Transcript A Land Cover Change intercomparison

A [simple] land cover change
intercomparison
A. Pitman, R. Betts, R. Pielke Sr. et al.
Background
• LCC affects ~45% of the terrestrial surface
(Vitousek et al., 1997)
• likely an underestimate (Williams, 2003)
• Globally distributed but regionally centred
Background
• Deforestation experiments demonstrate an
impact on regional climates
• But some are now attributing large changes in
climate remote from LCC to LCC via
teleconnections
• Mechanisms include Walker and Hadley cell
changes and Rossby wave propagation
Chase et al, 2000
Betts, 2000
Status
• The IPCC (2001) notes possible regional impact of LCC;
• some are interpreting GCM results as evidence of the
global scale impact of LCC;
• Others see LCC only in terms of radiative impacts
• Some see any remote effects of LCC as ‘model
variability’.
• either might be true - but it is something that we need to
know more confidently.
Status
• There are problems with the design of all
attempts to explore the climate impact of
LCC using GCMs
– Many use short (<20-year) simulations for
natural and current vegetation;
– Most perform single realizations;
– Many perform standard t-tests that do not
account for the autocorrelation in the data;
– Spatial resolution tends to be quite coarse.
Proposal
• A LCC intercomparison involving 10-15 groups
with:
– a common land cover perturbation (historical land
cover to current). We might do a future scenario too;
– AMIP-2 length simulations, using the AMIP-2 design;
– multiple realizations with each model (5-10);
– use appropriate statistics to determine whether there
are regional impacts of LCC.
Proposal
• a common land cover perturbation
(historical land cover to current, but we
might do a future scenario too);
– Crops + other [Betts/de Noblet]
– 1900 and 2000 snap-shots
– Static vegetation
– Modellers free to translate changes into pfts
– Future scenario not decided
Proposal
• AMIP-2 length simulations, using the AMIP-2 design;
–
–
–
–
Fixed SSTs
Limiting relevance but cheap and easy: inclusive
Easy for most groups
AMIP-2 standard output format (easy)
• We need to recognise that the set of people who are pushing LCC
as a major climate driver have limited overlap with core climate
modelling groups … limits the level of experimental complexity that
is possible.
• It is more politically important to include these groups that have a
larger sample of core climate modelling groups.
Proposal
• multiple realizations with each model (510);
– Advice from GLASS appreciate on the
number required;
– Advice welcomed on best way to perturb the
sample
Proposal
• use appropriate statistics to determine
whether there are regional impacts of LCC
– Again, advice encouraged.
Timeline
• We wanted to mesh with IPCC [not possible]
• Review paper from the community
• Data sets available by November/December 2004
• Simulations performed by October 2005
• Analysis over the subsequent six months.
• data will be made available to individual groups
Objectives
• We do not aim to “answer” the LCC question;
• We aim to start a process – if the LCC community
conduct these experiments and the answers are
interesting, we have a common foundation to build from
• Our experiments are limiting – but we have to balance
what is achievable by the specific community we are
trying to involve
• If GLASS thinks the experiments are too limiting then we
would prefer to know now !
Questions
• Is this worth doing ?
– relatively cheap, but it is limited in scope;
– too slow for IPCC 4th assessment
– would force some to confront model variability cf.
teleconnection issue
•
•
•
•
Is AMIP-2 ok as a framework ?
Advice on the LCC data ?
Realizations ?
Statistics ?