1. University of Oxford Uncertainty in climate science: what it means for the current debate Myles Allen Department of Physics, University of Oxford myles.allen@physics.ox.ac.uk

2. University of Oxford Three kinds of uncertainty Big picture uncertainty: what is the chance that climate scientists have got it all wrong? Large-scale projection uncertainty: how much global warming should we expect over the coming decades and centuries? Small-scale prediction uncertainty: what does this mean for flood risk in OX1/OX2?

3. University of Oxford Copenhagen Diagnosis Big picture: global emissions continue to rise

4. University of Oxford And the climate system continues to warm – on multi-decade if not sub-decade timescales

5. University of Oxford January 2010 – the warmest on record: but not everywhere

6. University of Oxford But can we trust the record?

7. University of Oxford Yes. The impact of the “climategate” revelations on the instrumental temperature record

8. University of Oxford Multiple indicators of “unequivocal” warming: Arctic sea ice in September 2005 & 2007

9. University of Oxford Uncertainty in projected warming over the next few decades Global temperature response to greenhouse gases and aerosols Solid: climate model simulation Dashed: recalibrated prediction using HadISST & CRUTEM data to August 1996 (Allen et al, 2000) 14 years

10. University of Oxford There was a time when people took 14-year climate forecasts seriously

11. University of Oxford The article in question

12. University of Oxford Uncertainty in projected warming over the next few decades Global temperature response to greenhouse gases and aerosols Solid: climate model simulation Dashed: recalibrated prediction using HadISST & CRUTEM data to August 1996 (Allen et al, 2000) 14 years

13. University of Oxford Uncertainty in projected warming over the next few decades Global temperature response to greenhouse gases and aerosols Solid: climate model simulation Dashed: recalibrated prediction using HadISST & CRUTEM data to August 1996 (Allen et al, 2000) Forecast verification 01/01/00 to 31/12/09

14. University of Oxford But what does this mean for flood risk in OX1? South Oxford on January 5th, 2003 Photo: Dave Mitchell

15. University of Oxford Models predict increasing winter rainfall in North West Europe over the next 80 years IPCC

16. University of Oxford Are recent UK floods affected by climate change? Pall et al 2010 Aim: to quantify the role of increased greenhouse gases in precipitation responsible for 2000 floods. Challenge: relatively unlikely event even given 2000 climate drivers and sea surface temperatures (SSTs). Approach: large (multi-thousand-member) ensemble simulation of April 2000 – March 2001 using forecast- resolution global model (90km resolution near UK). Identical “non-industrial” ensemble removing the influence of increased greenhouse gases, including attributable SST change, allowing for uncertainty.

17. University of Oxford Performing simulations using distributed computing: climateprediction.net >300,000 volunteers (50,000 active), 90M model-years

18. University of Oxford Autumn 2000 in the ERA-40 reanalysis… …and in one of the wetter members of our ensemble.

19. University of Oxford Risk of floods in the year 2000, with and without the influence of increased greenhouse gases 2x increase in risk

20. University of Oxford Uncertainties in the science: the current state of play Uncertainty in basic causes is climate change is relatively low: IPCC (2007) concluded human influence was “very likely” the cause of most of the warming over the past 50 years – meaning a 10% chance that it wasn’t. Uncertainty in large-scale trends still around a factor of two – meaning changes predicted for 2040 might occur in 2030 or 2050 – does this matter? Uncertainty in the impacts of climate change, and the costs for nations, organisations and individuals, are still very high.