> I suspect, but don't know, that climate is a balanced set of dynamic
> interdependencies that do not behave smoothly under perturbations.
well, even putting aside concern over non-smooth jumps, we also have to worry about the sign (direction) of the response .... see below...
Ian Murray said:
> With the evidence that old growth sequesters CO2 better than young
> forests, tree huggers are getting more militant. Expect to see whats
> going on in Oregon, Washington and California catch on amongst
> forest activists all across the globe [hopefully]. Also targeting of
> banks that finance deforestation. Meanwhile, start tree planting
> clubs and hook into Arbor Day rituals.
les schaffer
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Nature 408, 184 - 187 (2000) © Macmillan Publishers Ltd.
Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model
PETER M. COX*, RICHARD A. BETTS*, CHRIS D. JONES*, STEVEN A. SPALL* & IAN J. TOTTERDELL?
* Hadley Centre, The Met Office, Bracknell, Berkshire RG12 2SY, UK ? Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK
Correspondence and requests for materials should be addressed to P.M.C. (e-mail: pmcox at meto.gov.uk).
The continued increase in the atmospheric concentration of carbon dioxide due to anthropogenic emissions is predicted to lead to significant changes in climate[1]. About half of the current emissions are being absorbed by the ocean and by land ecosystems[2], but this absorption is sensitive to climate[3, 4] as well as to atmospheric carbon dioxide concentrations[5], creating a feedback loop. General circulation models have generally excluded the feedback between climate and the biosphere, using static vegetation distributions and CO2 concentrations from simple carbon-cycle models that do not include climate change[6]. Here we present results from a fully coupled, three-dimensional carbon-climate model, indicating that carbon-cycle feedbacks could significantly accelerate climate change over the twenty-first century. We find that under a 'business as usual' scenario, the terrestrial biosphere acts as an overall carbon sink until about 2050, but turns into a source thereafter. By 2100, the ocean uptake rate of 5 Gt C yr-1 is balanced by the terrestrial carbon source, and atmospheric CO2 concentrations are 250 p.p.m.v. higher in our fully coupled simulation than in uncoupled carbon models[2], resulting in a global-mean warming of 5.5 K, as compared to 4 K without the carbon-cycle feedback.
[snip]
[snip many caveats at end]
============
Nature 408, 187 - 190 (2000) © Macmillan Publishers Ltd.
Offset of the potential carbon sink from boreal forestation by decreases in surface albedo
RICHARD A. BETTS
Hadley Centre for Climate Prediction and Research, The Met Office , Bracknell, Berkshire RG12 2SY, UK
Correspondence should be addressed to the author (e-mail: rabetts at meto.gov.uk).
Carbon uptake by forestation is one method proposed[1] to reduce net carbon dioxide emissions to the atmosphere and so limit the radiative forcing of climate change[2]. But the overall impact of forestation on climate will also depend on other effects associated with the creation of new forests. In particular, the albedo of a forested landscape is generally lower than that of cultivated land, especially when snow is lying[3-9], and decreasing albedo exerts a positive radiative forcing on climate. Here I simulate the radiative forcings associated with changes in surface albedo as a result of forestation in temperate and boreal forest areas, and translate these forcings into equivalent changes in local carbon stock for comparison with estimated carbon sequestration potentials[10-12]. I suggest that in many boreal forest areas, the positive forcing induced by decreases in albedo can offset the negative forcing that is expected from carbon sequestration. Some high-latitude forestation activities may therefore increase climate change, rather than mitigating it as intended.
[snip]
The work I report here has focused on perturbations to the Earth's radiation budget, which is the fundamental driver of the climate system. Forestation may also influence the climate by modifying the fluxes of heat, moisture and momentum between the land surface and atmosphere. Whereas boreal forests warm their local climate through reduced albedo, tropical forests tend to cool and moisten their local climates by greatly enhancing evaporation. Both may also influence distant regional climates via the atmospheric circulation[9, 27]. Assessment of the effect of forestation on climate at a given time in the future will require simulations with a climate model that incorporates vegetation dynamics[25, 28] and other atmospheric, terrestrial and oceanic components of the carbon cycle[28], in which forest growth occurs at appropriate rates in relation to changes in atmospheric CO2 and snow cover. Nevertheless, my results suggest that high-latitude forestation would exert a positive radiative forcing through reduced albedo that in many places could outweigh the negative forcing through carbon sequestration. If afforestation and reforestation are required to decrease radiative forcing rather than simply to reduce net CO2 emissions, then changes in surface albedo must also be considered.