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Edited by Peter M. Cox, University of Exeter, Exeter, United Kingdom, and accepted by the Editorial Board September 24, 2013 (received for review February 8, 2013)
Significance
Whether the dry-season length will increase is a central question in determining the fate of the rainforests over Amazonia and the future global atmospheric CO2 concentration. We show observationally that the dry-season length over southern Amazonia has increased significantly since 1979. We do not know what has caused this change, although it resembles the effects of anthropogenic climate change. The global climate models that were presented in the Intergovernmental Panel on Climate Change’s fifth assessment report seem to substantially underestimate the variability of the dry-season length. Such a bias implies that the future change of the dry-season length, and hence the risk of rainforest die-back, may be underestimated by the projections of these models.
Abstract
We have observed that the dry-season length (DSL) has increased over southern Amazonia since 1979, primarily owing to a delay of its ending dates (dry-season end, DSE), and is accompanied by a prolonged fire season. A poleward shift of the subtropical jet over South America and an increase of local convective inhibition energy in austral winter (June–August) seem to cause the delay of the DSE in austral spring (September–November). These changes cannot be simply linked to the variability of the tropical Pacific and Atlantic Oceans. Although they show some resemblance to the effects of anthropogenic forcings reported in the literature, we cannot attribute them to this cause because of inadequate representation of these processes in the global climate models that were presented in the Intergovernmental Panel on Climate Change’s Fifth Assessment Report. These models significantly underestimate the variability of the DSE and DSL and their controlling processes. Such biases imply that the future change of the DSE and DSL may be underestimated by the climate projections provided by the Intergovernmental Panel on Climate Change’s Fifth Assessment Report models. Although it is not clear whether the observed increase of the DSL will continue in the future, were it to continue at half the rate of that observed, the long DSL and fire season that contributed to the 2005 drought would become the new norm by the late 21st century. The large uncertainty shown in this study highlights the need for a focused effort to better understand and simulate these changes over southern Amazonia.
Footnotes
- ↵1To whom correspondence should be addressed. E-mail: rongfu{at}jsg.utexas.edu.
Author contributions: R. Fu designed research; R. Fu, L.Y., W.L., and K.F. performed research; B.L., R. Fisher, and R.B.M. contributed new reagents/analytic tools; L.Y., W.L., P.A.A., L.H., and S.C. analyzed data; and R. Fu, R.E.D., and R. Fisher wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. P.M.C. is a guest editor invited by the Editorial Board.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1302584110/-/DCSupplemental.
Freely available online through the PNAS open access option.
Increased dry-season length over Amazonia
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