The reigning myth has it that climate change from anthropogenic greenhouse gas (GHG) emissions is supposed to increase intense precipitation. But a slew of research now points to other causes for this. As a result, assessments of climate change, such as the one used by the EPA in support of its 2009 Endangerment Finding (which is required to justify proposed GHG emissions limits for U.S. power plants), may be wrong when they primarily blame changed rain intensity on GHGs.
Here is an example of how this plays out across the Heartland of the United States.
“Heavy” or “intense” precipitation across the U.S. Midwest and Great Plains is rising slightly. We found this to be the case in our 2004 paper “Trends in Precipitation on the Wettest Days of the Year across the Contiguous USA” (Michaels et al., 2004). Balling and Goodrich identified it in their 2011 paper, “Spatial analysis of variations in precipitation intensity in the USA.” Groisman et al. earlier this year made a similar finding in their work “Changes in intense precipitation over the central U.S.” And most recently, Villiarini and colleagues (2012) confirmed it in their paper “Changing Frequency of heavy Rainfall over the Central United States.”
As to the reason for this increase, we did not speculate in our 2004 work, only to note that while heavy precipitation was not increasing disproportionately to total precipitation. Balling and Goodrich (2011) went a bit further, pointing out an apparent connection between precipitation intensity changes and the Atlantic Multidecadal Oscillation (AMO)— a large-scale circulation pattern operating in the Atlantic ocean. Balling and Goodrich also suggested that anthropogenic climate change may be involved, but were cautious:
We found some evidence that spatial and temporal variations and trends in precipitation intensity are related to the Atlantic multidecadal oscillation, although the AMO itself is conflated with anthropogenic climate change. Given the complexity of the spatial patterns in precipitation intensity trends along with a significant link to AMO, making any direct link between anthropogenic changes in atmospheric composition and increases in precipitation intensity must be done with caution.
Villarini et al. (2012) went even further, pointing out the area with the greatest increase in heavy precipitation roughly coincided with the area with the greatest rise in temperature, and concluding that the two must be related:
Based on these results and our understanding of the physical processes at play, it is reasonable to state that the observed increasing trends in heavy rainfall in the northern part of the domain are related to the observed increases in temperature over the recent years and highlights the need to better understand regional temperature trends.
Villarini et al. (2012), however, gave no indication as to why regional temperatures may be increasing. But Groisman et al. (2012) were a bit less shy, stating that:
“[T]here are good reasons to expect that some of the observed changes in intense precipitation over extratropical land areas (including the central United States) are part of the global climatic change.”
However, Villarini et al. and Groisman et al. point out that there is probably more to it than just temperature changes and anthropogenic global warming.
Vallarini et al. note:
[I]t is likely that changes in land use / land cover and agricultural practice over this part of the US are also going to play a role in increasing the amount of water vapor in the atmosphere
And Groisman et al. concur:
In parallel, there were large-scale land use changes over the central United States and its adjacent areas that could also shift the regional water budget in the same direction.
In support of these statements, both research teams cite the findings of DeAngelis and colleagues (2010) who studied precipitation changes observed over and downwind of the Ogallala aquifer that underlies much of the Great Plains (from Nebraska to northern Texas), and its relationship to the rapid post-WWII expansion of irrigation. DeAngelis et al. (2010) find that the increase in irrigation across a large-region of the Great Plains very likely has contributed to the increase in summertime precipitation there. In their own words:
A long‐term record of station and gridded precipitation observations covering the entire 20th century shows that July precipitation increased 15–30% in a broad region downwind of the Ogallala Aquifer, stretching from eastern Kansas through Indiana…. While the July precipitation increase was only statistically significant in a region far downwind of the Ogallala, the timing and spatial distribution of the broad precipitation increase is overall consistent with our hypothesis that Ogallala irrigation may have enhanced the regional precipitation.
Interestingly, DeAngelis et al. goes on to note that “there is no clear evidence that atmospheric circulation changes or modes of internal climate variability increased the July precipitation.”
Expanding irrigation is not the only mechanism which may produce precipitation increases that mimic those hypothesized to occur from anthropogenic greenhouse gas emissions. For example Degu et al. (2011) investigated the effect of water impoundments on precipitation characteristics. They found that, in certain climate types, large dams produce may enhance the atmospheric conditions that give rise to intense precipitation near the reservoirs. The authors report that “our findings point to the possibility of storm intensification in impounded basins of the Mediterranean and arid climate[s] of the United States.” And while, at first pass, this effect may seem limited in scope, the authors are quick to remind us that “Today, there are more than 70,000 dams in the US capable of storing a volume of water almost equaling one year’s mean runoff” and that “[g]iven that land cover is a first order forcing on local climate change, the historical chronology of irrigation patterns and other land cover types around multi‐purpose reservoirs needs to be investigated with an atmospheric model to understand how heavy storms are physically modified (become more/less frequent or altered in average intensity).”
Urban areas, too, have been shown to have a significant and large-scale influence on precipitation characteristics, including increasing the frequency and magnitude of intense precipitation events. Specific demonstrations of this have been made for cities in the central U.S. including Indianapolis (Nyogi et al., 2011), and Oklahoma City (Hand and Shepherd, 2009), as well as for in other parts of the country (Atlanta (Shem and Shepherd, 2008); Houston (Shepherd, 2010)) and other cities and countries around the world (e.g. Mitra et al., 2011). Ashley et al. (2011) note the consequences of continued urbanization into the future on convective precipitation events:
As urban cities continue to grow into the 21st Century, so will the convective feedbacks and, in return, enhanced thunderstorm risk they engender. When this risk is juxtaposed with elevated physical vulnerability created by urban infrastructure (e.g., impervious surfaces, outdated and aging storm drainage infrastructure, etc.), as well as the social vulnerability due to a concentration of millions of people and their assets into these centers, devastating consequences can result.
The related research area which is most rapidly growing concerns the influence of anthropogenic aerosol emissions (note that aerosols are not considered to be greenhouse gases) on the intensity of precipitation events. For example, Li et al. (2011) reported a strong association between atmospheric aerosol loading and extreme precipitation for the U.S. Great Plains. According to the press release accompanying the Li et al. (2011) article:
[t]he research provides the first clear evidence of how aerosols— soot, dust and other small particles in the atmosphere— can affect weather and climate; and the findings have important implications for the availability, management and use of water resources in regions across the United States and around the world,
[t]his work confirms what previous cloud modeling studies had suggested, that although clouds are influenced by many factors, increasing aerosols enhance the variability of precipitation, suppressing it when precipitation is light and intensifying it when it is strong. This complex influence is completely missing from climate models, casting doubt on their ability to simulate the response of precipitation to changes in aerosol pollution.
Clearly, this expanding collection of recent research indicates a complex interplay between non-GHG anthropogenic alteration to the environment, natural variability, and precipitation intensity than was recognized by the EPA in its support for its Endangerment Finding. EPA would therefore be well-advised to revisit and reassess its conclusions regarding human activities and their actual and potential impact on intense precipitation events.
The complete picture will be a long time coming. Consider the evaluation of the situation by Groisman et al. (2012):
More comprehensive studies will be required to perform a special study to separate climatic and local anthropogenic factors in any attribution of causality. A combination of global and regional climate and hydrological modeling driven by well-documented external anthropogenic forcing (that includes, in addition to global factors, regional land use and water management changes) can be a way to perform this attribution study.
Groisman et al. hasten to add that such a study is “easy to envision” but “ extremely laborious to do.”
Ashley, W.S., M.L. Bentley, and J. A. Stallins, 2011. Urban-induced thunderstorm modification in the Southeast United States. Climatic Change, doi:10.1007/s10584-011-0324-1
Balling, R.C., and G.B. Goodrich, 2011. Spatial analysis of variations in precipitation intensity in the USA. Theoretical and Applied Climatology, 104, 415-421, doi:10.1007/s00704-010-0353-0
DeAngelis, A., F. Dominguez, Y. Fan, A. Robock, M. D. Kustu, and D. Robinson, 2010. Observational evidence of enhanced precipitation due to irrigation over the Great Plains of the United States. Journal of Geophysical Research, 115, D15115, 14 pp., doi:10.1029/2010JD013892.
Degu, A. M., F. Hossain, D. Niyogi, R. Pielke Sr., J. M. Shepherd, N. Voisin, and T. Chronis, 2011. The influence of large dams on surrounding climate and precipitation patterns. Geophysical Research Letters, 38, L04405, doi:10.1029/2010GL046482.
Groisman, P. Ya., R. W. Knight, and T. R. Karl, 2011. Changes in intense precipitation over the central U.S. Journal of Hydrometeorology, 13, 47-66.
Hand, L., and J.M. Shepherd, 2009. An investigation of warm season spatial rainfall variability in Oklahoma City: Possible linkages to urbanization and prevailing wind. Journal of Applied Meteorology and Climatology, 48, 251–269.
Li, Z., F. Niu, J. Fan, Y. Liu, D. Rosenfeld, and Y. Ding, 2011. The long-term impacts of aerosols on the vertical development of clouds and precipitation. Nature Geoscience, doi: 10.1038/NGEO1313.
Michaels, P.J., P.C. Knappenberger, O.W. Frauenfeld, and R.E. Davis, 2004. Trends in precipitation on the wettest days of the year across the contiguous USA. International Journal of Climatology, 24, 1873-1882.
Mitra, C., M. Shepherd and T. Jordan, 2011. On the relationship between the pre-monsoonal rainfall climatology and urban land cover dynamics in Kolkata city, India. International Journal of Climatology, DOI: 10.1002/joc.2366
Niyogi, P. Pyle, M. Lei, S.Arya, C. Kishtawal, M. Shepherd, F. Chen, and B. Wolfe, 2011. Urban Modification of Thunderstorms: An Observational Storm Climatology and Model Case Study for the Indianapolis Urban Region. Journal of Applied Meteorology and Climatology, 50, 1129-1144. doi: 10.1175/2010JAMC1836.1
Shem, W., and M. Shepherd, 2008. On the impact of urbanization on summertime thunderstorms in Atlanta: Two numerical model case studies. Atmospheric Research, 92, 172–189.
Shepherd, J.M., W.M. Carter, M. Manyin, D. Messen, and S. Burian, 2010. The impact of urbanization on current and future coastal convection: A case study for Houston. Environment And Planning, 37, 284-304.
Villarini, G., J. Smith, and G. Vecchi, 2012. Changing Frequency of Heavy Rainfall Over the Central United States. Journal of Climate, doi:10.1175/JCLI-D-12-00043.1, in press.