1. © Crown copyright Met Office 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, 29 th October 2008

2. © 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 4 th Assessment Report Primary source of climate information/projections for UKCIP

3. © 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?

4. © 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?

5. © 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 Cant be sure yet whether recent faster rise will be sustained Source: IPCC 4 th Assessment Report

6. © Crown copyright Met Office 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. Source: IPCC 4 th Assessment Report

7. © Crown copyright Met Office 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

8. © Crown copyright Met Office Projected future changes in global sea level in IPCC AR4 Source: IPCC 4 th Assessment Report 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

9. © Crown copyright Met Office 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 Source: IPCC 4 th Assessment Report

10. © Crown copyright Met Office 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

11. © 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)

12. © 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?

13. © 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)

14. © 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) Precipitation (m/yr) Drying over Western Europe: ~15% N.B. Sea level rise of 20-50 cm around North Atlantic Climate model experiment with artificially-induced shutdown of MOC

15. © Crown copyright Met Office 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

16. © Crown copyright Met Office Projected response of MOC to climate change Models suggest gradual weakening of MOC through 21 st Century (0-50%) Weakening moderates rate of warming around the North Atlantic: this effect is already included in climate projections No rapid shutdown Source: IPCC 4 th Assessment Report So why worry?

17. © Crown copyright Met Office Reasons for concern 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 It must be possible – I saw it at the movies… From The Day After Tomorrow, 20th Century Fox 2004

18. © 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… Source: NERC RAPID …year-to-year variations may mask trend Need for continuous observations (and patience!)

19. © Crown copyright Met Office MOC summary Important for climate of whole northern hemisphere and sea level in North Atlantic IPCC AR4: Weakening very likely (>90% chance) in 21 st Century. Effect is already included in climate projections. IPCC AR4: Large-scale reorganisation/shutdown very unlikely (<10% chance) in 21 st 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

20. © 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?

21. © Crown copyright Met Office Record low sea ice extent in Summer 2007 Source: NSIDC September 2007March 2008

22. © Crown copyright Met Office 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

23. © Crown copyright Met Office Will the ice disappear faster than predicted? Low HadGEM1 model predicts both trend and level of year- to-year variations 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

24. © Crown copyright Met Office Sea ice summary Record low sea ice in 2007: long-term decline plus year-to- year 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

25. © 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.

26. © Crown copyright Met Office