Scientists start to meet a “grand challenge” and none too soon.

Whenever an extreme weather event occurs these days, the question almost inevitably asked is: Was it caused by global warming? For years, the less-than-satisfying scientificresponse went something like this: We don’t know; even though global warming will increase the likelihood of extreme events, directly linking a specific event to global warming is not possible.

What Had Been Impossible Approaches the Possible

But that was then and much has changed of late. While for some the words “not possible” mean don’t even bother trying, for others it is merely a challenge to go about proving the perceived impossible possible.

And so the World Climate Research Programme — ajoint project of the World Meteorological Organization and the International Council for Science whose“main objectives … are to determine the predictability of climate and to determine the effect of human activities on climate” — has identifiedpredicting extreme weather events and attributing them to climate change(or not) as one of six “grand challenges”in climate research.

In answer to that challenge, “Explaining Extreme Events of 2012 from a Climate Perspective,” thesecond [pdf]in aseries of papers edited by Thomas Peterson of NOAA’s National Climatic Data Center and colleagues,was released by theBulletin of the American Meteorological Society on September 5, 2013. (A similaranalysisof extreme events from 2011 was released last year.)

Connecting the Dots to Climate Change or Ruling Out a Connection in a Given Event

So how do scientists do the “impossible”? How do they establish a causal link between an extreme weather event and climate change? The methodology is probabalistic rather than deterministic, similar to the way weather predictions provide probabilities of rainfall (i.e., a 30 percent chance of rain) rather than “rain” or “no rain” absolutes.

In this case, the analyses determine how much more likely a given event is as a result of global warming. This is done using a model (most often a climate model) to simulate the extreme event for two cases: one with preindustrial conditions including preindustrial greenhouse gas concentrations and the other for current enhanced concentrations of greenhouse gases. Each case is run repeatedly, and the number of times the model produces an event like the one being studied is recorded. If the model with current conditions produces the event more often than that of preindustrial conditions, it is concluded that global warming made the event more likely and probably contributed to its occurrence.

Probably the first instance of this methodology was the study of the2003 heat wave in Europethat led to the premature deaths of more than 20,000 people. Writing in the journalNature, Peter Stott of the Hadley Centre for Climate Prediction and Research and co-authorsconcludedthat “it is very likely (confidence level >90%)that human influence has at least doubled the risk of a heatwave exceeding” a mean summer temperature for 2003.

There is a caveat to all this: these analyses are based on computer model simulations and as a result are only as good as the models’ ability to simulate the processes that led to the extreme event in the first place. This probably does not represent a huge caution for events involving extremes in temperature, but I am a lot less sanguine when it comes to events involving more complex phenomena like heavy rainfall.

A Look at Some of 2012s Most Extreme Events

The most recent study looked at 12 events analyzed by 18 different scientific teams (some events had multiple analyses). The choice of events studied was neither random nor pre-determined; it looks like scientists were encouraged to do analyses of events of their choosing from 2012 and submit them for review and publication. The ones that got included therefore reflect a combination of scientific interest and success in passing the peer-review bar.

Here’s a quick summary:

Event Finding Authors
1. Extreme Temperatures, Drought in the United States
1.1 Severe drought, central United States “[H]uman alteration … may have had little effect on the frequency of low-precipitation periods.” D. Rupp et al.
1.2 July heat, United States The “likelihood … is greater in the current forcing than in the preindustrial forcing.” N. Diffenbaugh and M. Scherer
1.3 Spring-summer heat, United States “The record hot spring …can be explained mainly by atmospheric dynamics.” Its connection to climate change “requires further research.” J.Cattiaux and P. Yiou
1.4 March-May heat, easternUnited States “The anthropogenic contribution … leads to a factor of 12 increase in the risk of such an event.” T. Knutson et al.
2. Mid-Atlantic Coast
2.1 Hurricane Sandy “[W]e estimate … a one- to two-thirds decrease in Sandy-level event recurrences.” i.e., more frequent.* W. Sweet et al.
3. Arctic Sea Ice
3.1 Sea ice minimum, Arctic “Our modeling results indicate that the exceptional 2012 sea ice loss was primarily due to the sea ice memory [partly related to long-term warming] and to the positive feedback of the warm atmospheric conditions [partly related to long-term warming], both contributing approximately equally.” Guemas et al.
3.2 Sea ice minimum, Arctic “The … [Arctic sea ice extent] is extremely unlikely to have occurred due to internal climate variability alone, … and has a much greater likelihood of occurrence in the ‘forced plus internal variability’ scenario.” R. Zhang and T Knutson
4. Europe
4.1Not enough ice in Holland for 11-City Tour despite frigid February temperatures “Snowfall on the thin ice that had just formed is shown to limit the ice growth more strongly than the effect of warming.” (See related post.) H. de Vries et al.
4.2 Extreme summer rain and drought, Europe “Both anthropogenic forcing and natural variability may have influenced [the event].” B. Dong et al.
4.3 Extreme summer rain in northwestern Europe “We find no evidence that declines in sea ice can explain these recent wet summers” and “the possibility remains that recent European summer precipitation anomalies are due to other drivers in the climate system rather than chance.” S. Tett
4.4 Summer rain, United Kingdom “[T]he model distributions studied suggest that any anthropogenic influence on these patterns was minimal.” S. Sparrow
4.5 Summer rain, northern Europe “This suggests a contribution of climate change to precipitation rate in northern Europe.” P. Yiou and J. Cattiaux
4.6 Winter drought 2011-2012, Iberian Peninsula “[T]he return period of such an extreme drought has decreased [i.e., the likelihood has increased] between the 1960s and 2000s decades. …[S]uch a trend has been partially driven by the anthropogenic emissions of greenhouse gases and aerosols.” R. Trigo et al.
5. Africa
5.1 Spring drought; eastern Kenya, southern Somalia “It seems likely that other factors, such as warming in the western Pacific … a recent transition in Pacific decadal variability.., and stronger western-to-central Pacific SST gradients may be contributing to the recent dryness.” C. Funk et al.
6. East Asia
6.1 Summer flooding, northern China “We are unable to confirm or reject the role of climate change in the 21–22 July 2012 rainfall event due to the inability of the models to accurately replicate observations in this region of China.” T. Zhou et al.
6.2 Summer rain, Japan “The extremeness of the Japanese heavy rainfall of 2012 was mainly caused by the oceanic natural variability and probabilistic atmospheric natural variability rather than by anthropogenic climate change.” Y. Imada et al.
7. Australia, New Zealand
7.1 2011-12 Extreme rain, southeastern Australia “[W]e detect limited evidence of a change in extreme rainfall … that may be attributed to anthropogenic climate change.” A. King et al.
7.2 Extreme rain March 2012, eastern Australia “[W]e find that the overall long-term effect of human influences on the climate increases the chances of above-average rainfall by 5%–15% (best estimate), although .… the extreme magnitude of the event appears to arise mainly from unforced internal climatic variations.” N. Christidis et al.
7.3 Extreme rain Dec. 2011; Golden Bay, New Zealand “This analysis indicates that the total moisture available for precipitation in the Golden Bay/Nelson extreme rainfall event of December 2011 was 1%–5% higher as a result of the emission of anthropogenic greenhouse gases.” S. Dean et al.


In their conclusion, Peterson et al. point out that the attribution business with the current methodology is a lot more complex than a pat yes or no answer. About half the papers found a global warming connection. Some, as I do, may find that to be a large percentage — it’s a lot harder to establish a positive result than a negative one. Perhaps more telling is the fact that in some cases different teams looked at the same event (such as the extreme summer rainfall in Northern Europe)and reached differing conclusions.

To aid readers’ understanding of this tricky business, the authors point to a driving analogy used by scientists at the University Corporation for Atmospheric Research in 2012:

“Adding just a little bit of speed to your highway commute each month can substantially raise the odds that you’ll get hurt some day. But if an accident does occur, the primary cause may not be your speed itself: it could be a wet road or a texting driver.” Similarly, while climate models may indicate a human effect is causing increases in the chances of having extremely high precipitation in a region (much like speeding increases the chances of having an accident), natural variability can still be the primary factor in any individual extreme event.”

Stay tuned for more studies like this one. It’s a safe bet that we’’ll see another series of papers appearing in the Bulletin of the American Meteorological Society about this time next year looking at extreme events in 2013. And there’ll be some fodder for scientists to chew on. Granted it’s been anespecially quiet hurricane seasonso far (with onlysix named stormsand no hurricanes) and thankfully nothing even approximating a Sandy this year. But the year has not been without its doozies:

And we still have some months to go before we close the books on 2013. So, as I said, stay tuned.


End Note

*Anew studyby Elizabeth Barnes of Colorado StateUniversity et al. suggests Sandy’s westward trajectory will be less likely to occur with climate change.