A new paper (alerted to me by Dev Niyogi of Purdue University) has appeared which documents the complexity of the climate system, and our limited understanding of consequences due to deliberate or inadvertent human climate intervention.
The paper published by Science on November 17, 2006 by Randerson et al in Science is entitled “The Impact of Boreal Forest Fire on Climate Warming” (subscription required) has the abstract,
“We report measurements and analysis of a boreal forest fire, integrating the effects of greenhouse gases, aerosols, black carbon deposition on snow and sea ice, and postfire changes in surface albedo. The net effect of all agents was to increase radiative forcing during the first year (34 Â± Watts per square meter of burned area), but to decrease radiative forcing when averaged over 80-year fire cycle (â2.3 Â± 2.2 Watts per square meter) because multidecadal increases in surface albedo had a larger impact than fire-emitted greenhouse gases. This result implies that future increases in boreal fire may not accelerate climate warming.”
The conclusion of the paper has the very important conclusion that.
“Future interactions between the land surface and climate in northern regions may involve both negative feedbacks within the boreal interior (via mechanisms outlined here) and positive feedbacks involving shrub and forest expansion in arctic tundra ecosystems and loss of snow cover. Our analysis illustrates how ecosystem processes that generate carbon sources and sinks have inseparable consequences for other forcing agents. To the extent that the contemporary Northern Hemisphere carbon sink originates from changes in northern forest cover and age, its value from a climate perspective requires a more nuanced view that encompasses all agents of radiative forcing. Important next steps include reducing uncertainties associated with direct and indirect aerosol effects and disturbance-linked changes in albedo, exploring the combined impacts of feedbacks of the forcing agents estimated here within climate models, and extending this approach to assess the radiative forcing associated with land-cover transitions in temperate and tropical ecosystems.”
The Randerson et al paper also has significant relevance in terms of the sequestration of carbon within vegetation and soils as a component of deliberate human climate intervention.
This new paper supports the perspective in a short essay that I wrote for the Bulletin of the American Meteorological Society in 2001. It is entitled “Carbon sequestration — The need for an integrated climate system approach“.
The essay reads as follows,
“The concern with respect to the anthropogenic input of carbon dioxide into the atmosphere (Houghton et al. 1995) has resulted in proposals for long-term removal programs of this gas based on forestation and agricultural procedures. Referred to as âcarbon sequestration,â? the value of this effort is defined by the amount of CO2 removal and the length of time before it would be reemitted into the atmosphere. The extraction of the CO2 from the atmosphere reduces its contribution as a radiatively active greenhouse gas. Landscapes that would be modified for this purpose have been referred to as âbiomass farms.â?
However, the alteration of the land surface is likely to result in other effects on the heat energy of the atmosphere. Any additional water vapor evaporated or transpired into the atmosphere, for instance, would increase the greenhouse gas warming effect and at least partially offset the benefit of carbon sequestration. Alternatively, a net reduction in water vapor input might enhance the benefit of carbon sequestration with respect to a reduction in greenhouse gas concentrations.
Since, in the atmosphere, however, a water vapor molecule has a much shorter lifetime than a carbon dioxide molecule, the evaluation of changes in transpiration or evaporation would have to consider its net effect over multiyear timescales. Changes in water vapor flux into the atmosphere can also alter cloud and precipitation, so that its net effect on the radiation budget is quite complex.
It is, therefore, somewhat more straightforward to evaluate the change in the long-term surface energy budget due to the landscape change associated with carbon sequestration. A darkening of the land surface, for example, would result in a lower albedo, which would contribute to atmospheric heating (Cotton and Pielke 1995) an effort contrary to the goals of carbon sequestration. Elevating the albedo would add to the goal of carbon sequestration. Just changing the surface albedo from 0.2 to 0.15, for example, can reduce the annual averaged insolation reflected back into space by 5 W mâ2 or more!
There has, unfortunately, been no attempt to evaluate the benefit of carbon sequestration as a means of reducing the concentrations of the radiatively active gas CO2 in the atmosphere, while at the same time, assessing the influence of this sequestration on the radiatively active gas H2O, and on the surface heat energy budget. Until these effects are factored in as part of an integrated climate assessment, a policy based on carbon sequestration as a means to reduce the radiative warming effect of increased atmospheric concentrations of CO2 could actually enhance this warming.
Cotton, W. R., and R. A. Pielke, 1995: Human Impacts on Weather
and Climate. Cambridge University Press, 288 pp.
Houghton, J. T., L. G. Meira Filho, B. A. Callendar, N. Harris,
A. Kattenberg, and K. Maskell, Eds., 1995: Climate Change”
Clearly, the issue of carbon sequestration, and the climate system in general, are not as well understood as claimed in climate assessments such as presented in the previous IPCC reports, nor in the draft AMS Statement on climate change (see). We will see soon if this complexity is reported on in the new IPCC assessment and in the revised version of the AMS draft Statement..