Climate change is a term used for numerous effects on long-term weather patterns caused by human activity. Climate change generally refers to the effects on Earth’s climate of greenhouse gases produced by human activities. Greenhouse gases trap heat from sunlight in Earth’s atmosphere, much as do the glass walls of a greenhouse (see Greenhouse effect). Human activities that contribute greenhouse gases to the atmosphere include the burning of fossil fuels and deforestation. The increase in heat-trapping gases in the atmosphere due to such activities has led to an increase in Earth’s average temperature of 1.8 Fahrenheit degrees (1 Celsius degree) from the mid 1800’s to the late 2010’s. This effect is known as global warming. Global warming does not simply result in rising temperatures, however. The long-term warming of Earth’s surface, atmosphere, and oceans affects the global climate in complex ways. Scientists call these effects climate change.
Climate change includes shifts in rainfall and snowfall amounts. It also involves increased frequency and severity of extreme weather events, such as heat waves, droughts, floods, and wildfires. Many of the impacts of climate change are more damaging to human society and natural ecosystems than would be a simple rise in temperature. 
Earth’s climate has changed many times throughout its history. However, the current changes are unlike anything seen previously, because they have been caused almost entirely by human activities. The changes have also been increasing rapidly since the 1960’s.
Temperature extremes
Not all locations on Earth are warming at the same rate or at the same time. Earth’s poles are warming much faster than the equator. Average temperatures at night are increasing more quickly than average temperatures during the day. Summers are warming faster than winters in the Northern Hemisphere.
Since 1950, the number of extreme cold periods has decreased and the number of extreme heat waves has increased. These changes in temperature extremes have reduced the average number of cold-related deaths and increased the average number of heat-related deaths among human populations.
Seasonal changes in temperature are also shifting, with warm spring weather occurring earlier in the year and cool fall weather coming later. Plants and animals around the globe have responded by shifting the timing of natural events that occur in the spring. Such events as flowering, egg-laying, migration, and the growth of new leaves happen earlier. For example, lilac bushes have flowered earlier in the spring across much of the United States. Butterflies have appeared earlier in the United Kingdom.
Precipitation changes
Climate change includes considerable and ongoing changes in precipitation (rainfall and snowfall) patterns that result from global warming. These precipitation changes are partly caused by changes in global weather patterns, including where and when storms occur. As a result, the total amount of rain and snow a place receives in a year, as well as the timing of when that rain and snow occur, has been changing and will continue to change. Scientists are still working to predict the exact precipitation changes for any particular location. However, it is generally agreed that wet places will receive more precipitation and dry places will receive less as climate change progresses.
Another well-documented effect of climate change is the amplification of the water cycle. The water cycle is the process by which water moves around Earth’s surface, including cycles of evaporation and precipitation. Rising global temperatures amplify the cycle, increasing the volume of water moving through it. Because warm air holds more water vapor than does cold air, the atmosphere holds more water vapor as the planet warms. More humid atmospheric conditions produce fewer but larger storms than do dryer atmospheric conditions. Therefore, rainy days are becoming less common, but more rain tends to fall at a time.
Both changes in weather patterns and the amplification of the water cycle lead to more extreme precipitation events. An increase in these events is one of the most damaging aspects of climate change. Both droughts—caused by a lack of precipitation—and floods—caused by too much precipitation over a short period—are becoming increasingly common across the globe. Droughts and floods are often characterized by how frequently they occur at any given location. For example, a 20-year drought or flood is one that occurs about once every 20 years. A more severe 100-year drought or flood occurs about once every century. As a result of global warming, extreme precipitation events such as 100-year droughts are becoming more common worldwide. By 2100, climatologists (scientists who study Earth’s climate) predict that some locations will experience what used to be 100-year events every 10 to 20 years.
Wildfires
Climate change is causing an increase in wildfires. The combination of warmer weather and more frequent and severe droughts associated with climate change makes large wildfires more likely, particularly in dry parts of the globe such as Australia and the western United States. Since 1990, researchers have documented changes in the number and size of wildfires in the United States and other parts of the world.
Effects on human society
Increased heat waves, floods, and droughts, as well as changes in the timing of the seasons, have major effects on the water and food resources on which humans rely. Many areas around the globe are experiencing greater and more frequent water shortages due to climate change. Farmers face increasingly uncertain weather for growing crops. Increased flooding also threatens more people and buildings. For example, flooding of the Mississippi River in the United States has increased over the last 100 years.
Global warming is also causing sea levels to rise. Millions of people live near the ocean around the world. Coastal flooding driven by an increase in extreme storms and the rise in sea level could threaten more than 200 million people by the year 2100.
Effects on wildlife
Climate change is also harming natural ecosystems. An ecosystem is a community of living things and the nonliving things that they depend on in their environment. Climate change is gradually altering ecosystems by causing plants and animals to shift where they live as temperatures rise and precipitation patterns shift. Extreme events, such as droughts and wildfires, are particularly damaging to natural ecosystems. For instance, severe wildfires in Australia in 2019 and 2020 may have killed over 1 billion wild animals and threatened multiple plant and animal species with extinction. 
Droughts and heat waves have recently caused rapid forest die-offs around the globe. A forest may take hundreds of years to grow, but a single hot and dry summer can kill millions of trees at once. The death of the trees that form the foundation of forests threatens the other species that live there. Forests store a large amount of carbon in wood and soils. When forests die off, they release this carbon into the atmosphere in the form of carbon dioxide and other greenhouse gases. These greenhouse gases worsen climate change, making further forest die-offs more likely. Scientists call such a process, in which a change to a system leads to further similar changes, a positive feedback loop.
Studying climate change
Studying climate change involves three main ways of gathering information. They are (1) observations of weather conditions, (2) evidence of climate from the distant past, and (3) global climate models.
Weather observations.
Humans have been documenting the weather for hundreds of years using thermometers, rain gauges, and wind-speed gauges. Modern weather observations stretch across the globe and are augmented (enriched) by multiple types of weather satellites. Scientists use these observations to understand the historical patterns of temperature and precipitation from year to year and place to place. They also use the observations to detect recent changes in such patterns. In many parts of the world, for instance, scientists since 1980 have detected a decrease in the number of rainy days—but an increase in the amount of rain in individual storms—by using rain gauge readings from the past 100 years.
Evidence of past climate.
Scientists place direct observations of weather in historical context by recreating Earth’s climate hundreds, thousands, and even millions of years ago. They do this by using climate proxies, natural indicators that enable researchers to reconstruct historical climate. Examples of climate proxies include bubbles of gas trapped in ice cores and the chemical makeup of ancient corals. Climate proxies have shown that modern climate change is occurring much more rapidly and more suddenly than most historical climate fluctuations.
Climate models.
Scientists also use global climate models to predict how ongoing human greenhouse gas emissions will alter Earth’s climate in the future. Global climate models are complex computer models that simulate atmospheric physics, the circulation of Earth’s oceans and atmosphere, and even human activities. These models incorporate our best knowledge about how Earth’s climate system works. They are our best tool for predicting future climate change.
