CESD 1 SAGES Scottish Alliance for Geoscience, Environment & Society Climate Change: Observing and Simulating the Past; Predicting the Future Simon Tett, Chair of Earth System Dynamics & Modelling With thanks to Gabi Hegerl, Ben Santer, Phil Jones, Keith Briffa, Peter Thorne, Philip Brohan, Nick Rayner, John Kennedy, Peter Stott, Myles Allen, Gareth Jones, John Mitchell, Geoff Jenkins, Chris Folland, David Parker, Jonathan Gregory, Bob Harwood, Richard Kenway and Claire Jones

CESD 2 What are we trying to understand? Image created by Reto Stockli with the help of Alan Nelson, under the leadership of Fritz Hasler How might the earth system evolve in the future? How and why did it evolve in the past?

CESD 3 What are we modelling? From Space Science and Engineering Center, University of Wisconsin-Madison

CESD 4 Overview Basic physics Modelling the climate system Observations of climate change Using climate models –Understanding 20 th century climate change –Role of natural drivers in natural variability –Predictions of future change –Importance of external drivers Concluding thoughts

CESD 5 Radiation – the driver of the climate system Key ideas –Lots of incoming shortwave radiation (“Visible”) from sun –Same total energy going out from Earth but peaks in Infra-red. (“Heat”) –Surface is warmer than you’d expect from simple radiation budget. The bit of the climate system that radiates energy to space is high up (where it is cooler). Atmosphere cools with height So surface is warmer the “greenhouse” effect Changing the height of the atmosphere where energy gets to space will then affect the surface temperature

CESD 6 Lapse Rate Temperature falls with height From om/ Tropical Pacific lapse rate

CESD 7 Feedbacks Act to amplify (or decrease) warming from changes in CO 2, other greenhouse gases and other climate drivers. –Blackbody – warmer planet emits more radiation and so cools. (Negative feedback) –Water vapour – warmer atmosphere can store more water vapour. Water vapour absorbs “heat” radiation so is a Greenhouse gas. Most important in the upper troposphere Warmer world will have more moisture in the atmosphere and so will trap more heat. Positive feedback. –Clouds Positive feedback – “trap” “heat” radiation. Particularly true for high clouds Negative feedback – reflect back solar radiation. Particularly true for low clouds –Ice/Albedo feedback. Ice is white and reflects lots of solar energy back to space. Melt ice and more solar radiation absorbed which in turn warms the climate..

CESD 8 Snow/Ice Feedback Image courtesy NASA/GSFC/JPL, MISR Team. See Team. Summer Winter

CESD 9 Climate Modelling Atmospheric modelling has long history – first attempts, using computers, made in 1950’s. General Circulation Models (GCM’s) developed from numerical weather prediction models –Take physical laws and apply them to atmospheric and oceanic motions. –Key is that GCM’s are built bottom up. –Interested in “Emergent Phenomenon”, such as statistics of data, rather than detailed evolution. Other approaches but not covered in this lecture.

CESD 10 Karl and Trenberth 2003 Modelling the Climate System Main Message: Lots of things going on!

CESD 11 General Circulation Models 3-D model of the circulation of the atmosphere and ocean Fundamental equations: Conservation of momentum Conservation of mass Conservation of energy Equation of state

CESD 12 Parameterized Processes Slingo From Kevin E. Trenberth, NCAR Unresolved motions and processes affect the large scale flow so their effect needs to be parameterized.

CESD 13 What are we trying to parameterize? What is there… How we parameterise

CESD 14 Numerical Modelling Cray Y-MP ~ 1990 HECToR – 2008 L. F. Richardson circa 1920 Since the 1960’s super-computer computational power increased by factor of 16 every decade. Over my career increased fold

CESD 15 Observing Climate Change Observing system not stable Climate changes slowly compared to observing system.

CESD 16 Global Mean Temperature From Brohan et al, 2006

CESD 17 The longer perspective Recent warming unprecedented

CESD 18 Changes in Upper Ocean temperatures From Palmer et al, 2007 The upper ocean is warming at, when looked one way, at roughly the same rate everywhere

CESD 19 Changes in the free atmosphere: Large Observational Uncertainty From Thorne et al, 2005 & Titchner et al, 2008 Left plot shows cooling in the tropical atmosphere. Contradicts climate models which predict largest warming in the tropics. Right hand plot shows range of possible temperature changes in tropical free atmosphere due to uncertainties in observations. Sometimes models are more reliable than observations!

CESD 20 Model Applications Understanding 20 th century climate change The role of natural and human drivers in climate variability Future scenarios Summary: external drivers important in explaining observed climate variability and future climate change

CESD 21 What might cause observed change?

CESD 22 Internal variability – variability generated within the climate system Recent tropical Pacific ocean temperatures from IRI source by Martin Visbeck The North Atlantic Oscillation

CESD 23 Natural Factors that might effect climate: Volcanoes Volcanic Aerosol depth Large tropical volcanoes inject sulphur dioxide into the Stratosphere where it stays for 2-3 years. Effect is to make an aerosol that scatters light and so cools climate.

CESD 24 Natural Factors that might effect climate: Solar Irradiance Solar activity (sunspots etc) & irradiance changes with 11- year solar cycle. There are long term changes in solar activity – the Maunder Minimum being one example. Converting this to changes in solar irradiance can be done though very uncertain. “Sun-like” starts which show activity variations have been used to estimate irradiance changes. Recent work (astronomical) and modelling (Lean et al) suggests there may be no significant long term variation in solar irradiance Sunspot Number

CESD 25 Human Factors that might affect climate: Aerosols Thanks to Met Office

CESD 26 Human Factors that might affect climate: Greenhouse gases 2000 Ice cores Flasks Year Mauna Loa Observatory Ice cores Year CO 2 MMR*10 6 CH 4 MMR*10 9 Greenhouse gas concentrations have changed over the last century. Their effect is to decrease the transmission of heat radiation by the atmosphere. So should warm climate.

CESD 27 Understanding and Attributing Climate Change in the 4 th Assessment Globe, Land, Ocean and individual continents all likely show human induced warming. Warming effect of greenhouses gases likely offset by other human and natural drivers

CESD 28 Modelling the last 500 years How important are external drivers compared to internal climate variability? Simulation with fixed drivers – “internal” variability alone. Simulation with only natural drivers –Sun & Volcanic eruptions Simulation with human and natural drivers –Natural + changes in greenhouse gases, aerosols, and land-surface properties

CESD 29 Natural Drivers Annual: Slow changes with large negative forcings (from volcanoes) 25-year Gaussian filter. Solar and Volcanic forcing as important as one another. “Maunder Minimum” includes volcanic contribution. Tambora is largest eruption of last 500 years. Late 20 th century is also a volcanically active period. Solar Volcanic

CESD 30 Effect of natural drivers Both hemispheres change together as does the land & ocean though there are some differences. Natural variability is about ±0.3K compared to “internal” variability of ±0.1K. So Natural forcings are an important driver of global-scale temperature variability SH has less variability (as more ocean) than does NH

CESD 31 Naturally driven variability Effect of natural drivers is to increase variability in the tropics

CESD 32 Adding human drivers Greenhouse gases Aerosols Volcanoes Sun Tot. Natural Total Human Aerosols and volcanoes offset some GHG and solar warming

CESD 33 Temperature Changes with human drivers included Effect of human drivers is to warm climate so that it warms outside envelope of natural variability by mid- late 19 th century in southern hemisphere land and by mid 20 th century in northern hemisphere

CESD 34 Effect of human drivers of climate Shows impact of human drivers on zonal-average temperature. Tropics warm first and warming is significant by mid 19 th century. Northern hemisphere warming delayed by aerosol cooling in simulation

CESD 35 Predicting the Future Material in this section from IPCC 4th assessment report. Results based on multi-model archive. Typically show average across all model simulations with uncertainties from range Scenarios used to drive models. Self- consistent atmospheric concentrations of CO2 and other greenhouse gases. Based on different human development paths

CESD 36 Projections of Future Changes in Climate Best estimate for low scenario (B1) is 1.8°C (likely range is 1.1°C to 2.9°C), and for high scenario (A1FI) is 4.0°C (likely range is 2.4°C to 6.4°C).

CESD 37 Projected warming in 21st century expected to be greatest over land and at most high northern latitudes and least over the Southern Ocean and parts of the North Atlantic Ocean Projections of Future Changes in Climate

CESD 38 Projections of Future Changes in Climate Precipitation increases very likely in high latitudes Decreases likely in most subtropical land regions

CESD 39 Is climate changing faster than we thought it would? Lot of argument has been about reality of climate change –Are observations good? –Is the sun responsible for warming? –Feedbacks are weak so that future warming not likely to be a great threat? General consensus (see 4 th Assessment report) is that climate is changing, likely due to human influences and agreement between different models as to likely warming. But could models be underestimating future climate change?

CESD 40 What does the future hold? Ensemble of “perturbed physics” models showing large uncertainty range of future warming. Which are right? Climate Sensitivity – measure of feedbacks. “Long tail” suggests there may be strong feedbacks.

CESD 41 Sea-ice (its ½ what is should be) Is this unexpected? Are we missing something fundamental in our understanding of the Earth system?

CESD 42 Sea-Ice NASA/GODDARD SPACE FLIGHT CENTER SCIENTIFIC VISUALIZATION STUDIO; (DATA) ROB GERSTON, GSFC

CESD 43 Circulation change important for regional changes Observations Model mean Human influence detected on Sea Level Pressure BUT magnitude under-simulated in Northern Hemisphere (e.g. Gillett et al., 2005) These problems will affect regional model simulations and regional predictions Multi-model archive From Gabi Hegerl NH SH

CESD 44 UK Extreme events Tewkesbury 2007Photograph: Daniel Berehulak/GettyImages Met Office figures show that May to July in the England and Wales Precipitation is the wettest in a record that began in We must learn from the events of recent days. These rains were unprecedented, but it would be wrong to suppose that such an event could never happen again…. (Hazel Blears, House of Commons, July 2007) Is it human induced climate change or natural variability?

CESD 45 UK changes Precipitation (blue) and temperature (red) for and 1981-date (dashed) High summer drying and warming. Rest warming and moistening

CESD 46 Change over last century Observations distinct from zero, consistent with all and inconsistent with natural. Implies human influence on UK climate. Does model underestimate high summer changes? Natural All Obs

CESD 47 Summary & Conclusions Basic understanding of the climate system explains greenhouse effect and why would expect warming in response to changing atmospheric composition Details of response come from feedbacks Climate models are built “bottom up” not top down. Uncertainties arise from need to parameterize unresolved phenomenon Interested in the emergent behaviour which is not easily predictable from basic physics in model. Instrumental observations of surface temperature back to mid 19 th century This, and other observations, show clear evidence of warming and climate change.

CESD 48 Summary & Conclusions Using models and observations to establish that: –20 th century climate change is likely to be human driven with greenhouse gas warming being offset by natural and other human drivers –That external drivers are an important driver of natural climate variability –That humans might have affected 19 th century tropical climate. –Climate change has already happened and will continue to happen regardless of what we do. –But will be large if emissions are not reduced. Models may be underestimating changes to come particularly those related to changes in atmospheric circulation. –This has important consequences for regional (i.e. UK) climate change.

CESD 49 The End! Thanks for listening Any Questions?

CESD 50 Extra Material

CESD 51 (Natural) Variability in Extreme events can be large From Allan et al, ’s 60’s 90’s

CESD 52 Trends since July-Aug CET Oct-May CET July-Aug EWP Oct-May EWP Obs0.42 K/Cent 0.50 K/Cent -8.2 % /Cent 6.3% /Cent Model0.35 K/Cent +/ K/Ce nt +/- 0.3 Biggest difference is +/ % /Cent +/- 9% 1.7% /Cent +/- 4% Biggest difference is 10% Model not capturing drying trend

CESD 53 Available observed weather data are limited before 1950 and almost non-existent before Many more observations exist, in logbooks, reports and other paper records (mostly in the UK). If we digitised them we could improve the climate record and extend it back to Hadley Centre digitised observations from Royal Navy Ships logbooks for WW2. These give a much-improved picture of 1940s climate. Digitisation as a source of new data

CESD & 2007

CESD 55 Another feedback: The Carbon cycle From Friedlingstein et al, Plots shows additional CO2 from feedbacks between climate change and carbon cycle. Values vary between 25 and 225 ppm at 2100 mostly due to land-carbon cycle feedbacks.

CESD 56 Relative Contributions Greenhouse gases Other human Natural

CESD 57 Assessing Recent Change Observed trend (K/decade) marked with X where outside maximum absolute 50-year trend from Natural. + where outside maximum trend. Recent changes are outside simulated natural variability over large parts of the world. Suggests that natural systems are already being affected by climate change

CESD 58 Fig 9.18b Observed (black) and simulated precipitation trends Observations Multi-model mean Model range Thin solid line  model’s all forcing response detected in obs Figure from IPCC WGI Ch9 (Hegerl, Zwiers et al) Zonal 20th century precipitation change Changes in rainfall over NH underestimated by models?

CESD 59 Ranking From John Kennedy & the Met Office

CESD 60 Radiation

CESD 61 Timeseries of UK records

CESD 62 Actual heat waves

CESD 63 Future changes in the Hydrological Cycle