Climate Variability & Change - Past & Future Decades Brian Hoskins Director, Grantham Institute for Climate Change, Imperial College London Professor of Meteorology, University of Reading

Observed climate system Projections for 21 st century Uncertainties in projections Targets for climate related policy Concluding comments

Some measurements of the climate system. x

Comparison of Observed T since 1990 with IPCC projections 19 point filter on ObservationsObserved variability added to IPCC

Mean Central England T 1772 to 22 July 2012

Some measurements of the climate system Global sea level. Arctic sea ice Sept min in Arctic sea ice area

Projections: globally averaged surface warming IPCC 2007 Different scenarios

B1 A1B A Surface T projections for different periods and scenarios IPCC 2003

Change in average maximum number of dry days in year, IPCC 2011

Normalised PDFs for 2100 Decadal T (A1B) Ed Hawkins

UNEP Emissions Gap Report Nov 2010 Cumulative carbon – Allen et al 2009

Matsuno & Maruyama, 2012 CO2 emissions CO2 concentration Impact of post 2100 global emissions level Atm CO 2 TSL Atm CO 2 SL T 450ppm CO2 stabilisation Zero emissions floor

Contributors to uncertainty in trends in decadal T Hawkins 2011 GlobalUK

T % Contributions to uncertainty in different decades

“Dangerous Climate Change” e.g. 2C Is 1.9C fine and 2.1C disastrous? Policy makers could do with a ceiling on ΔT or GHGs Atmospheric CO2 level e.g. 450ppm Cumulative emissions e.g. 1trillion t of carbon

Thresholds (“Tipping Points”)

Are extremes increasing at a surprising/damaging rate? Coumu & Rahmtorf 2012

Attribution of extremes Basic physics/theory More heat waves & less cold periods: shift of distribution & change in local T/D balance More heavy rainfall events can be expected – Clausius-Clapeyron More intense storms? Latent heating+, baroclinicity +/- Model attribution studies Analogues of synoptic situations e.g. Cattiaux et al 2010 for extreme cold European winter 2009/10

Summer 2010 Barriopedro day rainfall in July - ECMWF analysis Otto et al 2012 Model estimates of Return periods for T/Z

20 th Century Blocking in CMIP5 Models: NH Winter ERA40Mean of modelsSD of models

Concluding Comments Very strong evidence that we are performing a very dangerous experiment with our planet Very long time-scales of atmospheric CO 2 & the ocean imply a climate commitment, but also imply some very long term sensitivity to GHG emission floor next century Current models suggest emissions reductions in next few decades important only after 2050 Thresholds will probably exist in the climate system but we do not know where they are. For societies thresholds will certainly exist. Extreme weather events may be increasing more than simple shifts of distributions suggest but this has not yet been shown conclusively Uncertainties in climate models & their prediction of natural variability will be reducible to some extent Hard targets are not based on scientific evidence but softer ones to guide policy can be posed e.g. CCC 50% chance of not exceeding 2C by much & very small chance of reaching 4C

Fractional uncertainty & Fraction of Variance Globe British Isles

precip

S/N (b) & (d) assuming model uncertainty zero

Matsuno & Maruyama, 2012 CO2 emissionsCO2 concentration

Global temperatures and 10, 20 & 30 year running averages

Number of Monthly Record High Temperatures 17 stations, decadal smoothing Coumu & Rahmtorf 2012 from Benestad (2004)

21 st Century Blocking in CMIP5 Models: NH Winter for RCP8.5

Date at which signal emerges from “noise”