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in semi-stable global climate modes, with each mode having its own characteristic
regional distribution of local climate types. Consequently the stability of a global
climate mode is related to the stability of circulation patterns such as ocean thermo-
haline circulation (Clark et al., 2002). Perturb the climate a little too much and this
may affect thermohaline circulation (as discussed above). Perturb the thermohaline
circulation and this can affect ocean circulation, creating new regional patterns of
warm and cool localities. Create new regional temperature patterns and this will
affect atmospheric circulation, and so the Earth system will settle into a new semi-
stable state until further climatic perturbation takes place. Note that in addition to
immediate regional climatic impacts a change in the thermohaline circulation could
possibly disrupt marine methane hydrates (see section 6.6.4) and these would have
an additional and global climatic effect.
So, how likely is a circulation change? The IPCC's warming projections for the
21st century are fairly linear. Most of their scenarios result in near-linear projections;
projections that do not appear to contain a quantum jump. Yet the palaeoclimatic
record is full of sudden jumps and this alone suggests that we need to take changes
in oceanic and atmospheric circulation seriously.
If the question of rapid change deviating from the linear has any credence, and if
such fluctuations are discernable at the global level, then they should be seen within
major biosphere subsystems (remembering that the biosphere includes the geosphere).
In 2005 a team led by Chih-hao Hsieh of the Scripps Institution of Oceanography in
San Diego, California, published work suggesting that large-scale marine ecosystems
(such as oceans) are dynamically non-linear. As such they have the capacity for
dramatic change in response to stochastic fluctuations in basin-scale physical states.
This, they say, is exemplified by fish-catch data and larval fish abundance that are
prone to non-linear responses and do not simply track a dimension of physical change
such as sea-surface temperature. In short, ecological catastrophes (such as a species'
regional population crash) may be the result of a modest change in, say temperature,
at a critical point. This has implications for adopting the precautionary principle when
estimating ecosystem sustainable yields in a time of change.
Of course, as noted a number of times, biological systems feed back into climate.
The area of wetlands affects atmospheric methane, which in turn affects climate.
Vegetation cover, be it a change in or decline of cover, affects regional albedo and so
local climate. So, if the biological response to modest linear environmental change
is not necessarily itself linear, and biology affects environment, then environmental
change such as climate change will also be non-linear. On one hand this does not lend
confidence to the IPCC's near-linear forecasts. On the other, the IPCC do provide
high and low near-linear forecasts and so can be said to provide a window in which
non-linear responses can take place. This theoretical line of argument for non-linear
change is all very well, but what is happening in the real world at the macro scale?
There was much concern with regard to changes in atmospheric and ocean circu-
lation in the late 1990s and early 21st century. Computer modellers have undertaken
much of the work in possible atmospheric circulation changes. Conversely, work on
ocean thermohaline circulation has been undertaken more equally by those making
field observations whose results then feed into modellers' work. Changes in thermo-
haline circulation are of particular concern as field observations have shown that some
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