Temperature Changes

Surface Temperature Change

Figure 1: Annual Average Global Surface Temperature Anomalies 1880-2006. Courtesy NOAA (Surface temperature records such as the one shown here have been quality controlled to remove the effects of urbanization at observing stations in and around cities. Click on Thumbnail for full size image.

Records from land stations and ships indicate that the global mean surface temperature warmed by between 1.0 and 1.7°F since 1850 (see Figure 1). These records indicate a near level trend in temperatures from 1880 to about 1910, a rise to 1945, a slight decline to about 1975, and a rise to present (NRC, 2006). The Intergovernmental Panel on Climate Change (IPCC) concluded in 2007 that warming of the climate system is now “unequivocal,” based on observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level (IPCC, 2007).

According to the National Oceanic and Atmospheric Administration's (NOAA) 2006 State of the Climate Report and the National Aeronautics and Space Administration's (NASA) 2006 Surface Temperature Analysis:

• Since the mid 1970s, the average surface temperature has warmed about 1°F.
• The Earth’s surface is currently warming at a rate of about 0.32ºF/decade or 3.2°F/century.
• The five warmest years over last century have likely been: 2005, 1998, 2002, 2003, 2006. The top 10 warmest years have all occurred since 1990.

Additionally (from IPCC, 2007):

• The warming trend is seen in both daily maximum and minimum temperatures, with minimum temperatures increasing at a faster rate than maximum temperatures.
• Land areas have tended to warm faster than ocean areas and the winter months have warmed faster than summer months.
• Widespread reductions in the number of days below freezing occurred during the latter half of the 20th century in the United States as well as most land areas of the Northern Hemisphere and areas of the Southern Hemisphere.
• Average temperatures in the Arctic have increased at almost twice the global rate in the past 100 years.

United States Surface Temperature Trends

Observations compiled by NOAA’s National Climatic Data Center indicate that over the past century, temperatures rose across the contiguous United States at an average rate of 0.11°F per decade (1.1°F per century). Average temperatures rose at an increased rate of 0.56°F per decade from 1979 to 2005. The most recent eight-, nine-, and ten-year periods were the warmest on record.

Warming occurred throughout most of the U.S., with all but three of the eleven climate regions showing an increase of more than 1°F since 1901. The greatest temperature increase occurred in Alaska (3.3°F per century). The Southeast experienced a very slight cooling trend over the entire period (-0.04°F per century), but shows warming since 1979.

Figure 2: Annual Mean Temperature Anomalies 1901-2005. Click on Thumbnail for full size image. Data courtesy NOAA's National Climatic Data Center.

The IPCC has concluded that most of the observed warming in global average surface temperature that has since the mid-20th century is very likely a result of human activities (IPCC, 2007). During the first half of the last century, there was likely less human impact on the observed warming, and natural variations, such as changes in the amount of radiation received from the sun, likely played a more significant role.

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Tropospheric Temperature Change

Measurements of the Earth’s temperature taken by weather balloons (also known as radiosondes) and satellites from the surface to 5-8 miles into the atmosphere - the layer called the troposphere - also reveal warming trends. According to NOAA's National Climatic Data Center:

• For the period 1958-2006, temperatures measured by weather balloons warmed at a rate of 0.22°F per decade near the surface and 0.27°F per decade in the mid-troposphere. The 2006 global mid-troposphere temperatures were 1.01°F above the 1971-2000 average, the third warmest on record.
• For the period beginning in 1979, when satellite measurements of troposphere temperatures began, various satellite data sets for the mid-troposphere showed similar rates of warming — ranging from 0.09°F per decade to 0.34°F per decade, depending on the method of analysis.

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Stratospheric Temperature Change

Weather balloons and satellites have also taken temperature readings in the stratosphere – the layer 9-14 miles above the Earth’s surface. This level of the atmosphere has cooled. The cooling is consistent with observed stratospheric ozone depletion since ozone is a greenhouse gas and has a warming effect when present. It’s also likely that increased greenhouse gas concentrations in the troposphere are contributing to cooling in the stratosphere as predicted by radiative theory (Karl et al., 2006).

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Recent Scientific Developments

The U.S. Climate Change Science Program (CCSP) recently published the report “Product 1.1 Temperature Trends in the Lower Atmosphere: Steps for Understanding and Reconciling Differences,” which addresses some of the long-standing difficulties in understanding changes in atmospheric temperatures and the basic causes of these changes. According to the report:

• There is no discrepancy in the rate of global average temperature increase for the surface compared with higher levels in the atmosphere. This discrepancy had previously been used to challenge the validity of climate models used to detect and attribute the causes of observed climate change.
• Errors identified in the satellite data and other temperature observations have been corrected. These and other analyses have increased confidence in the understanding of observed climate changes and their causes.
• Research to detect climate change and attribute its causes using patterns of observed temperature change shows clear evidence of human influences on the climate system due to changes in greenhouse gases, aerosols and stratospheric ozone.
• An unresolved issue is related to the rates of warming in the tropics. Here, models and theory predict greater warming higher in the atmosphere than at the surface. However, greater warming higher in the atmosphere is not evident in three of the five observational data sets used in the report. Whether this is a result of uncertainties in the observed data, flaws in climate models, or a combination of these is not yet known.