Table of Contents

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Summary Table
1. Introduction to Explaining Extreme Events of 2014 from a Climate Perspective

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.

2. Extreme Fire Season in California: A Glimpse Into the Future?

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.

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3. How Unusual was the Cold Winter of 2013/14 in the Upper Midwest?

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.

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4. Was the Cold Eastern U.S. Winter of 2014 Due to Increased Variability?

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.

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5. The 2014 Extreme Flood on the Southeastern Canadian Prairies

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.

6. Extreme North America Winter Storm Season of 2013/14: Roles of Radiative Forcing and the Global Warming Hiatus

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.

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7. Was the Extreme Storm Season in Winter 2013/14 Over the North Atlantic and the United Kingdom Triggered by Changes in the West Pacific Warm Pool?

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.

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8. Factors Other Than Climate Change, Main Drivers of 2014/15 Water Shortage in Southeast Brazil

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.

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9. Causal Influence of Anthropogenic Forcings on the Argentinian Heat Wave of December 2013

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.

10. Extreme Rainfall in the United Kingdom During Winter 2013/14: The Role of Atmospheric Circulation and Climate Change

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.

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11. Hurricane Gonzalo and its Extratropical Transition to a Strong European Storm

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.

12. Extreme Fall 2014 Precipitation in the CĂ©vennes Mountains

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.

13. Record Annual Mean Warmth Over Europe, the Northeast Pacific, and the Northwest Atlantic During 2014: Assessment of Anthropogenic Influence

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.

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14. The Contribution of Human-Induced Climate Change to the Drought of 2014 in the Southern Levant Region

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.

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15. Drought in the Middle East and Central–Southwest Asia During Winter 2013/14

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.

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16. Assessing the Contributions of East African and West Pacific Warming to the 2014 Boreal Spring East African Drought

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.

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