Bob Henson ·

Above: Vehicles are submerged in floodwaters on July 9, 2016 in Wuhan, Hubei Province of China. Many parts of the Wuhan area were submerged in floodwaters during July as torrential rains affected the Yangtze River valley. A new study finds that climate change increased the risk of the rains that led to the flooding by 17 – 59%. Image credit: VCG/VCG via Getty Images.

“Impossible” is a fraught word, but a new set of studies concludes that at least three atmospheric and oceanic phenomena from 2016 wouldn’t have occurred had we not been adding greenhouse gases to the air for more than a century.

Record heat in Asia, record-high global temperature, and a marine “heat wave” in the far North Pacific are among 27 events analyzed in “Explaining Extreme Events in 2016 from a Climate Perspective.” This report, free to download in its entirety, is the sixth annual compilation of climate attribution studies published as a supplement to the Bulletin of the American Meteorological Society (BAMS). The 2016 report was released on Wednesday.

The BAMS series has become the leading venue for papers in the climate-science subfield called detection and attribution (D&A). The idea in such work is to identify a measurable weather/climate change (detection) and to determine what caused it (attribution). Most D&A studies end up yielding probabilities, typically the extent to which odds of a given event have been raised by climate change. This year, the BAMS report includes three events that go so far beyond any modern precedent that they can’t be explained without invoking human-produced greenhouse gases.

"This report marks a fundamental change," said Jeff Rosenfeld, editor-in-chief of BAMS. “For years scientists have known humans are changing the risk of some extremes. But finding multiple extreme events that weren’t even possible without human influence makes clear that we're experiencing new weather, because we've made a new climate."

Figure 1. The 27 locations and events analyzed in “Explaining Extreme Events in 2016 from a Climate Pespective.” Image credit: Courtesy Bulletin of the American Meteorological Society.

Those who caricature climate science as being a monolithic echo chamber might be surprised at the range of conclusions in attribution research. Out of the 130 studies published in the six years of BAMS supplements, about 32% found no link between a given extreme event and long-term climate change. In their introduction, the editors stressed: “…this publication prides itself as a venue that accepts papers without consideration for whether a role for climate change is found.” In fact, the editors have started to limit the number of heat-related papers only because climate change is so firmly and clearly implicated in recent global heat: “Given that the majority of heat papers now use a widely established and accepted methodology, the scientific value of continuing to include a large number of heat studies began to seem limited.”

By the same token, it’s heat-related findings that have risen to a new level of confidence, as demonstrated in three of this report’s papers.



Figure 2. Globally averaged temperatures hit consecutive record highs in 2014, 2015, and 2016. This year is likely to place just behind this group as one of the four warmest years on record. Image credit: NOAA/NCEI.

Record-high global temperature

Globally averaged surface temperatures hit a new high for the third consecutive year in 2016, leaving the much-disputed warming slowdown of the 2000s in the dust. A team led by Thomas Knutson (NOAA Geophysical Fluid Dynamics Laboratory) compared the 2016 value to all of the annual global temperatures calculated in an ensemble of more than 24,000 years of simulations from the Climate Model Intercomparison Project (CMIP5). All of these runs were for a “control” atmosphere—i.e., one in which greenhouse gases are kept at preindustrial levels.

The warmest peaks and valleys in the CMIP5 control runs were around 0.5°C above and below the midpoint. In contrast, the 2016 global temperature was roughly 1.3°C beyond the 1881-1920 average—thus more than twice as high as anything one would expect had we not been burning fossil fuels. Even if you add about 0.2°C to account for the volcano-induced cooling in 1881-1920, the 2016 warmth can’t be explained without climate change.

“Using the CMIP5 models, we don’t see any way you can get that size of that anomaly from natural variability alone,” said Knutson in an email. “We got a minor boost from El Niño…but it still wasn’t a strong enough influence.” He added that CMIP5 models differ in the amount of natural or “internal” variability they portray, but as noted above, the 2016 value transcended all of these differences.

The newly conclusive results are partly because the warmth of both 2015 and 2016 was so exceptional, but also due to some refinements in methodology. In the BAMS collection published last year, Knutson and colleagues found that natural variability was able to match the global warmth of 2014 in a few extreme model runs. However, that study included some CMIP5 models with only a single ensemble member available for runs that included only natural

forcing factors such as past volcanoes and solar variability. “This, in retrospect, was not a sound choice,” Knutson told me. “A single ensemble member can contain strong internal variability components that need to be averaged out to some degree by using multiple ensemble members.” Also, the team found that the global averaging techniques used last year artificially boosted the model-based global departures from average in the model.

With the improved and corrected methodology, “even the second-ranked year (2015) is beyond the reach of natural variability, according to the CMIP5 models,” said Knutson.

Figure 3. Hot weather takes its toll on June 5, 2016 in New Delhi, India. Delhiites were reeling under severe heat wave conditions as temperatures hit 42.6°C (108.7°F) with relatively high humidity. On May 19, 2016, the town of Phalodi in western India set an all-time national record with a high of 51°C (123.8°F). Image credit: Virendra Singh Gosain/Hindustan Times via Getty Images.

Asian heat

Numerous records were smashed in 2016 during prolonged heat waves across Asia. At least 580 people were killed by record-setting heat across India, and Thailand experienced its most prolonged heat wave in at least 65 years. A research team led by Yukiko Imada (Japan’s Meteorological Research Institute) found that extreme heat was more than twice as widespread across Asia in 2016 as it was during the runner-up year, 2010. The drying and warming effects of El Niño are partly to blame, but when calculating probabilities, the researchers found: “All of the risk of the extremely high temperatures over Asia in 2016 can be attributed to anthropogenic [human-caused] warming. In addition, the ENSO condition [El Niño] made the extreme warmth two times more likely to occur.”

North Pacific warming

Last year’s exceptional warmth in the Bering Sea and Gulf of Alaska cannot be reproduced in preindustrial climate, according to work led by John Walsh (University of Alaska Fairbanks). The warmth in both 2015 and 2016 had far-reaching impacts in and near Alaska, including widespread algae blooms, marine die-offs, and depleted sea ice. Although natural variability played a role, it was not enough by itself to produce the warmth observed in 2016. The authors hypothesize that some of the extraordinary heat was channeled into these regions from the remnants of the North Pacific “blob,” a persistent zone of unusually warm water that peaked in 2014-2015 and dissipated in 2016. They add: “Given the many impacts of the 2016 anomaly, the future climate projected here will result in a profound shift for people, systems, and species when such warm ocean temperatures become common and not extreme in the [Gulf of Alaska] and Bering regions.”

Figure 4. Flooded roads impede traffic in Wuhan, China, on July 6, 2016. An overnight storm following days of rain stopped traffic, cut power and water supplies, and trapped citizens in their homes in many parts of Wuhan, a metropolis on the Yangtze River. A new study finds that widespread flooding in the Yangtze River basin during 2016 was made more likely by human-produced climate change. Image credit: Wang He/Getty Images.

Climate change hiked the odds of 2016’s most expensive weather disaster

Among the other papers in this year’s BAMS report that found a climate-change link was a study led by Xing Yuan (Chinese Academy of Sciences). It examined the Yangtze River flooding in China, the year’s most expensive weather-related disaster. The floods cost $28 billion and claimed 475 lives. Yuan and colleagues found that human-caused climate change has increased the risk of the rains that led to the flooding by 17 – 59%. This risk increases further, to 37 – 91%, when El Niño is a factor, since the after-effects of an El Niño event in winter (as we saw in 2015-16) raise the odds of heavy rain in China in the subsequent summer.

Other summaries of climate change attribution research

Earlier this year, Carbon Brief published a map of 144 climate change attribution studies covered by 138 scientific papers published over the past 20 years, from Typhoon Haiyan to the 2011 – 2015 California drought. More than three-quarters of these papers came from the five years of BAMS studies published up to that point. The analysis suggested that 63% of the extreme weather events studied were made more likely or more severe by human-caused climate change. The 144 studies focused on extreme heat (34%), drought (23%) and rainfall/flooding (20%.) ­

Figure 5. Breakdown of the Carbon Brief analysis of 144 climate change attribution studies carried out in a 20-year period through early 2017. More than 75% of the heat-related studies found that climate change made the event more severe or more likely to occur. A majority of the studies of rain/flood events either found no discernable influence from climate change or found insufficient data to show any potential link. Attribution research on flooding can be complicated by land-use changes and water management decisions that affect where and how floods occur. Image credit: Carbon Brief, used with permission.

A separate analysis published this week by the Energy and Climate Intelligence Unit, a non-profit group that supports debate on climate change and energy issues, looked at 59 extreme weather/climate change attribution studies published in the two years since the Paris Climate Accord of 2015. The analysis found that 41 of the studies demonstrated that climate change had made extreme weather events more intense and/or longer-lived, and that a few of them had costs and death tolls that could be partially attributed to human-caused climate change. They estimated that 4,000 climate change-caused deaths could be attributed to extreme weather events over the past two years. These deaths primarily came from two events: the 2015 heatwaves in France (1,000 of the 3,275 total heat wave deaths attributable to climate change) and in South Asia (2,800 of the 3,200 deaths in India and Pakistan attributable to climate change.)

In addition, the analysis found that $8.1 billion in damages from five extreme weather events during the past several years could be attributed to climate change, as shown below:

The study concluded: “These figures cannot and must not be taken as estimates of the overall cost of climate change for this period, which will be far higher.”

Dr. Jeff Masters contributed to this post.