How we know the Earth is warming

Scientists are certain the Earth has been warming for 100 years. Here's how they know.

As far back as the 1850s, a small number of weather stations around the world were compiling temperature records. These numbers grew during the 20th century and today there are thousands of land-based weather stations and ocean buoys in every corner of the world monitoring temperatures.

Temperature records since 1850

These temperature records clearly show a warming of the Earth over the past century, with particularly rapid heating over the past few decades.

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  View graph Source: NYT

  Global temperature data

  Each of the four agencies that report global temperature trends—NOAA, NASA, HADCRU, and JMA—show the warming trend.

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  View graph Source: NASA

  Average U.S. temperatures

  Annual average temperatures in the United States since 1880 also show the warming trend.

Satellite measurements since 1979

Atmospheric temperature measurements taken from orbiting satellites also show warming. Weather satellites have been monitoring global atmospheric temperatures since 1979.

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  View graph Source: IPCC

  Atmospheric temperatures

  Trend in tropospheric temperatures (the lowest part of Earth's atmosphere) from 1979 to 2005 shows warming.

Sea level rise in the 20th century

During the 20th century, sea level rose an average of 7 inches after 2,000 years of relatively little change.

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  View graph Source: CSIRO

  Global sea level rise

  Satellite altimeter and coastal tide gauge data show rising sea levels since 1870.

Before 1850: proxy records

Proxy records are sophisticated ways of inferring surface temperatures over previous centuries and millennia. Taken together, these independent records show widespread warming over the 20th century, with a particularly sharp uptick in temperature over the last few decades.

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  View graph Source: NAP

  Temperature reconstructions

  Surface temperature reconstructions of the past 2,000 years from proxy records show a warming trend.

While proxy records are, by definition, not as accurate or precise as direct measurements, they provide a robust picture of thousands of years of the Earth's history. Three main types of proxy records used to create this picture are:

• Ice cores

  One proxy method is to drill into glaciers and ice sheets to extract ice samples. Since the ice was formed from snow that fell over the centuries, the deeper you drill, the farther back in time you are looking.

  The chemical composition of the ice correlates very strongly with temperature. Scientists have constructed temperature records from ice cores taken from Tibetan and Andean glaciers, an ice cap in the Canadian Arctic, and the Greenland and Antarctic ice sheets. These records show that, at low latitudes, 20th century climate was unusually warm compared to the previous 2,000 years.

  In the Canadian Arctic, warming over the past 150 years is unprecedented compared to the previous millennium. In Greenland and coastal Antarctica, there is clear evidence of warming over the past century. Ice cores from Antarctica's interior do not show warming over the past century.

• Tree rings

  In temperate regions, trees generally produce one ring a year, and some tree species are extremely long-lived. (A bristlecone pine, for example, can live more than 4,000 years.) Patterns in the width and density of tree rings provide year-by-year temperature information.

  Scientists have tree ring records from more than 2,000 sites on all inhabited continents, though most of the records are from temperate areas of the Northern Hemisphere. These records show that 20th century warming was unusual compared to at least the past 500 years.

• Coral reefs

  Corals build their hard skeletons with annual bands of calcium carbonate. The geochemical composition of each annual band varies depending on the temperature of the water at the time the band was formed. Scientists have coral proxy records from the Atlantic, Indian, and Pacific oceans, with most of these going back 400 years. Coral proxy records indicate sea surface warming in most tropical locations over the past century.