Human Impact on Growing Precipitation Extremes

Observed increases in intense precipitation events have been a focus of much research in recent years.  One matter of significant debate is to what extent humans have contributed to these precipitation events. Greenhouse gases and aerosols can affect the atmosphere in ways that cause increased temperatures globally.  The amount of water the atmosphere can hold in clouds increases on an exponential scale with temperature increase, leading to potential for dramatic dry spells in certain areas and equally violent release of precipitation in others.  Atmospheric circulation is also affected by global warming, altering the patterns of precipitation on a global scale.  Both of these systems can have dramatic impacts on global water resources, whether in the form of severe drought, flooding, or shifts in seasonality for agriculturalists.  However, because natural disasters and severe precipitation have always occurred, it is difficult to determine to what extent humans have impacted these events.  This study marks the first formal claim that humans have significantly contributed to the intensification of extreme precipitation events.
Min, S., Zhang, X., Zwiers, F., Hegerl, G. C., 2011.  Human contribution to more intense precipitation extremes.  Nature 470, 378-381.  [GSSS: human contribution precipitation]

            Min et al. (2011) compared observed and simulated changes in maximum precipitation to see how accurate the models were and determine if they would be useful for prediction in the future.  Three models incorporating historic data of natural events and anthropogenic factors like aerosols and greenhouse gases were generated.  Both single-day and five-day-consecutive precipitation maxima were used in order to see broad trends, and because these time periods tend to be characteristic of the types of dramatic precipitation events that heavily impact human society.  From the data and regressions, the models seemed to underestimate the level of intensity in precipitation, but were otherwise generally accurate. 
            When analyzing the results of the simulations, it is important to keep in mind that precipitation varies geographically.  Because of data scarcity, this study could only focus on regions in the Northern Hemisphere, capturing North America, Eurasia, and India in its direct studies.  In order to improve comparability of results and reduce uncertainty from this confined sample, Min et al. (2011) created a weighted index to standardize their measurements between geographical regions.  The indexed results showed increasing trends in heavy precipitation events in 61–65% of the total area studied.  When the regressions were run with just natural events, human impact and natural events, and human impact alone, it was very clear that humans are making a large difference in precipitation in these affected areas. 
            The global impact of increased dramatic precipitation events may be extreme, and this study shows compelling evidence that humans are playing a role in the frequency and magnitude of these disasters.  However, as always in a scientific study, the researchers were careful to mention that there was some uncertainty due to the sample size and potential omitted data.  Fortunately for those researchers looking for work in a tough economy, there is much room for further study on this topic. 

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