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Heavy Precipitation Over the US: Has it Increased as Some have Predicted it Should?
Reference
Mahajan, S., North, G.R., Saravanan, R. and Genton, M.G. 2012. Statistical significance of trends in monthly heavy precipitation over the US. Climate Dynamics 38: 1375-1387.

Background
Noting that "extreme events of precipitation have a potential for impacting our social and economic activities," Mahajan et al. state that it is "essential to determine if there has been a systematic change in the extremes over the past years and what awaits us in the future owing to global warming," especially in light of the fact that "climate model projection studies suggest that intense precipitation would be on the rise as global temperatures increase due to increased greenhouse gas forcings in the future (Meehl et al., 2000a,b; Cubasch et al., 2001; Zwiers and Kharin, 1998; Kharin and Zwiers, 2000)," which future, obviously, has already begun, as well over a decade has passed since the writing of the cited papers.

What was done
In the words of the four US researchers, "trends in monthly heavy precipitation, defined by a return period of one year, are assessed for statistical significance in observations and Global Climate Model (GCM) simulations over the contiguous United States using Monte Carlo non-parametric and parametric bootstrapping techniques."

What was learned
Mahajan et al. report that trends estimated from the two data sources they employed "straddle the margin of statistical significance, and hence a definitive answer to the question of increasing trend of heavy precipitation over the US cannot be arrived at by looking at observational data." And with nearly half (9 out of 20) of the GCMs employed in their study predicting trends that are "significantly different from the observations," they are forced to conclude that "the GCMs are not yet fully capable of simulating extremes of precipitation at a regional level," as they say that only "one GCM simulation of the twentieth century does demonstrate a statistically significant upwards trend in monthly heavy precipitation."

What it means
When all was said and done, the four scientists concluded the report of their research by writing "we cannot definitively answer if the heavy precipitation in the US has been increasing in the past century." Yet they continue to repeat the ominous refrain that "GCM integrations suggest that it is very likely to increase in the future." We shall see.

References
Cubasch, U., Meehl, G.A., Boer, G.J., Stouffer, R.J., Dix, M., Noda, A., Senior, C.A., Raper, S. and Yap, K.S. 2001. Projections of future climate change. In: Houghton, J.T., Ding, Y., Griggs, D.J., Noguer, M., van der Linden, P.J., Dai, X., Maskell, K. and Johnson, C.A. (Eds.), Climate Change 2001: The Scientific Basis. Contributions of Working Group 1 to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, UK.

Kharin, V.V. and Zwiers, F.W. 2000. Changes in the extremes in an ensemble of transient climate simulations with a coupled atmosphere-ocean GCM. Journal of Climate 13: 3760-3788.

Meehl, G.A., Karl, T., Easterling, D.R., Changnon, S., Pielke, R., Changnon, D., Evans, J., Groisman, P.Y., Knutson, T.R., Kunkel, K.E., Mearns, L.O., Parmesan, C., Pulwarty, R., Root, T., Sylves, R.T., Whetton, P. and Zwiers, F. 2000a. An introduction to trends in extreme weather and climate events: observations, socioeconomic impacts, terrestrial ecological impacts, and model projections. Bulletin of the American Meteorological Society 81: 413-416.

Meehl, G.A., Zwiers, F., Evans, J., Knutson, T., Mearns, L. and Whetton, P. 2000b. Trends in extreme weather and climate events: issues related to modeling extremes in projections of future climate change. Bulletin of the American Meteorological Society 81: 427-436.

Zwiers, F.W. and Kharin, V.V. 1998. Changes in the extremes of the climate simulated by CCC GCM2 under CO2 doubling. Journal of Climate 11: 2200-2222.

Reviewed 29 August 2012