Evidence for climate ChangeThe Working Group I Report of the

Intergovernmental Panel on Climate Change Fourth Assessment Report

Nathan Bindoff and othersACECRC, IASOS, CSIRO MAR

University of TasmaniaTPAC

Royal Society of Tasmania

Projected impacts of climate change1°C 2°C 5°C4°C3°C

Sea level rise threatens major cities

Falling crop yields in many areas, particularly developing regions

FoodFood

WaterWater

EcosystemsEcosystems

Risk of Abrupt and Risk of Abrupt and Major Irreversible Major Irreversible ChangesChanges

Global temperature change (relative to pre-industrial)0°C

Falling yields in many developed regions

Rising number of species face extinction

Increasing risk of dangerous feedbacks and abrupt, large-scale shifts in the climate system

Significant decreases in water availability in many areas, including Mediterranean and Southern Africa

Small mountain glaciers disappear – water supplies threatened in several areas

Extensive Damage to Coral Reefs

Extreme Extreme Weather Weather EventsEvents

Rising intensity of storms, forest fires, droughts, flooding and heat waves

Possible rising yields in some high latitude regions

Stern report (2006)

Why the concern about climate change?

Changing Atmosphere

Radiative change 1750-2005

• Role of aerosols

Global mean temperatures are rising faster with time

Warmest 12 years:1998,2005,2003,2002,2004,200

6, 2001,1997,1995,1999,1990,200

0

SPM-3a

Sea level is rising in 20th century

Rates of sea level rise:•1.8 + 0.5 mm yr-1, 1961-2003•1.7 + 0.5 mm yr-1, 20th Century•3.1 + 0.7 mm yr-1, 1993-2003

SPM-3b

Other evidence from observations•Oceans have warmed

•Oceans becoming more acidic

•Patterns of rainfall/evaporation are changing

•Evidence over both land and oceans

•Droughts are more frequent

•Extremes events are changing

•More warm nights

•More storm surges

•Strengthening westerlies

• Reduced snow, shrinking Arctic Sea-Ice

•Melting Glaciers, melting Greenland ice sheet, mass loss from Antarctica

“……evidence for climate change is unequivocal….”

Climate models, essential to hypothesis testing

Observations 1980-2000

Mean Model 1980-2000

• What is attribution?• Anthropogenic

greenhouse gas increases very likely caused most of the observed warming since mid-20th century

• extremely unlikely due to natural variation

Observations

Solar + volcanic

TS-23

Attribution to man

All forcing = GHG + Aerosols + solar + volcanic

Continental warming

likely shows a significant anthropogenic contribution over the past 50 years

Observations All forcing natural forcing

SPM-4

Scenarios of future change

Projections of Future Changes in Climate

Best estimate for low scenario (B1) is 1.8°C (likely range is 1.1°C to 2.9°C), and for high scenario (A1FI) is 4.0°C (likely range is 2.4°C to 6.4°C).

Broadly consistent with span quoted for SRES in TAR, but not directly comparable

• Spatial patterns: greater warming over land, greater warming at high latitudes

• Albedo changes in high latitudes, less snow and sea-ice.

Figure SPM-5,TS-28, 10.8, 10.28

Projections of Future Changes in Climate

HighEmissions

LowEmissions

• Precipitation increases are very likely in high latitudes in 2090-2099

• Decreases are likely in most subtropical land regions in 2090-2099

Figure SPM-6, TS-30, 10.9

Projections of Future Changes in Climate

Scenarios for sea-ice

North. Hem.Summer

South. Hem.Winter

Future Climate: Greenland Ice Sheets1900 36602170 2610 3030

=1.4m

Ice Sheets: a key risk for future climate

Post 2100 changes, Greenland:

• “…..and that the surface mass balance becomes negative at a global average warming (relative to 1961-1990) in excess of 1.2 to 3.9°C. If a negative surface mass balance were sustained for millennia, that would lead to virtually complete elimination of the Greenland ice sheet and a resulting contribution to sea level rise of about 7 m.”

Almost all marker scenarios exceed 1.2 to 3.9 °C tipping points.

• “.. If radiative forcing were to be stabilized in 2100 at A1B levels11, thermal expansion alone would lead to 0.3 to 0.8 m of sea level rise by 2300 (relative to 1980–1999). “

Implication, while not stated, is that there will be large sea level changes beyond 2100 (eg by 2300 something like 1.5 to 3.5m)

Projections of Future Climate: Ice Sheets

Scenarios for Extremes- frost, heat waves, growth

• “The balance of evidence suggests a discernible human influence on global climate.” (SAR, 1995)

• “There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.” (TAR, 2001)

• “Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.” (AR4, 2007)

• “Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns.” (AR4, 2007)

The IPCC WGI “Headlines”

• Past successes– Montreal Protocol (1988)– Pollution controls for Sulphur emissions

(acid rain)

• There are alternatives– Problem is the number of choices and

which is best?

• Important to act sooner than later– Already locking in future change– Harder to mitigate or adapt to

dangerous change– Important to act gradually– Its cheap

Causes for optimism

Global mean temperatures compared with past (NH)

Very likely that last 50 years was warmer than any period in last 500 years

Emission Scenarios•High – fossil fuel intensive future

•Low – greater alternative energy sources

•Mix of economics, technology change.

•Do not include mitigation

•Aerosols

High LowMedium

Scenarios for sea-ice

AntarcticWinter

ArcticSummer

1980-2000

2080-2100

Very likely that the Atlantic meridional overturning circulation (MOC) will slow down over the course of the 21st century.

Very unlikely that the MOC will undergo a large abrupt transition during the 21st century. Longer-term changes in the MOC cannot be assessed with confidence

Studies with additional fresh water from melting of the Greenland Ice Sheet suggest that this will not lead to a complete MOC shutdown in the 21st century.

Ch. 10, Fig. 10.15

Ice sheet contributions to sea level rise

Antarctic ice sheet loses mass mostly through increased glacier flowGreenland mass loss is increasingLoss: glacier discharge, melting

Mass loss of Greenland:• 0.05 ± 0.12 mm yr-1 SLE, 1961-2003• 0.21 ± 0.07 mm yr-1 SLE,

1991-2003

Mass loss of Antarctica:• 0.14 ± 0.41 mm yr-1 SLE,

1961-2003• 0.21 ± 0.35 mm yr-1 SLE,

1991-2003

125,000 years ago, higher Arctic temperatures likely resulted in sea level 4-6m above present - contributions may have come from both

Arctic Ice Fields (especially Greenland) and Antarctica

Simulated and observed Arctic warming at 125,000 yr B.P.

Estimated reduction in Greenland Ice Sheet Area and Thickness

A paleoclimate perspective

The most important spatial pattern (top) of the monthly Palmer Drought Severity Index (PDSI) for 1900 to 2002.

The time series (below) accounts for most of the trend in PDSI.

Drought is increasing most places

Example from South West Australia

AWBM

Assumption: no change in land use

Assumption: no change in land use

Tasmania Water Catchment Models

Factor of 1.0 represents no change in inflows

Factors <1.0 represents drying

Factors >1.0 represents wetter

Great Lake factors well below 1.0 and thus drying predicted

Others have drier Summers/Autumns and wetter Winters

Example: Hydro Tasmania Inflow Prediction

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Arthur's Lake

Great Lake

Trevallyn Dam

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Lake Burbury

Lake Mackintosh

Lake Rowallan

Most important lake