Twentieth Century Warming

Daily observations of temperature have been gathered from meteorological stations round the globe for many decades. Figure 1 shows global temperatures from 1850 to the present. The most obvious feature is the gradual warming of the planet, although you can also notice decades when the global temperature remained constant or even cooled. These variations reflect the competition between warming and cooling factors, with both natural and human origin. However, the most prominent signal of change has been the steady warming of the Earth since 1970: the warmest years on record (1998 and 2005), the warmest decade on record (the 1990s), and by 2007, eleven out of twelve of the warmest years have been in the past twelve years.1 Clearly, over the last century, our planet has been warming. Other signals of a warming planet or “fingerprints” of climate change have also been identified.2 Fingerprints include the melting of glaciers, the rise in sea level, and the changes in distribution of plant and animal species. Today most glaciers in the world are in retreat, some of them very rapidly.3 Sea-level rise has also been observed around the world.4 Plants and animals are also sensitive to temperature, and migrations in numerous insect and marine animal populations toward higher altitudes or higher altitudes (where it’s cooler) have been observed.5 Taken together, these collections of fingerprints present further evidence that the planet is warming.


Figure 1. Observed changes in (a) global average surface temperature, (b) global average sea level where tide-gauge data is in blue and satellite data is in red, and (c) Northern Hemisphere snow cover for March-April. All changes are relative to corresponding averages for the period 1961–1990. Smoothed curves represent decadal average values while circles show yearly values. The shaded areas represent the uncertainty.






1. P. Jones, “Global Temperature Record,” Climate Research Unit Information Sheet ( Norwich : University of East Anglia, 2007),
info/warming/, accessed Oct. 30, 2007.

2. A couple of papers on fingerprints can be found at
nature/links/030102/030102-3.html and a climate fingerprints hot map at, accessedMay 1, 2008.

3. The Grosser Aletsch Glacier in Switzerland , the longest glacier in the Alps, has retreated 8,500 feet (2,600 m) since 1980, and the Rongbut Glacier, which drains the north side of Mount Everest into Tibet , has been retreating 65 feet (20 m) per year over the last few decades. In 2006, the Swiss Glacier survey of 85 glaciers found 84 retreating and 1 advancing. Similarly, of the glaciers in the Italian Alps, only about a third were in retreat in 1980, while by 1999, 89 percent of these glaciers were retreating. In 2005, the Italian Glacier Commission found that 123 glaciers were retreating, 1 advancing and 6 stationary,
messntz/glacierlist.html, accessed May 1, 2008.

4. On the Pacific island of Tonga , the sea level appears to have risen about 0.3 inches (8 mm) a year over the last fifteen years. Although this may not seem like much, during times of storms, low-lying islands are expected to see an increased threat of flooding within the next few decades. Sea level rise is not constant, and depends on local variations in ocean temperature and winds. Further information on Pacific Islands and sea level can be found at
pacificsealevel/index.shtml, accessed May 1, 2008.

5. C. Parmesan and G. Yohe, “A Globally Coherent Fingerprint of Climate Impacts across Natural Systems,” Nature, 21 (2003), 37–42. 18. IPCC, Climate Change 2007: The Physical Science Basis: Contribution of Working Group 1 to the Fourth Assessment Report on the Intergovernmental Panel on Climate Change, S. Solomon, et al., eds. (Cambridge: Cambridge University Press, 2007), Fig SPM.3,, accessed May 8, 2008.


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