Transcript Global Warming: Earth evolution or man made? The case for being

Global Warming: Earth evolution
or man made?
The case for being man made.
Nathan Bindoff
ACECRC, IASOS, CSIRO MAR
University of Tasmania
TPAC
Why the concern about climate change?
0°C
Food
Water
Global temperature change (relative to pre-industrial)
1°C
2°C
3°C
4°C
5°C
Projected impacts of climate change
Falling crop yields in many areas, particularly
developing regions
Falling yields in many
Possible rising yields in
developed regions
some high latitude regions
Small mountain glaciers
disappear – water
supplies threatened in
several areas
Significant decreases in water
availability in many areas, including
Mediterranean and Southern Africa
Sea level rise
threatens major cities
Ecosystems
Extensive Damage
to Coral Reefs
Rising number of species face extinction
Extreme
Rising intensity of storms, forest fires, droughts, flooding and heat waves
Weather
Events
Risk of Abrupt and
Increasing risk of dangerous feedbacks and
Major Irreversible
abrupt, large-scale shifts in the climate system
Changes
Stern report (2006)
Changing Atmosphere
Radiative change 1750-2005
• Role of aerosols
Global mean temperatures are rising faster with time
Warmest 12 years:
1998,2005,2003,2002,2004,2006,
2001,1997,1995,1999,1990,2000
SPM-3a
Global mean temperatures compared with past (NH)
Very likely that last 50 years was warmer than any period in last 500 years
Sea level is rising in 20th century
SPM-3b
Rates of sea level rise:
•1.8 + 0.5 mm yr-1, 1961-2003
•1.7 + 0.5 mm yr-1, 20th
Century
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
Attribution to
man
• 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
All forcing = GHG + Aerosols +
solar + volcanic
Observations
Solar + volcanic
TS-23
Continental warming
SPM-4
Observations
All forcing
natural forcing
likely shows a significant anthropogenic contribution over the past 50 years
Summary for the affirmative case
•
Affirmative case has shown
– many strands of evidence for climate change over a broad range of
variables, that are consistent with conceptual/qualitative models of
climate change
– that known man induced changes in atmosphere are very likely to
explain global and continental surface temperature records for the last
century
– it is extremely unlikely that solar variations can explain global mean
observations
– The above results are quantitative, formally rigorous, and robust,
combining both observations and physical models
– There are many more formal attribution and detection studies for
other climate variables (precipitation, ocean warming) that support the
above case.
A paleoclimate perspective
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
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
I am not the only one, IPCC WGI “Headlines”
• “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)
Emission Scenarios
Medium High
Low
•High – fossil fuel intensive
future
•Low – greater alternative
energy sources
•Mix of economics,
technology change.
•Do not include mitigation
•Aerosols
Role of volcanoes on radiative forcing
Projections of Future Changes in Climate
Low
Emissions
High
Emissions
• 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
Figure SPM-6, TS-30, 10.9
• Precipitation increases are very likely in high latitudes in 2090-2099
• Decreases are likely in most subtropical land regions in 2090-2099
Scenarios for sea-ice
North. Hem.
Summer
South. Hem.
Winter
Scenarios for sea-ice
1980-2000
Arctic
Summer
Antarctic
Winter
2080-2100
Other observed changes in the climate system
Future Climate: Greenland Ice Sheets
1900
2170
=1.4m
Ice Sheets: a key risk for
future climate
2610
3030
3660
Projections of Future Climate: Ice Sheets
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)
Scenarios for Extremes- frost, heat waves, growth
Causes for optimism
• Past successes
– Montreal Protocol (1987,1989)
– 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
Global atmospheric CO2 budget
Pep Canadell, Corinne LeQuere, Mike Raupach, Gregg Marland,
Skee Houghton, Tom Conway, Philippe Ciais
Atmospheric accumulation =
FFoss + FLUC + FLandAir + FOceanAir
Projections of Future Changes in Climate
The IPCC is a “remarkable
example” of mobilizing expert
analysis to inform policymakers
Jeffrey Sachs (Nature, 12 August 2004)
The IPCC assessments are “dull as
dishwater”
Tim Flannery, The Weather Makers
The structure
of the IPCC for
the Fourth
Assessment
Report
Co-chairs:
Susan Solomon, USA
Dahe Qin, China
Ch. 10, Fig. 10.15
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.
Glacier contribution to sea-level since 1961
Increased glacier retreat
since the early nineties
Mass loss from glaciers
and ice caps:
• 0.5 ± 0.18 mm yr-1, 19612003
• 0.77 ± 0.22 mm yr-1, 19912003
Ice sheet contributions to sea level rise
Mass loss of Greenland:
• 0.05 ± 0.12 mm yr-1 SLE,
1961-2003
• 0.21 ± 0.07 mm yr-1 SLE,
1991-2003
Antarctic ice sheet loses mass
mostly through increased glacier
flow
Greenland mass loss is
increasing
Loss: glacier discharge, melting
Mass loss of Antarctica:
• 0.14 ± 0.41 mm yr-1 SLE,
1961-2003
• 0.21 ± 0.35 mm yr-1 SLE,
1991-2003
Attribution
• are observed
changes consistent
with expected
responses to
forcings
inconsistent with
alternative
explanations
All forcing = GHG + Aerosols +
solar + volcanic
Observations
Solar+volcanic
TS-23
Climate models, and climate model credibility
Observations 1980-2000
Mean Model 1980-2000
Heat content change: time evolution
Key points for 19612003:
•consistency of products
• oceans absorbed 0.21 ± 0.04
W m–2 (0-3000m) over the
earth’s surface.
•70% of this energy is
absorbed in top 700 m
•0.1°C warming (0-700m)
•1993-2003 has higher rates
of warming (0.50 ± 0.18 W m–2)
•decadal variability, cooling
since 2003