Paleo Perspective on Global Warming

Are We Seeing Global Warming?
K. Hasselmann
Science 276, 914-916 (1997), with hypernotes

Warning: links not current, sorry.

The author is at the Max-Planck-Institut für Meteorologie, D-20146 Hamburg, Germany[HN1]. E-mail:

The measured increase in global mean surface temperature since the last century is about 0.5oC [HN2]. This value is consistent with the predictions of state-of-the-art climate models [HN3](see figure, top), but an order of magnitude smaller than the climate variations experienced year for year in any given region of Earth (1). Regional climate fluctuations are largely due to shifts in air masses and tend to cancel when averaged over the globe or over a longer time period. Thus, attempts to detect anthropogenic global warming have focused on global scales and long-term trends. Despite considerable progress, the question of whether the observed gradual increase in global mean temperature over the last century is indeed caused by human activities or is simply an expression of natural climate variability on larger spatial and temporal scales remains a controversial issue [HN4].

Figure 1
Patterns of climate change. (Top) Evolution of observed (4) and computed global mean temperatures. Curves GHG (1) and GHG + SO4 (1) for greenhouse gas-plus-aerosol forcing are from Hegerl et al. (6). The corresponding curves GHG (2) and GHG + SO4 (2) are from computations with the improved model of Roeckner et al. (11), see also (7). (Bottom) Pattern correlation between observed 30-year trends and climate change signal simulated for the greenhouse gas only case and the greenhouse gas-plus-aerosol case (green). Also shown are correlations after subtraction of the spatial mean from the patterns (red). [Adapted from Hasselmann et al. (12), see also (7)]

To answer this question we need to (i) predict the anthropogenic climate change signal, (ii) determine the natural climate variability noise, and (iii) compute the signal-to-noise ratio and test whether the ratio exceeds some predefined statistical detection threshold. The last problem is the easiest one. It can be solved by generalizing standard signal analysis methods developed for the detection of time-dependent signals in noisy time series to the space-time-dependent case (2). A space-time filter, or fingerprint, is used to enhance the less noise-contaminated components.

The first two problems are more difficult ones. Are climate models sufficiently reliable to predict the climate change signal that we wish to extract from the natural variability noise? And do we know the space-time structure of natural climate variability well enough to meaningfully apply the fingerprint technique?

Modern climate models consist of coupled ocean-atmosphere general circulation models (CGCMs) that integrate basic fluid dynamical equations [HN5]. They simulate the time-dependent three-dimensional flow fields and associated transports of mass, heat, and other fluid properties at a resolution of typically a few hundred kilometers. Processes below this resolution (such as clouds and ocean eddies) cannot be represented explicitly and must be parameterized, that is, expressed in terms of the resolved larger scale motions [HN6]. This is the major source of uncertainty of CGCMs. Advances in supercomputers enabling higher model resolution have helped reduce these uncertainties, and the latest greenhouse warming simulations by different modeling groups show a scatter of only 20% in the predicted global mean temperature, compared with a value of typically 50% a few years ago (3). But significant differences still exist in the predicted patterns of tempera ture change, or in other distributions such as precipitation or sea level rise.

Similar uncertainties apply to the estimation of natural climate variability [HN7] on the decadal-to-century time scales relevant for anthropogenic climate change detection. The instrumental record for global surface temperatures extends back over little more than a hundred years (4), insufficient for useful estimates of climate variability except at short decadal time scales. The instrumental record can be augmented by longer paleoclimatic records [HN8] from tree rings, corals, or deep-ocean cores (5), but such proxy data also have numerous problems of interpretation.

Nevertheless, combining these independent analyses, various groups have produced best-guess estimates of the space-time structure of natural climate variability and have applied fingerprint methods to test whether the global warming pattern predicted by state-of-the-art climate models can be detected in the observed temperature data (6-8). The general conclusion of these efforts (adorned by numerous caveats), in the cautious words of the Intergovernmental Panel on Climate Change (IPCC) [HN9], is that "the balance of evidence suggests a discernible human influence on climate" [(3), p. 4].

The hesitant reversal of the original negative detection assessment of the first 1990 IPCC report (9) was the fruit not only of improved models and the application of more advanced fingerprint techniques, but also a shift of focus from the 100-year temperature trends to shorter 30-year trends, whose noise statistics can be more reliably determined, and which exhibit higher signal-to-noise levels--a consequence of the accelerated warming in recent decades (see figure, top). Another important factor was the availability of new global warming predictions including both greenhouse gases and aerosols, which gave better agreement between the observed and predicted temperature patterns (see figure, bottom). However, the impact of aerosols [HN10] is still poorly known, and the pattern correlations for the greenhouse gas-plus-aerosol forcing shown in the figure, although generally higher in the last decades than for the greenhouse gas-only case, are still relatively low. A s tatistically significant climate change signal was nevertheless detected, as this is dominated by the pattern-independent global mean temperature (6).

A reduction in the present uncertainties would significantly improve our confidence not only in the detection of climate change, but also in its attribution to anthropogenic greenhouse warming (10). This requires further research not only on the impact of aerosols, but also on the sources of the discrepancies in the global warming patterns predicted by different CGCMs. Foremost among these are the role of clouds [HN11] , the interactions between the tropical ocean and the global atmospheric circulation [HN12] , the coupling between the atmosphere, ocean, and sea-ice in high latitudes [HN13] , and the snow and soil water budget (3).

However, the inherent statistical uncertainties in the detection of anthropogenic climate change can be expected to subside only gradually in the next few years while the predicted signal is still slowly emerging from the natural climate variability noise. It would be unfortunate if the current debate over this ultimately transitory issue should distract from the far more serious problem of the long-term evolution of global warming once the signal has been unequivocally detected above the background noise.

See also 16 May 1997 Science special section on
Tropospheric Processes, pp. 1093-1088.

References and Notes

  1. The warming for a continued uncontrolled increase in greenhouse gas emissions is predicted to become comparable to the present regional warm extremes within the next century.
  2. K. Hasselmann, Meteorology of Tropical Oceans, D. B. Shaw, Ed. (Royal Meteorological Society, Reading, Berkshire, UK,1979), p. 251; J. Climate 6, 1957 (1993).
  3. J. T. Houghton et al., Eds., Climate Change (Cambridge Univ. Press, Cambridge, UK, 1995).
  4. P. D. Jones, Geophys. Res. Lett. 21, 1149 (1994); J. Climate 7, 1794 (1994); line and K. R. Briffa, Holocene 2, 165 (1992).
  5. T. P. Barnett et al., Holocene 6, 255 (1996).
  6. G. C. Hegerl et al., Climate Dyn., in press.
  7. J. F. B. Mitchell et al., Nature 376, 501 (1995).
  8. B. D. Santer et al., Climate Dyn. 12, 77 (1995); G. C. Hegerl et al., J. Climate 9, 2281 (1996); B. D. Santer et al., Nature 382, 39 (1996); S. F. B. Tett et al. Science 274, 1170 (1996).
  9. J. T. Houghton et al., Eds., The 1990 Report of the IPCC Scientific Assessment Working Group (Cambridge Univ. Press, Cambridge UK, 1990).
  10. K. Hasselmann, Climate Dyn., in press.
  11. E. Roeckner et al., Climate Dyn. 12, 737 (1996).
  12. K. Hasselmann et al., Aksel Wiin Nielsen Symposium Proceedings (European Centre for Medium Range Weather Forecasts, Reading, Berkshire, UK, 1996).

Related Resources on the World Wide Web

The United Nations has several Web pages devoted to climate change. The home page for the UN Environment Programme (UNEP) Conference on Climate Change offers information and links about the science of climate change as well as official documents. A climate change information kit is available courtesy of UNEP's Information Unit for Conventions (IUC).

NASA's Global Change Master Directory (GCMD) Web site has information about satellite and in situ Earth science data, with broad coverage of the atmosphere, hydrosphere, oceans, solid earth, and biosphere. NASA also has a fact sheet on earth science topics, including global warming.

The National Oceanic and Atmospheric Administration provides Web pages for the National Climate Data Center and the Climate Diagnostics Center, with online data access and interactive visualizations of climate data.

The Smithsonian Institution and NASA offer a basic introduction to climate change issues in their Ocean Planet Web site.

The Glossary of Oceanography and the Related Geosciences by S. K. Baum of Texas A&M University provides definitions of key terms. NASA has created a teacher's reference guide with glossary, and NSF has a glossary of atmospheric sciences.

Numbered Hypernotes

  1. The Deutsches Klimarechenzentrum, which includes the Max-Planck-Institut für Meteorologie, has many links to data, research projects, and information on climate studies.

  2. The UNEP Information Unit for Conventions (IUC) has a fact sheet on the signs of climate change, as part of their information kit. The NASA Goddard Institute for Space Studies Web site offers temperature data sets.

  3. Climate models are discussed in UNEP IUC fact sheet number 14. A general discussion of climate modeling is available in a report by E. Barron of the U.S. Global Change Research Office. A page of links to climate models and modeling groups is also available.

  4. The UNEP has a fact sheet on the issues involved in deciding whether climate change is a result of natural variability or anthropogenic emissions. Climate change is a controversial topic, with a large range of opinions provided by such groups as the World Resources Institute, the Hoover Institution, Worldwatch, the Science and Environmental Policy Project, Greenpeace, and the Electric Power Research Institute.

  5. The Department of Mathematics, University of New South Wales, Australia, has a tutorial on the equations of fluid dynamics.

  6. A definition of parameterization, with linked examples, is given at Texas A&M University.

  7. The Deutsches Klimarechenzentrum offers links to climate data that illustrate natural variability. The UNEP also has links to a wide range of data sets.

  8. An introduction to paleoclimatology has been written by J. Overpeck for Reviews of Geophysics. An index of data sets at Texas A&M University includes paleoclimate data, such as tree rings. Notes from a workshop on decadal climate variability discuss the problems of using paleoclimate data.

  9. The full text of the 1995 Second Assessment report of the UNEP Intergovermental Panel on Climate Change is available, along with summaries of the working group reports, at the IPCC home page.

  10. NASA provides a fact sheet on aerosols and their impact on climate change.

  11. The influence of clouds on the energy balance of the climate system is discussed on a Web site of the NASA Goddard Space Flight Center. GSFC also has a fact sheet about the impact of clouds.

  12. W. R. Holland, A. Capodonti, and M. M. Holland discuss advances in ocean modeling for climate change research in an article in Reviews of Geophysics. The Geophysical Fluid Dynamics Laboratory has a Web site with information about their ocean general circulation model. Information about IPCC atmosphere-ocean model simulations is available at NASA's Goddard Institute for Space Studies.

  13. The Goddard Space Flight Center has a fact sheet on sea ice and its effect on global warming and climate. A Web page is available with information on the parallel climate modeling collaboration between Los Alamos National Laboratory, the Naval Postgraduate School, and the National Center for Atmospheric Research.

Copyright © 1997 by the American Association for the Advancement of Science.

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