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Extreme Seasons

 

One of the most visible consequences of a warming world is an increase in the intensity and frequency of extreme weather events. The National Climate Assessment finds that the number of heat waves, heavy downpours, and major hurricanes has increased in the United States, and the strength of these events has increased, too.

A measure of the economic impact of extreme weather is the increasing number of billion-dollar disasters, which is shown below. The map shows all types of weather disasters, some of which are known to be influenced by climate change (floods, tropical storms) and some for which a climate influence is uncertain (tornadoes).

Drought:

Climate change increases the odds of worsening drought in many parts of the United States and the world in the decades ahead. Regions such as the U.S. Southwest are at particular risk.

There are a number of ways climate change may contribute to drought. Warmer temperatures can enhance evaporation from soil, making periods with low precipitation drier they would be in cooler conditions. Droughts can persist through a “positive feedback,” where very dry soils and diminished plant cover can further suppress rainfall in an already dry area. A changing climate can also alter atmospheric rivers (narrow streams of moisture transported in the atmosphere), which can especially disrupt precipitation patterns in the Western United States. A combination of shifting atmospheric rivers and warmer temperatures can also affect Western snowpack and melt, potentially decimating the water supply.

Estimates of future changes in seasonal or annual precipitation in a particular location are less certain than estimates of future warming. However, at the global scale, scientists are confident that relatively wet places, such as the tropics, and higher latitudes will get wetter, while relatively dry places in the subtropics (where most of the world’s deserts are located) will become drier.

Recent U.S. droughts have been the most expansive in decades. In 2011, Texas experienced its driest 12 months ever. At the peak of the 2012 drought, an astounding 81 percent of the contiguous United States was under at least abnormally dry conditions. The state of California experienced a particularly drawn out drought extending from December 2011 to March 2019, broken in part by the wettest winter in the United States.

Threats Posed by Drought

The United States is historically susceptible to drought. Paleoclimate studies show major droughts in the distant past, while some more recent dry periods are still within living memory, such as the Dust Bowl of the 1930s or the drought of the 1950s. These historic examples serve as guideposts to highlight our vulnerabilities to drought as we move into a warmer and, in some places, drier future.

Severe drought can affect:

Agriculture: Droughts affect livestock and crops, including cornerstone commodities like corn, soybeans, and wheat. At the height of the 2012 drought, the U.S. Department of Agriculture declared a natural disaster over 2,245 counties, 71 percent of the United States. Globally, drought struck several major breadbasket regions simultaneously in 2012, adding to food price instability. In countries already facing food insecurity, cost spikes can lead to social unrest, migration, and famine.
Transportation: Droughts can affect water levels on rivers of commerce like the Mississippi. Transport barges need at least nine feet of water, and to maintain this level, the U.S. Army Corps of Engineers had to blast, dredge, and clear obstructions on a key stretch of the Mississippi in 2013. Also, drought and heat can buckle roadways. A 2011 drought in Texas caused $26 million in pavement distress.
Wildfires: Drought conditions and record heat have fueled damaging and sometimes deadly wildfires in the U.S. West. Millions of forested acres and thousands of homes have been lost over the past decade due to fires thriving in dry, stressed forests and the proximity of communities to fire-prone forests.
Energy: Droughts can raise concerns about the reliability of electricity production from plants that require cooling water to maintain safe operations. Hydroelectric power may also become unavailable during droughts. When heat waves coincide with droughts, electricity demands can grow, compounding stress on the grid.
How to Build Resilience

Governments and businesses must identify their vulnerability to drought and improve resilience. Actions like conserving water, enhancing water efficiency throughout landscapes, city plans, water infrastructure, identifying alternative water supplies, emergency planning for drought, and planting drought-resistant crops will help prepare for both future droughts and climate change.

Other actions that improve resilience to other stressors, like deploying green infrastructure for stormwater management or increasing energy efficiency in buildings (thereby using less water-cooled power), can improve resilience to drought as a co-benefit.

These steps will be most effective if they are combined with reductions in greenhouse gases that can minimize the ultimate magnitude of climate change.

Heat:

Extreme heat can increase the risk of other types of disasters. Heat can exacerbate drought, and hot dry conditions can in turn create wildfire conditions. In cities, buildings roads and infrastructure can be heated to 50 to 90 degrees hotter than the air while natural surfaces remain closer to air temperatures. The heat island effect is most intense during the day, but the slow release of heat from the infrastructure overnight (or an atmospheric heat island) can keep cities much hotter than surrounding areas. Rising temperatures across the country poses a threat to people, ecosystems and the economy.

Human Health

Extreme heat is one of the leading causes of weather-related deaths in the United States, killing over 600 per year more than all other impacts (except hurricanes) combined. The Billion Dollar Weather Disasters database compiled by the National Oceanic and Atmospheric Administration lists heat waves as four of the top 10 deadliest U.S. disasters since 1980.

Heat stress occurs in humans when the body is unable to cool itself effectively. Normally, the body can cool itself through sweating, but when humidity is high, sweat will not evaporate as quickly, potentially leading to heat stroke. High humidity and elevated nighttime temperatures are likely key ingredients in causing heat-related illness and mortality. When there’s no break from the heat at night, it can cause discomfort and lead to health problems, especially for those who are low income or elderly, if access to cooling is limited.

Hot days are also associated with increases in heat-related illnesses including cardiovascular and respiratory complications, kidney disease, and can be especially harmful to outdoor workers, children, the elderly, and low-income households.

In extreme temperatures, air quality is also affected. Hot and sunny days can increase ozone levels, which in turn affects NOX levels. In addition, greater use of heating and cooling of indoor spaces requires more electricity and, depending on the electricity source, can emit more of other types of pollution, including particulates. These increases in ozone and particulate matter can pose serious risks to people, particularly the same vulnerable groups directly impacted by heat mentioned above.

Agriculture

High temperatures at night can be particularly damaging to agriculture. Some crops require cool night temperatures, and heat stress for livestock rises when animals are unable to cool off at night. Heat-stressed cattle can experience declines in milk production, slower growth, and reduced conception rates.

Energy

While higher summer temperatures increase electricity demand for cooling, at the same time, it also can lower the ability of transmission lines to carry power, possibly leading to electricity reliability issues during heat waves. Although warmer winters will reduce the need for heating, modeling suggests that total U.S. energy use will increase in a warmer future. In addition, as rivers and lakes warm, their capacity for absorbing waste heat from power plants declines. This can reduce the thermal efficiency of power production, which makes it difficult for power plants to comply with environmental regulations regarding their cooling water.

Extreme precipitation:

The most immediate impact of heavy precipitation is the prospect of flooding. This risk can be heightened in urban areas where impervious pavements force water to quickly run off into sewer systems. Among recent examples:

In August 2017, Hurricane Harvey dumped 48 inches of rain on Houston, breaking the U.S. record for most rainfall from a single event and demonstrating that the biggest threat from tropical cyclones is not always coastal flooding and wind damage.

In July 2016, more than six inches of rain fell in two hours in historic Ellicott City, Maryland, causing more than $22 million in damages.

In September 2013, Boulder, Colorado, received almost a year’s worth of rainfall (17 inches) in four days. The resulting flooding destroyed homes, shut down thousands of oil and gas wells, and damaged crops.

In 2010, almost 20 inches of rain fell in the Nashville, Tennessee, area over three days. Losses in Nashville alone totaled over $1 billion.

In addition to flooding, heavy precipitation also increases the risk of landslides. When above-normal precipitation raises the water table and saturates the ground, slopes can lose their stability, causing a landslide. Washington state, which has a particularly high risk of landslides due to its terrain, is anticipating more frequent landslides as a result of climate change-induced increases in heavy downpours.

Excessive precipitation can also degrade water quality, harming human health and ecosystems. Stormwater runoff, which often includes pollutants like heavy metals, pesticides, nitrogen, and phosphorus, can end up in lakes, streams, and bays, damaging aquatic ecosystems and lowering water quality for human uses.

Many cities in the United States use a combined sewer system, where both stormwater and wastewater are mixed, treated, and released. Heavy rainfall can overwhelm such systems, sending excess stormwater, wastewater, and untreated sewage directly into the environment, risking public health and disrupting local fisheries.

Hurricanes:

Although scientists are uncertain whether climate change will lead to an increase in the number of hurricanes, there is more confidence that warmer ocean temperatures and higher sea levels are expected to intensify their intensity and impacts. Stronger hurricanes will be far more costly in terms of damages and deaths without action to make coastal (and inland) areas more resilient.

Hurricanes are subject to a number of climate change-related influences:

Warmer sea surface temperatures could intensify tropical storm wind speeds, potentially delivering more damage if they make landfall. Based on complex modeling, NOAA has suggested that an increase in Category 4 and 5 hurricanes is likely, with hurricane wind speeds increasing by up to 10 percent. Warmer sea temperatures also are causing hurricanes to wetter, with 10-15 percent more precipitation from cyclones projected in a 2 degree C scenario. Recent storms such as Hurricane Harvey in 2017 (dropping over 60 inches in some locations), Florence in 2018 (with over 35 inches) and Imelda in 2019 (44 inches) demonstrate the devastating floods that can be triggered by these high-rain hurricanes.

Sea level rise is likely to make future coastal storms, including hurricanes, more damaging. Globally averaged, sea level is expected to rise by 1-4 feet in low and moderate emissions scenarios during this century, which will amplify coastal storm surge. For example, sea level rise intensified the impact of Hurricane Sandy which caused an estimated $65 billion in damages in New York, New Jersey, and Connecticut in 2012. Much of this damage was related to coastal flooding.

Areas affected by hurricanes are shifting poleward. This is likely associated with expanding tropics due to higher global average temperatures. The changing patterns of tropical storms (a shift northward in the Atlantic) could put much more property and human lives at risk, but much more research is required to build a better understanding of how these patterns might change.

The connection between climate change and hurricane frequency is less straightforward. It is likely the number of storms will remain the same or even decrease, with the primary increase being of the most extreme storms. For the 21st century, some models project no change or a small reduction in the frequency of hurricanes, while others show an increase in frequency. More recent work shows a trade-off between intensity and frequency – that as warmer oceans bolster hurricane intensity, fewer storms actually form.

Tornadoes:

The link between tornadoes and climate change is currently unclear. Current data on tornadoes is inconsistent because measuring the presence of tornadoes relies on eyewitness accounts and aftermath damage assessments rather than quantifiable data. Additionally, it is difficult to identify long-term trends in tornado records, which only date back to the 1950s in the U.S., because the population in many areas affected by tornadoes has grown, contributing to increased eyewitness reports and greater property in harm’s way. Improved technology, such as advanced radar, also helps us “see” tornadoes that may not have been detected decades ago.

An additional challenge is the physics associated with tornadoes. Researchers are working to better understand how the building blocks for tornadoes – atmospheric instability and wind shear – will respond to global warming. It is likely that a warmer, more humid world would allow for more frequent instability. However, it is also possible that a warmer world would lessen chances for wind shear. Climate change also could shift the timing of the tornadoes or the regions that are most likely to be hit, with less of an impact on the total number of tornadoes.

An added difficulty in trying to determine future tornado frequency and intensity based on changes in the climate, is that tornadoes are too geographically small to be well simulated by climate models. Models can simulate some of the conditions that contribute to forming severe thunderstorms that often spawn tornadoes. Multiple studies find that the conditions that produce the most severe thunderstorms are more likely in a warmer world.

It will remain challenging to estimate any climate change influence on tornadoes until scientists can improve their physical understanding of the processes that cause tornadoes and the observational record of tornado frequency.

Wild Fire:

Climate change has been a key factor in increasing the risk and extent of wildfires in the Western United States. Wildfire risk depends on a number of factors, including temperature, soil moisture, and the presence of trees, shrubs, and other potential fuel. All these factors have strong direct or indirect ties to climate variability and climate change. Climate change causes forest fuels (the organic matter that burns and spreads wildfire) to be more dry, and has doubled the number of large fires between 1984 and 2015 in the western United States.

Research shows that changes in climate that create warmer, drier conditions, increased drought, and a longer fire season are boosting these increases in wildfire risk. For much of the U.S. West, projections show that an average annual 1 degree C temperature increase would increase the median burned area per year as much as 600 percent in some types of forests. In the Southeastern United States modeling suggests increased fire risk and a longer fire season, with at least a 30 percent increase from 2011 in the area burned by lightning-ignited wildfire by 2060.

Once a fire starts—more than 80 percent of U.S. wildfires are caused by people—warmer temperatures and drier conditions can help fires spread and make them harder to put out. Warmer, drier conditions also contribute to the spread of the mountain pine beetle and other insects that can weaken or kill trees, building up the fuels in a forest.

Land use and forest management do affect wildfire risk. Changes in climate add to these factors and are expected to continue to increase the area affected by wildfires in the United States.