Explaining Extreme Events of 2015

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Human-caused climate change very likely increased the severity of heat waves that plagued India, Pakistan, Europe, East Africa, East Asia, and Australia in 2015 and helped make it the warmest year on record, according to new research published today in a special edition of the Bulletin of the American Meteorological Society.

The fifth edition of “Explaining Extreme Events from a Climate Perspective” presents 26 peer-reviewed research papers that examine episodes of extreme weather of 2015 over five continents and two oceans. It features the research of 116 scientists from 18 countries analyzing both historical observations and changing trends along with model results to determine whether and how climate change may have influenced the event.

The strongest evidence for a human influence was found for temperature-related events—the increased intensity of numerous heat waves, diminished snowpack in the Cascades, record-low Arctic sea ice extent in March, and the extraordinary extent and duration of Alaska wildfires. The extreme El Niño in 2015 provided a unique challenge for researchers, since El Niño  itself can affect the probability of these events. In each of these cases, however, the scientists found a unique contribution from climate change over and above that of the El Niño.

“After five years of the BAMS Explaining Extreme Events report, we’re seeing mounting evidence that climate change is making heat waves more extreme in many regions around the world,” said lead editor Stephanie C. Herring, a scientist with NOAA’s National Centers for Environmental Information. "As we get better at distinguishing the influence of climate change from natural variability, the local significance and impacts of this global phenomenon are becoming clearer."

Numerous other events of 2015 were made more extreme by climate change, the report found. The probability of “sunny day” tidal flooding in the Miami area, like the one that inundated coastal areas that September, has risen 500 percent since 1994, according to one study. Human-induced climate change likely contributed to the record high intensity of west North Pacific typhoons and the record amount of winter sunshine in the United Kingdom.

But, researchers found no evidence of an overall climate change signal in the delayed onset of the Nigerian spring rainy season or in the extreme daily rainfall totals that inundated Chennai, India, in December. There was likewise no evidence that the extreme cold winter conditions over the northeast United States in 2015 were made more likely by human-induced climate change.

Over the past five years, more than 100 papers examining extreme events have been accepted for publication in this special report. Of the more than 100 events analyzed, around 65% were found to be influenced by human-caused climate change. While there’s mounting evidence in the role of climate change in amplifying the severity of heat waves, evidence of a climate change signal has not been found in a majority of extreme precipitation event papers published in the annual report since 2012, Herring said.

However, she cautioned that the lack of clear evidence of a climate signal did not necessarily mean climate change played no role in an event. A “null” result could mean the event fell within the bounds of natural variability. It could also mean that the framing of the research question or the method of analysis chosen requires further refinement and development.

Contributing authors choose the event they wish to study, so the new studies are neither a random sample nor a comprehensive survey of extreme weather events. They do illustrate how various methods can be applied to different extreme event attribution analysis, and in cases where multiple groups look at the same event, it allows different approaches to be compared.

“With this report, we continue to document scientists’ growing skill in identifying how climate change influences today’s weather,” said Jeff Rosenfeld, editor-in-chief of the Bulletin of the American Meteorological Society. “These accessible and brief papers show the scientific community and the public that once seemingly impossible insights about climate impacts are now within the capability of timely, rigorous science.”

The American Meteorological Society independently conducted the peer reviews for studies included in this special report.

Evidence for human-influenced climate change was identified for:

  • Ten extreme heat events, including heat waves in Europe, India, Pakistan, China, Indonesia, Japan, and Australia
  • The record average global temperature in 2015
  • Record-low Arctic sea ice in March
  • Alaska’s intense wildfire season
  • Extreme drought in southwestern Canada
  • Extreme May rainfall in southeast China
  • Florida’s “sunny day” flood in September
  • Record winter sunshine in the United Kingdom

No climate change signal was found for:

  • Outbreaks of extreme cold in the eastern United States and Canada
  • The late onset of Nigeria’s spring rainy season
  • Heavy daily precipitation in December over Chennai, India

Many lines of evidence, such as summarized in the IPCC and U.S. National Climate Assessments, reveal significant global trends in extremes such as a rise in heat waves and an increased frequency of heavy precipitation events since 1950 have been influenced by increasing levels of greenhouse gasses in the atmosphere. Papers published in this report take a place-based and event-specific approach to identifying the role of change, and they answer the question of how much a particular recent event’s likelihood or intensity has changed in a historical climate perspective.

Five NOAA scientists served as editors of Explaining Extreme Events of 2015 from a Climate Perspective: Herring, James Kossin, and Carl Schreck III of NOAA’s National Centers for Environmental Information along with Martin P. Hoerling and Andrew Hoell with NOAA’s Earth Systems Research Laboratory. Peter A. Stott with the U.K. Met Office Hadley Centre also served as an editor.

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