Present climate and biological change - Climate Change: Biological and Human Aspects - page 204

flip to a new state. It is important to note that there are other parts of the Earth system
that may harbour a critical transition point: the MOC is just one (see also section
6.6.8).
In short, irrespective of whether or not critical transitions are involved, the future
sees us entering a climate mode that bucks the natural trend and which has not been
seen during the past 2 million years of glacials and interglacials. Either way, the
Holocene interglacial is not likely to end (geologically) soon (within a few thousand
years). Conversely, not only is it likely for the interglacial to continue for another
10 000 or even 25 000 years (as it would without anthropogenic warming), but
with human-induced warming it may possibly (in the context of the last chapter's
discussion of the early-Eocene carbon event) last for well over 100 000 years: here at
the very least the next Milankovitch-paced glacial would be skipped. Meanwhile, the
next few centuries are likely to see global warming with temperatures unprecedented
during previous interglacials, let alone our current one, which saw the rise of early
human civilisation.
5.1.5 Holocenesummary
Whereas the Holocene represents a period of warmth as well as of climatic stability,
the past 10 000 years have not seen a static climate. For example, the Holocene
climatic maximum saw considerable shifts in biomes, both before and after that time.
More recently there was a second medieval (warm) anomaly and then the Little Ice
Age cooling. This last in particular serves to demonstrate that there are many factors
involved in the forcing of the global climate and not just greenhouse gases, be they
natural or anthropogenic. Indeed other factors, such as volcanic and solar activity,
play their parts.
So far we have only considered climate change in the broadest of contexts and with
little detail. There is not the space in a topic that introduces the interface between
climatology, biology and human ecology to dwell at length on the intricacies of
climate. For instance, one could spend much time debating where weather variability
ends and climate change begins. Nonetheless, such intricacies do exist and the reader
needs to be aware of this. For example, there are several oscillations taking place in
the oceans and atmosphere that determine dry or wet, warm or cool, or combinations
thereof in the seasons in any year.
For instance, in the South Pacific there is an oscillation, the El Nino Southern
Oscillation (ENSO), that results in El Nino years. The El Nino is a current that flows
along the coasts of Ecuador and Peru between January and March and which termin-
ates the local fishing season. In some years it is warmer and lasts longer, so preventing
a nutrient-rich upwelling. It is part of the bigger Southern Oscillation that sees the
Pacific waters build up due to the prevailing winds in the western South Pacific. This
build-up takes time and is released both when the build-up is sufficient and met-
eorology allows. There are a number of other key oscillations. One affecting North
America and Europe is the Arctic Oscillation. Another, affecting the Antarctic, is the
Antarctic Oscillation, which is sometimes known as the Southern Annular Mode.
A question researchers have posed in recent years is whether these oscillations are
themselves modulated over a longer time frame. For example, one recent analysis of