What Do the Latest Climate Assessments Tell Us about Nor’easters?

An ever-lengthening procession of winter storms has marched across the mid-Atlantic and Northeast over the last few years. Even their names have grown more prolific and colorful, from Snowmaggedon of January 2010 to this week’s Blizzard of 2015, also known as Winter Storm Juno. Do these monikers imply the storms themselves are getting more fierce? The naming trend can be explained largely by the demands of social media. However, recent overviews of national and global climate indicate that, as a whole, the most intense rainstorms and snowstorms in the Northeast U.S. are growing even more intense. Our understanding of a warmer climate with wetter extremes arises from both observations of past trends and model-based projections of future climate. It’s also supported by basic physics: in a warmer global environment, more water vapor evaporates from oceans and lakes, where it can be steered into rain- and snow-producing storms.

The most recent findings from the Intergovernmental Panel on Climate Change appear in its fifth major assessment, released in 2013-14. According to Chapter 2 of the Working Group 1 report, many parts of the world are reporting more frequent and intense bouts of extreme precipitation over the last few decades, though a few areas (such as western Asia and southern Australia) are bucking the trend. Notably, the IPCC found that “evidence is most compelling for increases in heavy precipitation in North America, Central America and Europe.” (FAQ 2.2, p. 218). The report also confirms earlier findings that the Northern Hemisphere jet stream continues a long-term poleward migration. That trend is widely expected to continue in the next several decades, though not all studies agree.

Within the United States, the region where nor’easters prowl is also where we find the most pronounced turn toward extreme rainfall and snowfall. The most comprehensive report to date on our nation’s climate is the latest U.S. National Climate Assessment, released early in 2014. It found that the trend toward intensified precipitation is stronger in the Northeast than in any other part of the country (Fig. 2.18 in the report, and Figure 1 above). For the period 1958 – 2012, this region saw a 71% increase in the amount of precipitation that fell on the wettest 1% of all days. “In the mid-latitudes, where most of the continental U.S. is located, there is an upward trend in extreme precipitation in the vicinity of fronts associated with mid-latitude storms,” noted the report. “Projections of future climate over the U.S. suggest that the recent trend towards increased heavy precipitation events will continue. This is projected to occur even in regions where total precipitation is projected to decrease, such as the Southwest.” (full report, Chapter 2, p. 37).



Figure 1. The map shows percent increases in the amount of precipitation falling in very heavy events (defined as the heaviest 1% of all daily events) from 1958 to 2012 for each region of the continental United States. These trends are larger than natural variations for the Northeast, Midwest, Puerto Rico, Southeast, Great Plains, and Alaska. The trends are not larger than natural variations for the Southwest, Hawai‘i, and the Northwest. The changes shown in this figure are calculated from the beginning and end points of the trends for 1958 to 2012. Image credit: U.S. National Climate Assessment, Fig. 2.18.


For a more specific take on U.S. trends in heavy snow, we can call on a 2013 paper led by Ken Kunkel (NOAA/North Carolina State University) that appeared in the Bulletin of American Meteorological Society (BAMS). It uses the recently developed Regional Snow Index, a spinoff of the Northeast Snowfall Impact Scale that ranks snowfalls by depth and coverage and by the population of affected areas. Kunkel and a large set of collaborators analyzed the 50 strongest snowstorms observed from 1900-01 to 2009-10 in each of six climate regions east of the Rockies. Among the six regions, the Northeast (see Figure 2) saw the second-greatest increase (13%) in the number of extreme snowstorms per decade across the century-plus period. Overall, said the report, “the greater number of extreme storms in recent decades is consistent with other findings of recent increases in heavier and more widespread snowstorms.”




Figure 2. Number of extreme snowstorms (top 10% of all snowstorms, 1900 - 2010) occurring each decade within the six U.S. climate regions in the central and eastern U.S., based on an analysis of the 50 strongest storms for each of the six climate regions from Oct 1900 to Apr 2010. Shown in the map for each region are (left) temperature and (right) precipitation trends, calculated as departures from the 20th-century average for all snow seasons in which each storm occurred. Snow seasons are defined as Dec–Mar for the South and Southeast regions and Nov–Apr for the other four regions. Image credit: Kunkel et al., 2013, BAMS/American Meteorological Society.


Given the potential for disaster inherent in the worst winter storms, even a 13% increase in their frequency could pack a notable punch. As evident in Figure 2, however, there is sharp variability from decade to decade in the frequency of such extreme snowstorms, something that should come as no surprise to any longtime weather observer. Which brings us back to the last decade and its bumper crop of Northeastern storms. At New York City’s Central Park, five of the 10 biggest storm-total snowfalls on record have occurred since 2000; in Boston, it’s four of the top 10, and in Washington, three of the top 10. This 21st-century onslaught could be related to the longer-term boost provided to extreme rains and snows by a warming planet, as well as a shorter-term jump in the frequency of Northeast storms related to cycles in global and regional climate. In addition, a growing amount of research suggests that Arctic amplification and sea-ice loss could be influencing jet-stream behavior and fostering outbreaks of cold and snow in North America and Eurasia (see the subsection "Is the jet stream getting weird?" in this Jeff Masters post from last November). Variations in snow-measuring practice over time may also play a role in local trends and records, although these can generally be factored out of broader-scale studies. For example, the study above led by Kunkel drew on a subset of U.S. reporting stations where confidence in the long-term quality of snow reports was highest.

And finally, the latest version of a query that can never be answered with razor-sharp precision: to what extent was the Blizzard of 2015 a product of climate change? All storms are now unfolding in a warmer, moister global climate. To get the closest thing to a quantitative answer on this particular storm, you would need to carry out an “attribution” study, using computer models to simulate the storm with and without such factors as the warmer-than-normal North Atlantic ocean temperatures that helped fuel the blizzard. Such studies have shown that events such as the deadly European heat wave of 2003 and Russian heat wave of 2010 became considerably more likely in a greenhouse-warmed climate. For the last three years, BAMS has released special reports, each featuring a set of attribution studies on extreme events from the previous year. The BAMS report on 2013 analyzed a violent midlatitude storm in Germany and Denmark, with inconclusive results. To my knowledge, no attribution work has yet been done specifically on nor’easters. Friederike Otto (University of Oxford) has put together an informal writeup explaining how attribution studies are carried out, using England’s destructive flooding of 2000–01 as an example.

Bob Henson