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An update on some ‘hot topics’ Sea level and coastal changes; ‘Gulf Stream’; Arctic sea ice Richard Wood Head, Climate, Cryosphere and Oceans SDC Climate Change Seminar, Edinburgh, 29th October 2008 © Crown copyright Met Office Met Office Hadley Centre • Established 1990 as UK focus for climate change science • Policy-relevant science, but not policy • Around 100 staff, mostly research scientists • Funded primarily by Defra/DECC and MoD through the ‘Integrated Climate Programme’ • Other funding sources, e.g. EC, increasing contracts with specific customers for tailored information (e.g. EA re Thames Estuary) • A major player in global climate science, e.g. provided 9 lead authors to the IPCC 4th Assessment Report • Primary source of climate information/projections for UKCIP © Crown copyright Met Office Some ‘hot topics’ in climate change (and some progress since IPCC AR4) • Sea level and coastal changes • The ‘Gulf Stream’ and climate change: heading for a cooler future? • Arctic sea ice: disappearing faster than expected? © Crown copyright Met Office Some ‘hot topics’ in climate change (and some progress since IPCC AR4) • Sea level and coastal changes • The ‘Gulf Stream’ and climate change: heading for a cooler future? • Arctic sea ice: disappearing faster than expected? © Crown copyright Met Office Global sea level has been rising • From 1993-2003 rising at 3 mm/yr • Longer term rate is 1.8 mm/yr • Other periods in past show similar rapid rise • Can’t be sure yet whether recent faster rise will be sustained © Crown copyright Met Office Source: IPCC 4th Assessment Report Causes of sea level rise In decreasing order of their contribution to recent sea level rise: • Thermal expansion: warmer water expands • Melting land ice: glaciers and small ice caps • Melting of Greenland and Antarctic ice sheets. Recent evidence of fast changes that are not included in current models • Melting sea ice: NO EFFECT ON SEA LEVEL since ice is already floating • The relative contributions of the different components is likely to change over time, so need to model each of them in some detail. © Crown copyright Met Office Source: IPCC 4th Assessment Report Some land ice has been on the move • Some outlet glaciers have been moving faster than previously thought • Physics is not well understood • Temporary blip or long term acceleration? • Effect is not included in current ice sheet models © Crown copyright Met Office Projected future changes in global sea level in IPCC AR4 • Scientific knowledge is insufficient to make ‘best estimate’ of rapid ice sheet component • Illustrative scenarios only for these components • Scaling up recent imbalance with rising temperature adds another 0-20cm © Crown copyright Met Office Source: IPCC 4th Assessment Report Sustained warming above some threshold could lead (eventually) to complete loss of the Greenland ice sheet Threshold warming probably somewhere in range 1.9 – 4.6 deg C Global sea level rise about 7m Melting takes centuries to millennia © Crown copyright Met Office Source: IPCC 4th Assessment Report Impacts of sea level rise are likely to be felt through extremes: storm surges • Short lived increases in local sea level • Driven by low atmospheric pressure and strong winds in shelf seas • Need to model changes in mean sea level and in winds and storminess uncertainty in response • Improved scenarios for UKCIP08 © Crown copyright Met Office Sea level summary • Driven primarily by ocean expansion and by melting land ice • Land ice processes not well understood: science is controversial and evolving. • Demand for policy-relevant predictions is ahead of scientific understanding. Better models coming on stream in a few years. • Impacts through extremes such as surges • Impact somewhat reduced for Scotland due to upward land movement • UKCIP08: probabilistic scenarios and ‘worst plausible case’ scenario for contingency planning (may help to avoid unnecessary adaptation cost) © Crown copyright Met Office Some ‘hot topics’ in climate change (and some progress since IPCC AR4) • Sea level and coastal changes • The ‘Gulf Stream’ and climate change: heading for a cooler future? • Arctic sea ice: disappearing faster than expected? © Crown copyright Met Office The Atlantic ‘Meridional Overturning Circulation’ (a.k.a. MOC, thermohaline circulation, THC, conveyor belt, ‘Gulf Stream’) • Cooling in North Atlantic drives sinking and southward flow • ‘Sucks’ warm water northwards to North Atlantic • Warms whole of northern hemisphere climate • Potential to weaken quickly or irreversibly (evidence from geological past and from a range of models) © Crown copyright Met Office Climatic impact of a hypothetical MOC shutdown Cooling over UK: 2-5 °C (greatest over Scotland and Northern Ireland) Temperature (°C) Drying over Western Europe: ~15% N.B. Sea level rise of 20-50 cm around North Atlantic Precipitation (m/yr) © Crown copyright Met Office • Climate model experiment with artificially-induced shutdown of MOC Combined surface temperature effect of a hypothetical THC shutdown and global warming Anomalies (vs. preindustrial climate) in 1st decade after a hypothetical THC collapse in 2049 (under IS92a greenhouse forcing) N.B. This is a ‘what-if?’ experiment, not a prediction, projection or scenario © Crown copyright Met Office Projected response of MOC to climate change • Models suggest gradual weakening of MOC through 21st Century (0-50%) • Weakening moderates rate of warming around the North Atlantic: this effect is already included in climate projections • No rapid shutdown So why worry? © Crown copyright Met Office Source: IPCC 4th Assessment Report Reasons for concern ‘It must be possible – I saw it at the movies…’ From ‘The Day After Tomorrow’, 20th Century Fox 2004 Some more serious reasons: Evidence of rapid switches (on and off) in the past – but last event 8200 years ago and seems to need large supply of fresh water from melting ice sheets (not around today) Many models suggest there is a threshold beyond which the MOC may shut down irreversibly – most but not all this evidence comes from simplified climate models Irreversibilty is important if we ‘overshoot’ on the way to stabilising greenhouse gases © Crown copyright Met Office Has the MOC been changing recently? • Directly observed 5 times in history • Decreasing trend or just poorly sampled wiggles? • Monitoring system now in place (since 2004). First year shows… • …year-to-year variations may mask trend • Need for continuous observations (and patience!) © Crown copyright Met Office Source: NERC RAPID MOC summary • Important for climate of whole northern hemisphere and sea level in North Atlantic • IPCC AR4: Weakening very likely (>90% chance) in 21st Century. Effect is already included in climate projections. • IPCC AR4: Large-scale reorganisation/shutdown very unlikely (<10% chance) in 21st Century • Monitoring system now in place (until 2013). No evidence of slowdown yet (consistent with model predictions) • Concerted research effort over next 4-5 years to: • Assess conditions under which shutdown/irreversible change are likely • Develop ‘early warning’ prediction system © Crown copyright Met Office Some ‘hot topics’ in climate change (and some progress since IPCC AR4) • Sea level and coastal changes • The ‘Gulf Stream’ and climate change: heading for a cooler future? • Arctic sea ice: disappearing faster than expected? © Crown copyright Met Office Record low sea ice extent in Summer 2007 September 2007 © Crown copyright Met Office March 2008 Source: NSIDC Was summer 2007 a sign of accelerating ice loss? Source: NSIDC • Summer 2008 slightly higher than 2007 • Long term trend with year-to-year variations superimposed • Most climate models underestimate the trend – so should we expect the ice to disappear sooner than predicted? • Predictions of imminent disappearance based on extrapolation from past few years © Crown copyright Met Office Will the ice disappear faster than predicted? • Low • HadGEM1 model predicts both trend and level of yearto-year variations © Crown copyright Met Office • Running into future, summer ice disappears around 2060s (mid-range emissions scenario) • The other model that verfies well against observations (CCSM3) predicts disappearance around 2040s Sea ice summary • Record low sea ice in 2007: long-term decline plus year-toyear variation. Some recovery in 2008. • Ice recovered in winter 08/09 – but thinner. Long-term impact? Irreversible/ratchet effect? • Models that can reproduce observed behaviour suggest summer ice will be lost around 2040s-2060s • However models may not reproduce the processes of recent change in detail – so future of Arctic still uncertain © Crown copyright Met Office Closing thoughts • Some global aspects of climate change are well understood and documented with increasing confidence in the IPCC Assessment Reports (e.g. climate is warming, human influence, projected future changes). • Other aspects (e.g. those shown here) are less well understood. Current predictions are more uncertain (but still useful). Generally, uncertainty increases as the level of detail required increases. • Science advances over time, but sometimes bumpily and sometimes slower than policy. • Important not to jump to conclusions from isolated observations or individual modelling studies. Value of IPCC process. • Science agenda is moving towards support for adaptation decisions (predict 1-30 years ahead, more reliable regional detail) – as well as pinning down key uncertainties for mitigation. © Crown copyright Met Office © Crown copyright Met Office