Surface Temperature Change
Records from land stations and ships indicate that the global mean surface temperature warmed by between 0.7 and 1.5ºF during the 20th century (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). Warming is now occurring over most of the globe and is consistent with the global retreat of mountain glaciers, reduction in snow-cover extent, the earlier spring melting of ice on rivers and lakes, and increases in sea-surface temperatures and ocean heat content (NRC, 2001).
According to the National Oceanic and Atmospheric Administration's (NOAA) 2005 State of the Climate Report and the National Aeronautics and Space Administration's (NASA) 2005 Surface Temperature Analysis11:
- Since 1900, the average surface temperature has warmed by about 1.2-1.4 ºF.
- Since the mid 1970s, the average surface temperature has warmed about 1 ºF.
- The Earth’s surface is currently warming at a rate of about 0.32ºF/decade or 3.2 ºF/century.
- The five warmest years over last century have likely been: 2005, 1998, 2002, 2003, 2004. The top 10 warmest years have all occurred since 1990.
Additionally (from IPCC, 2001):
- 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.
United States Surface Temperature Trends
Observations compiled by NOAA’s National Climatic Data Center indicate that most regions of the U.S. have warmed by at least 1ºF (0.6ºC) since 1901 although the Southeast (climate region 2) has cooled (see Figure 2). Warming in excess of 1.8°F (1°C) has occurred in the West (climate region 8) and Alaska (climate region 10).
The last five five-year periods (1999-2003, 1998-2002, 1997-2001, 1996-2000,1995-99) were the warmest for the duration of national records (which began in 1901). The most recent six-year-(1998-2003), seven-year (1997-2003), eight-year (1996-2003), nine-year (1995-2003), and ten-year (1994-2003) periods were also the warmest on record for the United States.
The general scientific consensus is that most of the observed warming in global average surface temperature that has occurred over the past 50 years is likely a result of human activities (IPCC, 2001; NRC, 2001). However, the National Research Council cautioned it cannot be ruled out that some significant part of the warming is also a reflection of natural variability (NRC, 2001). 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.
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 Center13:
- For the period 1958-2005, 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 2005 global mid-troposphere temperatures were 1.28 °F above the 1961-1990 average, the second 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 0.22 ºF per decade to 0.32ºF per decade.
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).
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.
- IPCC, 2001: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton, J.T., Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell, and C.A. Johnson (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 881pp.
- National Research Council (NRC), 2001. Climate Change Science: An Analysis of Some Key Questions. National Academy Press, Washington, DC
- National Research Council (NRC), 2006. Surface Temperature Reconstructions For the Last 2,000 Years. National Academy Press, Washington, DC.
- Temperature Trends in the Lower Atmosphere: Steps for Understanding and Reconciling Differences. Thomas R. Karl, Susan J. Hassol, Christopher D. Miller, and William L. Murray, editors, 2006. A Report by the Climate Change Science Program and the Subcommittee on Global Change Research, Washington, DC.