Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 331

contemplated? If either natural or human systems do not behave in the future as
anticipated then there will be unforeseen consequences.
Such unforeseen circumstances could take us outside of the IPCC forecasts in
either a positive or a negative way. At the most fundamental level, with regard to
natural systems, we know that in the past atmospheric carbon dioxide has been both
markedly higher and lower than the IPCC window. However, since the Industrial
Revolution the trend has been upwards, so it would be prudent to consider surprises
that increase climate change. Indeed, in 2004 it was realised from direct measure-
ments that atmospheric carbon dioxide was increasing faster around the turn of the
millennium than had previously been contemplated. The current question revolves
around whether this is an atypical blip or part of an unforeseen trend.
6.6.3 Sea-levelrise
The IPCC predictions for sea-level rise have, if anything, become more conservative
with successive reports (see the summary in Table 5.1). To some, in the UK scientific
community at least, this has been itself surprising as a body of research since the late
1980s (the time from which the IPCC first drew on existing peer-reviewed academic
literature) has pointed to higher sea levels of up to 6 m during the warmest part of
the last (Sangamon or Eemian) interglacial, even though much of that interglacial
was climatically comparable to the present one and only part of it marginally warmer
than today. Further, there is recent evidence that Antarctic ice flows in some places
are faster than thought and that Greenland's ice cap is melting more rapidly than
predicted. Having said this, there are also possible counteracting factors but there is
uncertainty, and hence the possibility of surprise.
It has to be said again that the IPCC provide a consensus view; it came to its
conclusions through a committee process and that its committees were not always
as unified as might be preferred, even if a good proportion of its members were in
general accord. It is not appropriate, or possible, for us to explore exactly how they
came to their conclusions, although we can be certain that their conclusions do arise
from a variation in opinion and/or confidence in forecasts (hence one reason for the
upper and lower estimates in addition to the best estimates the IPCC provide). We also
have to acknowledge, with a significant proportion of the global population living
in areas likely to be affected by sea-level rise, that flooding attributable to climate
change could have major political implications. In other words, irrespective of the
risk (probability), the potential hazard is high. So, given the comparative reassurance
of a low forecast on one hand, and potentially high costs in the event of high sea levels
on the other, it may be tempting for policy-makers to play down sea-level concerns
when in fact there should be a greater apprehension and attention to the precautionary
principle. Psychological perceptions and political drivers are one thing but scientific
understanding is another, even if the latter can contribute to the former (which is what
was hoped with the formation of the IPCC).
We know that during the last interglacial sea levels were up to 6 m higher than
today. This therefore might be considered a realistic scenario of concern that would
be currently foreseen to take place beyond the 2100 horizon frequently used by
policy-makers. Conversely, the total sea-level rise due to mass contributions if all