Tipping Points or Gradual Climate Change?

1
Tipping Points or Gradual Climate Change?
Tim Lenton1,2 1School of Environmental Sciences,
University of East Anglia, Norwich, UK 2Tyndall
Centre for Climate Change Research, UK Hermann
Held3, Elmar Kriegler3,4, Jim Hall2,5, Wolfgang
Lucht3, Stefan Rahmstorf3, John
Schellnhuber2,3 3Potsdam Institute for Climate
Impact Research (PIK), Potsdam,
Germany 4Department of Engineering and Public
Policy, Carnegie Mellon University, Pittsburgh,
USA 5School of Civil Engineering and Geosciences,
University of Newcastle-upon-Tyne, UK
2
Methods

• UK-German workshop
• British Embassy, Berlin, 5-6 October 2005
• 36 participants
• International expert elicitation exercise
• Led by Elmar Kriegler and Jim Hall
• 43 participants
• Literature review
• Hundreds of papers

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Little things can make a big difference

• Tipping element
• A component of the Earth system, at least
  sub-continental in scale (1000km), that can be
  switched under certain circumstances into a
  qualitatively different state by a small
  perturbation.
• Tipping point
• The corresponding critical point in forcing and
  a feature of the system at which the future
  state of the system is qualitatively altered.

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Defining a tipping element

• For components of the Earth system (S)
• The parameters controlling the system can be
  transparently combined into a single control r,
  and there exists a critical control value rcrit
  from which a small perturbation by dr gt 0 leads
  to a qualitative change F in a crucial system
  feature F, after some observation time T gt 0,
  i.e.
• F(r rcritdrT) F(rcritT) F gt 0

Lenton et al. (submitted) PNAS
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D-O events and Indian monsoon
Indian Ocean cave stalagmite proxy for monsoonal
precipitation
(Ages now shifted 2.3kyr younger)
Greenland ice core proxy for local temperature
Transition at onset of interstadial 12 occurs
within 25 years!
Burns et al. (2003) Science 301 1365-1367.
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Policy relevant tipping elements

• Add to the definition the following conditions
• Human activities are interfering with the system
  S such that decisions taken within a political
  time horizon (TP gt 0) can determine whether
  rcrit is reached.
• The time to observe a qualitative change plus the
  time to trigger it lie within an ethical time
  horizon (TE tcrit T).
• A significant number of people care about the
  fate of the system S.
• We use TP 100 years, TE 1000 years

Lenton et al. (submitted) PNAS
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Location of potential policy relevant tipping
elements
ENSO El Nino Southern Oscillation
Lenton et al. (submitted) PNAS
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Arctic sea-ice
Credit NASA
Line represents average ice edge for 1979-2004
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Potential policy relevant future tipping elements
in the climate system
Already tipped
Lenton et al. (submitted) PNAS
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Some candidates eliminated
Decline of summer Arctic sea-ice quasi-linear
and/or threshold already passed Threshold for
year-round sea-ice loss inaccessible this century
(IPCC AR4 models)
Lenton et al. (submitted) PNAS
11
Some candidates eliminated
Decline of summer Arctic sea-ice quasi-linear
and/or threshold already passed Threshold for
year-round sea-ice loss inaccessible this century
(IPCC AR4 models)
Only one model study showing collapse of AABW
formation, more research required
Lenton et al. (submitted) PNAS
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Some candidates eliminated
Decline of summer Arctic sea-ice quasi-linear
and/or threshold already passed Threshold for
year-round sea-ice loss inaccessible this century
(IPCC AR4 models)
Permafrost melt and tundra loss
quasi-linear Methane release feedback only modest
Only one model study showing collapse of AABW
formation, more research required
Lenton et al. (submitted) PNAS
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Policy relevant future tipping elements in the
climate system
Lenton et al. (submitted) PNAS
Results from literature review and workshop
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Ranking of six tipping elements
Lenton et al. (submitted) PNAS
Results from expert elicitation
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Melt of Greenland ice sheet
Recent mass balance ( global sea-level) 1996
-9131km3 ice/yr (0.230.08 mm/yr) 2000
-13831km3 ice/yr (0.350.08 mm/yr) 2005
-22441km3 ice/yr (0.570.1 mm/yr)
Increased summer melting
Credit NASA
Accelerating glacier retreat (Jakobshavn Isbrae)
Alley et al. (2005) Science 310 456-460
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Reorganisation of thermohaline circulation
Atlantic MOC (Sv)
Atlantic MOC (Sv)
GENIE-1 model, data assimilated with Ensemble
Kalman Filter, for IPCC AR4, 3C climate
sensitivity
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Instability of West Antarctic ice sheet
Velicogna Wahr (2006) Science 311 1754-1756
Shepherd Wingham (2007) Science 315 1529-1532
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Increase in ENSO amplitude
Normal
El Niño

• Increase in ENSO amplitude in most realistic
  models under 3-6C warmer stabilised climate
• No clear change in El Niño frequency

Guilyardi (2006) Climate Dynamics 26 329-348
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Amazon rainforest dieback

• Occurs under 3-4C global warming due to more
  persistent El Niño in HadCM3LC GCM with carbon
  cycle
• Regional climate model (60km) coupled to CCCMA
  GCM

2080-2100 Simulated
1981-2000 Observed
rainforest
rainforest
savanna
savanna
6
shrubland
4
7
3
2
barren
cropland
grassland

• Could be triggered by land-use and/or climate
  change

Cook and Vizy (submitted) Journal of Climate
Cox et al. (2000) Nature 408 184-187
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Boreal forest dieback

• Occurs under 3C global warming (7C local
  warming)
• LPJ Dynamic Global Vegetation Model driven by
  climate change from SRES A2 forcing of HadCM3 GCM

Change in deciduous (DEC) and evergreen (EVG)
woody vegetation from 2000 to 2100
Change in vegetation (VEGC) and soil (SOILC)
carbon content from 2000 to 2100
Joos et al. (2001) Global Biogeochemical Cycles
15 891-907
Lucht et al. (2006) Carbon Balance and Management
1 6
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Interactions between tipping events
Tipping events are connected A?B if at least 5
experts judged that triggering A had a direct
effect on the probability of triggering B
thereafter
Based on results from expert elicitation
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West African Monsoon and future Sahara/Sahel
greening?
Gulf of Guinea JJAS SST anomaly (K)

• Collapse of the monsoon leads to increased inflow
  of moist air from West
• Requires 3K warming of Gulf of Guinea SSTs
• Potential for increased food production in the
  Sahel region

Sahelian JJAS precipitation change (mm day-1)
Cook and Vizy (2006) Journal of Climate 19
3681-3703
Claussen et al. (2003) Climatic Change 57 99-118
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Indian summer monsoon

• Land cover change or aerosol forcing could
  trigger switch to arid regime
• CO2 forcing would encourage return
• Potential roller coaster with huge social
  impacts

Zickfeld et al. (2005) Geophysical Research
Letters 32 L15707.
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Can an approaching tipping point be detected?

• Consider the longest system-immanent timescale, t
• Bifurcations are universally characterised by t?8
  
• In principle, t can be reconstructed through time
  series analysis
• Degenerate fingerprinting method is based on
  diagnosing the decay rate of perturbations

Kleinen, Held Petschel-Held (2003) Ocean
Dynamics 53 53-63
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Test of the detection method
CLIMBER-2 model Linear increase in CO2 from 280
to 800 ppmv Stochastic perturbation of freshwater
forcing
Limitation Demands long, high-resolution
observational record
Held Kleinen (2004) Geophysical Research
Letters 31 L23207
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A tipping point in the human system?
Control variable Carbon price Response
Increased share of renewables Non-linearity Shar
e increases despite carbon price declining
Edenhofer et al. (2006) The Energy Journal
Special Issue 57-108
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Conclusions

• Society may be lulled into a false sense of
  security by projections of gradual change
• Tipping elements in the climate system could be
  triggered this century by anthropogenic forcing
• It is unclear that any would be tipped by our
  present warming commitment alone
• To prevent eventual melt-down of the Greenland
  ice sheet melt would require stringent mitigation
  
• In principle, a tipping point can be anticipated
  before it is reached, but in practice
  sufficiently high-resolution, long records are
  lacking
• Improved understanding is needed to help policy
  makers avoid the unmanageable and manage the
  unavoidable

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Implications for Cost Benefit Analysis
CBA ignoring tipping elements
Leads to an optimum at 4 K warming
Lenton et al. (submitted) PNAS
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Implications for Cost Benefit Analysis
CBA with high-impact tipping element and certain
threshold
Optimum temperature switches to a lower target of
2.5 K warming
Lenton et al. (submitted) PNAS
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Implications for Cost Benefit Analysis
Insurance premium for a 2C policy
CBA with high-impact tipping element and
uncertain threshold
Trade-off has to be dealt with by society
Potential loss coming with a 3C policy
Optimist target
Pessimist target
Lenton et al. (submitted) PNAS