Forests and climate: a warming paradox.

![Figure][1] Yatir forest. In their Report, Rotenberg and Yakir studied the semi-arid Yatir forest in Israel. CREDIT: THE WEIZMANN INSTITUTE E. Rotenberg and D. Yakir (“Contribution of semi-arid forests to the climate system,” Reports, 22 January, p. [451][2]) showed that forestation may not be an effective tool for climate change mitigation. They found that in a semi-arid landscape, the warming potential of a forest due to changes in the surface albedo and the longwave radiation emission far outweighs the cooling effect due to carbon sequestration. However, their analysis did not address the fact that the radiation balance of the surface is not the same as the radiation balance of the climate system. The atmosphere retains a significant portion of the longwave radiation emitted and the shortwave radiation reflected by the surface. Globally, only 10% of the surface longwave radiation escapes the atmosphere to the outer space ([ 1 ][3]). The escape fraction over Rotenberg and Yakir's site is probably higher due to low cloud cover, but not by much: The outgoing longwave radiation for a clear sky at the top of the atmosphere suggests a maximum of 20% ([ 2 ][4]). Similarly, because of atmospheric absorption and cloud reflection, the local albedo at the top of the atmosphere is lower than the surface value. By not taking into account this energy redistribution, Rotenberg and Yakir may have substantially overestimated the warming effect of forestation (and the cooling effect of desertification). A deeper issue, also related to energy redistribution, is whether it is accurate to combine the CO2 radiative forcing and the surface radiation change for the purpose of analysis. To help policy discussions, the greenhouse effects are often expressed as climate sensitivity ([ 3 ][5]), estimated at ∼0.8°C increase in the surface temperature per W m−2 increase in the radiative forcing ([ 4 ][6]). The surface exchange process does not work that way. Rotenberg and Yakir's paradoxical result—that the forest, being an efficient convector, is much cooler despite more radiation loading than the shrubland—provides a powerful argument against combining the two quantities. In humid climates, forests also cool the surface by removing its latent heat, which is released above the atmospheric boundary layer by cloud condensation. 1. [↵][7]1. K. E. Trenberth 2. et al ., Bull. Am. Meteorol. Soc. 90, 331 (2009). [OpenUrl][8] 2. [↵][9]1. S. K. Yang 2. et al ., J. Clim. 12, 477 (1999). [OpenUrl][10][CrossRef][11] 3. [↵][12]1. V. Ramanthan, 2. G. Carmichael , Nat. Geosci. 1, 221 (2008). [OpenUrl][13][CrossRef][14] 4. [↵][15]1. S. Solomon IPCC, Summary for Policymakers (SPM), in Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, S. Solomon et al ., Eds. (Cambridge Univ. Press, New York, 2007). [1]: pending:yes [2]: /lookup/doi/10.1126/science.1179998 [3]: #ref-1 [4]: #ref-2 [5]: #ref-3 [6]: #ref-4 [7]: #xref-ref-1-1 "View reference 1 in text" [8]: {openurl}?query=rft.jtitle%253DBull.%2BAm.%2BMeteorol.%2BSoc.%26rft.volume%253D90%26rft.spage%253D331%26rft.atitle%253DBULL%2BAM%2BMETEOROL%2BSOC%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [9]: #xref-ref-2-1 "View reference 2 in text" [10]: {openurl}?query=rft.jtitle%253DJ.%2BClim.%26rft.volume%253D12%26rft.spage%253D477%26rft_id%253Dinfo%253Adoi%252F10.1175%252F1520-0442%25281999%2529012%253C0477%253AEOTERB%253E2.0.CO%253B2%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [11]: /lookup/external-ref?access_num=10.1175/1520-0442(1999)012 2.0.CO;2&link_type=DOI [12]: #xref-ref-3-1 "View reference 3 in text" [13]: {openurl}?query=rft.jtitle%253DNat.%2BGeosci.%26rft.volume%253D1%26rft.spage%253D221%26rft_id%253Dinfo%253Adoi%252F10.1038%252Fngeo156%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [14]: /lookup/external-ref?access_num=10.1038/ngeo156&link_type=DOI [15]: #xref-ref-4-1 "View reference 4 in text"