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.
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.
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 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 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.
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 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.
Southeast Brazil experienced profound water shortages in 2014/15. Anthropogenic climate change is not found to be a major influence on the hazard, whereas increasing population and water consumption increased vulnerability.
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.
Extreme winter rainfall in the United Kingdom becomes eight times more likely when the atmospheric circulation resembles winter 2013/14, whereas anthropogenic influence is only discernible in extremes with a shorter duration.
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.
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.
According to CMIP5 models, the risk of record annual mean warmth in European, northeast Pacific, and northwest Atlantic regions--as occurred in 2014--has been greatly increased by anthropogenic climate change.
A combined modeling and observational study suggests that the persistent rainfall deficit during the 2014 rainy season in southern Levant was made more likely due to anthropogenic climate change.
Of three identified proximate drought factors, climate change does not appear important for two. The third factor, western Pacific SSTs, exhibits a strong warming trend but attribution is an open question.
Anthropogenic warming contributed to the 2014 East African drought by increasing East African and west Pacific temperatures, and increasing the gradient between standardized western and central Pacific SST causing reduced rainfall, evapotranspiration, and soil moisture.