Land use change and global warming

An interesting tidbit that showed up over at Nature's "Nature Reports: Climate Change" in a story on what might be the discovery of the missing carbon sink.

Albedo effect

Other scientists have also recently come to the conclusion that northern forests, although critically important in maintaining biodiversity, might be less important in slowing climate change than tropical forests. Govindasamy Bala and Ken Caldeira found that tropical forests help cool the Earth in two ways: by storing carbon and also by reflecting the suns warming rays back to space⁵. "Unlike tropical forests, high latitude forests darken the Earth's surface, causing the earth to absorb more sunlight, an effect that is most pronounced in snowy regions. This darkening of the surface has a warming influence that can be stronger than the cooling influence of carbon storage in these forests," says Caldeira. This suggests that removing high-latitude forests would have a net cooling effect on the planet, whereas removal of tropical forests would result in warming.

What is interesting here is what has been happening for the past 30 to 50 years. Northern forests have been growing and tropical forests have been shrinking. Thus, according to the above, this should lead to warming. How much of this land use change impact is included in the models of climate change? Not sure.

New Model: Some near term offset of anthropogenic warming

A new model, published in Science, that includes more information about the internal variability of the Earth system (e.g. El Ninos, etc.) predicts some potential for ameliorating anthropogenic warming in the next tens years (yet about 50% of the years after 2009 are still predicted to be warmer than 1998 (the warmest so far)).

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Science 10 August 2007:

Vol. 317. no. 5839, pp. 796 - 799
DOI: 10.1126/science.1139540

Improved Surface Temperature Prediction for the Coming Decade from a Global Climate Model
Doug M. Smith,* Stephen Cusack, Andrew W. Colman, Chris K. Folland, Glen R. Harris, James M. Murphy

Previous climate model projections of climate change accounted for
external forcing from natural and anthropogenic sources but did not
attempt to predict internally generated natural variability. We
present a new modeling system that predicts both internal variability
and externally forced changes and hence forecasts surface temperature
with substantially improved skill throughout a decade, both globally
and in many regions. Our system predicts that internal variability
will partially offset the anthropogenic global warming signal for the
next few years. However, climate will continue to warm, with at least
half of the years after 2009 predicted to exceed the warmest year
currently on record.


Fig. 4. Globally averaged annual mean surface temperature
anomaly (relative to 1979–2001) forecast by DePreSys starting from June
2005. The CI (red shading) is diagnosed from the standard deviation of
the DePreSys ensemble, assuming a t distribution centered on
the ensemble mean (white curve). Also shown are DePreSys and ensemble
mean NoAssim (blue curves) hindcasts starting from June 1985 and June
1995, together with observations from HadCRUT2vOA (black curve).
Rolling annual mean values are plotted seasonally from March, June,
September, and December. The mean bias as a function of lead time was
computed from those DePreSys hindcasts that were unaffected by Mount
Pinatubo (SOM text) and removed from the DePreSys forecast (but not the
hindcasts).


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