© crown copyright met office an update on some hot topics sea level and coastal changes; gulf...
TRANSCRIPT
© 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, 29th October 2008
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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
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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?
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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
Source: IPCC 4th Assessment Report
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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 4th Assessment Report
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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
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Projected future changes in global sea level in IPCC AR4
Source: IPCC 4th 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
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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 CGlobal sea level rise about 7mMelting takes centuries to millennia
Source: IPCC 4th Assessment Report
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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
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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)
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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?
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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)
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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
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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
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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 shutdownSource: IPCC 4th Assessment ReportSo why worry?
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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
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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!)
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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?
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Record low sea ice extent in Summer 2007
Source: NSIDC
September 2007 March 2008
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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
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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
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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
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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.
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