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Can we really predict future climate change? Ellie Highwood Department of Meteorology, University of Reading BA Thames Valley February2004 Outline • Before we start… • Are we qualified to predict future climate? • The science of climate prediction • Future predictions and uncertainty • Ensemble forecasting (and how you can help!) • Global warming vs ice age theory BA Thames Valley February2004 Is climate change responsible? • Record hot temperatures in Europe during 2003? • Flooding and climate change? BA Thames Valley February2004 Scorchio! Summer 2003 Record summer temperatures happened in Kent on 10 August 2003… And, summer 2003 was very warm across Europe compared to historical temperatures BA Thames Valley February2004 Floods… e.g. Abingdon, January 2003 Photo by Guy Shepherd BA Thames Valley February2004 But is it global warming? “Climate” is very variable, day to day, year to year and place to place, and some of these variations are due to internal chaotic interactions between components of the climate system - “Climate is what we expect, weather is what we get “ (Apologies to Ed Lorenz, 1976) So, it’s not easy to say if any one event, such as a hot spell or a flood is due to global warming, But… BA Thames Valley February2004 “Record hot” events are more likely in a generally warmer world BA Thames Valley February2004 Flooding and climate change? • Little change in flood occurrence, although high volume flows have lasted longer in the past 30-50 years (Tyndall centre). • Increased frequency in “intense” events in mid and high latitude areas (IPCC) UK: increase in frequency of intense rainfall events in the winter and a decrease in the summer (Tim Osborn, CRU) • But, flood occurrence isn’t just about rainfall – catchment changes and channel changes are also important. Impact vs. occurrence? BA Thames Valley February2004 What is climate anyway? • Expected values of temperature and rainfall? • Includes seasonal cycles, year-toyear and decade to decade variations, and place-to-place variations BA Thames Valley February2004 What causes “variations” in climate? Internal chaotic interactions e.g. El Nino + Natural but external forcing of the climate system (e.g. changes in the Sun, volcanic eruptions) This “natural variability” is here to stay! But then we add Human activities… BA Thames Valley February2004 Are we qualified to predict? • Do we know enough about the fundamental processes determining the Earth’s climate? • Do we understand (and can we reproduce) any changes that have occurred in the past? • Are we capable of building models to simulate climate, and of being confident in their results? BA Thames Valley February2004 Fundamental processes • Greenhouse effect • Clouds • Oceanatmosphere interactions • Land surface + vegetation + hydrological cycle • Snow and ice • Humans? BA Thames Valley February2004 THE GREENHOUSE EFFECT: identified in 1896! ..most escapes to outer space and cools the earth... SUN …but some IR is trapped by some gases in the air, thus reducing the cooling…. Sunlight passes through the atmosphere.. ..and warms the earth. BA Thames Valley February2004 Arrhenius 1896 Infra-red radiation is given off by the earth... Major Players in the Greenhouse Effect Water Vapour: The natural greenhouse gas which makes our planet habitable Human influences on the greenhouse effect climate change Carbon dioxide Fossil fuels, deforestation 100 years Methane 10 years Agriculture, natural gas Other gases (nitrous oxide, CFCs, ground-level ozone..) 2 weeks Aerosols BA Thames Valley February2004 Power generation, transport 2 weeks CLIMATE CONFUSION •The greenhouse effect is a natural phenomenon which makes life on Earth possible •The enhanced greenhouse effect is expected to lead to global warming •Detecting a change in climate is not the same as attributing it to a particular cause (e.g. human activity) BA Thames Valley February2004 Do we understand past changes? There is evidence that the Earth has been very warm in the past 10 years... BA Thames Valley February2004 Although it’s really about global change… some places have seen very large warming, other regions have actually cooled. BA Thames Valley February2004 The evidence for global warming is mounting! INCREASE • Length of freeze free season • Length of growing season • More frequent heat waves • Wetter winters/Drier summers • Sea level rise BA Thames Valley February2004 DECREASE • Extent of mountain glaciers • Sea-ice amounts and thickness • Fewer Frosts Climate change is not unprecedented The earth has been capable of rapid transitions in the past. But ….. BA Thames Valley February2004 The northern hemisphere is probably the warmest it’s been for at least 1000 years… Mann et al., Science 1999 (Northern Hemisphere only) BA Thames Valley February2004 … the rise in temperature is strongly correlated with a rise in the emission of certain gases into the atmosphere by human activities…. Concentrations of these gases are at record levels in the atmosphere BA Thames Valley February2004 … And the present trends in CO2 and methane are outside those experienced on earth for at least the last 400,000 years….. BA Thames Valley February2004 More greenhouse gases leads to more trapping of IR and a warmer surface (As has been observed) SUN Roughly 2 Wm-2 more energy is absorbed by the Earth than it emits Sunlight passes through the atmosphere.. ..and warms the earth. BA Thames Valley February2004 CO2 CO2 CO2 Infra-red radiation is given off by the earth... Arrhenius 1896 Image: BMRC Circumstantial evidence? Eventually we have to use computer models, to understand, and to predict The Science of Climate Prediction Computer models are our laboratory. BA Thames Valley February2004 What is a climate model anyway? • Huge computer codes based on fundamental mathematical equations of motion, thermodynamics and radiative transfer. • These govern: – Flow of air and water - winds in the atmosphere, currents in the ocean. – Exchange of heat between the atmosphere and the earth’s surface – Release of latent heat by condensation during the formation of clouds and raindrops – Absorption of sunshine and emission of thermal radiation BA Thames Valley February2004 The Met Office The earth is represented by a grid of squares, typically of length 250 km, and by a stack of layers. This gives us a 3-D picture of the circulation of the atmosphere and oceans BA Thames Valley February2004 The earth system is also very complex….. Climate models must address the possible interactions and feedbacks between the various components. And they must consider human behaviour and our response to climate change. BA Thames Valley February2004 Are our models qualified to predict? First we have to make sure our models are getting the present climate about right Figures from the IPCC 2001 report BA Thames Valley February2004 We test different combinations of potential causes against observations… NATURAL ONLY NATURAL + MAN-MADE BA Thames Valley February2004 MAN - MADE ONLY “Most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentration” (IPCC 2001) And look for “fingerprints” in data and model results BA Thames Valley February2004 We finally make predictions … How quickly the climate will change in the future depends on: • How much greenhouse gas emissions grow –depends on population growth, energy use, new technologies, etc (“scenario” uncertainty) • How sensitive the climate system is to emissions –how clouds, ice, oceans etc respond to the extra heating (“structural” uncertainty in climate models and impact models) Uncertainty in the predictions BA Thames Valley February2004 Climate changes over the next few decades are predicted to be much larger than we have seen so far, but there is quite a range of “uncertainty” in the predictions… By 2100: •Increase in global mean temperature of 1.4°C - 5.8°C. •Sea level rise by a further 0.09m – 0.88m. BA Thames Valley February2004 IPCC (2001) We can also produce very detailed predictions of climate change, with little idea of how reliable they might be. 2080 temperature change (K) 2080 precipitation change (%) Source: Mat Collins, Hadley Centre We need to be able to judge the probability of a particular change, and to quantify the likely uncertainty in our predictions. BA Thames Valley February2004 Sources of uncertainty in climate prediction • Uncertainties in models (“physics” ensembles) • uncertainties in emissions (“boundary condition” ensembles) • Uncertainties in initial conditions (very important for Weather forecasts) “Ensemble” prediction: run many simulations with small changes to the model setup each time, giving a probablistic forecast BA Thames Valley February2004 Why is there model uncertainty? • Many simplifications and assumptions must be made to make useable models of the climate system • There are often different ways of doing this and sometimes there isn’t a “best” method. Some of these are based on observations – which have an inherent uncertainty • These variations leads to differences in the feedbacks in models and differences in the future predictions BA Thames Valley February2004 You can help us produce the forecast! Be part of a huge ensemble experiment using spare processor capacity of millions of home/work/school PCs! Run your own unique version of the model, helping us to understand the causes of uncertainty in our climate predictions. http://www.climateprediction.net BA Thames Valley February2004 Standard Visualisation Package What will you see? www.climateprediction.net BA Thames Valley February2004 What will we end up with? BA Thames Valley February2004 What about the “UK ice age” theory? “Shut down” of thermohaline circulation leads to cooling of UK BA Thames Valley February2004 Is it likely? IPCC 2001 BA Thames Valley February2004 So far, no model has shown total shutdown before 2100, but climate change may make it more likely in successive centuries From Vellinga and Wood, 2002 Temperature response 20 years after THC collapse If it did shut down, it would produce a cooling tendency But, if you include forcing from increases in greenhouse gases in the model up until 2100, EU temperatures still warm overall BA Thames Valley February2004 Communicating Uncertainty “Uncertainty” conveys a lack of knowledge, lack of understanding or disagreement between scientists. But, as we’ve seen… Our predictions can be uncertain despite actually knowing a lot about the climate system and climate change BA Thames Valley February2004 Communicating uncertainty? • Often it is the least certain (extreme weather) or least likely (thermo-haline shutdown) phenomena that are used as evidence of climate change or predictions of the future • Including probabalities will help balance the discussion. As long as we can communicate the meaning adequately – To customers? – To the public? This is not just a climate change issue! BA Thames Valley February2004 “ If we begin with certainties, we shall end in doubts, but if we begin with doubts, and are patient in them, we shall end in certainties” Francis Bacon “ Real knowledge is to know the extent of one’s ignorance” Confucius BA Thames Valley February2004 BA Thames Valley February2004 Climateprediction.net design Expt 1: UM with simple, thermodynamic ocean. (HadSM3) Aim: To identify parameter combinations which have little effect on the mean climate but a large effect on climate sensitivity. Double CO2 15 yr, 2 x CO2 Calibration 15 yr spin-up Derived fluxes 15 yr, base case CO2 Diagnostics from final 8 yrs. Control Expt 2: Fully coupled model. (HadCM3 - the “coupled” model) Distribute pre-packaged simulations of 1950 -2050. Downweight or eliminate runs which compare badly with observations. Re-distribute the surviving versions to simulate 2000-2050. BA Thames Valley February2004