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Global Climate Change DigestArchives of the
Global Climate Change Digest

A Guide to Information on Greenhouse Gases and Ozone Depletion
Published July 1988 through June 1999


Item #d99jun1

Northern Hemisphere Temperatures During the Past Millennium; Inferences, Uncertainties, and Limitations,” M. E. Mann, R. S. Bradley, and M. K. Hughes, Geophys. Res. Lett. 26 (6), 759-762 (1999).

A close examination of natural archives, such as tree rings and ice cores, which record climate variations each year, coupled with sophisticated computer analysis and statistics to reconstruct yearly temperatures and their statistical uncertainties, going back to the year AD 1000, strongly suggests that the 1990s were the warmest decade of the millennium, with 1998 the warmest year so far. Three sets of 1,000-year-long tree-ring records from North America were used along with tree rings from northern Scandinavia, northern Russia, Tasmania, Argentina, Morocco, and France. Ice cores from Greenland and the Andes mountains were also used in the climate reconstruction. In addition, the record suggests that the warming in the 20th century counters a 1000-year-long cooling trend.

Item #d99jun2

“Surface Air Temperature and Its Changes over the Past 150 Years,” P. D. Jones et al., Reviews of Geophysics 37 (2), 173-199 (1999).

A study of actual temperature readings for the mid-nineteenth century to the present and of proxy data for earlier years indicated that the average annual surface temperature of the whole world is 14.0oC. It is a little warmer (14.6oC) in the northern hemisphere, where there is more land, than in the southern hemisphere (13.4oC), where there is more water. The global annual cycle reflects the land-dominated northern seasons, with a July average maximum of 15.9oC and a January average minimum of 12.2oC. Other conclusions reached by the study include:

  • Annual global surface temperatures warmed by 0.57oC from 1861 to 1997. From 1901 to 1997, the gain was 0.62oC. Over both periods, the gain was greater in the southern than in the northern hemisphere.
  • The warmest years on record occurred in the 1990s: 1998, 1997, 1995, and 1990 in descending order. Their average temperatures ranged from 0.57oC above the 1961 to 1990 average in 1998 to 0.35oC above in 1990.
  • Most warming in the 20th century has occurred in two distinct periods: 1925 to 1944 and 1978 to 1997. In both periods, warming was greatest over the northern continents and during the December-February and March-May seasons.
  • Arctic temperatures have warmed slightly on an annual basis, with statistically significant increases from 1961 to 1990 during the months of May and June.
  • Much of the recent increase in average temperature has occurred at night. From 1950 to 1993, the minimum nighttime temperature warmed by 0.18oC per decade, while the maximum daytime temperature increased 0.08oC per decade.
  • The coldest year of the millennium was 1601. The coldest century was the 17th, which was on average only 0.5 to 0.8oC cooler than the 1961 to 1990 average. The warmest century of the millennium was the 20th.
  • “Global Changes in Intensity of the Earth’s Magnetic Field During the Past 800 kyr,” Yohan Guyodo and Jean-Pierre Valet, Nature 399, 249-252 (1999).

Iron-rich minerals laid down in sediments indicate the direction and intensity of the Earth’s magnetic field at the time of their deposition. The dates of those depositions can be determined by analysis of the sedimentary record, and the history of Earth’s magnetic field can thereby be reconstructed. Thirty-three sediment reports were used to develop such a history that goes back 800,000 years and includes the most recent polar reversal 780,000 years ago. The results of the study indicate that the intensity of the Earth’s magnetic field declines for several thousand years just before a polar reversal. It would be expected that such drastic changes in the planet’s magnetic field would affect its other processes, such as climate, plate tectonics, volcanism, and ocean circulation. For example, reductions in the magnetosphere would increase the exposure of the Earth’s surface to solar winds and lead to mass extinctions and other effects on the environment and biota. But no correlations with mass dieoffs or climate changes (like ice ages) can be seen in the paleorecords.

Item #d99jun3

“Hydroclimatic Trends and Possible Climatic Warming in the Canadian Prairies,” T. Y. Gan, Water Resources Res. 34 (11), 3009-3015 (1998).

Kendall’s test was applied to precipitation, temperature, streamflow, and evapotranspiration data. The analysis indicated that the Canadian prairies have become warmer and drier during the past 40 to 50 years. Warming trends are more frequent and more geographically widespread than drying trends in the data. No correlation between precipitation and maximum temperature was observed.

Item #d99jun4

“Simulating Climate Change Effects in a Minnesota Agricultural Watershed,” M. P. Hanratty and H. G. Stefan, J. Env. Qual. 27, 1524-1532 (1998)

A computer model calibrated to the Cottonwood River near New Ulm, Minn., was used to estimate the effects of climate change on the quality and quantity of water running off the area’s agricultural watershed. The variables tracked were mean monthly streamflow, phosphorus yield, ammonia yield, nitrate yield, and sediment yield. The climate changes by the Canadian Climate Centre’s GCM for a doubling in CO2 were used as the input. Decreases were projected for all the variables tracked, with significant changes during many months of the year. The study concluded that land use, land cover, and land-management practices would have a greater influence on water quality than would climate change.

Item #d99jun5

“Complexity and Climate,” D. Rind, Science, 284 (5411), 105-107 (1999).

Science is entering a new phase in which systems that are driven by massive numbers of variables, many of which are interacting, are being investigated with computer modeling and analysis to determine whether the behavior of these complex systems can be understood and predicted. In this review article, Rind considers climate as a complex system and its amenability to analysis. He finds that climate is governed by both ordered forcing and chaotic behavior, producing a system with characteristics of both types of variable. To date, analysis and prediction of climate have focused on the responses to ordered forcings on the system, and Rind points out that the chaotic component may be just as important and will definitely be more difficult to predict. However, at the present time, the magnitude of the contribution of that chaotic component is unknown.

Item #d99jun6

“High-Resolution Holocene Environmental Changes in the Thar Desert, Northwestern India,” Y. Enzel et al., Science, 284 (5411), 125-128 (1999).

Analysis of the sediments of a dry lake in northwestern India reveals indications of fluctuations in lake level and water table height during the Holocene. These fluctuations are attributed to changes in the monsoon rains that fell on the area. During the early Holocene, the lake was shallow and fluctuated frequently; 6300 years ago, its depth increased dramatically before dessicating completely about 4800 years ago. At the same time as the dessication, dunes were severely destabilized, and saltation occurred. Surprisingly, human civilization flourished only after the dessication.

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