The all-time record number of storms over the British Isles in winter 2013/14 cannot be linked directly to anthropogenic-induced warming of the tropical west Pacific.
The extreme 2013/14 winter storm season over much of North America was made more likely by the multiyear anomalous tropical Pacific winds associated with the recent global warming hiatus.
The near-record number of extremely cold days during winter 2014 in the eastern United States cannot be attributed to trends or variability changes. Daily temperature variability is actually decreasing, in contrast to CMIP5 simulations and projections.
The frigid 2013/14 Midwestern winter was 20–100 times less likely than in the 1880s due to long-term warming, while winter temperature variability has shown little long-term change.
The fire season in northern California during 2014 was the second largest in terms of burned areas since 1996. An increase in fire risk in California is attributable to human-induced climate change.
This special supplement on explaining extreme events has now published 79 papers over the past four years. Over half of these papers have shown that human-caused climate change influenced an event's frequency and/or intensity in a substantial manner. It could be argued that because all of these events occurred in the context of a warmer world, there are impacts on all extremes whether or not the influence is detectable with current methods and available observations. While potentially true, to make attribution results informative to adaptation decisions, scientists must take on the questions of whether the risk or magnitudes of such events have increased or decreased, by how much, and what level of confidence supports the claims. This is the challenge the authors who have contributed to this report have taken on. The summary table (Table 34.1) is provided to give readers a general overview of their results. However, it is a highly simplified categorization of the results and does not include information about the size of the signal detected and the confidence in the results. This information is present within each individual report, and provides essential context for understanding and interpreting results for any individual event.
The January 2014 floods paralyzed nearly all of Jakarta, Indonesia. The precipitation events that lead to these floods were not very unusual but show positive trends in the observed record.
The record dry spell over Singapore–Malaysia was caused by the southward contraction of the intertropical convergence zone. Within present evidence, there is no clear attribution to climate change.
Anthropogenic forcing may have contributed to an 11-fold increase in the chance of the 2014 hot spring in northern China.
Extreme daily fall precipitation in the Cévennes mountains has very likely intensified. The probability of amounts witnessed in 2014 is estimated to have tripled since 1950, with large uncertainties.
After transitioning from a hurricane to an extratropical storm, Gonzalo tracked unusually far, achieving exceptional strength over Europe; however, it was within the historical range of such transforming storms.
The Argentinian heat wave of December 2013 was likely caused in part by anthropogenic forcings. These forcings have increased the risk of such an event occurring by a factor of five.
The collective effects of anthropogenic climate change and artificial pond drainage may have played an important role in producing the extreme flood that occurred during early summer 2014 on the southeastern Canadian Prairies.
Understanding how long-term global change affects the intensity and likelihood of extreme weather events is a frontier science challenge. This fourth edition of explaining extreme events of the previous year (2014) from a climate perspective is the most extensive yet with 33 different research groups exploring the causes of 29 different events that occurred in 2014. A number of this year’s studies indicate that human-caused climate change greatly increased the likelihood and intensity for extreme heat waves in 2014 over various regions. For other types of extreme events, such as droughts, heavy rains, and winter storms, a climate change influence was found in some instances and not in others. This year’s report also included many different types of extreme events. The tropical cyclones that impacted Hawaii were made more likely due to human-caused climate change. Climate change also decreased the Antarctic sea ice extent in 2014 and increased the strength and likelihood of high sea surface temperatures in both the Atlantic and Pacific Oceans. For western U.S. wildfires, no link to the individual events in 2014 could be detected, but the overall probability of western U.S. wildfires has increased due to human impacts on the climate.
Challenges that attribution assessments face include the often limited observational record and inability of models to reproduce some extreme events well. In general, when attribution assessments fail to find anthropogenic signals this alone does not prove anthropogenic climate change did not influence the event. The failure to find a human fingerprint could be due to insufficient data or poor models and not the absence of anthropogenic effects.
This year researchers also considered other humancaused drivers of extreme events beyond the usual radiative drivers. For example, flooding in the Canadian prairies was found to be more likely because of human land-use changes that affect drainage mechanisms. Similarly, the Jakarta floods may have been compounded by land-use change via urban development and associated land subsidence. These types of mechanical factors reemphasize the various pathways beyond climate change by which human activity can increase regional risk of extreme events.