weather, climate & society atmo 336 natural climate variability

Weather, Climate & SocietyATMO 336

Natural Climate Variability

Perspective: Time Scales

13.7 b.y.

4.6 b.y. 2.1 b.y.

3.5 b.y.

65 m.y.

Avg. human life span=0.15 s

21 s

http://ocw.mit.edu/OcwWeb/Earth--Atmospheric--and-Planetary-Sciences/12-301Fall-2006/LectureNotes/index.htm

1 lt.y.=1016 m

13.7 Ga (+/- 1%)

http://ocw.mit.edu/OcwWeb/Earth--Atmospheric--and-Planetary-Sciences/12-301Fall-2006/LectureNotes/index.htm

Perspective: Astronomic Distance Scales

http://en.wikipedia.org/wiki/Earth

What is Climate Change?

• Climate change - A significant shift in the mean state and event frequency of the atmosphere.

• Climate change is a normal component of the Earth’s natural variability.

• Climate change occurs on all time and space scales.• It was stated that “a plethora of evidence exists that

indicates the climate of the Earth has changed.” What is that evidence? How do we know what we know?

Determining the Past Climate

Paleoclimatology - the study of past climates.• Past 100-200 years (weather observations)• Must use indirect climate measures, proxies, to

examine further into the past. Some proxies:

- Tree rings (1,000+ years before present BP)

- Trapped pollen (10,000+ years BP)

- Glacial ice cores (100,000+ years BP)

- Ocean sediment cores (1 Million+ years BP)

- Geology (1 Billion+ years BP)

Detecting Change With Proxies

Scientifically, the best way to detect change is to directly measure it.

Unfortunately for the timescales of interest in climate science, we were not always able (interested in?) to measure quantities such as temperature, precipitation, wind speed, direction, greenhouse gas levels, etc.

But, obviously we want to know what these properties were and how they changed in the past to test our understanding of how climate changes.

The study of past climate is known as paleoclimate science.

Courtesy J. Thornton, U Wash

Record: 1000 ~ Present day“Length” of growing seasonGood versus stressed yearsMajor fires

Unlocking “Stored” Climate Change

• Modern Instrument Record

• Tree Rings

• Ice Cores

• Sediment cores

• Rock formations/typesCourtesy J. Thornton, U Wash

Record: ~ 1Mya to ~ 20th centInert gases (CO2, CH4, N2O,…)Particulates (soot, ash, etc)Temperature??

Unlocking “Stored” Climate Change

• Modern Instrument Record

• Tree Rings

• Ice Cores

• Sediment cores

• Rock formations/typesCourtesy J. Thornton, U Wash

Record: ~ 200 MyaMicrofossils (ocean T),Volcanic glassOrganic detritusMagnetic pole location

Unlocking “Stored” Climate Change

• Modern Instrument Record

• Tree Rings

• Ice Cores

• Sediment cores

• Rock formations/types

Courtesy J. Thornton, U Wash

Unlocking “Stored” Climate Change

• Modern Instrument Record

• Tree Rings

• Ice Cores

• Sediment Cores

• Rock formations/types

Record: ~ 4.5 GyaGeologic formation Geochemical analysisMagnetic poleContinental LocationFossil record

Banded Iron Formations BIFs tied up oceanic O2 Prevented atmospheric O2 Date no later than ~2 GYA

Detecting Change With Proxies

Another property/qty that is a function of property of interest.

The measured property is a PROXY for the one of interest.

Think approximate

Courtesy J. Thornton, U Wash

18O/16O low

18O/16O high

18O/16O lower18O/16O lower still

Water Cycle – Water Isotope T Proxy

Courtesy J. Thornton, U Wash

Vostok Ice Core Record

T based on water isotope proxy

Last Ice Age

During last ice age (18,000 years ago)Temps 6oC colder CO2 levels 30% lower

CH4 levels 50% lower

(Sea level was higher)

than pre-industrial interglacial values

Courtesy J. Thornton, U Wash

O18 analysis of ocean sediments can be used to construct past

temperatures

http://en.wikipedia.org/wiki/%CE%9418O

Warm

Cold

600 Million Years of Climate

http://www.scotese.com/climate.htm

The past climate of the Earth can be deduced “by mapping the distribution of ancient coals, desert deposits, tropical soils, salt deposits, glacial material, as well as the distribution of plants and animals that are sensitive to climate, such as alligators, palm trees & mangrove swamps.”

542 Mya

65 Mya

490 Mya

200 Mya

145 Mya

251 Mya

299 Mya

417 Mya

359 Mya

444 Mya

1.8 Mya

540 Mya of Climate Change from O18

http://en.wikipedia.org/wiki/Oxygen_isotope_ratio_cycle

http://www.snowballearth.org/images/geologic_column.gif

Snowball Earth!

http://nai.nasa.gov/newsletter/03182005/snowball.jpg

Snowball Earth: Some Evidence

dropstones

http://www-eps.harvard.edu/people/faculty/hoffman/Snowball-fig11.jpg

Basic physics are understood: Runaway ice-albedo feedback

How does earth thaw? CO2?

Simple climate models exhibit hysteresis between ice-no ice states

http://www.snowballearth.org/slides/Ch7-6.gif

Delayed response between CO2 forcing and rapid jumps between ice free and snowball earth.

Snowball cycles show freeze-thaw-hothouse

http://www.snowballearth.org/images/snwbltvst.gif

Is snowball earth a mechanism for jump starting evolutionary leaps?

http://www.snowballearth.org/slides/Ch7-6.gif

Metazoa and plants appear after last episode

http://si.wikipedia.org/wiki/%E0%B6%BB%E0%B7%96%E0%B6%B4%E0%B6%BA:Oxygen_atmosphere.png

Life is responsible for the “recent” rise of oxygen

Use of sunlight for metabolism. Oxygen is a byproduct.

Use of oxygen for metabolism. Water and CO2 are byproducts.

Multicell organisms

Cambrian explosion

Atmospheric constituents have changed radically through the ages

http://www.ozh2o.com/atmos.jpg

Long-Term Climate Change

250 million years ago, the world’s landmasses were joined together and formed a super continent termed Pangea.

As today’s continents drifted apart, they moved into different latitude bands.

This altered prevailing winds and ocean currents.

NAE-A

AfSAIndia

NAIndiaAf

SA

E-A

AntAus

Ant

Aus

180 M BP Today Ahrens, Fig 13.6

Long-Term Climate Change• Circumpolar seaway

leads to large latitudinal temperature gradient.

Current S.H. Situation

• Circumequatorial seaway leads to small latitudinal temperature gradient.

Situation 50 MYahttp://www.ace.mmu.ac.uk/eae/Climate_Change/Older/Continental_Drift.html

Long-Term Climate Change• Circumpolar ocean

current formed around Antarctica 40-55 MY ago once Antarctica and Australia separated.

• This prevented warm air from warmer latitudes to penetrate into Antarctica.

• Absence of warm air accelerated growth of the Antarctic ice sheet.

http://www.ace.mmu.ac.uk/eae/Climate_Change/Older/Continental_Drift.html

Most Recent Ice Age

Extend of continental glaciers 18,000 years BP.

Sea level was 100-125 m lower than present.

Bering land bridge between Siberia and Alaska.

Aguado and Burt, Fig 16-4

http://en.wikipedia.org/wiki/Image:Recent_Sea_Level_Rise.png

http://earthobservatory.nasa.gov/Study/BorealMigration/Images/pleistocene_modern.jpg

SST 18,000 years BP

Much cooler over the North Atlantic Ocean.

Ocean currents were undoubtedly different.

North Atlantic Drift was probably much weaker.

18,000 BP TodayAhrens, Fig 13.2

Temperatures since the last Ice Age… the time that humans have

flourished

Rapid warming occurred at end of Younger-Dryas period.

Ice cores indicate that Ice Age conditions ended in 3 years!

Glacial retreat Rapid melt

Glacial advance

Apline advance

Ahrens, Fig 13.3

Climate affects human societies

Temperatures for Europe during the last 1200 years.

Viking settlements lost in GreenlandViking colonization

in Greenland

Ahrens, Fig 13.4

Evidence of Climate Change

Surface temperatures based on meteorological observations.Is the warming of the past century due to human activities?

0.6oC warming past century

Ahrens, Fig 13.5

Controversial “Hockey Stick”

Key Points: Climate Change

• Proxy data are used to infer the past climate.

• Data show that the Earth’s Climate

Has changed in the past

Is changing now

And will continue to change

• Key question is determining whether recent changes are due to natural causes or man.

Key Points: Climate Change

• The climate system is very complex.

Contains hundreds of feedback mechanisms

Feedbacks are not completely understood.

• Three general climate change mechanisms:

Astronomical

Atmospheric composition

Earth’s surface