Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 355

6.6.9 Theprobabilityofsurprises
It may seem oxymoronic to discuss the probability of surprises, for if it were possible
to calculate any such events with accuracy then strictly the surprise would not be a
surprise. However, we do know a few things about the surprises discussed so far.
First, regarding vulcanism, we know that over a period of centuries there is a certain
level of volcanic activity that includes large eruptions of standard-type volcanoes and
that over far longer timescales the eruption of volcanic traps occurs. That, for example,
a Yellowstone eruption is overdue on a geological timescale of hundreds of thousands
of years translates to such an eruption in the next 100 years as being not particularly
surprising. We also know that in the past other surprises, such as methane hydrate
releases, have happened. With further elucidation of their mechanisms we may be
able to make better estimations of the probability of future happenings.
Second, we know we are not in a steady-state situation. We are not just talking
about the early 21st-century world being warmer than the 19th century, but a world
that is continuing to get warmer.
If the second point was not significant enough there is a third, that anthropogenic
releases of carbon have been rising almost exponentially since the Industrial Revolu-
tion (a graph of historic carbon releases on a logarithmic scale against time is nearly
linear). We are therefore steadily accelerating the greenhouse forcing of climate year
on year: the increase in warming noted in the previous paragraph is not linear but has
an exponential component.
Fourth, some greenhouse surprises (for example ocean circulation changes and
methane from hydrate releases) may be linked. Surprises can compound and some-
times in unexpected ways, resulting in a critical transition and a climate threshold
being crossed.
Finally, we are reaching the limitations of Quaternary experience. The 20th century
saw the global climate warm to levels not seen for more than 500 years. Early in
the first quarter of the 21st century we are likely to see temperatures not previously
reached for 750 years since the MCA (MWP). Sometime before the middle of the 21st
century could well see temperatures rise to those of the Holocene maximum or above,
which took place some 8000-5000 years ago (Chapter 4). Shortly after that, sometime
around the middle of the century, we are likely to reach a temperature comparable
to that experienced at the height of the last interglacial. Some decades before the
end of the 21st century (should the IPCC forecasts be largely correct) the Earth is
likely to experience a greenhouse regime broadly analogous to a time at least 1.0-3.5
mya, and by the century's end there could be conditions not seen since the Pliocene.
('Broadly analogous' because these palaeo-analogues did not have quite the same
greenhouse forcing as today due to the circulation reasons discussed in Chapter 4.)
The passing of each of these successive stages is likely to increase the prospects of a
climatic surprise.
One purpose of current climate research is to try to ensure that IPCC surprises don't
surprise us. Climate-related policy needs to reflect this. Furthermore, just because the
IPCC focused on a 100-year forecast does not mean that greenhouse problems will
cease or become static in the year 2100. Global fossil fuel reserves (especially dirty