Effects of Climate Change on Precipitation: A Summary

Precipitation is the result of an intricate atmospheric global process, and a disruption at any step can lead to dramatic alterations.  Global climate change will almost certainly cause such a disturbance, and many individual scientists have studied the possible impacts that such a change might produce.  In this paper, Trenberth (2011) summarizes the various conclusions of researchers who have explored the effects of climate change on global water resources, and uses their studies to draw some inferences of his own.  The most probable consequence is increased and more intense drought worldwide.  Sea surface temperature change and wind shifts will also likely disrupt storm patterns, leading to more extreme events in certain regions of the world.  In addition, more precipitation falling as rain instead of snow will decrease water “storage” in ice and snow and may lead to severe water shortages in some areas dependent on spring runoff.  Like Min et al. (2011), Trenberth claims that the probability of human impact on these results cannot be ignored, and that further studies must be conducted in order to determine exactly the nature of our impact, and how we might be able to mitigate it before the consequences on global water sources become too severe.  Nora Studholm
Trenberth, K. E., 2011.  Changes in precipitation with climate change.  National Center for Atmospheric Research.  Symposium manuscript of Technical Conference on Changing Climate and Demands for Climate Services for Sustainable Development.

            The global water cycle depends on the heat of the sun and the composition of the atmosphere.  As these factors continue to change, the character and amount of precipitation globally will alter as well.  Increased heating of the earth leads to greater evaporation and surface drying, which will likely lead to increased intensity and duration of drought.   Already, droughts have been observed to be increasing in frequency and severity over the 20th century, and regions defined as “very dry” on the Palmer Drought Severity Index have more than doubled in extent since the 1970s. 
Redistribution of rainfall worsens this phenomenon.  The water-holding capacity of air goes up by approximately 7% per 1°C increase in temperature.  As sea surface temperatures rise faster than atmospheric temperatures over land, the water vapor in the air above the oceans increases more than that over terrestrial areas.  This leads to more intense precipitation events over the ocean, and less rainfall over land areas, furthering the risks of drought in vulnerable regions. 
Relocation of precipitation also leads to risks in flooding, ironically.  In regions that are not affected by drought, rainfall and extreme events may actually increase dramatically.  Furthermore, more precipitation is likely to fall as rain rather than snow, and snowmelt in high altitude regions will occur earlier in the season.  This not only makes ineffectual the role of snow as a method of water “storage” for times of decreased precipitation, but also leads to increased runoff from heavier rainfall combined with snowmelt in the spring.  The combination leads to an increased likelihood of flooding in the spring and drought in the summer.  Floods can cause billions of dollars of damage and take thousands of lives, and droughts can have similar economic and ecological impacts, with the added risk of increased wildfires.  Effects of droughts and floods can both be mitigated by human intervention, such as improved drainage and irrigation, but there must be adequate plans in place to implement such changes.
            A final impact Trenberth discusses is the possibility of modest changes in winds, which may change patterns of precipitation and make wet areas wetter and dry areas drier.  A shift in the storm track may result in varied atmospheric circulation patterns, and as subtropical high-pressure systems move poleward, peak wind speeds in tropical storms and hurricanes will continue to increase.  The total number of storms worldwide appears to be decreasing, but the average intensities of the events are going up. 
            As suggested by Min et al. (2011), human influence in these recent changes is difficult to deny.  As humans add more carbon dioxide and particulates to the atmosphere, greenhouse gasses trap outgoing infrared radiation and warm the planet.  Water vapor increase in the atmosphere creates a positive feedback cycle for climate change, as increased vapor enhances the greenhouse effect.  Radiative forcing, changes in irradiance levels between levels of the atmosphere, also increases surface heating.  All of these effects are almost certainly direct or indirect results of human actions. 
            However, it is difficult to test exactly what the changes in precipitation will be in the future.  As Trenberth points out, there are many different conflicting models of future climate change and precipitation, some of which claim that there will be no changes whatsoever.  Hughes et al. (2011) experienced this difficulty in their study of the Okavango River Basin, where their seven models gave widely disparate accounts of future possibilities.  Another difficulty is that precipitation can be hard to measure.  Gauges used for data collection are affected by winds, especially when measuring light snow and rain.  Precipitation data are inherently messy because of the intermittency and variability of precipitation even under normal circumstances.  Furthermore, there are large variations on year-to-year and decadal scales, and between different geographies.  Local features like vegetation, soil, and topography play a large role in the effects of precipitation events, although generally regions in higher latitudes can be assumed to have increased overland precipitation.  In addition, vague definitions of terms make comparisons between studies difficult, as it is hard to know exactly what is meant by a “high impact” or “extreme” precipitation event without a universal definition for these terms.  Researchers have attempted to gain confidence in studies by using both remotely-sensed and gauge measured precipitation devices, and by looking at a range of variables including atmospheric moisture, soil moisture, and stream flow.  However, uncertainties still exist and past studies conflict strongly, ranging from claiming near zero global changes to significant predicted trends depending on the dataset and model used.   This report is inclined to emphasize those studies that claim the significant effects, but it is careful to mention that other viewpoints do exist.
            Even with disagreement among scientists about the precise effects of climate change on global precipitation, it seems evident from the body of studies that Trenberth has gathered that there will be significant impacts of some kind on the hydrological cycle in the future.  The most likely results are increased extremes in precipitation events, from flooding to droughts, which will make managing and using water resources more challenging and more essential in our uncertain future.
Works Cited
Hughes, D. A., Kingston, D. G., Todd, M. C., 2011.  Uncertainty in water resources availability in the Okavango River basin as a result of climate change.  Hydrology and Earth System Sciences 15, 931–941. 
Min, S., Zhang, X., Zwiers, F., Hegerl, G. C., 2011.  Human contribution to more intense precipitation extremes.  Nature 470, 378-381.  

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