Danger signal
JOHN HOUGHTON
describes the new consensus on humanity's effect on the
climate atmosphere
Recent changes in climate, the balance of the evidence
suggests, can now be attributed to human activities. This was the
conclusion, in late November 1995, of the official scientific group set up
by the World Meteorological Organization and UNEP to assess the available
information on climatic change.
Delegates from 96 countries came to Madrid for the meeting of Working
Group 1 of the Intergovernmental Panel on Climate Change (IPCC) to agree
the latest science on global warming and finalize an official summary of
their assessment to guide policy makers. The summary, in turn, forms part
of the IPCC's second Assessment Report which was adopted by the
representatives of the world's governments in Rome in December 1995.
It has been very important that this Report be not only scientifically
comprehensive and accurate, but also balanced in expressing both the
issues that are well understood and those about which there is large
uncertainty. Despite great pressures at the Madrid meeting to alter the
balance - especially from some of the oil-producing states and some of the
industrial non-governmental organizations - the final policy makers'
summary compromises neither this nor the science. It has been widely
acclaimed as a clear and accurate expression of current scientific
knowledge.
Remarkably, consensus was achieved in Madrid, and the process of intense
debate and discussion during the meeting added to the clarity of the
document, and improved it. This was due, in no small part, to the way the
scientists present joined in the process, struggling to find the right
words.
The most important new scientific result was on whether the effects of
human activities on the climate could yet be identified. Any human-induced
effect on the climate will be superimposed on natural variations that are
taking place all the time. The question is whether the 'signal' of such
anthropogenic climate change has yet been detected through the 'noise' of
this natural variability. The meeting concluded: 'The balance
of evidence suggests a discernible human influence on global climate.'
It took several hours finally to formulate that sentence, but everyone
agreed it and believed it to be a suitably cautious expression of the
scientific work on the recent trends and patterns of change. It represents
a large advance on previous IPCC Assessments, which concluded that climate
change over the last few decades could all have been due to natural
variability.
There are two main reasons for this scientific advance. First, better
estimates have now been made of the influence of particles of sulphate
aerosols - dust particles mainly resulting from the burning of fossil
fuels - in cooling the atmosphere around major industrial regions. This is
important because it substantially alters the pattern of the expected
warming. Secondly, much more careful studies have been carried out to
compare the patterns of temperature change that have actually been
observed over the past decades with those that have been generated by
computer models that include the effects both of these aerosols and of the
greenhouse gases. These patterns get closer as time goes on, which is what
would be expected as a human-induced 'signal' increased in strength.
Rising temperatures
The summary adds that the limited evidence available suggests that the
average global temperature of the 20th century is at least as warm as in
any other century since at least 1400 AD (earlier data are too sparse to
allow average global temperatures to be calculated reliably). It says that
most of the studies of the statistical significance of the trend in
average global temperatures over the last century have detected a
significant change and show that the warming that has taken place is
unlikely to be entirely natural in origin.
It stresses, however, that our ability to qualify human influence on
climate is limited because the expected 'signal' is still emerging from
the 'noise' of the natural variability, and because there are
uncertainties in such key factors as the magnitude and patterns of this
variability in the long term and the pattern, over time, of the effects of
aerosols and greenhouse gases.
We do know that concentrations of greenhouse gases in the atmosphere have
grown significantly since the mid-18th century. By 1992, carbon dioxide,
methane and nitrous oxide had increased by nearly 30 per cent, 145 per
cent and 15 per cent respectively from their levels before the industrial
revolution, tending to warm the surface of the Earth and produce other
changes in the climate. These changes can largely be attributed to the use
of fossil fuels, agriculture and such changes in land use as the felling
of forests.
Many greenhouse gases remain in the atmosphere for a long time - carbon
dioxide and nitrous oxide stay there from many decades to centuries - and
have a similarly long-term effect. If carbon dioxide emissions continue at
present levels, concentrations in the atmosphere would increase at a
nearly constant rate for two centuries: they would reach about 500 parts
per million by volume (ppmv) - approaching double the pre-industrial level
of 280ppmv by the end of the next century.
Studies indicate that carbon dioxide can only be stabilized at 450ppmv in
the atmosphere if man-made emissions worldwide dropped to 1990 levels in
about 40 years time, and then fell substantially lower. If it took 110
years to return emissions to what they were in 1990, concentrations could
be stabilized at 650ppmv; if it took 240 years they would only level off
at 1,000ppmv - nearly four times what they were before the industrial
revolution. The higher the emissions are in early decades, the lower they
will have to be later on. Meanwhile, stabilizing methane concentrations at
today's levels would involve reducing anthropogenic emissions by 8 per
cent; doing the same for nitrous oxide would mean cutting them by more
than half.
Average global surface temperature has increased by about 0.3šC to
0.6šC over the last century, and recent years have been amongst the
warmest since 1860. Global sea level has risen by between 100mm and 125mm
over the past 100 years, and much of this may be related to the
temperature increase. Night-time temperatures over land have generally
increased more than daytime ones.
Some regional changes have also become evident. The recent warming has
been greatest, for example, over the mid-latitude continents in winter and
spring, but there have been a few areas of cooling, such as the North
Atlantic ocean. Precipitation has increased over high latitudes in the
Northern Hemisphere, particularly in winter.
A warmer
future
There is clear evidence of changes in some extremes of climate in some
regions: the proportion of rain falling in heavy storms over the
contiguous states of the United States of America has increased, and
several large areas of the world now have fewer frosts. In some areas the
weather has become more variable, but in others it has grown less so. And
there is not enough evidence to determine whether there have been
consistent changes in the variability of the climate or in extreme weather
on a global scale over the 20th century.
Temperatures will rise substantially in the future. The IPCC's 'best
estimate' - if policies to reduce carbon dioxide emissions from current
levels are not implemented - is that the average global surface
temperature will rise by about 2šC between 1990 and 2100: its lowest
and highest estimates give a range of about 1šC to about 3.5šC.
In every case the average rate of warming would probably be greater than
any seen in the last 10,000 years, but the actual changes over years and
decades would include considerable natural variability. Because of the
inertia of the oceans, which take time to heat up, temperature will
continue to rise beyond the year 2100 even if concentrations of greenhouse
gases in the atmosphere have been stabilized by then.
Average sea level is also expected to rise because the oceans expand as
they get warmer and because glaciers and ice-sheets will melt. The 'best
estimate' projects a rise of about 500mm between now and 2100: the lowest
estimate is 150mm, the highest 950mm. Sea levels would continue to rise at
a similar rate in future centuries even if greenhouse gas concentrations
were stabilized by the year 2100, and would carry on doing so even after
global average temperatures stabilized.
Warmer temperatures will also produce a more vigorous hydrological cycle.
This means that there will be more evaporation and more precipitation.
This in turn will lead to more severe floods and droughts in some places,
and there may be heavier rain storms. We do not yet know enough to say
whether or not tropical cyclones and other severe storms will increase,
decrease or change in their geographical distribution.
Very little can yet be said about likely local or regional changes, partly
because we do not yet know enough about the effects of aerosols, and how
they may change in the future: the IPCC will encourage more work on this.
But some features are predictable. The land will warm more than the sea in
winter: the greatest warming will be in the northern latitudes in winter,
but there will be little warming of the Arctic in summer. All these
changes are associated with identifiable physical mechanisms.
There are many factors still limiting our ability to project and detect
future climate change, and more research is needed to reduce
uncertainties. And there may yet be unpleasant surprises. Future
unexpected, large and rapid changes in the climate system are by their
nature difficult to predict. But they have happened in the past, and the
rapid forcing effects of greenhouse gases could bring them about again,
with enormous implications for human well-being.
Sir John Houghton is Chairman of the IPCC's Working Group 1 and
Chairman of Britain's Royal Commission on Environmental
Pollution.
