Understanding the Link Between Climate Change and Extreme Weather

Heat Waves

• Why does it matter? Heat waves can have serious health consequences, particularly for older adults, young children, the poor, and people with certain pre-existing health conditions, like asthma or heart disease.^[1] Excessive heat can also kill or injure crops and livestock, and it can lead to power outages as heavy demand for air conditioning strains the power grid.
• How does it relate to climate change? Even a small rise in average temperature brought on by climate change can boost the odds of extreme heat and heat waves.
• What's happening? Climate change has increased the likelihood of more frequent and more severe heat waves. Heat waves have generally become more frequent and intense across the United States in recent decades, particularly in the western United States (including Alaska).^[1] The impacts of heat waves are greatest in the Northeast and Midwest,^[2] and in urban areas, where the urban heat island effect increases vulnerability to heat-related health impacts.
• What's ahead? Heat waves are expected to become more frequent, longer, and more intense in the years ahead.^[2] The number of extremely hot days is projected increase throughout the United States.^[1]
• How sure is the science? Scientists are highly confident^[2] that heat waves and other extreme heat events have and will continue to become more frequent and intense due to climate change.

Droughts

• Why does it matter? Droughts can mean crop and livestock failures for farmers, which in turn can cause higher food prices and possibly even food shortages. Droughts can also stress water supplies and contribute to wildfires.
• How does it relate to climate change? As temperatures rise because of climate change, more water evaporates from land and water bodies. Along with changes in precipitation patterns, this can contribute to unusual dryness in some areas.
• What's happening? In recent decades, some regions have experienced more intense and longer droughts, while other regions have seen less frequent, less intense, or shorter droughts.^[3] There has been no general trend in the overall extent of drought across the contiguous United States since 1900.^[1] However, large portions of the Southwest have experienced drought conditions since weekly Drought Monitor records began in 2000.^[4] Droughts are expected to be a normal condition for the southern and central United States in the next century.^[1]
• What's ahead? Summer droughts are expected to intensify almost everywhere in the continental United States due to longer periods of dry weather and more extreme heat. A number of studies project that widespread drought will become more common over much of the southern and central United States, with amplified drought severity. A reduction in soil moisture, which exacerbates heat waves, is projected for much of the western and central United States.^[1]
• How sure is the science? Scientists are highly confident^[1] that droughts have become more frequent and intense in some regions of the United States, and that climate change increases the likelihood of these severe droughts in the future, particularly in the Southwest. Factors such as changes in a region's land use or high draws on water supplies can also play a role, especially at the local level.

Wildfires

• Why does it matter? Wildfires and their associated air pollution can cause deaths, injuries, and eye, respiratory, and cardiovascular illnesses. Large wildfires can disrupt or displace communities and have significant economic costs related to fire suppression, property damage, and losses in recreation, tourism, forestry, and related industries, They can also threaten wildlife and degrade local ecosystems.
• How does it relate to climate change? Climate change can contribute to dry conditions through higher temperatures, increased rates of evaporation, and earlier spring snowmelt (resulting in longer dry seasons), which all increase the risk of wildfires.
• What's happening? In recent decades, the extent of wildfires appears to have increased, particularly in the western United States.^[1][4] Increased warming, drought, and insect outbreaks, all caused by or linked to climate change, have increased wildfires and impacts to people and ecosystems in the Southwest.^[1]
• What's ahead? There is very high confidence^[1] that under projected climate change, forests and communities in the western United States will be increasingly affected by more frequent, larger, and more intense wildfires.
• How sure is the science? Scientists are highly confident^[1] that wildfires have increased in size in the western United States and very highly confident^[1] that dry conditions associated with climate change increase the likelihood of large wildfires in the future. Other factors, such as land management, land use changes, and impacts of pests, can also play a role in forest vulnerability.

Extreme Rainfall

• Why does it matter? Extreme rainfall events can damage crops, erode soil, and increase flooding. In addition, runoff from precipitation can degrade or contaminate water quality as pollutants deposited on land wash into water bodies used by people for drinking, irrigation, and other activities.
• How does it relate to climate change? Warmer temperatures cause more water to evaporate into the air. This moisture-laden air can produce more intense precipitation in the form of extreme rainfall events and storms.
• What's happening? In recent years, a larger percentage of precipitation has come in the form of intense single-day events.^[4] The amount of rain falling on the heaviest rain days has also increased over the past few decades.^[1]
• What's ahead? It is likely that the frequency and intensity of extreme precipitation events will increase over most of the United States and many other areas of the globe.^[5] A trend towards increase heavy precipitation will continue to occur, even in areas where total precipitation is projected to decrease.^[1]
• How sure is the science? Scientists are highly confident^[1] that across most of the United States, the heaviest rainfall events have become more intense and frequent, especially in the Midwest and Northeast, and that the frequency and intensity of extreme rainfall events will further increase in the future for most areas in the United States. Other factors, such as weather systems and ocean cycles like El Niño, can also play a role.

Extreme Winter Precipitation

• Why does it matter? Heavy winter storms can disrupt transportation, the flow of goods, and emergency and medical services. A buildup of snow can collapse roofs, knock down trees and power lines, and cause flooding when it melts.
• How does it relate to climate change? Warmer temperatures cause more water to evaporate into the air. If the temperature is still below freezing, this moisture-laden air can produce more intense precipitation in the form of unusually heavy snow, sleet, and freezing rain events, even in years when total snowfall is lower than average.
• What's happening? Winter storms have increased in frequency and intensity since the 1950s, and their tracks have shifted northward over the United States.^[1]
• What's ahead? It is likely that the frequency and intensity of extreme winter precipitation events will increase in some areas of the United States, particularly in the northern states.^[1]
• How sure is the science? Scientists have medium confidence^[1] that winter storms have increased slightly in frequency and intensity.

Hurricanes

• Why does it matter? Powerful hurricanes that make landfall can cause significant numbers of deaths and injuries, and disrupt or displace communities. When combined with coastal waters made higher by sea level rise, strong winds can create damaging storm surges. Hurricanes are also among the most costly extreme weather events, with severe storms causing billions of dollars in economic losses.^[6][7] Even hurricanes that don't make landfall can influence waves, currents, and storm tides, which can result in property damage, flooding, and coastal erosion.
• How does it relate to climate change? Increases in hurricane activity are linked to warming ocean temperatures because hurricanes draw more energy from warmer water. But the link between ocean temperature and hurricanes is complex, and other factors can also play a role in the formation and intensity of these storms.^[8]
• What's happening? There has been an increase in the intensity, frequency, and duration of hurricanes and in the number of strong (Category 4 and 5) hurricanes in the North Atlantic Ocean since the early 1980s.^[1] However, changes in observation methods over time make it difficult to know for certain whether tropical storm activity has shown a long-term increase.^[8]
• What's ahead? The intensity of the strongest hurricanes is projected to continue to increase as the oceans continue to warm, bringing stronger winds and heavier rains. Whether changes in hurricane frequency or landfall will occur is less certain.^[1]
• How sure is the science? Scientists have medium confidence^[1] that hurricane intensity and associated heavy rainfall will continue to increase under a changing climate, but significant uncertainties remain.

Tornadoes

• Why does it matter? Strong tornadoes can cause deaths and injuries, disrupt or displace communities, and inflict severe damage to crops, trees, buildings, and infrastructure.
• How does it relate to climate change? A warming climate can lead to stronger and more frequent thunderstorms, and these storms can spur tornadoes.^[8] But scientists do not yet fully understand all of the ways in which tornadoes may be linked to climate change.
• What's happening? Although the number of tornado reports has increased with better observation practices, there has been little change in the frequency of the strongest tornadoes over the past 55 years in the United States.^[9]
• What's ahead? With a warming climate, the thunderstorms and weather conditions that give rise to tornadoes could increase in some areas. But challenges in observing and modeling these events result in many uncertainties about how the frequency and intensity of tornadoes will change.^[1][10]
• How sure is the science? Scientists have low confidence^[1] in projections of trends in severe storms, including the intensity and frequency of tornadoes, hail, and damaging thunderstorm winds. This is in part due to a lack of long-term and the fact that such small and often remote storms are difficult to monitor and model.

Floods

• Why does it matter? Flooding can cause disease, deaths, and injuries; damage property and critical infrastructure such as sewer systems and wastewater treatment facilities; and disrupt or displace communities.
• How does it relate to climate change? Heavy rainfall events, more intense storms, and changes in the timing of snowmelt can lead to more frequent or intense flooding in some areas.^[1]
• What's happening? Many regions of the United States are experiencing significant changes in the magnitude of river flooding. When averaged over the entire nation, however, the increases and decreases cancel each other out and show no national level trend. For instance, there has been an increase in flooding events in the Midwest, Great Plains, and Northeast in the last several decades, where the largest increases in heavy rain amounts have occurred. But flooding has decreased in the Southwest.^[1]
• What's ahead? Heavy rainfall events and more intense storms in some regions could lead to more frequent or intense flooding in many United States regions, even in areas where total precipitation is projected to decline.^[1] There is medium confidence^[10] that projected increases in heavy rainfall would contribute to increases in local flooding in some regions.
• How sure is the science? The link between floods and climate change is a relatively new area of research, and many other factors, like land use and management practices, can trigger these events or influence how damaging they become. Scientists have high confidence^[1] that there have been regional trends in floods. However, scientists have low confidence^[10][1] in projections of future changes in flood frequency and intensity, because the causes of regional changes are complex.