Home

About Transportation and Climate Change

The Science of Climate Change

Greenhouse Gases | Warming | Precipitation | Sea Levels | Impacts

Warming

Global temperatures are rising. Observations collected over the last century suggest that the average land surface temperature has risen 0.45-0.6°C (0.8-1.0°F) in the last century. The surface of the ocean has also been warming at a similar rate. Studies that combine land and sea measurements have generally estimated that global temperatures have warmed 0.3-0.6°C (0.5-1.0°F) in the last century. About two-thirds of this warming took place between 1900 and 1940. Global temperatures declined slightly from the 1940s through the 1970s; but have risen more rapidly during the last 25 years than in the period before 1940.

Surface temperatures are not rising uniformly. Night-time low temperatures are rising on average about twice as rapidly as daytime highs. The winters in areas between 50 and 70° North Latitude (the latitude of Canada and Alaska) are warming relatively fast, while summer temperatures show little trend. Urban areas are warming somewhat more rapidly than rural areas, because of both the changes in land cover and the consumption of energy that take place in densely developed areas (a feature known as the "urban heat island" effect).

Red circles reflect warming/blue circles reflect cooling.
Note: cooling in Southeast U.S. may be due
to sulfate aerosol influence.

In the United States, temperatures in the last 50 years have cooled in the East while warming in the West. Over the last 100 years, the pattern is similar, except that New England is warmer than 100 years ago because it warmed more in the first half of the 20th century by more than it cooled in the second half. This pattern of warming and cooling may be part of a worldwide pattern: while most of the earth has warmed, the regions that are downwind from major sources of sulfur dioxide emissions have generally cooled (see the discussion on sulfates in the Atmospheric Change section). This pattern is evident when one compares the two world maps below.

Trends of Surface Temperature (1951-1993)
Global Historical Climate Network (GHCN)

The first map of the world shows the areas that have warmed and cooled from 1951-93. The second map of the world shows the amount of incoming solar radiation blocked by the cloud of atmospheric sulfates downwind from industrial emissions of sulfur dioxide.

Solar Energy Blocked by Atmospheric Sulfates
(Radiative Forcing in Watts per Square Meter)

Source: based on IPCC (1994)

Although scientists have incontrovertible evidence that the surfaces of the land and oceans have been warming, some scientists are not yet convinced that the atmosphere is also warming. The satellite data do not show a warming trend; however, the 1979-97 data series may be too short to show a trend in atmospheric temperature.

Part of the reason that satellites do not show a warming trend may be a coincidence regarding the year that NASA began to collect this data. Balloon data, which shows the same absence of warming over the 1979-97 period, shows a significant warming trend from 1958 to 1997. Measurement errors associated with the new technology, and cyclical variations in temperature due to El Ninos, may also be responsible for the lack of a warming trend. Nevertheless, to many scientists, the absence of a warming trend in the satellite data provides an important caution that there is still much to learn about the global climate.

* Note: measurements are relative to the average of 1850-70 (surface) and 1979-89 (satellite) temperatures.
Data Sources: Phil Jones, University of East Anglia (UK) and David Parker, United Kingdom Meteorological Office (surface data); John Christy, University of Alabama and Roy Spencer, National Aeronautics and Space Administration (satellite data).