New Paper “Terrestrial Biogeochemical Feedbacks In The Climate System” By Arneth Et Al 2010

I have been alerted to a new paper [h/t to Bill DiPuccio] which provides yet further documentation on how complex the climate system is.  The paper is

Arneth et al, 2010: Terrestrial biogeochemical feedbacks in the climate system. Nature Geoscience 3, 525 – 532 (2010)
Published online: 25 July 2010 | doi:10.1038/ngeo905.

The abstract reads

“The terrestrial biosphere is a key regulator of atmospheric chemistry and climate. During past periods of climate change, vegetation cover and interactions between the terrestrial biosphere and atmosphere changed within decades. Modern observations show a similar responsiveness of terrestrial biogeochemistry to anthropogenically forced climate change and air pollution. Although interactions between the carbon cycle and climate have been a central focus, other biogeochemical feedbacks could be as important in modulating future climate change. Total positive radiative forcings resulting from feedbacks between the terrestrial biosphere and the atmosphere are estimated to reach up to 0.9 or 1.5 W m−2 K−1 towards the end of the twenty-first century, depending on the extent to which interactions with the nitrogen cycle stimulate or limit carbon sequestration. This substantially reduces and potentially even eliminates the cooling effect owing to carbon dioxide fertilization of the terrestrial biota. The overall magnitude of the biogeochemical feedbacks could potentially be similar to that of feedbacks in the physical climate system, but there are large uncertainties in the magnitude of individual estimates and in accounting for synergies between these effects.”

A significant conclusion that they report is that

“Given the limited number of quantitative experiments available, our estimates of the total radiative forcing from biogeochemical feedbacks that operate in the climate– chemistry system can only serve as a very rough guide. Nonetheless, we conclude that the climate effect of changes in element cycles and atmospheric chemistry is comparable in magnitude to that of physical feedbacks such as water vapour or clouds.”  [underline added]

Figure 1 in their paper is quite interesting. Its caption reads

“Figure 1 [R]adiative forcing from terrestrial biogeochemistry feedbacks in response to anthropogenic atmospheric and climate changes. Bars indicate the approximated minimum-to-maximum feedback range over the twenty-first century. The carbon cycle CO2 and climate feedbacks include average and standard deviations (white)….Totals include top-end estimates, without and with (black rectangle) C–N interactions. Estimates are based on a number of assumptions that had to be made and clearly point to the need for more interdisciplinary studies. Confidence in scientific understanding is at best low (L), in most cases very low (VL).”

This figure is with respect to a global average radiative forcing and does not consider the heterogenous effect of these feedbacks on regional scale climate. Even so, they conclude “at best”  a low, and in most cases, a very low level of scientific understanding. The paper, unfortunately, separates “atmospheric changes” from “climate changes” in their caption which is an error as the atmosphere is, of course, part of the climate system (see Figure 1-1 in NRC, 2005).

This paper further reinforces our conclusion in

Pielke Sr., R., K. Beven, G. Brasseur, J. Calvert, M. Chahine, R. Dickerson, D. Entekhabi, E. Foufoula-Georgiou, H. Gupta, V. Gupta, W. Krajewski, E. Philip Krider, W. K.M. Lau, J. McDonnell,  W. Rossow,  J. Schaake, J. Smith, S. Sorooshian,  and E. Wood, 2009: Climate change: The need to consider human forcings besides greenhouse gases. Eos, Vol. 90, No. 45, 10 November 2009, 413. Copyright (2009) American Geophysical Union.

with respect to the diversity of climate forcings and feedbacks, most of which were inadequately assessed (or even reported on) in the 2007 IPCC WG1 report.

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