Geoscience Reference
In-Depth Information
land, sea and ice, ocean currents, albedo, plant cover and atmospheric composition.
The further back in time we go, the more the boundary conditions differ from those
of today. Perhaps the most useful aspect of studies of past climate from the point of
view of providing insights into possible future impacts is in providing a spectrum of
natural climatic variability against which to assess current trends.
Using climatic trends based on monitored observations of temperature, precipita-
tion and evaporation may at first seem to be the most reliable of these three approaches
to assessing the possible impacts of climate change. When confronted with extreme
events such as prolonged droughts and widespread floods, which appear to be beyond
the limits of natural variability, there is a natural tendency for climate scientists and
natural resource managers to invoke 'global warming' or 'global climate change' as
the culprit. This raises the question of what we mean by 'natural variability'. There is
no objective or absolute measure of this concept, because it depends entirely on the
spatial and temporal scale adopted. It would be useful if those who use the term 'nat-
ural variability' were to define it relative to different time scales, which they seldom, if
ever, do. One lesson from the past is that we can match modern extreme events simply
by examining the Holocene record of past climatic and hydrologic fluctuations. That
being the case, all we can safely say when dealing with modern extreme events is that
they are consistent with present global climatic warming trends but are not necessarily
caused by them.
Another wild card involved in assessing future climatic impacts in the drier regions
of the world concerns possible future changes in ENSO events, which are presently
responsible for substantial interdecadal and interannual variability in precipitation
(
Chapter 23
). We know the areas that are most influenced by ENSO events today
and over the past five hundred years and more, and can probably safely assume that
this spatial pattern is unlikely to change very much in coming decades (Collins et al.,
2010
). However, we cannot predict with any confidence whether progressively warmer
sea surface temperatures will lead to more frequent or more severe ENSO events than
in the historic past (IPCC,
2007a
). Some authors have proposed that change is likely,
while others are less convinced. Vecchi et al. (
2006
) considered that in the future,
the atmospheric circulation in the tropical Pacific would weaken, leading to weaker
ENSO events, and Power and Smith (
2007
) found that the average June-December
values of the Southern Oscillation Index between 1977 and 2006 were the lowest
on record, indicating a weakening of the Walker Circulation (
Chapter 22
)atatime
when the tropical sea surface temperatures were the highest on record. An important
conclusion from this work was the need to take into account global warming when
assessing links between ENSO events and future climatic variations across the globe.
We saw at the start of this chapter that there have been claims of an increase in
the magnitude and frequency of extreme events such as floods and droughts as global
temperatures on land and sea become progressively warmer. Here again, a measure of
healthy scepticism is required. Consider the question of extreme droughts. In North
