Transcript edinburgh_SDC_wood_081029_final

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?
© 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
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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.
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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
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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
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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
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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
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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)
© 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)
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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)
Drying over Western Europe:
~15%
N.B. Sea level rise of 20-50 cm
around North Atlantic
Precipitation (m/yr)
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•
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
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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 shutdown
So why worry?
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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
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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…
• …year-to-year variations
may mask trend
• Need for continuous
observations (and
patience!)
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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
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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
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Will the ice disappear faster than predicted?
• Low
• HadGEM1 model predicts
both trend and level of yearto-year variations
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• 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
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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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