© Crown copyright Met Office An update on some hot topics Sea level and coastal changes; Gulf...

© 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


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


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?


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


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.



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


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


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



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


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)





future?



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)


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


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


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?


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


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!)


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





future?



Record low sea ice extent in Summer 2007

Source: NSIDC

September 2007 March 2008


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


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


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


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 An update on some hot topics Sea level and coastal changes; Gulf...

Documents

Transcript of © Crown copyright Met Office An update on some hot topics Sea level and coastal changes; Gulf...