Weather & Climate

Weather versus climate

• Weather is the atmospheric conditions in one place over a short time, from minutes to hours or days.^[1]
• Climate is the long-term average (usually over at least 30 years) of temperature, humidity and rainfall patterns, for a region or the whole globe.^[1]

A single cold day tells you about the weather. Decades of records tell you about the climate.

What is climate change?

Climate change is a long-term change in the average weather patterns that define Earth’s local, regional and global climates.^[1] Global warming refers more narrowly to the long-term heating of Earth’s surface observed since the pre-industrial period, due to human activities, mainly burning fossil fuels.^[1]

What’s causing today’s change?

The changes observed in Earth’s climate since the mid-20th century are driven by human activities, particularly fossil fuel burning, which increases the levels of heat-trapping greenhouse gases.^[1] Earth’s average temperature has risen by about 1 °C (1.8 °F) since the 1850–1900 period, and it is currently rising by more than 0.2 °C per decade.^[1]

The evidence

NASA lists several independent lines of evidence:^[2]

• Global temperature has risen about 1 °C since the late 19th century.
• The ocean is warming, and it stores about 90% of the extra energy.
• Ice sheets are shrinking: Greenland lost an average of 279 billion tons of ice a year between 1993 and 2019, and Antarctica about 148 billion tons a year.
• Glaciers are retreating almost everywhere in the world.
• Spring snow cover in the Northern Hemisphere has decreased over the past five decades.
• Sea level rose about 20 cm in the last century, and the rate is accelerating.
• Arctic sea ice has declined rapidly in extent and thickness over recent decades.
• Extreme heat records in the United States have been increasing since 1950.
• The ocean is becoming more acidic: surface waters are about 30% more acidic than before industrialization.

How scientists know

Some evidence comes from the distant past, locked in ice cores drilled from Greenland, Antarctica and tropical mountain glaciers. Today, Earth-orbiting satellites help scientists see the big picture.^[2]