Climate change
Lots of factors influence any single extreme weather event. As the climate warms, though, we’re seeing trends in many weather extremes–like more frequent heat waves, heavier rains in some areas, and more severe droughts and hurricanes.
Scientists look at the historical record of weather in a region and call events “extreme” if they produce unusually high or low levels of rain or snow, temperature, wind, or other effects. Typically if an event is unlike 90% to 95% of similar weather events that have occurred in the area, it’s considered extreme.
Yes. When we talk about trends, we mean that over time we’re seeing a change in either the number of extreme weather events or the intensity of them. Sometimes the trend shows changes in both.
Worldwide, we’re experiencing more extremely hot days and nights. In the United States, heat waves are happening more often than they used to in major cities across the nation. During the 1960s there were an average of two heat waves per year. In the 2010s, there were more than six per year. The increase in the frequency of extremely hot conditions matches what we would expect to see with an increase in Earth’s average temperature.
Many people, particularly children, the elderly, economically disadvantaged groups, and other vulnerable populations, have a tough time handling high temperatures. Heat cramps, heat exhaustion, heatstroke, and hyperthermia are all possible effects of exposure. Heat can also worsen chronic health conditions such as cardiovascular, respiratory, and diabetes-related diseases. So more extreme heat is expected to lead to an increase in illness and deaths.
We’re seeing fewer extreme cold events worldwide, and those that do occur tend to be less severe than in previous decades. Day-to-day and year-to-year changes in weather patterns will still produce some unusually cold days and nights—and winters and summers—even as the climate warms. The good news is that fewer people are likely to become sick or die from exposure to extreme cold.
The increase in greenhouse gas emissions is warming Earth’s lower atmosphere and making it moister. This additional moisture means that, when it rains or snows, it is more likely to come in heavy downfalls. That increases the risk of flooding in affected areas.
Trends in extreme rainfall and snowfall vary from region to region, though. The most pronounced increases are in North America and parts of Europe, especially in winter.
Yes. Higher air temperature can make droughts worse because it increases surface evaporation, leaving the land surface drier than it would be at a cooler temperature. Climate change may also alter rainfall patterns, which could further exacerbate droughts in some regions.
Droughts are complex phenomena, though, involving various combinations of precipitation and temperature patterns, the storage of moisture in soils and snowpack, and impacts from the ways we manage our land and water. So it can be difficult to determine how climate change contributed to any individual drought.
Climate change has extended fire seasons and worsened drought conditions that make many areas vulnerable to fire. In recent decades, we’ve seen increased costs and impacts of wildfires. As with droughts, however, many factors are involved. The expansion of communities into wilderness areas has exposed more people—and their homes and businesses—to risk from wildfire.
The characteristics of an ecosystem, including its temperature, soil moisture, wind speed, and vegetation, also affect an area’s wildfire vulnerability. Since these factors vary from region to region, climate change impacts on wildfire also vary. In the western United States, for example, climate change has dramatically increased the number of fires and the area burned in recent decades. The Arctic has also become more susceptible to fire, due in part to higher temperatures and shortened snow cover seasons as a result of climate change.
The evidence tells us we can expect an increase in the intensity of hurricanes because of climate change. That evidence comes from climate models and our understanding of basic physical principles. A warmer and moister atmosphere over the oceans makes it likely that the strongest hurricanes (when they occur) will be more intense, produce more rainfall, and possibly be larger. In addition, rising sea level contributes to stronger storm surges, worsening the impact.
The impact of climate change on hurricane frequency remains a subject of ongoing study. There’s considerable uncertainty about how overall frequency is changing because of the large natural variability and the incomplete observational record.
Yes. As the climate warms, we’re seeing trends in many weather extremes, as described above. But we also know that any individual extreme weather event is influenced by many factors—such as daily and seasonal weather patterns, natural climate patterns such as El Niño or La Niña, and the decisions people make about land management and infrastructure—in addition to climate change.
Increasingly, the answer is yes—at least for certain kinds of events. The science of attributing extreme weather events to climate change has advanced rapidly in recent years. It’s bringing new insights into the ways that human-caused climate change can influence the magnitude or frequency of some extreme weather events.
Confidence is strongest for extreme events that are influenced by climate change through a well-understood mechanism—for example, the more frequent heat waves that are closely connected to human-caused increases in global temperature.
Attributing a weather event to human-caused climate change doesn’t mean that droughts or torrential rains wouldn’t have occurred without human activity. Weather events result from a combination of natural and human-influenced factors that together produce the specific conditions for a particular event.
Event attribution studies can answer questions about how much climate change influenced the probability or intensity of a specific weather event. Scientists do these studies through climate modeling or by comparing a particular event to similar events in the historical record to see what has changed, and in many cases they combine these two approaches.
Because it can help inform choices about managing risks. For example, if a community knows that increased precipitation from climate change has turned what was previously a “500-year flood” into a “100-year flood” (or more accurately: a flood that has a 1-in-100 chance of happening every year), they may make different choices about things like how to manage land or whether to build a floodwall.
Still looking for more? Check out Attribution of Extreme Weather Events in the Context of Climate Change.
