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 (5-10);

– 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 ?