Climate Change: The Evidence Mounts Up
Michael C. MacCracken, Director Office of the US Global Change Research Program

Nature, August 24, 1995, Vol. 376, pp. 645-646

Our present climate is unusually warm, and the pattern of warming over the past century strongly suggests an anthropogenic influence from greenhouse gas and sulphate aerosols. That was the message emerging from a week-long symposium examining climate variability over the past 1,000 years, which brought together results from a growing array of observational techniques, analyses of natural records and model results ("Climate Variability and Forcing Over the Past Millennium", XXI General Assembly of the International Union of Geodesy and Geophysics, Boulder, Colorado, 3-8 July 1995).

Very precise measurement of the vertical profile of air temperature in boreholes drilled up to a few thousand metres deep indicate how the near-surface ground temperature has changed over the past few decades, over the past one or two centuries, and since the early part of this millennium (H. N. Pollack, Univ. Michigan). Papers were presented on results from Europe, North America, Africa, Asia, New Zealand and Australia; virtually all measurements indicate that there was an extended cool period a few centuries ago and that ground temperatures during the present century are on average about a degree warmer than during the last century and, more importantly, than earlier this millennium.

Whereas borehole temperatures provide a direct but increasingly smoothed record, ice cores, tree rings and coral growth layers provide indirect, but year-by-year (and even season-by-season) estimates of the temperature and precipitation over much of the globe. Ice-core records provide information about volcanic eruptions, specifically the amount of sulphate aerosol injected (as deposited aerosols are trapped in the ice) and the cooling that it induced, which can be inferred from changes in oxygen isotope ratios (G. A. Zielinski, Univ. New Hampshire). Tree-ring evidence suggests that the coldest summers were 1601, 1641, 1669, 1699, 1783, 1816 and 1912---with all but 1699 associated with known volcanic eruptions (P. D. Jones, Univ. East Anglia). The latest results suggest that the sulphate content of material ejected in the 1883 Krakatoa eruption was relatively low, leading to only minor global cooling.

Combined land and ocean records indicate that there has been a global warming of 0.3 to 0.6 K since the last century, albeit with cooling in the North Atlantic and some parts of China and North America (N. Nicholls, Bureau of Meteorology Research Centre, Melbourne). Data on retreat of mountain glaciers from the tropics to high latitudes reinforce this evidence (M. F. Meier, Univ. Colorado). Although there is evidence that the seasonal cycle is also shifting towards earlier winters (D. J. Thomson, Bell Labs), there was considerable dispute about how certain we could be that greenhouse gas increases were accelerating the shift that is itself due to changes in the Earth's orbital elements.

Several investigators (G. C. Reid, National Oceanic and Atmospheric Administration, Colorado; D. V. Hoyt, Research and Data Systems, Maryland) reported statistical linkages between climate fluctuations and variations in solar irradiance, but the evidence that variations in the Sun's energy output can be large enough to cause the changes will be at best circumstantial until a plausible mechanism is quantified. A diagnostic model aiming to match the climate record of the past few centuries indicated that solar variations could only have had an important effect in the unlikely event that the cooling influence of sulphate aerosols is largely compensating for the warming influence of greenhouse gases. For the future, all indications are that the greenhouse gas effect will increasingly govern the behavior of the climate (M. E. Schlesinger, Univ. Illinois).

Interestingly, the warming pattern since the mid-1970s looks rather like the footprint of El Nino events, although this does not explain the strong warming over parts of Eurasia (C. K. Folland, Hadley Centre, UK). These quasi-periodic events warm the eastern tropical Pacific Ocean, which sets off a chain of events that cool the central North Pacific, warm northwest Canada, increase precipitation in the southeastern United States and intensify the large-scale pressure pattern across the Pacific, North America and the North Atlantic Ocean that alters mid-latitude storm tracks. Does this finding mean that the recent warming is due primarily to an increased (but not understood) frequency of El Nino events, or is this the pattern by which greenhouse-gas-induced warming is becoming evident? We don't know. Climate models are only just starting to achieve the high resolution and verisimilitude needed to reproduce, although often not strongly enough, the observed El Nino signature. Some model simulations suggest that the warming pattern will be similar to a persistent El Nino, but that oscillations will continue, superimposed on the higher average temperature (G. A. Meehl, National Center for Atmospheric Research, Colorado).

A variety of model simulations for the nineteenth and twentieth centuries are being conducted. Simulations that couple ocean, atmosphere and land surface aerosols show better agreement with the historical record if they include the increasing concentrations of both greenhouse gases and sulphate aerosols than if they take into account just greenhouse gases (J. F. B. Mitchell, Hadley Centre; U. Cubasch, Max-Planck-Institut fur Meteorologie, Hamburg; see J. F. B. Mitchell et. al. Nature 376, 501-504; 1995). This is evident both in the records of the global average temperature and in the geographical patterns of changes in surface and tropospheric temperatures (B. D. Santer and K. E. Taylor, Lawrence Livermore National Lab.). The chief discrepancy found in analyses of the vertical temperature pattern is that the simulated warming extends up into the lower stratosphere whereas the observations show cooling in this region. This discrepancy is probably a result of not including the effects of stratospheric ozone depletion, which other modeling studies indicate causes cooling due to reduced absorption of solar and terrestrial radiation.

In addition to lowering projections of overall global warming, including sulphates in the simulations has regional effects, including reversing the projected intensification of the Asian summer monsoons found in greenhouse-gas-only calculations (Mitchell). This occurs because the high (but uncertain) projections of future sulphur emissions (mainly from energy generation) for eastern Europe, India and China create a cooling pall over southern Asia. Interestingly, the change in the Earth's orbit since 6,000 years ago had a radiative influence of similar character, and the modelled and observed result of the change is a reduction of the summer monsoon and the aridification of much of the Middle East and northern Africa (suggesting that the models are responding as would nature).

Another important issue is understanding the internal variability of the atmosphere-ocean(-glacier) system. The picture slowly emerging from observations seems to fit reasonably well (at least in some cases) with mechanisms of decadal to interdecadal variability found in simulations with coupled ocean-atmosphere models, which are becoming more realistic (U. Mikolajewicz, Max-Planck-Institut fur Meteorologie).

In the vernacular popular in the United States today, one could say that the DNA (here for Distinguishing Natural and Anthropogenic) evidence is becoming quite compelling. Although greenhouse gases and aerosols are not yet convicted beyond all reasonable doubt, the case is becoming steadily stronger.