Climate Change A Statistician’s Perspective Dennis Trewin Statistical Consultant, Australia

Conclusions of the IPCC Support their conclusions on past climate change and the impact of human activity Applaud the transparency of the report Compliment the degree of international collaboration Concerned that there may be statistical flaws in the projections of future climate change

Conclusions from the IPCC Summary for Policy Makers Increase of 0.5 degrees to 2030 (0.3 to 0.75) for all scenarios Accumulation of greenhouse gases is important (suggesting earlier action) Best estimates of increase to the end of the century range from 1.8 (B1 scenario) to 4.0 (A1 scenario) degrees

Climate Change Scenarios A1 - very rapid economic growth, global population peaking in mid century, economic convergence, rapid introduction of new technologies A1FI – fossil intensive A1T – non-fossil energy sources A1B – balanced A2 – heterogeneous world, regional based development continually increasing population

Climate Change Scenarios B1 – Similar to A1 but change towards a service and information economy with reductions in material intensity B2 – Similar to A2 but population increases at a slower rate and intermediate levels of economic development

My Concerns Non-use of purchasing power parities (Castles/Henderson criticism) Convergence assumptions in A1 and B1 High population growth assumptions in A2 and B2 No estimates of robustness Insufficient information on distribution of projections (addressed in the latest round)

Comparative Growth Rates Scenario/SourceAnnual Growth Rate (%) A13.1 B12.4 US Dept of Energy1.9 B21.8 World Bank1.8 IEA1.5 A21.0

Population Projections B2 is 13-19% higher than UN projections A2 is even higher A1 and B1 broadly consistent with UN projections

Validity of the Scenarios Best Estimates at 2100 and Range A1- unlikely because of very high economic growth assumptions, A1FI – 4.0 (2.4, 6.4) A1B – 2.8 (1.7, 4.4) A1T (1.4, 3.8) A2 – unlikely because of very high population growth assumptions, 3.4 (2.0, 5.4) B1 – most likely but high economic growth because of convergence assumptions, 1.8 (1.1, 2.9) B2 – unlikely because of very high population growth assumptions, 2.4 (1.4,3.8)

B1 Estimates Best EstimateLikely Range Temp Change to degrees Temp Change to degrees Sea Level Rise to 2100 na m

What would I advise policy makers? Climate is extremely variable Global warming since 1900 with associated sea rises; human activity is the major contributing factor Distributional impacts of global warming Unless greenhouse gas emissions further climate change and associated impacts can be expected

What would I advise policy makers? Best estimate until 2030 is temperature increase of 0.5 degrees, sea level increase of 8 cms; distributional impacts may be important B1 group of scenarios is most consistent with authoritative assumptions on economic and population growth A lot of uncertainity remains so take an adaptive approach with some hedging for risk

Conclusion Should the OECD(?) develop climate change projections using scenarios based on authoritative economic and population projections with appropriate use of PPPs?