Summary. This is the second of two posts concerning the relationship between global warming and extremes of rainfall. The first post, immediately preceding this one, presents a simplified explanation of how increasing atmospheric temperature leads to increased moisture in the air, and discusses the interplay between water vapor, clouds, heating and cooling of the air, and winds.
This post discusses three recent articles in the scientific journal Nature. The first two present new results that for the first time directly link extremes of rainfall and flooding to the warming of the average global temperature as a result of greenhouse gas emissions. Statistical analyses of rainfall and flooding were used to correlate observed rainfall patterns or flooding resulting from heavy rains to the predictions of several climate models. The third article is a commentary appearing in the same issue of the journal. According to the commentary, the increased levels of greenhouse gases in the atmosphere, arising from human activity, are likely already adversely affecting the intensity of rainfall and increasing the likelihood of serious damage from flooding.
Introduction. Humans around the planet continue to burn fossil fuels as their primary energy source at an ever-increasing pace. Upon combustion, these fuels release carbon dioxide, CO2, a principal greenhouse gas. Other, more potent greenhouse gases also arise from natural processes and other human activities. The increased level of CO2 in the atmosphere already added by human activity has led to a global increase in the long-term, planet-wide, average temperature of about 0.7 ̊ C (1.3 ̊ F) above the level that prevailed before the industrial revolution began. Global warming models predict that higher global temperatures will affect the climate of different parts of the planet in different ways, with some areas experiencing greater amounts of rainfall, and others experiencing drought.
The seemingly increasing severity of extreme weather patterns around the globe in recent years suggests that there may be a correlation with the documented increase in global temperature. Increased rainfall with consequent flooding has been observed in some regions, while increased aridity, sometimes accompanied by increased numbers and extents of wildfires, has also been observed. Because of the anecdotal nature of such observations, it has been difficult to draw direct correlations between warming of global temperature and localized, somewhat short-term, more extreme excursions of weather.
Analysis of Data Over 49 Years Shows That Global Warming Is Responsible for Extremes of Rainfall Over the Northern Hemisphere. Min and coworkers (Nature, 2011, Vol. 470, pp. 378-381, doi:10.1038/nature09763; see Note 1) present the first objective identification that human-induced global warming contributes to increased occurrence of extreme precipitation. They studied rainfall records collected over the 49 year period from 1951-1999 from most regions of the Northern Hemisphere including India Asia . They compared the data with predictions from climate simulations prepared from several distinct models.
Their analysis shows that “human-induced increases in greenhouse gases have contributed to the observed intensification of heavy precipitation events found over approximately two-thirds of … Northern Hemisphere land areas.” Additionally, the authors observed that the models generally predicted less extreme precipitation than was actually reported in the data, indicating that future projections of the effects of global warming using these climate models may underestimate the occurrence of extreme precipitation. This underestimate could have serious consequences in planning, both with respect to policies and practices, for future effects of global warming.
The results are consistent with results of earlier workers that show that the extremes of precipitation are in accord with increased water vapor content that warmer air holds (see the preceding post). Significantly, the observation period under scrutiny ended in 1999. This date precedes the even more extreme planetary warming that has been recorded in the last decade, as well as the numerous anecdotal extreme weather events that have been newsworthy in recent years.
Details. The authors obtained the daily records of rainfall observed at 6,000 stations in the Northern Hemisphere from 1951-1999. Climate model simulations incorporating the effects of human-induced warming from several computer-driven models were also prepared. Using a statistical method that identifies extreme deviations of the observed data over the full time interval from the aggregated model predictions, global maps of the Northern Hemisphere were prepared showing the geographic distribution of positive and negative extremes over the entire time interval of the study
Next, occurrences of extreme precipitation summed over the full Northern Hemisphere were prepared at five-year intervals and compared with the various model predictions. The effects of human-induced warming show increasing trends of occurrence of extreme events over the period, but the predictions were lower in extent than the observations, a trend especially apparent for the last 5-year point.
Finally, using a statistically rigorous optimal detection method, the authors prepared a single number with its uncertainty range, for each of four sub-regions in the Northern Hemisphere. This number reflects whether characteristics of extreme precipitation occur in the sub-region over the full time period. The results show that human-induced influences on climate are detected in the observed time-based, and the observed time- and geographic-based, deviations giving rise to extremes of precipitation.
Extreme Rainfall and Catastrophic Flooding in England Wales England Wales
Details. The authors sought to obtain the probability of a correlation between rainfall observed, the modeled climate conditions predicted from human-induced global warming, and the extent of flooding. To begin with, they modeled the rainfall based on the increased humidity predicted from global warming such as described in the preceding post, and compared that with the amount that would have occurred in the absence of the warming of the atmosphere. It was determined that excess precipitation occurred with a probability of 33%.
The authors then modeled autumn conditions across the Northern Hemisphere for the years 1957-1999, just preceding 2000, compared to four model calculations for the absence of global warming based on conditions presumed to have occurred in 1900. With the results they developed a three-dimensional representation over the hemisphere up through the atmosphere. (This type of time comparison has been explained in an early Warmgloblog post.) This result was compared to a similar calculation for the year 2000. The pattern differed considerably from that of the preceding decades.
Next, as a substitute for measuring the flooding itself, the authors modeled the runoff of rain falling in the watersheds (catchbasins) of the rivers in the area, using geological and climate simulations, on a day-by-day basis. The amount of computation that this required was so huge that they accomplished it using distributed computing on volunteered computers throughout the Internet. For the single model including global warming, and for each of the four models for the absence of global warming, over 2000 simulations were computed. For each of the models for no warming, scatter plots of simulated results portraying daily runoff were displayed, and compared to the single complete model including warming. The points computed for the complete model showed runoff values higher than all computed points for three of the no warming models, and for a large majority of computed points for the fourth no warming model. These differences became especially pronounced for computed simulations that correspond to times for recurrence of rare events (i.e., the expected interval between recurrences) of greater than 10 autumns.
Smoothed histograms of a “fraction of risk attributable to twentieth century [human-induced] greenhouse gases” display the probabilities for the occurrence of the excess runoff. The four separate histograms were then combined into a single composite. The composite shows that the increase in risk that the floods under consideration were due to increased human-induced greenhouse gas emissions is very likely, i.e., found in 9 out 10 cases, with a risk of more than 20%, and to be likely, i.e., found in 2 out of 3 cases, with a risk of more than 90%.
A Commentary on the Two Articles Described Above Is Provided by Allan, who was not involved in either study, in the same issue of Nature; (see Note 1) . Allan points out the important role played by the increase in water vapor capacity of air with increasing temperature. He also notes that water vapor itself is thought by some to affect atmospheric behavior, acting to lessen precipitation due to radiative cooling of the atmosphere. As a consequence the ultimate increase in potential global rainfall may be less than 6% per ̊ C implied from the water vapor capacity alone. Allan also points to the need for more study on possible local effects in storms and otherwise in more limited regions that can act to increase or to diminish the temperature dependence of rainfall resulting from changes in the water vapor capacity of air.
Conclusion. The two research articles discussed here are the first to show rigorously, using computational statistical methods and planetary climate models, that increased occurrence of heavy rainfall arises as a direct consequence of the increase in atmospheric concentration of greenhouse gases due to human activity. This increased rainfall includes severe extremes that inflict major damage to human society. It is important to extend these types of statistical analysis to other warming-related phenomena.
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Note 1. Abstract available online free, or the full article for a fee or through personal or institutional subscription. Many public libraries, and university libraries open to the public, receive the journal.
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