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

FROM VOLUME 10, NUMBER 1, JANUARY 1997

PROFESSIONAL PUBLICATIONS...
TEMPERATURE TRENDS

Item #d97jan24

"The Little Ice Age and Medieval Warm Period in the Sargasso Sea," L.D. Keigwin (Woods Hole Oceanog. Inst., Woods Hole MA 02543), Science, 274(5292), 1504-1508, Nov. 29, 1996.

Climate data from marine sediments generally exhibit millenial-scale fluctuations, but lack sufficient time resolution to be compared directly to instrumental records. The gap in resolution occurs at the century time scale, the very scale on which anthropogenic warming is thought to be occurring. This paper reports a study of sediments from the Bermuda Rise in the northern Sargasso Sea, a location where century-scale resolution is possible. Results show that about 400 years ago (the Little Ice Age) and 1700 years ago sea surface temperature was about 1° C cooler than today, while about 1000 years ago (the Medieval Warm Period) it was about 1° C warmer than today. Thus, at least some of the warming since the Little Ice Age appears to be part of a natural oscillation.

Item #d97jan25

"Regional Trends of Surface and Tropospheric Temperature and Evening-Morning Temperature Difference in Northern Latitudes: 1973-93," R.J. Ross (ARL/NOAA, 1315 East-West Hwy., Silver Spring MD 20910; e-mail: rross@arlrisc.ssmc.noaa.gov) J. Otterman et al., Geophys. Res. Lett., 23(22), 3179-3182, Nov. 1, 1996.

Examines the consistency of surface temperature trends and corresponding upper-air temperature trends in eight north-latitude regions. In most seasons, surface temperature trends were positive in Eurasia and western North America but were negative in central North America and eastern Canada. Regions with sizable and statistically significant surface trends usually had significant tropospheric trends up to 500 mb and sometimes 300 mb. The significant tropospheric trends showed a tendency to decrease in magnitude with height.

Item #d97jan26

"Geographic Analysis of Differences in Trends Between Near Surface and Satellite-Based Temperature Measurements," R.C. Balling Jr. (Off. Climatol., Arizona State Univ., Tempe AZ 85287; e-mail: robert.balling@asu.edu), ibid., 23(21), 2939-2941, Oct. 15, 1996.

Over the past 16 years of overlapping records (1979-1994), satellite-based measurements of tropospheric temperature show a linear cooling of 0.10° C, while the near-surface air temperature record developed by Jones (1994) shows a warming of 0.18° C, a significant discrepancy that is the source of much debate. This study shows that most of the difference comes from tropical ocean areas, with a much smaller contribution from the mid-latitude land areas of the Northern Hemisphere. Discusses the implications of these results to the many explanations that have been offered for the differential trend.

Item #d97jan27

"Detecting Greenhouse-Gas-Induced Climate Change with an Optimal Fingerprint Method," G.C. Hegerl, (Max Planck Inst. Meteor., Bundestr. 55, D-21046 Hamburg, Ger.), J. Clim., 9(10), 2281-2306, Oct. 1996.

This study attempts to detect anthropogenic climate change in the observational record of near-surface temperatures by trying to identify as a "fingerprint" the spatial and temporal changes in temperature that are expected to characterize anthropogenic change, should it be present. A coupled ocean-atmosphere general circulation model is used, both to estimate the fingerprint and to estimate natural climatic variability. The estimate of natural variability is then used to statistically modify (optimize) the fingerprint to reduce the confounding influence of random fluctuations.

The null hypothesis, that the latest observed 20-year and 30-year trend of near-surface temperature (ending in 1994) is part of natural climate variability, is rejected with a risk of less than 2.5%-5%. However, to attribute the observed warming uniquely to anthropogenic greenhouse gas forcing, more information is needed on the climate's response to other forcing mechanisms (e.g., changes in solar radiation, or volcanic or anthropogenic sulfate aerosols) and their interactions. Furthermore, the estimate of internal (natural) climate variability is still uncertain. With these caveats, the conclusion remains that a statistically significant, externally induced warming has been observed.

Item #d97jan28

"Observed Changes in the Diurnal Temperature and Dewpoint Cycles Across the United States," P.C. Knappenberger (Dept. Environ. Sci., Univ. Virginia, Charlottesville VA 22903; e-mail: pck4s@virginia.edu), P.J. Michaels, P.D. Schwartzman, Geophys. Res. Lett., 23(19), 2637-2640, Sep. 15, 1996.

Several recent studies have examined differences in maximum and minimum surface temperatures at various stations for signs of climate change. However, more frequent hourly data must be used to detect any change in the pattern of heating throughout the day. This study examines the diurnal temperature cycles of 15 stations across the U.S. since the late 1940s and finds an ubiquitous shift in the daytime cycle of warming towards later in the day. Stations were divided into eastern and western subsets as a first approximation for high and low sulfate aerosol conditions. Results were consistent with a sulfate effect on overall temperature, but inconsistent with modeled estimates of the effect of sulfates on the intradiurnal course of temperature.

Item #d97jan29

"Effects of Climate Change on Mountain Ecosystems — Upward Shifting of Alpine Plants," H. Pauli (Dept. Vegetation Ecol., Univ. Vienna, Althanstr. 14, 1091 Vienna, Austria), M. Gottfried, G. Grabherr, World Resource Review, 8(3), 382-390, Sep. 1996.

Presents empirical evidence of the upward movement of vascular plants in the European Alps. Comparison of data collected in 1992-1993 from 30 high summits with historical data shows an increase in species at 70% of the summits. Approximate rates of upward movement for common alpine plants were calculated to be between zero and four meters per decade. This result may already be a "measurable" result of global warming since the 19th century.

Item #d97jan30

"Recent Changes in Tropical Freezing Heights and the Role of Sea Surface Temperature," H.F. Diaz (ERL/CDC/NOAA, 325 Broadway, Boulder CO 80303), N.E. Graham, Nature, 383(6596), 152-154, Sep. 12, 1996.

A widespread retreat of alpine glaciers and melting of tropical ice-cap margins has been observed in recent decades, over which time a general climate warming at lower altitudes has been documented. This study compared high-altitude air-temperature measurements for the past few decades, to temperatures predicted by a climate model forced by the observed pattern of (rising) sea-surface temperatures in a 19-year simulation. Results strongly indicate that high-altitude warming is related to a decadal increase in tropical sea temperatures, and consequent enhancement of the tropical hydrological cycle.

Item #d97jan31

"Nineteenth-Century Marine Temperature Date: Comments on Observing Practices and Potential Biases in Marine Datasets," M. Chenoweth (Elkridge, U.S.A.), Weather, 51(8), 280-285, Aug. 1996.

Analyzes the difficulties of determining the reliability of early marine air and sea temperature data collected since the mid-1850s, which rests on accurate knowledge of details about how the data were collected (metadata, i.e., data or information about the data). Concludes that considerable work needs to be done before nineteenth-century marine data can be considered as reliable as modern data.

Item #d97jan32

"Removal of the El Niño-Southern Oscillation [ENSO] Signal from the Gridded Surface Air Temperature Data Set," P.M. Kelly (Clim. Res. Unit, Univ. E. Anglia, Norwich NR4 7TJ, UK; e-mail: M.Kelly@uea.ac.uk), P.D. Jones, J. Geophys. Res., 101(D14), 19,013-19,022, Aug. 27, 1996.

Previous attempts to remove the ENSO component of variability from climate records at the global scale have been based on relatively simple regression analyses using time lag relationships. This study used the statistical technique of empirical orthogonal analysis, which can recognize the spatial patterns of fluctuation characteristic of ENSO. Concludes that this approach presents an effective means of isolating and removing specific climate signals.

Item #d97jan33

"Mongolian Tree Rings and 20th Century Warming," G.C. Jacoby (Tree Ring Lab., Lamont-Doherty Earth Observ., Rte. 9W, Palisades NY 10964), R.D. D'Arrigo, T. Davaajamts, ibid., 771-773.

A 450-year tree-ring width chronology of Siberian pine shows wide annual growth rings for the recent century. Ecological site observations and comparisons with instrumental temperature records indicate that the ring widths of these trees are sensitive to temperature. This chronology shows that the recent warming indicated is unusual relative to temperatures of the past 450 years.

Item #d97jan34

"Trends of Maximum and Minimum Daily Temperatures in Central and Southeastern Europe," R. Brazdil (Dept. Geog., Masaryk Univ., Brno, Czech Rep.), M. Budikova et al., Intl. J. Climatol., 16(7), 765-782, July 1996.

European data are largely missing in the recent study on trends in daily maximum and minimum temperatures by Karl et al. (1993); this study is intended to fill some of the existing gaps in European analysis. The increase in the annual maximum daily temperature in central Europe over the period 1951-1990 was slightly lower than that of the minimum daily temperature, resulting in a small decrease in the daily temperature range of -0.08° C.

Item #d97jan35

"Recent Trends in Maximum and Minimum Temperature Threshold Exceedences in the Northeastern United States," A.T. DeGaetano (Northeast Regional Clim. Ctr., 1123 Bradfield Hall, Cornell Univ., Ithaca NY 14853), J. Clim., 9(7), 1646-1660, July 1996.

Assesses changes from 1959 to 1993 in the annual number of daily maximum and minimum temperature threshold exceedences for 22 primarily rural sites. Overall, a significant trend toward fewer cold minimum exceedences (minimum temperature less than or equal to 10° , 5° , or 0° F) is detected.

Item #d97jan36

"Climate: The Elements," J.A.T. Bye (Flinders Inst. for Atmos. & Marine Sci., Flinders Univ., GPO Box 2100, Adelaide 5001, Australia), ibid., 1546-1560.

Presents a simplified analytical climate model with these characteristics: the atmosphere is a simple oscillator for all periods over one year; the ocean stores heat; the ocean exchanges momentum with the atmosphere; and random forcing exists due to atmospheric thermodynamics and oceanic dynamics. Statistical characteristics gleaned from simulations with this model imply that random walk lengths (intervals of temperature excursions due to natural fluctuations) have a time scale of at least a century.

Item #d97jan37

"Climatic Process as a Random Walk," N.A. Bagrov (Hydrometeor. Sci. Res. Ctr. of the Russian Federation), Russian Meteor. & Hydrol., No. 9, 28-36, 1995.

Results of this analysis of the statistical characteristics of the atmosphere indicate that the observed increase in global temperature over the past century is unlikely to be merely a random trend; a change in the properties of the climate system itself, related to the biosphere, is likely to be involved.