Change: The Evidence Mounts Up
Michael C. MacCracken, Director Office of the US Global
Change Research Program
Nature, August 24, 1995, Vol. 376,
Change: The Evidence Mounts Up by Michael MacCracken was published
in Nature on August 24, 1995 and summarized the presentations
of a six-day symposium held 3-8 July 1995 on Climate Variability and
Forcing Over the Past Millennium at the XXI General Assembly of the
International Union of Geodesy and Geophysics in Boulder CO.
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
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
Michael C. MacCracken
is in the Office of the US Global Change Research Program, 300 D Street
SW, Washington DC 20024, USA.