"Two seminal Nature papers join growing body of evidence that human emissions fuel extreme weather, flooding that harm humans and the environment"
Here we show that human-induced increases in greenhouse gases have contributed to the observed intensification of heavy precipitation events found over approximately two-thirds of data-covered parts of Northern Hemisphere land areas. These results are based on a comparison of observed and multi-model simulated changes in extreme precipitation over the latter half of the twentieth century analysed with an optimal fingerprinting technique.
Changes in extreme precipitation projected by models, and thus the impacts of future changes in extreme precipitation, may be underestimated because models seem to underestimate the observed increase in heavy precipitation with warming
That’s from the first of two seminal studies in Nature, “Human contribution to more-intense precipitation extremes” (subs. req’d). The second looked at “Anthropogenic greenhouse gas contribution to flood risk in England and Wales in autumn 2000” (subs. req’d):
Occurring during the wettest autumn in England and Wales since records began in 1766 these floods damaged nearly 10,000 properties across that region, disrupted services severely, and caused insured losses estimated at £1.3 billion….
Here we present a multi-step, physically based ‘probabilistic event attribution’ framework showing that it is very likely that global anthropogenic greenhouse gas emissions substantially increased the risk of flood occurrence in England and Wales in autumn 2000.
… in nine out of ten cases our model results indicate that twentieth-century anthropogenic greenhouse gas emissions increased the risk of floods occurring in England and Wales in autumn 2000 by more than 20%, and in two out of three cases by more than 90%.
Scientists have predicted for decades that human-caused global warming would increased extreme weather events that cause severe harm to humans, property, and the environment. These two studies are but the latest in a growing body of scientific literature demonstrating that these predictions are coming true now.
They should help lay to rest the myth that human-caused global warming will contribute to grievous harm only in some far-off future. They also strongly support the view that the human-induced increases in greenhouse gases have contributed to the devastating extreme events that hit Australia and other parts of the world in the past several months, helping to drive up food prices (see how extreme weather, climate change drive record food prices).
The NYT has a great headline on this story, “Research Links Heavy Rains and Snow to Humans.” It is all heavy precipitation that humans are intensifying.
Of course, many of our top climate scientists have been documenting and explaining these types of conclusions for a while. I’ll list a bunch of the papers below. Kevin Trenberth, head of NCAR’s Climate Analysis Section, has a new paper out, “Changes in precipitation with climate change” that is well worth reading. So I asked him for a comment on these two studies. He told me:
These studies are very reasonable, and the main mechanism is well understood: it relates to the increased moisture in the atmosphere with higher temperatures and warmer oceans. However, the studies may well be conservative as the tools available (the climate models), do not simulate precipitation and all of its characteristics (intensity, frequency etc) as well as we would like to see.
As the first study makes clear, future changes in extreme precipitation are likely to be worse than the models suggest.
In an extended interview last year on the subject, Trenberth explained:
“I find it systematically tends to get underplayed and it often gets underplayed by my fellow scientists. Because one of the opening statements, which I’m sure you’ve probably heard is “Well you can’t attribute a single event to climate change.” But there is a systematic influence on all of these weather events now-a-days because of the fact that there is this extra water vapor lurking around in the atmosphere than there used to be say 30 years ago. It’s about a 4% extra amount, it invigorates the storms, it provides plenty of moisture for these storms and it’s unfortunate that the public is not associating these with the fact that this is one manifestation of climate change. And the prospects are that these kinds of things will only get bigger and worse in the future.”
Let’s hope these new studies helped put an end to the underplaying of the link between human caused emissions and the extreme weather events we are experiencing now.
The Washington Post has a good piece on the two studies, “Greenhouse gases led to increase in deluges, researchers say,” with more quotes:
“Human influence on the climate system has the effect of intensifying precipitation extremes,” said Francis Zwiers, a climate researcher at Environment Canada in Toronto and lead researcher on the first study.
Zwiers and his team gathered 50 years of rainfall statistics, and compared those observations to predictions made by computer simulations of the 20th century climate.
Those simulations included the warming impact of the billions of tons of carbon dioxide human society has pumped into the atmosphere.
The study found that observed increase in deluges “cannot be explained by natural internal fluctuations of the climate system alone,” said Zwiers. In other words, only the addition of greenhouse gases to the atmosphere explains why the United States and Canada have experienced a dramatic increase in heavy downpours.
“Large [rainfall] events are becoming larger,” Zwiers said. His work found that from 1951 to 1999, the probability of heavy downpours becoming even more extreme grew by about 7 percent, a figure he characterized as “really substantial.”
Richard Allan, a climate scientist at the University of Reading in England who was not part of the study, called the method employed by Zwiers “very rigorous.”
He added, “There’s already been quite a bit of evidence showing that there has been an intensification of rainfall” events across the globe.
But until now “there had not been a study that formally identified this human effect on precipitation extremes,” Zwiers said. “This paper provides specific scientific evidence that this is indeed the case.”
Note that these studies do not extend beyond the year 2000, so they miss the hottest decade on record and the wettest year on record.
You can see some of the amazing photos from the 2000 UK floods here.
Here’s some more quotes from leading scientists via Seth Borenstein’s AP story:
Both studies should weaken the argument that climate change is a “victimless crime,” said Myles Allen of the University of Oxford. He co-authored the second study, which connected flooding and climate change in the United Kingdom. “Extreme weather is what actually hurts people.”
Jonathan Overpeck, a University of Arizona climate scientist, who didn’t take part in either study, praised them as sensible and “particularly relevant given the array of extreme weather that we’ve seen this winter and stretching back over the last few years.”
… “Put the two papers together and we start to see an emerging pattern,” said Andrew Weaver of the University of Victoria, who wasn’t part of either study. “We should continue to expect increased flooding associated with increased extreme precipitation because of increasing atmospheric greenhouse gas. And we have no one to blame but ourselves.”
Let’s run through some of the other recent studies that support the conclusion that human-caused global warming is making weather more extreme:
A new study by a Duke University-led team of climate scientists suggests that global warming is the main cause of a significant intensification in the North Atlantic Subtropical High (NASH) that in recent decades has more than doubled the frequency of abnormally wet or dry summer weather in the southeastern United States….
The models – known as Coupled Model Intercomparison Project Phase 3 (CMIP3) models – predict the NASH will continue to intensify and expand as concentrations of carbon dioxide and other greenhouse gases increase in Earth’s atmosphere in coming decades.”This intensification will further increase the likelihood of extreme summer precipitation variability – periods of drought or deluge – in southeastern states in coming decades,” Li says.
And one of my commenters posted this list in response to the nonsensical quote by Pielke in the Wall Street Journal, “There’s no data-driven answer yet to the question of how human activity has affected extreme weather”¦.” The first three may be the most relevant.
Some observational, modelling and observational and modelling studies concerning the trends in temperature extremes and precipitation intensity. I have limited myself to material that has been released since 2007 (when the last IPCC report was released); this is by no means a complete list. Not all of them directly challenge the claim about human activities affecting extreme weather, but I included them because they point to a coherent picture.
Zhang et al. (2007): Detection of human influence on twentieth-century precipitation trends. (Nature)
“We show that anthropogenic forcing has had a detectable influence on observed changes in average precipitation within latitudinal bands, and that these changes cannot be explained by internal climate variability or natural forcing. We estimate that anthropogenic forcing contributed significantly to observed increases in precipitation in the Northern Hemisphere mid-latitudes, drying in the Northern Hemisphere subtropics and tropics, and moistening in the Southern Hemisphere subtropics and deep tropics. The observed changes, which are larger than estimated from model simulations, may have already had significant effects on ecosystems, agriculture and human health in regions that are sensitive to changes in precipitation, such as the Sahel.”
Christidis et al. (2011): The role of human activity in the recent warming of extremely warm daytime temperatures. (J. Climate).
“Our analysis is the first that attempts to partition the observed change in warm daytime extremes between its anthropogenic and natural components and hence attribute part of the change to possible causes. Changes in the extreme temperatures are represented by the temporal changes in a parameter of an extreme value distribution. Regional distributions of the trend in the parameter are computed with and without human influence using constraints from the global optimal fingerprinting analysis. Anthropogenic forcings alter the regional distributions, indicating that extremely warm days have become hotter.”
Zwiers et al. (2010): Anthropogenic Influence on Long Return Period Daily Temperature Extremes at Regional Scales. (J. Climate).
“We therefore conclude that the influence of anthropogenic forcing has had a detectable influence on extreme temperatures that have impacts on human society and natural systems at global and regional scales. External influence is estimated to have resulted in large changes in the likelihood of extreme annual maximum and minimum daily temperatures. Globally, waiting times for extreme annual minimum daily minimum and daily maximum temperatures events that were expected to recur once every 20 years in the 1960s are now estimated to exceed 35 and 30 years respectively. . In contrast, waiting times for circa 1960s 20-year extremes of annual maximum daily minimum and daily maximum temperatures are estimated to have decreased to less than 10 and 15 years respectively.”
Krishnamurthy et al. (2009): Changing Frequency and Intensity of Rainfall Extremes over India from 1951 to 2003. (J. Climate).
“Statistically significant increasing trends in extremes of rainfall are identified over many parts of India, consistent with the indications from climate change models and the hypothesis that the hydrological cycle will intensify as the planet warms. Specifically, for the exceedance of the 99th percentile of daily rainfall, all locations where a significant increasing trend in frequency of exceedance is identified also exhibit a significant trend in rainfall intensity.”
Teixeira and Satyamurty (2011): Trends in the Frequency of Intense Precipitation Events in Southern and Southeastern Brazil during 1960-2004. (J. Climate).
“In both regions, annual heavy and extreme rainfall event frequencies present increasing trends in the 45-year period. However, only in Southern Brazil is the trend statistically significant. Although longer time series are necessary to ensure the existence of long term trends, the positive trends are somewhat alarming since they indicate that climate changes, in terms of rainfall regimes, are possibly under way in Brazil.”
Ding et al. (2009): Changes in hot days and heat waves in China during 1961-2007. (Int. J. Clim.)
“Over most of China except northwestern China, the frequency of HDs was high during the 1960s-1970s, low in the 1980s, and high afterwards, with strong interannual variations. A remarkable increasing trend of HDs occurred after the 1990s in all regions. ”
Rodda et al.(2009): A comparative study of the magnitude, frequency and distribution of intense rainfall in the United Kingdom. (Int. J. Clim).
“Most noticeably, increases up to 20% have occurred in the north-west of the country and in parts of East Anglia. There have also been changes in other areas, including decreases of the same magnitude over central England. The implications of these changes are considered.”
Kysel½ (2009): Recent severe heat waves in central Europe: how to view them in a long-term prospect? (Int. J. Clim).
“Owing to an increase in mean summer temperatures, probabilities of very long heat waves have already risen by an order of magnitude over the recent 25 years, and are likely to increase by another order of magnitude by around 2040 under the summer warming rate assumed by the mid-scenario. Even the lower bound scenario yields a considerable decline of return periods associated with intense heat waves. Nevertheless, the most severe recent heat waves appear to be typical rather of a late 21st century than a mid-21st century climate. ”
Gallant and Karoly (2010): A Combined Climate Extremes Index for the Australian Region (J. Climate)
“Over the whole country, the results show an increase in the extent of hot and wet extremes and a decrease in the extent of cold and dry extremes annually and during all seasons from 1911 to 2008 at a rate of between 1% and 2% decade21. These trends mostly stem from changes in tropical regions during summer and spring. There are relationships between the extent of extreme maximum temperatures, precipitation, and soil moisture on interannual and decadal time scales that are similar to the relationships exhibited by variations of the means. However, the trends from 1911 to 2008 and from 1957 to 2008 are not consistent with these relationships, providing evidence that the processes causing the interannual variations and those causing the longer-term trends are different.”
Romps (2011): Response of Tropical Precipitation to Global Warming. (J. Atmos. Sci.)
“There are many properties of convection that can change as the atmosphere warms, each of which could produce deviations from CC scaling. These properties include the effective water-vapor gradient, cloud pressure depth, and cloud velocity. A simple theory is developed that predicts the changes in these properties consistent with CC scaling. Convection in the cloud-resolving simulations is found to change as predicted by this theory, leading to an ~20% increase in local precipitation fluxes when the CO2 concentration is doubled. Overall, an increase in CO2 leads to more vigorous convection, composed of clouds that are wider, taller, and faster.”
Wentz et al. (2007): How Much More Rain Will Global Warming Bring? (Science).
“Climate models and satellite observations both indicate that the total amount of water in the atmosphere will increase at a rate of 7% per kelvin of surface warming. However, the climate models predict that global precipitation will increase at a much slower rate of 1 to 3% per kelvin. A recent analysis of satellite observations does not support this prediction of a muted response of precipitation to global warming. Rather, the observations suggest that precipitation and total atmospheric water have increased at about the same rate over the past two decades.”
Allan et al. (2010): Current changes in tropical precipitation. (Environmental research letters).
“Analysing changes in extreme precipitation using daily data within the wet regions, an increase in the frequency of the heaviest 6% of events with warming for the SSM/I observations and model ensemble mean is identified. The SSM/I data indicate an increased frequency of the heaviest events with warming, several times larger than the expected Clausius-Clapeyron scaling and at the upper limit of the substantial range in responses in the model simulations.”
Allan and sodden (2008): Atmospheric Warming and the Amplification of Precipitation Extremes. (Science).
“We used satellite observations and model simulations to examine the response of tropical precipitation events to naturally driven changes in surface temperature and atmospheric moisture content. These observations reveal a distinct link between rainfall extremes and temperature, with heavy rain events increasing during warm periods and decreasing during cold periods. Furthermore, the observed amplification of rainfall extremes is found to be larger than that predicted by models, implying that projections of future changes in rainfall extremes in response to anthropogenic global warming may be underestimated.”
Lenderink and Meijgaard (2008) Increase in hourly precipitation extremes beyond expectations from temperature changes. (Nature).
“Indeed, changes in daily precipitation extremes in global climate models seem to be consistent with the 7% increase per degree of warming given by the Clausius-Clapeyron relation3, 4, but it is uncertain how general this scaling behaviour is across timescales. Here, we analyse a 99-year record of hourly precipitation observations from De Bilt, the Netherlands, and find that one-hour precipitation extremes increase twice as fast with rising temperatures as expected from the Clausius-Clapeyron relation when daily mean temperatures exceed 12″‰°C. In addition, simulations with a high-resolution regional climate model show that one-hour precipitation extremes increase at a rate close to 14% per degree of warming in large parts of Europe. Our results demonstrate that changes in short-duration precipitation extremes may well exceed expectations from the Clausius-Clapeyron relation. “
Of course, don’t miss Table 3.8 in the IPCC’s Fourth Assessment, Working Group I: The Physical Science Basis.
Finally, we have one of my favorites, Record high temperatures far outpace record lows across U.S.:
Spurred by a warming climate, daily record high temperatures occurred twice as often as record lows over the last decade across the continental United States, new research shows. The ratio of record highs to lows is likely to increase dramatically in coming decades if emissions of greenhouse gases continue to climb.
So yes, key weather events are becoming more extreme — especially deluges, heat waves, and droughts — as climate scientists have long predicted they would if atmospheric concentrations of greenhouse gases kept rising. And now we have solid attribution of that increase in extreme weather to human emissions in multiple independent studies.
So let’s move on from that “debate” and focus on how we best minimize the damage from future warming — aggressive greenhouse gas mitigation plus adaptation.
- The year of living dangerously. Masters: “The stunning extremes we witnessed gives me concern that our climate is showing the early signs of instability”; Munich Re: “The only plausible explanation for the rise in weather-related catastrophes is climate change“
- Hansen: Would recent extreme “events have occurred if atmospheric carbon dioxide had remained at its pre-industrial level of 280 ppm?” The “appropriate answer” is “almost certainly not.”