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Searching for a Greenhouse Gas Signal in Guatemalan Drought Records

Paper Reviewed
Anchukaitis, K.J., Taylor, M.J., Leland, C., Pons, D., Martin-Fernandez, J. and Castellanos, E. 2015. Tree-ring reconstructed dry season rainfall in Guatemala. Climate Dynamics 45: 1537-1546.

Persistent drought can have severe negative consequences for the health and well-being of both natural and managed ecosystems, especially in less affluent countries that lack the resources to mitigate its impacts. That is why drought is one of the most studied features of climate, especially given the fact that numerous general circulation models consistently project there will be a future rise in both the frequency and intensity of drought in many regions of the globe in consequence of greenhouse gas-induced global warming.

One such region of concern is Central America, which was singled out by the Intergovernmental Panel on Climate Change in its Fourth Assessment Report as being 'likely' (>66% probability) to experience declines in rainfall during both the dry and wet seasons (Christensen et al. 2007); and such projections have been upheld in the recent Fifth Coupled Model Intercomparison Project (Scheff and Frierson 2012; Bony et al. 2013).

Against this backdrop, Anchukaitis et al. (2015) set out to examine if these model-based projections have any basis in observational reality. To do so, they reconstructed a 300-year history of winter-spring (January through March) rainfall for the Central American country of Guatemala. Their chronology consisted of 72 cores from 30 Guatemalan fir (Abies guatemalensis) trees from the Cuchumatanes Mountains of western Guatemala and spanned the period 1689-2010.

Subjecting their proxy rainfall record to a series of statistical analyses, Anchukaitis et al. were able to "examine recent rainfall variability in a long-term context and characterize precipitation anomalies at interannual to multidecadal timescales." Results of those analyses revealed, in the words of the six scientists, that "recent winter-spring rainfall from the region is not yet exceptional in the context of the last several centuries, has a significant yet variable decadal component, is associated with large-scale modes of ocean-atmosphere variability, and reveals evidence of past multiyear droughts." And as for the model-based projections of increasing drought, the researchers report that "the expected signal of anthropogenic influence on the precipitation regime of the region has not unambiguously emerged," which is the politically-correct way of saying the observations do not align with the model projections.

In light of the above findings, Anchukaitis et al. write in the concluding section of their paper that "national or international efforts to respond to, ameliorate, or mitigate the consequences of drought in the region need to incorporate the potential for larger variability and sustained dry conditions in their planning, although single year droughts still dominate the record." Indeed they should, for drought is clearly a recurring phenomenon and a natural feature of climate to which all of society should be prepared to respond.

Bony, S., Bellon, G., Klocke, D., Sherwood, S., Fermepin, S. and Denvil, S. 2013. Robust direct effect of carbon dioxide on tropical circulation and regional precipitation. Nature Geoscience 6: 447-451.

Christensen, J.H., Hewitson, B., Busuioc, A., Chen, A., Gao, X., Held, I., Jones, R., Kolli, R.K., Kwon, W.T., Laprise, R., Magaña-Rueda, V., Mearns, L., Menéndez, C.G., Räisänen, J., Rinke, A., Sarr, A. and Whetton, P. 2007. Regional climate projections. In: Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M. and Miller, H.L. (eds) Climate change 2007: the physical science basis. Contribution of working group I to the fourth assessment report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom.

Scheff, J. and Frierson, D.M.W. 2012. Robust future precipitation declines in cmip5 largely reflect the poleward expansion of model subtropical dry zones. Geophysical Research Letters 39: doi:10.1029/2012GL052910.

Posted 26 October 2016