It is very likely that several of the unprecedented extremes of the past decade would not have occurred without anthropogenic global warming.
That’s the conclusion of a major new analysis of the scientific evidence in Nature Climate Change, “A decade of weather extremes” (subs. req’d). The research is by Dim Coumou and Stefan Rahmstorf of Germany’s Potsdam Institute for Climate Impact Research.
The study includes this table of extreme events — “The selection criterion for this (incomplete) list was that the event was documented to be record-breaking (that is, unprecedented) in a long measurement series”:
The study points out how devastating some of these events have been for people:
The Moscow heatwave and Pakistan flooding that year illustrated how destructive extreme weather can be to societies: the death toll in Moscow has been estimated at 11,000 and drought caused grain-harvest losses of 30%, leading the Russian government to ban wheat exports. At the same time Pakistan was hit by the worst flooding in its history, which affected approximately one-fifth of its total land area and 20 million people.
It explains that the context for these events is unprecedented human-caused global warming:
The unprecedented meteorological events listed in Table 1 occurred in a decade that was likely the warmest globally for at least a millennium
A number of these individual events have been the subject of “attribution” analysis making clear that they would have been extremely unlikely to have happened without human caused global warming (see Hansen et al: “Extreme Heat Waves … in Texas and Oklahoma in 2011 and Moscow in 2010 Were ‘Caused’ by Global Warming” and ”Study Finds 80% Chance Russia’s 2010 July Heat Record Would Not Have Occurred Without Climate Warming,” also by Rahmstorf and Coumou).
In addition, we’ve had analyses link recent drought and dust-bowlification to warming — see NOAA Bombshell: Human-Caused Climate Change Already a Major Factor in More Frequent Mediterranean Droughts:
“The magnitude and frequency of the drying that has occurred is too great to be explained by natural variability alone,” said Martin Hoerling, Ph.D. of NOAA’s Earth System Research Laboratory in Boulder, Colo., lead author.
The new study was of course completed before the blow-out, all-time record winter/spring heat wave, which many leading experts have said bears the fingerprints of human-caused warming.
Clearly, the more unprecedented climate extremes that we see, the stronger the case grows for the overall human fingerprint (see also Eight Must-Have Charts Summarize the Evidence for a “Human Fingerprint” on Recent Climate Change).
The powerful conclusion of this study builds off of Hansen’s famous 1988 testimony:
In 1988, Jim Hansen famously stated in a congressional hearing that “it is time to stop waffling so much and say that the evidence is pretty strong that the greenhouse effect is here”. We conclude that now, more than 20 years later, the evidence is strong that anthropogenic, unprecedented heat and rainfall extremes are here — and are causing intense human suffering.
What is most worrisome is that manmade warming is causing intense human suffering now, and we have only warmed 1.4°F in the past century. We’re projected to warm more than 5 times that this century if we are foolish enough to stay anywhere near our current emissions path.
Climate Progress has written many times about the 2009 study on the increase in the ratio of heat records to cold records in the United States, “spurred by a warming climate.”
The new study has an good analysis and chart on the increase in the number of monthly heat records worldwide:
Monthly heat extremes document the most persistent and thus destructive heatwaves. Their number increases faster with climate change than do daily extremes, because more-aggregated data has smaller variance and the number increases in proportion to the ratio of warming trend to variance. The number of observed local monthly heat records around the globe is now more than three times as high as expected in a stationary climate (Fig. 2). This observed increase is consistent with that expected from a simple stochastic model including the warming trend. For Moscow, which has experienced strong warming in the past 30 years, this model even gives a fivefold increase in the expected number of monthly heat records. Extremely hot summers (exceeding three standard deviations) are now observed in about 10% of the global land area, compared with only about 0.1–0.2% for the period from 1951 to 1980….
Results from modelling attribution studies are consistent with these observations: the risk of a heatwave of the magnitude of the 2003 European event has at least doubled but probably quadrupled (best estimate) as a result of human influence on climate.
Ten-year running averages of the number of unprecedented records in monthly mean temperature in 204 time series are shown, namely 17 weather stations from around the world for each calendar month, given as the ratio of the observed number of extremes to that expected in a stationary climate.
The study has an extended discussion in the conclusion about media coverage of extreme events and the famous “loaded dice” analogy:
Many climate scientists (including ourselves) routinely answer media calls after extreme events with the phrase that a particular event cannot be directly attributed to global warming. This is often misunderstood by the public to mean that the event is not linked to global warming, even though that may be the case — we just can’t be certain. If a loaded dice rolls a six, we cannot say that this particular outcome was due to the manipulation — the question is ill-posed. What we can say is that the number of sixes rolled is greater with the loaded dice (perhaps even much greater). Likewise, the odds for certain types of weather extremes increase in a warming climate (perhaps very much so). Attribution is not a ‘yes or no’ issue as the media might prefer, it is an issue of probability. It is very likely that several of the unprecedented extremes of the past decade would not have occurred without anthropogenic global warming. Detailed analysis can provide specific numbers for certain types of extreme, as in the examples discussed above.
Climatologist Kevin Trenberth takes a somewhat different view in his new Climatic Change piece, which I reposted over the weekend. He also thinks the question is ill-posed:
The answer to the oft-asked question of whether an event is caused by climate change is that it is the wrong question. All weather events are affected by climate change because the environment in which they occur is warmer and moister than it used to be….
The air is on average warmer and moister than it was prior to about 1970 and in turn has likely led to a 5–10 % effect on precipitation and storms that is greatly amplified in extremes. The warm moist air is readily advected onto land and caught up in weather systems as part of the hydrological cycle, where it contributes to more intense precipitation events that are widely observed to be occurring.
I had asked Rahmstorf about the dice analogy when I interviewed him for his earlier study on the Russian heat wave. I specifically asked him about whether one could say that we had not just loaded the dice but painted higher numbers on the dice. He said:
Painting higher numbers on the dice is a good analogy. There are new records that we haven’t seen before.
Rahmstorf’s new study has collected a variety of those — and we keep experiencing new ones, such as the “spring heat wave like no other in U.S. and Canadian history.”
Finally, the Potsdam Institute’s news release explains some of the background of the study for those who don’t subscribe to the journal:
Three pillars: basic physics, statistical analysis and computer simulations
The scientists base their analysis on three pillars: basic physics, statistical analysis and computer simulations. Elementary physical principles already suggest that a warming of the atmosphere leads to more extremes. For example, warm air can hold more moisture until it rains out. Secondly, clear statistical trends can be found in temperature and precipitation data, the scientists explain. And thirdly, detailed computer simulations also confirm the relation between warming and records in both temperature and precipitation.
With warmer ocean temperatures, tropical storms – called typhoons or hurricanes, depending on the region – should increase in intensity but not in number, according to the current state of knowledge. In the past decade, several record-breaking storms occurred, for example hurricane Wilma in 2004. But the dependencies are complex and not yet fully understood. The observed strong increase in the intensity of tropical storms in the North Atlantic between 1980 and 2005, for example, could be caused not just by surface warming but by a cooling of the upper atmosphere. Furthermore, there are questions about the precision and reliability of historic storm data.
To repeat the bottom line: “The evidence is strong that anthropogenic, unprecedented heat and rainfall extremes are here — and are causing intense human suffering.”
The time to act is now.
- Two seminal Nature papers join growing body of evidence that human emissions fuel extreme weather, flooding that harm humans and the environment
- Study: Global warming is driving increased frequency of extreme wet or dry summer weather in southeast, so droughts and deluges are likely to get worse
- Russian President Medvedev: “What is happening now in our central regions is evidence of this global climate change, because we have never in our history faced such weather conditions in the past”