Extreme weather

Extreme weather refers to atmospheric events that are unusual for a specific location and time of year.

Scientists identify these events by looking at a location's recorded weather history. An event is often classified as extreme if it reaches the 10th or 90th percentile of a probability density function. This means the event is as rare as, or rarer than, most observations in that area. While extreme weather describes these rare occurrences, "severe weather" is a specific term used by the World Meteorological Organization. Severe weather refers to any weather aspect that poses risks to life or property. It may also require the intervention of authorities to manage the situation.

Extreme weather can take many different forms depending on the region. Common types include heat waves, cold waves, droughts, and heavy precipitation. Storm events, such as tropical cyclones or tornadoes, also fall into this category.

These events can trigger natural hazards like floods and landslides. They also create significant social costs by impacting human health and the economy. Some human activities can make these impacts even worse. For example, building homes on floodplains or destroying wetlands can increase the damage from storms. Poor urban planning can also exacerbate the effects of extreme weather on society.

Heat waves are periods characterized by abnormally high temperatures and a high heat index. They can be extremely dry or accompanied by high humidity. These events can lead to several biological and environmental problems. To conserve water, plants will shut their stomata, which are small pores in their leaves. While this protects the plant from dehydration, it also stops the plant from absorbing nutrients effectively. This process can lead to higher levels of ozone and pollution in the air. In the United Kingdom, extra pollution during the summer of 2006 was estimated to have cost 460 lives.

In the summer of 2003, European heat waves caused an estimated 30,000 excess deaths due to heat stress and air pollution.

Historical data shows the massive scale of heat-related mortality. The most severe heat wave in the United States occurred in 1936 and killed more than 5,000 people directly. In Australia, the worst heat wave happened during 1938–39 and resulted in 438 deaths. Heat waves also impact infrastructure and ecosystems. They can cause power outages because the demand for air conditioning increases electricity use. In cities, the "urban heat island" effect can cause temperatures to stay high even overnight. Additionally, dry vegetation increases the likelihood of wildfires, and evaporating water bodies can destroy marine habitats.

Cold waves are distinguished by a rapid cooling of the air. The U.S. National Weather Service defines a cold wave as a rapid fall in temperature within a 24-hour period. These waves are formed by large, cool air masses that move over certain regions.

Extreme cold can cause serious damage to property and life. For instance, water in poorly insulated pipes can freeze and expand, causing them to rupture. This can also break water mains, making it harder for firefighters to use water during fires. For animals, cold waves can lead to death from hypothermia or starvation if heavy snow prevents them from reaching food. In humans, prolonged exposure can result in frostbite and damage to internal organs.

Extreme cold has also caused major historical food shortages. In 1816, an event known as the "Year Without a Summer" occurred. Volcanic eruptions had reduced the amount of incoming sunlight, causing freakish summer cold snaps. These snaps led to widespread crop failures and famines. While some research suggests climate change might lead to milder winters and fewer cold-related deaths, the topic remains complex. A 2023 study noted that while weak extreme cold events are decreasing in the Northern Hemisphere, strong events are actually increasing in places like Siberia and Canada.

Understanding the causes of extreme weather requires looking at complex global systems. Generally, a single extreme weather event cannot be blamed on just one cause. Instead, they are influenced by natural climate variability, such as the El Niño-Southern Oscillation (ENSO) or the North Atlantic Oscillation (NAO).

However, system-wide changes can increase how often or how intense these events become. Climate change is a major factor that scientists study through extreme event attribution. Rising global temperatures can increase the amount of water vapor in the atmosphere. This can make heavy rainfall and precipitation events much more severe. Scientists use climate models to compare current events to models of climates without man-made drivers to understand these connections.