Human Induced Climate Change: The IPCC Fourth Assessment AKE-Programme Annual Conference the German Physical Society (DPG) Regensberg, March Bill Hare, Visiting Scientist, PIK

IPCC AR4 - Physical Science report in context Climate Change 2007: The Physical Science Basis –Accepted Paris, Feb 2007 Climate Change 2007: Impacts, Adaptation and Vulnerability –Under final government review, to be adopted Brussels, April Climate Change 2007: Mitigation of Climate Change –Under final government review, to be adopted Bangkok, April 30-3 May 2007 AR4 Synthesis Report –Under preparation, final government review begins May 15, 2007 and report to be adopted in Valencia, November 2007

Overview Context of IPCC AR4 –Unless otherwise stated figures are from IPCC Summary for Policy Makers and/or Chapters wuth references in notes to slides. –SPM = refers to Summary for Policy Makers –TS = Technical Summary and numbers eg Fig X.Y refer to Chapters and figure number Projections –Emissions to climate response –Uncertainty in climate sensitivity –Global and regional projections Coupled carbon cycle Perspective on AR4 sea level rise projections Beyond 21st century climate change

Emissions to climate response Improved understanding of gas cycles Improved understanding radiative forcing due to gases, aerosols, land use change, volcanic activities and solar cycles Improved understanding global and regional climate system response to forcing Improved understanding of coupled carbon cycle climate interactions Figure 10.1.

Radiative forcing - response Energy balance of the perturbed climate system Radiative forcing at the top of troposphere Surface temperature change Total global mean feedback parameter Total heat content perturbation of the ocean Climate sensitivity defined at equilibrium for a radiative forcing equivalent to a doubling of CO2 concentrations

Improved understanding of radiative forcing The understanding of anthropogenic warming and cooling influences on climate has improved since the Third Assessment Report (TAR) Very high confidence that the globally averaged net effect of human activities since 1750 has been one of warming, with a radiative forcing of +1.6 [+0.6 to+2.4] W m-2.

Radiative forcing of the climate system FIGURE SPM-2. Global-average radiative forcing

Climate sensitivity Global mean warming at equilibrium for a radiative forcing equivalent to a doubling of CO2 concentration Likely: 2 to 4.5°C Likelihood range for first time Best estimate: 3°C Best estimate in 1990 and 1995 was 2.5°C Very unlikely to be less than 1.5°C. Values higher than 4.5°C cannot be excluded Cloud feedbacks remain the largest source of uncertainty.

Uncertainty in climate sensitivity Box 10.2, Figure 1

Improved climate change projections Large number of simulations available from a broader range of models. Quantitative basis for estimating likelihoods for many aspects of future climate change. Comparison with projections since 1990 strengthens confidence in near-term projections. Next two decades warming of about 0.2°C per decade is projected for range of SRES emission scenarios. Even if the concentrations of all greenhouse gases and aerosols had been kept constant at year 2000 levels, a further warming of about 0.1°C per decade would be expected. Comment: Caveat on sea level rise projections

IPCC Projections vs observations Figure TS-26. Model projections of global mean warming

Ranges of surface warming to 2100

AOGCM projections of surface temperature FIGURE SPM-6. Projected surface temperature changes for

Sea Ice Changes Figure Multi model simulated anomalies in sea ice extent

Precipitation projections FIGURE SPM-7.

North America snow Figure Percent snow depth changes in March

Changes in extremes

Projections of extreme weather events

Projected changes in extremes

Frosts, heatwaves, growing season

Regional projections There is now higher confidence in projected patterns of warming and other regional-scale features, including changes in wind patterns, precipitation, and some aspects of extremes and of ice.

Regional changes Temperature anomalies with respect to 1901–1950 for 1906–2005 (black line) and as simulated (red envelope) and as projected for 2001–2100

Carbon cycle climate coupling TAR and AR4: Warming tends to reduce land and ocean uptake of atmospheric carbon dioxide, increasing the fraction of anthropogenic emissions that remains in the atmosphere. Climate carbon cycle coupling is expected to add carbon dioxide to the atmosphere as the climate system warms, but the magnitude of this feedback is uncertain

Carbon cycle climate coupling Stronger assessed climate-carbon cycle feedbacks increases upper range of temperatures –eg IPCC SRES A2 scenario global average warming at 2100 by more than 1°C. Decreases CO2 emissions required to achieve a particular CO2 stabilisation level –For 450 ppm CO2 stabilization stronger climate-carbon cycle feedbacks reduce cumulative allowed emissions over 21st century be from approximately 670 GtC to approximately 490 GtC

Oceanic acidification

IPCC sea level projections vs observations Rahmstorf et al 2007 Science Observations – solid red (tide gauge) and blue (satellite) with non linear trend IPCC TAR SRES range IPCC TAR SRES range: Land ice (ice sheet) uncertainty range Sea level rise trend is at top of IPCC TAR range

Sea Level Rise (m at relative to ) CaseModel-based range excluding future rapid dynamical changes in ice flow B1 scenario0.18 – 0.38 A1T scenario0.20 – 0.45 B2 scenario0.20 – 0.43 A1B scenario0.21 – 0.48 A2 scenario0.23 – 0.51 A1FI scenario0.26 – 0.59 Sea level rise projections

Non uniform sea level rise Figure Local sea level change (m) due to ocean density and circulation change relative to the global average

Sea level rise: Observations vs Models Source of sea level rise Observed mm/yr Modeled mm/yr Thermal expansion Glaciers and ice caps Greenland ice sheet Antarctic ice sheet Sum of contributions Observed total SLR3.1 Difference Within error estimates of sum of contributions 35% of observed SLR unexplained? Opposite sign to observations : ice dynamics

Total sea level projections Antarctic ice sheet reduces sea level over 21s century Ad Hoc adjustment for ice sheet dynamics not included in models mean trend extrapolated to 2100

Uncertainty in AR4 sea level rise projections Models substantially underestimate past SLR Uncertain implications for future projections but of concern for risk assessment For both ice sheets recent accelerations in ice flow have contributed significantly to recent SLR but this is not included in the models. Full AOGCM temperature range with carbon cycle feedback not included –warming up to ca 5oC considered but not 6.4oC Ice sheet dynamics unable to be modeled at present –Ad Hoc adjustment is just that - Ad Hoc. VIEWPOINT: Likely that IPCC AR4 sea level rise projections are biased low due to these factors.

Beyond the 21st century Anthropogenic warming and sea level rise would continue for centuries due to the timescales even if greenhouse gas concentrations were to be stabilized. Inertia in climate system: –Past and future carbon dioxide emissions will continue to contribute to warming and sea level rise for more than a millennium. –Further increase in global average temperature of about 0.5°C (Radiative forcing stabilized in 2100) –Sea level rise from thermal expansion alone would lead to 0.3 to 0.8 m of sea level rise by 2300 and continue for many centuries (Radiative forcing stabilized in 2100)

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Additional slides if needed for questions

Inertia in the climate system

Energy content changes

Projected global mean temperature

Surface mean temperature change for 2100

Probabilistic assessments

Uncertainty in climate sensitivity

IPCC Assessments Year Observed global mean change 0.6ºC (1880s to 1990) ºC (1880s to 1990) ºC (1880s to mid 1990s 0.8°C ( °C) (1880s to 2001– 2005) Human influence due to anthropogenic increase in greenhouse gas concentrations Unequivocal detection “not likely for a decade or more.” “Balance of evidence suggests a discernable human influence.” “likely” that “most of the observed warming over the last 50 years” “very likely” “Most of the observed increase in globally averaged temperatures since the mid-20th century” Projected warming to ºC1-3.5ºC ºC (SRES range) ºC (SRES range) Projected sea level rise to cm13-94cm9-88cm (SRES range) cm (SRES range)

Regional changes

African changes

African precipitation

Carbon cycle

Thermal SLR commitment

Greenland ice sheet