How Likely are Major or
Abrupt Climate Changes, such as Loss of Ice Sheets or Changes in Global Ocean
Abrupt climate changes, such as the collapse of the West
Antarctic Ice Sheet, the rapid loss of the Greenland Ice Sheet or largescale
changes of ocean circulation systems, are not considered likely to occur in the
21st century, based on currently available model results. However, the
occurrence of such changes becomes increasingly more likely as the perturbation
of the climate system progresses.
Physical, chemical and biological analyses from Greenland ice
cores, marine sediments from the North Atlantic and elsewhere and many other
archives of past climate have demonstrated that local temperatures, wind
regimes and water cycles can change rapidly within just a few years. The
comparison of results from records in different locations of the world shows
that in the past major changes of hemispheric to global extent occurred. This
has led to the notion of an unstable past climate that underwent phases of
abrupt change. Therefore, an important concern is that the continued growth of
greenhouse gas concentrations in the atmosphere may constitute a perturbation
sufficiently strong to trigger abrupt changes in the climate system. Such
interference with the climate system could be considered dangerous, because it
would have major global consequences.
Before discussing a few examples of such changes, it is useful
to define the terms 'abrupt' and 'major'. 'Abrupt' conveys the meaning that the
changes occur much faster than the perturbation inducing the change; in other
words, the response is nonlinear. A 'major' climate change is one that involves
changes that exceed the range of current natural variability and have a spatial
extent ranging from several thousand kilometres to global. At local to regional
scales, abrupt changes are a common characteristic of natural climate
variability. Here, isolated, short-lived events that are more appropriately
referred to as 'extreme events' are not considered, but rather large-scale
changes that evolve rapidly and persist for several years to decades. For
instance, the mid-1970s shift in sea surface temperatures in the Eastern Pacific,
or the salinity reduction in the upper 1,000 m of the Labrador Sea since the
mid-1980s, are examples of abrupt events with local to regional consequences,
as opposed to the larger-scale, longer-term events that are the focus here.
One example is the potential collapse, or shut-down of the
Gulf Stream, which has received broad public attention. The Gulf Stream is a
primarily horizontal current in the north-western Atlantic Ocean driven by
winds. Although a stable feature of the general circulation of the ocean, its
northern extension, which feeds deep-water formation in the
Greenland-Norwegian-Iceland Seas and thereby delivers substantial amounts of
heat to these seas and nearby land areas, is influenced strongly by changes in
the density of the surface waters in these areas. This current constitutes the
northern end of a basin-scale meridional overturning circulation (MOC) that is
established along the western boundary of the Atlantic basin. A consistent
result from climate model simulations is that if the density of the surface
waters in the North Atlantic decreases due to warming or a reduction in
salinity, the strength of the MOC is decreased, and with it, the delivery of
heat into these areas. Strong sustained reductions in salinity could induce even
more substantial reduction, or complete shut-down of the MOC in all climate
model projections. Such changes have indeed happened in the distant past.
The issue now is whether the increasing human influence on
the atmosphere constitutes a strong enough perturbation to the MOC that such a
change might be induced. The increase in greenhouse gases in the atmosphere
leads to warming and an intensification of the hydrological cycle, with the
latter making the surface waters in the North Atlantic less salty as increased
rain leads to more freshwater runoff to the ocean from the region's rivers.
Warming also causes land ice to melt, adding more freshwater and further
reducing the salinity of ocean surface waters. Both effects would reduce the
density of the surface waters (which must be dense and heavy enough to sink in
order to drive the MOC), leading to a reduction in the MOC in the 21st century.
This reduction is predicted to proceed in lockstep with the warming: none of
the current models simulates an abrupt (nonlinear) reduction or a complete
shut-down in this century. There is still a large spread among the models'
simulated reduction in the MOC, ranging from virtually no response to a
reduction of over 50% by the end of the 21st century. This crossmodel variation
is due to differences in the strengths of atmosphere and ocean feedbacks
simulated in these models.
Uncertainty also exists about the long-term fate of the MOC.
Many models show a recovery of the MOC once climate is stabilised. But some
models have thresholds for the MOC, and they are passed when the forcing is
strong enough and lasts long enough. Such simulations then show a gradual
reduction of the MOC that continues even after climate is stabilised. A
quantification of the likelihood of this occurring is not possible at this
stage. Nevertheless, even if this were to occur, Europe would still experience
warming, since the radiative forcing caused by increasing greenhouse gases
would overwhelm the cooling associated with the MOC reduction. Catastrophic
scenarios suggesting the beginning of an ice age triggered by a shutdown of the
MOC are thus mere speculations, and no climate model has produced such an
outcome. In fact, the processes leading to an ice age are sufficiently well
understood and so completely different from those discussed here, that we can
confidently exclude this scenario.
Irrespective of the long-term evolution of the MOC, model
simulations agree that the warming and resulting decline in salinity will
significantly reduce deep and intermediate water formation in the Labrador Sea
during the next few decades. This will alter the characteristics of the
intermediate water masses in the North Atlantic and eventually affect the deep
ocean. The long-term effects of such a change are unknown.
Other widely discussed examples of abrupt climate changes are
the rapid disintegration of the Greenland Ice Sheet, or the sudden collapse of
the West Antarctic Ice Sheet. Model simulations and observations indicate that
warming in the high latitudes of the Northern Hemisphere is accelerating the
melting of the Greenland Ice Sheet, and that increased snowfall due to the
intensified hydrological cycle is unable to compensate for this melting. As a
consequence, the Greenland Ice Sheet may shrink substantially in the coming
centuries. Moreover, results suggest that there is a critical temperature
threshold beyond which the Greenland Ice Sheet would be committed to
disappearing completely, and that threshold could be crossed in this century.
However, the total melting of the Greenland Ice Sheet, which would raise global
sea level by about seven metres, is a slow process that would take many
hundreds of years to complete.
Recent satellite and in situ observations of ice streams
behind disintegrating ice shelves highlight some rapid reactions of ice sheet
systems. This raises new concern about the overall stability of the West
Antarctic Ice Sheet, the collapse of which would trigger another five to six
metres of sea level rise. While these streams appear buttressed by the shelves
in front of them, it is currently unknown whether a reduction or failure of
this buttressing of relatively limited areas of the ice sheet could actually
trigger a widespread discharge of many ice streams and hence a destabilisation
of the entire West Antarctic Ice Sheet. Ice sheet models are only beginning to
capture such small-scale dynamical processes that involve complicated
interactions with the glacier bed and the ocean at the perimeter of the ice
sheet. Therefore, no quantitative information is available from the current
generation of ice sheet models as to the likelihood or timing of such an event.