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ISIS Special Miniseries
Abrupt Climate Change Happening
‘Climate change’ conjures up a picture of a gradual process
occurring in the timescale of the earth, hundreds if not thousands of
years. Not anymore. Since the mid 1990s, scientists have been asking if
climate change might be abrupt, in other words, it could happen suddenly,
over a matter of decades or even years, and be global in extent. Dr. Mae-Wan Ho
The picture most people and most policy-makers have of climate change
is the one produced by the Intergovernmental Panel on Climate Change
(IPCC), which gives smooth projections of global warming ranging from 1 to
6 C by the end of this century, depending on the computer models used. All
the models assume processes occur smoothly and linearly, however, and do
not predict abrupt change.
Real earth processes, however, are nonlinear, often involving positive
feedback and threshold effects that give rise to abrupt, catastrophic
jumps or swings between different states.
As more and more data on ancient climate accumulate, it has become
clear that abrupt climate change is a reality on many differentscales, and
has occurred many times in the past.
One way to test a climate model that predicts the future is to see how
well it post-dicts the past. For example, nine major ice ages
have been found in the geological record, alternating with periods of
abrupt global warming. Can the model predict those given reasonable
This close correlation between temperature and carbon
dioxide concentration is one of the reasons palaeoclimatologist Richard
Alley at Pennsylvania State University, USA, who studies ancient climate,
believes human activities affect climate. The recent rate of increase in
atmospheric carbon dioxide simply has no geological precedence in the
known history of our earth (Fig. 2). A newly published study from the
University of East Anglia’s climatic research unit confirms that since
1980, we have been experiencing the hottest climate for the past 2 000
How well do current climate models of the IPCC post-dict the
past? They are much better than people often give them credit for, Alley
says. Climate changes show up in the right places at the right times, but
they just don’t produce the large, abrupt changes seen in the real world.
At the very end of the ‘Younger Dryas’, a global cooling
event between 13 000 and 11 500 years ago, average temperatures increased
by about 6 C within a decade in some places. A huge quantity of melt-water
from North America poured into the North Atlantic in a big hurry, and it
got cold again. Average temperatures fell by 6 C within a
And that could be due to effects on the thermohaline
circulation – a huge convection system that transports warm water from the
tropics to the poles and send cool water back through the depths of the
oceans (see “Global warming and then the big freeze”, this series).
Robert Dickson, hydrographer studying water movements at the
Centre of Environment, Fisheries and Aquaculture Science in Lowestoft,
Suffolk, believe that abrupt climate change is already happening. Dickson
has been monitoring changes in the North Atlantic over the past 40 years.
Within this period, all 10 of the warmest years since records began
happened between 1990 and 2002. It also includes extremes in the “North
Atlantic Oscillation” – the wide swings in atmospheric pressures between
the polar and mid-latitude regions – that’s responsible for climate
variability in the region.
Dickson expects these changes to slowdown the THC and
accelerate the global water cycle between the atmosphere and the oceans.
Isolated measurements have already indicated an increased
flux of freshwater into the north Atlantic. At the same time, there is an
increase in saltiness of water further south, through to the equatorial
south Atlantic, which can only be explained by increased evaporation; and
that would accelerate the water cycle between the surface of the earth and
its atmosphere. The Pacific and Indian oceans also show increased salinity
in the tropics and freshening near both poles between the 1950s and 1990s.
Water vapour is itself a greenhouse gas, which would contribute to further
warming, a positive feedback that could precipitate abrupt