A New Take on the Human Factor in Recent Extreme Events

Much debate in the last few years has centered on persistent kinks in the polar jet stream and the extreme weather they’ve helped produce, such as the record snowfalls in New England last February. Top researchers differ on how much a changing climate might be involved with this jet-stream “weirding.” However, there’s no question that sea levels have risen and global temperatures have warmed. Those unassailable facts may serve as the most direct link between climate change and extreme events, according to the Perspectives article Attribution of climate extreme events, published on Monday in Nature Climate Change. The authors include Kevin Trenberth and John Fasullo (National Center for Atmospheric Research) and Theodore Shepherd (University of Reading, England).

Trenberth is a leading expert in the global flows of energy and water around the world. Because warmer temperatures and increased water vapor have influenced the whole of Earth’s atmosphere, Trenberth and colleagues start with the premise that every storm is influenced by climate change to at least some extent. “The environment in which all weather events occur is not what he used to be,” their new paper states. At the same time, they agree that no storm is entirely a result of climate change: “...it is not possible to attribute a single climate extreme event, which by definition is unique and which has a large element of chance in its occurrence, to a specific cause.”


Figure 1. Temperature anomalies for July 2003 in western Europe, as calculated by observations from NASA’s Terra satellite. Image credit: Reto Stockli and Robert Simmon, based upon data provided by the MODIS Land Science Team.


Recognizing this quandary, many researchers who look into climate change and extreme events use models and observations to gauge how much of the risk of a particular extreme can be attributed to a warming planet. A landmark study led by Peter Stott (UK Hadley Centre) found a greater-than-90-percent chance that a European heat wave on par with the 2003 disaster that killed an estimated 70,000 people had become at least twice as likely due to human-produced greenhouse gases. Such studies often call on large-scale circulation, such as the flow at 500 millibars, as a key index of the extreme event. But there is a great deal of natural variability in where upper-level highs and lows set up, so an attribution study focused on circulation might find no evidence that climate change helped create a extreme event, even if there is unprecedented rainfall or heat--the variables that actually cause impact--associated with it. The new paper suggests that a more useful question might be: “Given the weather pattern, how were the temperatures, precipitation and associated impacts influenced by climate change?” The paper goes on to look at four recent events (see below) and how they would look through this lens.

Dáithí Stone, an attribution expert at Lawrence Berkeley National Laboratory and leader of the Weather Risk Attribution Forecast, sent me this take on the paper: "Recent studies exploring the role of greenhouse gas emissions in extreme weather events tend to be conservative by working under the 'innocent until proven guilty' paradigm, but this paper argues it would also be useful to work under the 'guilty until proven innocent' paradigm, or something in between. This is really the precautionary principle and can certainly make sense for adaptation decisions: even though residents of a coastal city might not have been measuring sea level, they may still think it wise to assume it is rising. But looking at things in the innocent-until-proven-guilty approach can be wise too, as in the Western legal systems designed to prevent witch hunts. So which paradigm is better depends on the purpose."

It remains to be seen which scientists will follow the lead of the new paper and focus more on thermodynamics (heat, moisture) and sea level rise, as opposed to circulation change. Jennifer Francis (Rutgers University) is among those who’ve found evidence for a link between depleted Arctic sea ice and unusual jet-stream behavior. In an email to me, she agreed with the overall conclusion of Trenberth and colleagues: “One should focus on climate changes that are irrefutable--such as rising sea levels, warmer tropospheric temperatures, increased water vapor, warmer SSTs, and changing soil moisture--all on a case-specific basis. Given a particular circulation pattern or weather system, these changes will affect the impacts of that system.” At the same time, she maintains that the question of how cutoff lows, blocking highs, and other jet-stream configurations may be changing is equally important. “Addressing this question requires a different approach that identifies and measures changes in these types of patterns,” said Francis. “For example, knowing whether the frequency of strong ridging in the eastern Pacific will change depending on certain factors--such as Pacific sea-surface temperature (SST) patterns and/or Arctic sea-ice loss--will be tremendously valuable in planning for water resources in western states.” She added: “Changes in dynamics are harder to pin down, but ultimately they have a farther-reaching impact on probabilities of particular extremes.”

Below are summaries of the four events linked by Trenberth and colleagues to thermodynamic and sea-level changes. The full paper can be viewed from a link at this Guardian blog post by John Abraham, thanks to a nature.com content sharing initiative. At Mashable, Andrew Freedman provides additional perspective on the new paper. A matrix created in 2012 for UCAR/NCAR AtmosNews outlines several different ways that scientists have approached the attribution of extreme events to climate change.

Jeff Masters and Bob Henson


Figure 1. Hurricane Sandy at 10:10 am EDT October 28, 2012. Image credit: NASA/GSFC.

1) Hurricane Sandy, 2012
Hurricane Sandy, the most powerful and second most destructive Atlantic hurricane in recorded history, barreled into New Jersey on October 29, 2012, bringing hurricane-force winds, torrential rains, heavy snow, and a massive storm surge. Sandy's catastrophic storm surge was responsible for the majority of the storm’s 131 deaths and $62 billion in damage in the United States. While papers have been published arguing that climate change could be expected to make Sandy’s unusual 1-in-700 year track west-northwestwards into new Jersey more or less likely, the authors of Monday’s study argue that the increased sea surface temperatures (SSTs) along its track due to global warming likely led to a bigger, more intense storm, stronger winds, and greater precipitation. Sandy traversed a broad strip of SSTs that were 1 - 1.5 °C warmer than average along the U.S. East Coast, and a 2014 model study using the European model by Magnusson et al., Evaluation of medium-range forecasts for Hurricane Sandy, found that these warmer SSTs decreased Sandy’s central pressure by 7.6 mb, increased the winds by 8 mph (3.6 m/s), and increased the precipitation by 35%. The authors of Monday’s study write, “Moreover, the storm was riding on sea levels that were higher by about 7.5” (19 cm) because of global warming. Although perhaps only one-half to one-third of the SST increase can be blamed on global warming from human activities, it is readily apparent that the storm surge and associated damage was considerably influenced by climate change. It is quite possible that the subways and tunnels might not have flooded without the warming-induced increases in sea level and in storm intensity and size, putting a potential price tag of human climate change on this storm in the tens of billions of dollars.” Indeed, Lloyd’s of London estimated that the amount of sea level rise due to global warming over the past century led to an additional $8 billion in damage from Sandy’s storm surge in New York. Here is another analysis (from UCAR/NCAR AtmosNews) on the factors that went into Sandy’s surge.


Figure 2. Damage to Highway 34 along the Big Thompson River, on the road to Estes Park, Colorado in September, 2013. Image credit: Colorado National Guard.

2) Boulder, Colorado floods, 2013
In September 2013, records rains over the Front Range of the Colorado Rockies fed rampaging floods that killed at least nine people and did $2 billion in damage. An assessment published in the Bulletin of the American Meteorological Society last September concluded concluded that the flood was not made more likely or more intense by climate change, given that models were just as likely to produce heavy September rain when run for the period 1870–1900 as for 1983–2012. However, the authors of the new study write, “Extremely high SSTs off the west coast of Mexico and the associated record atmospheric water vapor amounts that flowed into Colorado as a result were instrumental in the event, and it probably would not have occurred without human-caused warming. Such an increase in atmospheric water vapor becomes concentrated when focused by topography, as it did in Boulder, and further amplified on the ground as water drains into channels and rivers. This suggests an important role for human-caused warming in those Boulder floods.”


Figure 3. There's a car under here somewhere! A Maryland resident digs out after Snowmageddon. Image credit: wunderphotographer chills.

3) Snowmaggedon, 2010
On February 5 - 6, 2010, an incredible snowstorm dubbed “Snowmaggedon” hammered Washington DC and the mid-Atlantic states, burying them under 1 - 3 feet of snow. While the blizzard was not an exceptionally strong storm--the central pressure was a rather unimpressive 986 mb at the height of the blizzard--it was an exceptionally wet storm. The melted equivalent precipitation for the blizzard exceeded three inches along its core snow belt, a phenomenal amount of moisture for a winter storm. The blizzard formed a very unstable region aloft where thunderstorms were able to build, and there were many reports of thundersnow with snowfall rates of 2 - 3 inches per hour. The authors claim that unusually high SSTs in the tropical Atlantic Ocean (1.5 °C above normal) led to an exceptional amount of moisture flowing into the storm, which resulted in very large amounts of snow. While the storm was in the right place at the right time to generate a large amount of snow, the new paper argues that the extreme snowfall amounts were magnified by ocean temperatures made warmer by climate change.


Figure 4. An infrared VIIRS image of the eye of Haiyan taken at 16:19 UTC November 7, 2013. At the time, Haiyan was at peak strength with 195 mph sustained winds. Image credit: NOAA/CIRA.

4) Super Typhoon Haiyan, 2013
Super Typhoon Haiyan hit the Central Philippines on November 8, 2013, as one of the strongest tropical cyclones in world history, with peak surface winds estimated at 195 mph by the Joint Typhoon Warning Center. Haiyan killed over 7,700 people and did at least $13 billion in damage, making it the costliest and deadliest disaster in Philippine history, and Earth's deadliest natural disaster of 2013. The new study notes that oceanic heat content (OHC) and sea level had both risen significantly in the region since 1998 as a result of the negative phase of the Pacific Decadal Oscillation. “Consequently, as the typhoon approached the Philippines, it was riding on very high SSTs with very deep support through the high OHC, and the strong winds and ocean mixing did not cause as much cooling as would normally be experienced, probably helping the storm to maintain its tremendous strength,” write the authors. “Moreover, the storm surge was undoubtedly exacerbated considerably by the sea levels, which were some 30 cm [12”] above 1993 values. Although natural variability through the PDO played a major role, there is also a global component through increased OHC from the Earth’s energy imbalance."