The document discusses the upcoming IPCC Working Group I assessment report (AR5) on the physical science of climate change. Key findings from the previous AR4 report are reviewed, including that warming trends are unequivocal and due primarily to greenhouse gas increases. New observations and projections to be assessed in AR5 include rising CO2 levels, ice sheet thinning, and sea level rise. The report will also reaffirm AR4 conclusions and the unprecedented changes occurring in the climate system over recent centuries and millennia.

1. IPCC Working Group I: The Physical Science Basis
The Physical Science Basis of Climate Change:
Latest Findings to be Assessed by WGI in AR5
Thomas Stocker Gian-Kasper Plattner
Co-Chair Working Group I Deputy Head, Director of Science
University of Bern, Switzerland TSU WGI, University of Bern

2. Working Physical Science Basis: Latest Findings toFifth Assessment Report
The Group I Contribution to the IPCC be Assessed by WGI in AR5
IPCC (2007):
Warming in the
climate system
is unequivocal...
(IPCC, 2007, Fig. SPM-3)

3. Working Physical Science Basis: Latest Findings toFifth Assessment Report
The Group I Contribution to the IPCC be Assessed by WGI in AR5
IPCC (2007):
Most of the observed increase in global averaged temperature
... is very likely due to ... increase in GHG concentrations.
(IPCC, 2007, Fig. WGI-SPM-4)

4. Working Physical Science Basis: Latest Findings toFifth Assessment Report
The Group I Contribution to the IPCC be Assessed by WGI in AR5
IPCC (2007):
Continued GHG emissions ... would induce many changes ...
that would very likely be larger than those observed ...
2020-2029 2090-2099
(IPCC, 2007, Fig. WGI-SPM-6)
Change in Temperature (°C), Scenario A2

5. Working Physical Science Basis: Latest Findings toFifth Assessment Report
The Group I Contribution to the IPCC be Assessed by WGI in AR5
Some policy-relevant findings to be assessed in AR5:
Observations (3 examples)
Projections (3 examples)

6. Working Physical Science Basis: Latest Findings toFifth Assessment Report
The Group I Contribution to the IPCC be Assessed by WGI in AR5
1. CO2: Higher levels and more rapid increase
2009: 387 ppm
(Siegenthaler et al., 2005; Lüthi et al., 2008, NOAA)

7. Working Physical Science Basis: Latest Findings toFifth Assessment Report
The Group I Contribution to the IPCC be Assessed by WGI in AR5
2. Extensive thinning on the margins of Greenland and Antarctica
(Pritchard et al. 2009)

8. Working Physical Science Basis: Latest Findings toFifth Assessment Report
The Group I Contribution to the IPCC be Assessed by WGI in AR5
3. Persistent sea-level rise consistent with earlier estimates
Merrifield et al., 2009:
[1993–2008] (3.5 ± 0.4) mm/yr
IPCC 2007:
[1993–2003] (3.1 ± 0.7) mm/yr
(Merrifield et al., 2009, in BAMS)

9. Working Physical Science Basis: Latest Findings toFifth Assessment Report
The Group I Contribution to the IPCC be Assessed by WGI in AR5
4. Rapid loss of Arctic sea ice
(Wang and Overland, 2009)
ice-free

10. Working Physical Science Basis: Latest Findings toFifth Assessment Report
The Group I Contribution to the IPCC be Assessed by WGI in AR5
5. Long-term commitment and irreversibility of CO2 perturbation
(IPCC, 2007, Chapter 10, simplified, Plattner et al. 2008)

11. Working Physical Science Basis: Latest Findings toFifth Assessment Report
The Group I Contribution to the IPCC be Assessed by WGI in AR5
6. Abrupt climate changes caused by geoengineering
10 Mio t Sulphur / Year
OFF
ON
(Robock et al., 2008)
Termination
Problem
Year

12. Working Physical Science Basis: Latest Findings toFifth Assessment Report
The Group I Contribution to the IPCC be Assessed by WGI in AR5
Conclusions:
IPCC WGI firmly stands behind the conclusions of AR4, the
community of researchers and its individuals providing the scientific
basis, and the procedures of IPCC Assessments;
The combination of observations and paleoclimate information shows
unprecedented changes in the climate system, both in amplitude
and rate for 100‘s to many 1000‘s of years;
Wide-spread melting of ice margins is observed in Greenland and
Antarctica with implications for sea level rise;
Emitted CO2 remains in the atmosphere for 1000‘s of years causing
irreversible changes in the climate and in ocean chemistry;
Geoengineering methods have an inherent termination problem
and do not mitigate the direct effects of CO2 increase.