After 14 billion dollar weather disasters in 2011 in the U.S. (not to mention additional extreme weather in 2012), questions about their connection to climate change keep arising. When confronted with these questions, the answers scientists give are usually qualified, and thus seldom satisfying to those seeking a simple response. But that’s the reality, and the reasons are spelled out in great detail in a new study by many of the country’s leading climate and weather researchers.

The author list reads like a who’s who in climate science and meteorology (including names like Kunkel, Karl, Brooks, Kossin, Lawrimore, Arndt, Bosart, Changnon, and more.)

The study (abstract): “Monitoring and Understanding Trends in Extreme Storms: State of Knowledge”, to be published in a forthcoming issue of the Bulletin of the American Meteorological Society, states its objective plainly:

Due to the intense media coverage of and great public interest in the 2011 disasters, we suspect that many [meteorologists] have received inquiries or have a personal interest about the nature of these events in the context of long-term trends and potential climate change. This paper is meant to present a clear record that can be used by meteorological professionals about what is known and unknown and why.

Of the various severe storm categories, scientists have the most knowledge and understanding of changes in heavy precipitation and least knowledge/understanding of thunderstorms and ice. (Bulletin of the American Meteorological Society)

The paper examines changes that have been observed (or not observed) in four categories of severe storms - severe thunderstorms, extreme precipitation, hurricanes, and snow storms and ice storms - and attempts to explain their causes. (The study notes extreme precipitation cuts across the three other categories, in addition to being its own.)

In short, the study suggests extreme precipitation is the only (yet important) severe storm category changing in a meaningful way and for which increasing greenhouses provide a strong explanation for recent trends.

As for linking other severe storm categories to climate change - such as hurricanes and thunderstorms - the study essentially says scientists haven’t (yet) obtained the data and understanding.

To make the results of this study digestible, what follows is a break down of the storm categories with summaries of what the study said (in my own words) complemented by key excerpts (from the study in italics)...

Severe thunderstorms (including large hail, damaging winds, and tornadoes)

Summary: Good long-term records of thunderstorms don’t exist and there’s no clear physical reason why to expect they would have changed.

Key study excerpts:

* Due to the changing [observing] practices and the nature of rare events, we have little confidence in the accuracy of trends in the meteorological occurrence of severe thunderstorms (including hailstorms) and tornadoes.

* Although some ingredients that are favorable for severe thunderstorms have increased over the years others have not, so that, overall, changes in the frequency of environments favorable for severe thunderstorms have not been statistically significant.

Extreme precipitation (i.e. heavy downpours, snowfall rates)

Summary: Reliable data indicate heavy precipitation events are increasing and rising amounts of water vapor in the atmosphere due to human-caused warming offer a good, but not necessarily complete, explanation.

Key study excerpts:

* The U.S. observing network is better suited for the assessment of changes in very heavy precipitation than for any other class of extreme storm.

* Many studies have found a statistically significant increase in the number and intensity of extreme precipitation events of durations ranging from hourly to a few days

* A number of analyses have documented significant positive trends in water vapor concentration and have linked these trends to human [greenhouse gas] fingerprints in both changes of surface and atmospheric moisture.

* While the role of water vapor as a primary cause for the increase in extreme precipitation events is compelling, the possibility of changes in the characteristics of meteorological systems cannot be ruled out. There may also be regional influences from the temporal redistribution of the number of El Nino events versus La Nina events and from land use changes...

Tropical storms and hurricanes (tropical cyclones)

Summary: Records of past tropical storm and hurricane activity are likely incomplete. While the frequency of Atlantic storms has increased since 1970 when observations have been more reliable, the cause of this uptick is not agreed upon.

Key study excerpts:

* Detection of long-term changes in tropical cyclone (TC) activity has been hindered by a number of issues with the historical records.

* Robust detection of trends in Atlantic ... is significantly constrained

* While data issues confound robust long-term (i.e., ~40-years or more) trend detection, trends in Atlantic TC frequency [an increase] are robustly observed in the modern satellite period from around 1970 to present.

* Attribution of past TC [i.e. tropical storms, hurricanes] changes is ... challenged by a lack of consensus on the physical linkages between climate forcing and TC activity. As a result, attribution of any observed trends in TC activity in these basins to anthropogenic [manmade] forcing remains controversial.

Severe snowstorms and ice storms

Summary: There’s indication big snowstorms have increased over the last 60 years but no clear explanation for the change. The increase in big snowstorms has occurred even while the number of years with very little snow has stayed the same or increased. And there’s no evidence of meaningful changes in ice storms.

Key study excerpts:

* An analysis based on the area receiving snowfall of various amounts shows there were more than twice the number of extreme regional snowstorms from 1961-2010 as there were in the previous 60 years.

* Areal coverage of extremely low seasonal snowfall has been steady or slightly increasing since [the mid-1970s]

* The two most dominant factors that influence U.S. winter storm characteristics (trajectory, frequency, intensity) are the El Niño/Southern Oscillation (ENSO) and the North Atlantic Oscillation/Arctic Oscillation (N)AO phenomena.

* The decadal scale variability of storm properties associated with each phenomenon [ENSO and the NAO] can appear in observed records as a “trend,” illustrating a need for caution before attribution to anthropogenic climate change.

* There is no distinguishable trend in the frequency of ice storms for the U.S. as a whole since 1950.

Concluding thoughts

The study finds the most clear, but not 100 percent conclusive, link between climate change and extreme storms is in the increasingly heavy precipitation they have been producing. But it’s premature to paint a broad brush in conclusively linking climate change and tornadoes, hailstorms, ice storms, snow storms, and hurricanes.

The study stresses the importance of improving and maintaining observational datasets so we have more reliable, long-term records from which to examine trends moving forward. It also emphasizes the need to improve our physical understanding of complicated phenomena like hurricanes.

The time may well come when we can more reliably describe the causes for trends we see in severe weather. But, for the time being, we’re all on firmer ground when drawing connections between climate change and increasing warm weather, ice melt, glacier changes, sea level rise, etc. And this study - in combination with others - makes a strong case to include heavy precipitation on the list.