in atmospheric transmission in line with reduced emissions

“dimming” “brightening”

Atmospheric clear-sky transmission

Data source: Stern, 2005

Global AnthropogenicSulfur Emissions

From dimming to brightening: CausesChanges in cloud-free atmosphere 1950-2000


- +1950s to 1980s• global dimming counter-

balances increasing longwave downward radiation

• Surface radiative heating is not increasing

Wild et al. (2004) GRL 32

- +-xsince 1980s• Absence of global

dimming no longer masks longwave greenhouse effect

• Surface radiative heating increases significantlyWild et al. (2005) Science 308Wild et al. (2007) GRL

Global dimming versus greenhouse warming

Dimming BrighteningDimming / brightening modulates decadal warming rates

Observed global meanTemperature change

Data source:CRU

2m T

empe

ratu

re a

nom

alie

s (C

)

Deviation from 1960-1990

0.02�/ decade 0.2�/decade


Global dimming versus greenhouse warming

Wild 2012, BAMSWild 2016, WIREs Clim Change

Asymmetric hemispheric pollution

Emissions show trend reversal in NH, but not in SH

Anthropogenic sulfur emission 1950-2000

Source: Stern (2005)

Northern Hemisphere

Southern Hemisphere

Globe

Wild, BAMS 2012Radiation and Climate Change FS 2018 Martin WildWild 2016, WIREs Clim Change

2m T

empe

ratu

re (C

)“dimming” “brightening”

Temperature changeNorthern Hemisphere

2m T

empe

ratu

re (C

)

Wild 2016, WIREs Clim Change

Temperature changeSouthern Hemisphere

Sulfur Emissions since 1950

Reversal in Temperature trends in Northern Hemisphere, but not in Southern Hemisphere > fits to emissions

Deviations from 1960-1990 Deviations from 1960-1990

Asymmetric hemispheric warming

Negative surface net radiation

Positive surface net radiation

Observed precipitation NH mean landData source:

GHCN

Wild, BAMS 2012

Impact on the global water cycle


Variations in precipitation quantitatively consistentwith variations in surface net radiation


Impact on mountain glaciers

Swiss Glaciers area reduction:

“Dimming phase“1973 - 1985: -1 %“Brightening phase“1985 – 2000s:-18 %Paul et al. 2004, GRL

Swiss glaciers only retreated

after transition from dimming to brightening

Impact on carbon uptake and plant growth

Associated with cloud cover & anthropogenic aerosol (scattering & absorbing) changes

Total radiation= Direct + Diffuse

Diffuse fraction= Diffuse / Total

Dimming total radiation => less photosyntheis diffuse fraction => more photosynthesis

Brightening total radiation => more photosyntheis diffuse fraction => less photosyntheis

Effects of diffuse fraction dominates=> Increased carbon uptake and plant growth during dimming

Global dimming and brightening affects quantity & quality of radiation

Mercado et al. 2009Nature

Total

Direct

DiffuseOdessa, Abakumova et. al. 1996

total radiation --- reduces plant photosynthesis

diffuse fraction --- enhances plant photosynthesis

Global-Dimming period

Mercado et al. 2009 Nature

Direct radiation: Can only be used by uppermost leaves for photosynthesis

Diffuse radiation: penetrates deeper into canopy - can be more effectively used for photosynthesis

Modeling studies: Effect of diffuse fraction increase dominatesÞ Increased carbon uptake and plant growth during dimming

Impact on carbon uptake and plant growthWhy is plant productivity enhanced with more diffuse?



Additional reading


radiation basics lecture9 18may2018final forpdf · • global dimming counter-balances increasing...

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