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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 12, DECEMBER 1997

PROFESSIONAL PUBLICATIONS...
DETECTING ANTHROPOGENIC INFLUENCE

Item #d97dec35

"El Niño and Climate Change," K.E. Trenberth (NCAR, POB 3000, Boulder CO 80307; e-mail: trenbert@ncar.ucar.edu), T.J. Hoar, Geophys. Res. Lett., 24(23), 3057-3060, Dec. 1, 1997.

The authors update their 1996 analysis, which concluded that the recent behavior of El Niño-Southern Oscillation (ENSO) events may indicate a human influence. The recent evolution of ENSO, with a major new El Niño event underway in 1997, reinforces the evidence that the tendency for more El Niño and fewer La Niña events since the late 1970s is highly unusual, and very unlikely to be accounted for solely by natural variability.

Item #d97dec36

"Anomalous ENSO Occurrences: An Alternate View," B. Rajagopalan (Lamont-Doherty Earth Observ., POB 1000, Palisades NY 10964; e-mail: rbala@rosie.ldgo.columbia.edu), U. Lall, M.A. Cane, J. Clim., 10(9), 2351-2357, Sep. 1997.

Nonparametric statistical methods for time series are applied to an 1882-1995 seasonal Darwin sea level pressure anomaly time series. Return periods for the duration of the unusual 1990-1995 event of sustained high pressure are estimated to be considerably smaller than those obtained by Trenberth and Hoar (1996). Their conclusion that this event may be an effect of greenhouse gas-induced warming should be tempered by a recognition of the natural variability in the system.

Item #d97dec37

"Greenhouse Warming, Decadal Variability, or El Niño? An Attempt to Understand the Anomalous 1990s," M. Latif (Max Planck Inst. Meteor., Bundesstr. 55, D-20146 Hamburg, Ger.; e-mail: latif@dkrz.de), R. Kleeman, C. Eckert, J. Clim., 10(9), 2221-2239, Sep. 1997.

Investigates the dominant modes of variability in the tropics, and contrasts them with unusual El Niño behavior observed during the last few years. Analyzes anomalous sea surface temperature in the band 30° S-60° N, observed tropical surface wind stress fields, and a multidecadal run with an atmospheric GCM using the observed sea temperatures. A decadal mode in the data is found which has a complicated relationship with the ENSO cycle. Certain results are in conflict with those of Trenberth and Hoar (1996).

Item #d97dec38

"Observed Hemispheric Asymmetry in Global Sea Ice Changes," D.J. Cavalieri (Lab. for Hydrospheric Processes, NASA-Goddard, Code 971, Greenbelt MD 20771), P. Gloersen et al., Science, 278(5340), 1104-1106, Nov. 7, 1997.

Satellite observations indicate that from 1978 to 1996, the areal extent of sea ice decreased by 2.9% per decade in the Arctic and increased by 1.3% per decade in the Antarctic. The hemispheric asymmetry in these trends is consistent with one (but not all) modeled responses to CO₂-induced warming.

Item #d97dec39

"Surface Observed Global Land Precipitation Variations During 1900-88," A. Dai (NCAR, POB 3000, Boulder CO 80307; e-mail: adai@ucar.edu), I.Y. Fung, A.D. Del Genio, J. Clim., 10(11), 2934-2962, Nov. 1997.

The authors assembled an improved gridded data set of monthly precipitation for the period 1900-1988. Statistical analysis reveals a linear increasing trend of about 24 mm per decade in global precipitation. The spatial pattern of the trend and rate of increase are generally consistent with those in GCM projections of increased CO₂.

Item #d97dec40

"The Continuing Search for an Anthropogenic Climate Change Signal: Limitations of Correlation-Based Approaches," D.R. Legates (Dept. Geog. & Anthropology, Louisiana State Univ., Baton Rouge LA 70803; e-mail: legates@cwarren.ccg.ou.edu), R.E. Davis, Geophys. Res. Lett., 24(18), 2319-2322, Sep. 15, 1997.

Some recent studies (such as Santer et al., 1995 and 1996, and Hegerl et al., 1996 and next paper) claim to have found a "fingerprint" or evidence of anthropogenic climate changes by statistical comparison of observed and modeled temperature fields. The centered correlation statistic employed in these studies is shown here to be inappropriate for these situations, and should not be used to draw inferences about climate changes.

Item #d97dec41

"Multi-Fingerprint Detection and Attribution Analysis of Greenhouse Gas, Greenhouse Gas-Plus-Aerosol and Solar-Forced Climate Change," G.C. Hegerl (Joint Inst. for the Study of Atmos. & Ocean, Univ. Washington, Box 354235, Seattle WA 98195), K. Hasselmann et al., Clim. Dynamics, 13(9), 613-634, 1997.

Extends an earlier study, this time by searching for the presence of multiple "fingerprints" of climate change related to different external forcings (such as greenhouse gases, anthropogenic aerosols, or fluctuations in solar forcing), in climate model simulations compared to observations. Despite considerable uncertainties related to model characteristics and other factors, the authors conclude that observed climate change is consistent with a combined greenhouse gas plus aerosol forcing, but inconsistent with greenhouse gas or solar forcing alone.

Item #d97dec42

"The Observed Global Warming Record: What Does It Tell Us?" T.M.L. Wigley (NCAR, POB 3000, Boulder CO 80307), P.D. Jones, S.C.B. Raper, Proc. Natl. Acad. Sci., 94(16), 8314-8320, Aug. 5, 1997.

This review describes global, near-surface temperature data sets and their development, and interprets the changes that have occurred in terms of anthropogenic and natural causes. Inclusion of aerosol and solar forcing improves the fit between modeled and observed temperature changes, and brings into agreement estimates of climate sensitivity based on the observations and on models. However, various uncertainties that surround data/model comparisons make it impossible to use observed global-scale temperature changes to narrow the uncertainty range in the climate sensitivity below that estimated directly from climate models.

Item #d97dec43

"Climate Variability of a Coupled Ocean-Atmosphere-Land Surface Model: Implication for the Detection of Global Warming," S. Manabe (NOAA/GFDL, POB 308, Princeton NJ 08542), R.J. Stouffer, Bull. Amer. Meteor. Soc., 78(6), 1177-1185, June 1997.

Analysis of a 1000-year model simulation of global mean surface air temperature indicates that the observed warming trend of about 0.5° C in this century was not generated internally through the major components of the climate system. Rather, this means that it is likely to have been induced by a sustained change in the thermal forcing, such as that resulting from changes in atmosphere greenhouse gas concentrations, solar irradiance, and aerosol loading.

Item #d97dec44

"Comparison of Statistically Optimal Approaches to Detecting Anthropogenic Climate Change," G.C. Hegerl (Joint Inst. for the Study of Atmos. & Ocean, Univ. Washington, Box 354235, Seattle WA 98195), G.R. North, J. Clim., 10(5), 1125-1133, May 1997.

Although the core of the three statistical approaches that have been proposed is identical, by using different approaches they may allow us to better understand the properties of optimal detection. Discusses some practical concerns that arise in applications. The methods offer the prospect of increasingly more significant detection of forced climate change, but good hypothesized signals and good information on climate variability are essential.

Item #d97dec45

"Climate Forcing by Stratospheric Ozone Depletion Calculated from Observed Temperature Trends," W. Zhong (Dept. Phys., Imperial College, London SW7 2BZ, UK), R. Tuomi, J.D. Haigh, ibid., 23(22), 3183-3186, Nov. 1, 1996.

Previous studies have concluded that ozone depletion leads to a net negative radiative forcing, estimated by Ramaswamy et al. to be -0.08 W m-2 during the period 1979-1990 (global-annual average). This effect has been considered possible compensation for positive radiative forcing due to anthropogenic greenhouse gases over the same period. More realistic radiation calculations presented here show a much lower value of -0.025 W m-2, although inclusion of some omitted factors would raise this value somewhat.