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