By Jay Landers
Climate change is increasing the levels of extreme daily precipitation in both wet and dry regions around the world, but the predictions for drier regions may complicate efforts by engineers and others to ensure that long-lasting infrastructure is designed and constructed to accommodate uncertain future conditions.
More extreme precipitation and flash floods are likely to occur in those regions around the world that are typically arid, such as Gobi, Mongolia, as the global climate continues to change. Wikimedia Commons/Qfl247
April 12, 2016—Using observations and global climate models, researchers from the University of New South Wales Climate Change Research Centre and the Massachusetts Institute of Technology recently assessed changes in total annual precipitation and extreme daily precipitation in the world's dry and wet regions. In a paper titled "More Extreme Precipitation in the World's Dry and Wet Regions," published online March 7 by the journal
Nature Climate Change
, the researchers conclude that dry regions have been experiencing higher total precipitation amounts and greater precipitation extremes each year since 1950.
The researchers relied on observational data from dry and wet regions around the world for the study, although spatial coverage was limited in some areas. For example, data was available only for the following dry regions: northeast Asia, central Australia, northwestern North America, and north and southwestern Africa. For wet regions, observational data was available for Southeast Asia, India, the southeastern United States, Europe, eastern tropical Africa, southeastern Africa, small regions in northern tropical and eastern coastal Australia, and portions of eastern South America that include Brazil, Uruguay, and eastern Argentina. Observational data was not available for Africa's most significant dry region, the Sahara, or South America's notable wet region, the Amazon. However, these areas were included as part of the climate model simulations, the results of which were comparable to the findings based on the observational data.
For observational data, the researchers used HadEX2, a global dataset that consists of 27 indices of temperature and precipitation from 1901 to 2010. Of these 27 indices, two were used for the study: total precipitation and annual-maximum daily precipitation, a measure of extreme precipitation. Observational data showed statistically significant increases in total precipitation and precipitation extremes in the dry regions, according to the article. In fact, in the dry regions both indices were found to have increased on the order of one to two percent per decade during the period from 1950 to 2010. Although daily precipitation totals in wet regions were found to have increased at a similar rate, no statistically significant increase was found for total precipitation in wet regions.
For the purposes of modeling future conditions, the researchers used precipitation indices included as part of the fifth, and most recent, phase of the Coupled Model Intercomparison Project, a framework used to coordinate various climate model experiments. Using these data, the researchers found that extreme daily precipitation and total precipitation can be expected to intensify in dry regions in the future.
To refine their findings, the researchers analyzed the precipitation response to two different climate change scenarios. Under one scenario, emissions of greenhouse gases were assumed to decline, leading to a stabilization of atmospheric concentrations of greenhouse gases in the second half of the 21st century. In the second scenario, no such decline in emissions was assumed and greenhouse gas concentrations were assumed to increase significantly.
The modeling results indicated a "robust tendency toward greater precipitation totals and annual extremes on average over the regions of the world where each is [currently] least," according to the article. "Total precipitation amounts and annual extremes in the dry regions are also projected to continuously increase over the twenty-first century, and there is a statistically significant relationship between the magnitude of global warming and the intensification of precipitation," the article states.
By contrast, the modeling showed that total precipitation and precipitation extremes can be expected to increase to a lesser extent in wet regions. Compared to dry regions, increases in total precipitation and precipitation extremes in wet regions are "less robustly related to warming climate," according to the article.
Overall, the results are consistent with the theory that a warmer atmosphere can hold more water and therefore can be expected to result in more intense extreme precipitation events, said Markus Donat, Ph.D., a research fellow at the University of New South Wales Climate Change Research Centre and the lead author of the article in
Nature Climate Change
. Donat provided written responses to questions from
. One "positive surprise" from the study's findings was that its results were "so robust across observations and models," Donat said. "In many cases, when looking at regional-scale precipitation, there are large differences between observations and models. Our results show that when aggregating over the dry and wet regions of the globe there is good agreement."
If left unaddressed, more intense rainfall in the future would increase the potential for certain untoward outcomes. "This intensification has implications for the risk of flooding as the climate warms, particularly for the world's dry regions," the article states.
Even though extreme precipitation events occur rarely, their potential for disruption requires that steps be taken to address their effects, Donat said. Because dry regions may be less prepared to accommodate more intense rainfalls, measures must be taken in such areas to protect against such threats as flash flooding, he noted.
However, flooding is not the only potential risk, says Richard Wright, Ph.D., Dist.M.ASCE, the chairman of ASCE's Committee on Adaptation to a Changing Climate. Faced with the potential for larger extreme events, water managers in drier areas may need to increase the capacity of their storage systems to capture runoff from such events, Wright notes.
More generally, the research findings from Donat and his colleagues highlight the need for civil engineers to attempt to account for a great deal of potential climate-related variability in the future as they design long-lasting infrastructure today, Wright says. Engineers and others must realize that they can no longer rely simply on past climate and weather behavior when designing infrastructure that has a long design life. "We're going to have to deal with a great deal of uncertainty," he says.
To account for such uncertainty, the Committee on Adaptation to a Changing Climate advocates greater use of the principles of adaptive risk management as part of the design process. By seeking to account for as many future scenarios as possible and designing systems and structures that can be adapted in response to unforeseen circumstances, engineers can help to mitigate the potential future risks associated with climate change, Wright says. "This means we need to build into these systems the ability to adapt to extremes which may be more severe than we can justify initially."
Such an approach was highlighted in a white paper released by ASCE's Committee on Adaptation to a Changing Climate in May 2015. Titled
Adapting Infrastructure and Civil Engineering Practice to a Changing Climate
, it is available for download online at http://ascelibrary.org/doi/book/10.1061/9780784479193.