A heat wave, sometimes described as extreme heat, is a period of abnormally hot weather. Definitions vary but are similar. A heat wave is usually measured relative to the usual climate in the area and to normal temperatures for the season. Temperatures that humans from a hotter climate consider normal, can be regarded as a heat wave in a cooler area. This would be the case if the warm temperatures are outside the normal climate pattern for that area. High humidity often occurs during heat waves as well. This is especially the case in oceanic climate countries. Heat waves have become more frequent, and more intense over land, across almost every area on Earth since the 1950s. Heat waves occur from climate change.
A high pressure system in the upper atmosphere traps heat near the ground, forming a heat wave (for North America as an example)
Heat stroke treatment at Baton Rouge during the 2016 Louisiana floods
The National Weather Service risk categories for NWS HeatRisk.
The new California Academy of Sciences building in San Francisco's Golden Gate Park has a green roof that covers 2.5 acres (10,000 m2). According to the Academy's fact sheet on the building, the building consumes 30–35 percent less energy than required by code.
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to Earth's climate. The current rise in global average temperature is primarily caused by humans burning fossil fuels. Fossil fuel use, deforestation, and some agricultural and industrial practices add to greenhouse gases, notably carbon dioxide and methane. Greenhouse gases absorb some of the heat that the Earth radiates after it warms from sunlight. Larger amounts of these gases trap more heat in Earth's lower atmosphere, causing global warming.
Sea ice reflects 50% to 70% of incoming sunlight, while the ocean, being darker, reflects only 6%. As an area of sea ice melts and exposes more ocean, more heat is absorbed by the ocean, raising temperatures that melt still more ice. This is a positive feedback process.
Different levels of global warming may cause different parts of Earth's climate system to reach tipping points that cause transitions to different states.
Ecological collapse. Coral bleaching from thermal stress has damaged the Great Barrier Reef and threatens coral reefs worldwide.
Extreme weather. Drought and high temperatures worsened the 2020 bushfires in Australia.