The record-setting heat wave in Australia last year was “largely attributable” to human-caused climate change, according to a synthesis report released Monday. Heat waves in Japan, Korea, China and Europe were also “substantially influenced” by global warming, the report found.
For the synthesis report, published in the Bulletin of the American Meteorological Society, 22 groups of scientists looked at 16 extreme weather events that took place in 2013. The paper concludes that at this time, it is more difficult to discern the human impact on other extreme events, such as the drought in California, extreme rainfall in Colorado, and an early-season blizzard in South Dakota.
This is the third annual report on the connections between individual extreme weather events and anthropogenic (originating from human activity) climate change, led by scientists from the National Oceanic and Atmospheric Administration.
“Every extreme event raises questions about how anthropogenic climate change affected its intensity and its probability of occurrence,” the researchers explain. “This question arises for many reasons, but often it is the implications for the future that is of greatest concern.”
If anthropogenic influences caused changes in an event’s intensity or likelihood, this has implications for the impacts of future events and the actions that may be required to adapt to changes, the team adds.
Public health implications of extreme weather events
Globally, the number of reported weather-related natural disasters has more than tripled since the 1960s, resulting in an estimated 60,000 deaths annually, about 2,000 of which take place in the U.S. Likewise, the incidence of heat waves, drought, and flooding rains are on the rise, as are damage and losses from weather-related events.
These extreme weather events have both direct and indirect effects on human health. In addition to injuries and exposure-related morbidity and mortality (e.g., hypothermia, heat stroke, drowning, etc.), extreme weather can also threaten human health in a number of other ways, such as:
- Reducing the availability of fresh food and water.
- Interrupting communication, utility, and health care services.
- Contributing to carbon monoxide poisoning from portable electric generators used during and after storms.
- Increasing the incidence of infectious disease, particularly stomach and intestinal illness, among evacuees.
- Contributing to mental health disorders such as depression and post-traumatic stress disorder (PTSD).
The impacts of extreme weather events on health will depend on many factors. These factors include the effectiveness of a community’s public health and safety systems to address or prepare for the risk and the behavior, age, gender, and economic status of individuals affected. In general, those most vulnerable to the effects of extreme weather include women, young children, older adults, people with medical conditions, and the poor. The effects will likely vary by region, the sensitivity of populations, the extent and length of exposure to climate change impacts, and society’s ability to adapt to change.
Global warming influences ‘all manner of weather events’
In November 2011, the United Nations Intergovernmental Panel on Climate Change released a special report concluding that human activity had not just caused warming of the earth’s atmosphere, but also an increase in extreme weather and climatic events around the world.
The science linking extreme weather and the observed increase in average global temperature is extremely complex and not completely understood; regional and local extreme weather events are the result of the interplay of numerous variables that change with time and location. Statistical data, however, strongly point to a link between increasing temperature and extreme weather.
An overly simplified explanation is that heat is a form of energy, and higher temperatures increase the energy available to weather events. Also, warmer air holds more water vapor (a greenhouse gas,) that can result in increased precipitation (snow, rain, etc.).
Above, a bell curve distribution of temperatures shows that a small upward shift in the average global temperature increases the chances of extreme heat and related weather events.
Although it is impossible to link one weather event to climate change, there is a clear trend of increasing severe weather – and it’s expected to take a toll on human health. For instance, in a 2012 report from the Natural Resources Defense Council (NRDC), scientists estimated that the number of heat-related deaths will triple over the next 100 years as a result of climate.
Global warming is “influencing all manner of weather events,” said Dr. Marty Hoerling, the report’s co-editor and a scientist at NOAA’s Earth System Research Laboratory. What the report seeks to explore, however, is not if climate change influenced weather events, but by how much and in what direction –- whether it is making a particular weather event more or less likely, and more or less severe.
“Results from this report not only add to our body of knowledge about what drives extreme events, but what the odds are of these events happening again-and to what severity,” said Dr. Stephanie Herring, the report’s lead editor and a scientist at NOAA.
Mixed results on CA drought; conclusive evidence on global heat waves
To reach their conclusions, the researchers reviewed the existing scientific literature – including both observational and modeling analyses – examining 16 extreme weather events across four continents:
The team found mixed results about the California drought, which saw the driest 12-month period on record from 2013 into early 2014. Three studies looking at the links between Pacific Ocean surface temperatures and anomalies in the atmosphere were unable to find a direct tie to human-caused climate change, while another paper found that climate change was affecting atmospheric pressure patterns. Thus, the paper concludes, “implications for extremely low precipitation in California remain uncertain.”
Similarly, the report looked at the five days of heavy rainfall in Colorado in September 2013 that caused massive flooding and concludes that the probability of an event like it occurring has “likely decreased due to climate change.” Another study of extreme precipitation events in the U.S. found evidence that while most of the increase in such events was due to natural variations, there also was evidence the increase was due, at least in part, to climate change. Both those studies stressed the need for additional research.
The evidence was much more conclusive when the scientists analyzed extreme heat events: Of the five heat waves studied in the report, human-caused climate change-primarily through the burning of fossil fuels-was found to have clearly increased the severity and likelihood of those events. “The findings indicate that human-caused climate change greatly increased the risk for the extreme heat waves assessed in this report,” the researchers wrote.
“I think the key message is that given what we know today, it’s a lot easier for us to associate heat extremes with human influence than it is other extremes,” said Thomas Karl, director of NOAA’s National Climatic Data Center.
Climate change detection and attribution
The synthesis report highlights a relatively new area of climate science known as detection and attribution research, which explores what is known about the links between extreme weather events and anthropogenic climate change:
Detection of climate change is the process of demonstrating that climate has changed in some defined statistical sense, without providing a reason for that change. Attribution of causes of climate change is the process of establishing the most likely causes for the detected change with some defined level of confidence.
Using traditional approaches, unequivocal attribution would require controlled experimentation with our climate system. However, with no spare Earth with which to experiment, attribution of anthropogenic climate change must be pursued by:
(a) detecting that the climate has changed (as defined above);
(b) demonstrating that the detected change is consistent with computer model simulations of the climate change ‘signal’ that is calculated to occur in response to anthropogenic forcing;
(c) demonstrating that the detected change is not consistent with alternative, physically plausible explanations of recent climate change that exclude important anthropogenic forcings.
The application of this type of research to specific, localized weather events is still in its early stages, the scientists noted. But, they added, this new study offers one of the most reliable estimates yet, thanks to the rigorous methods. By pooling the results from numerous independent investigations, the researchers were able to reduce the error rate associated with the findings, allowing them to be more confident in their conclusions. “There is great scientific value in having multiple studies analyze the same extreme event to determine the underlying factors that may have influenced it,” said Dr. Herring.
‘Invaluable’ environmental intelligence
Confidence in the role of climate change about any one event is increased when multiple groups using independent methods come to similar conclusions – a process called triangulation. For example, in this report, five independent research teams looked at specific factors related to the record heat in Australia in 2013. Each team consistently found that human-caused climate change increased the likelihood and severity of that event.
However, for the California drought, which was investigated by three teams from the United States, human factors were not likely to have influenced the lack of rainfall, the researchers said. One team found evidence that atmospheric pressure patterns increased due to human causes (which could have made it more likely for a “ridge” of high pressure to linger over the western United States, keeping storms and rain away from California), but the influence on the California drought is not yet clear.
Rather than viewing such conflicting evidence as a weakness, though, the scientists say that it demonstrates the fact that detection and attribution studies are still in an experimental and evolving phase. “As models and methods improve, we could end up with new results in future studies,” said Karl. “That’s how science evolves.”
Research linking climate change to extreme weather is still evolving, with stronger evidence linking human activities to weather events like heat waves and flooding, compared to other events. Wide variation in natural patterns of precipitation and tropical storms makes it far more difficult to detect the impact of global warming.
When human influence for an event cannot be conclusively identified with the scientific tools available today, this means that if there is a human contribution, scientists have not yet separated those effects from natural climate variability. Isolating the human role in particular events over small geographic areas remains an extraordinarily difficult task for researchers; this is especially challenging when assessing precipitation, which has very large natural variations.
Despite the challenges, scientists say this type of analysis is remarkably helpful. “The science remains challenging, but the environmental intelligence it reveals for decision-makers is invaluable,” said Karl. “There is a lot of demand out there. We think this is an important activity.”