Results from the best available climate models do not predict abrupt changes in such
systems (often referred to as tipping points) in the near future. However, as warming
increases, the possibilities of major abrupt change cannot be ruled out. The composition of the atmosphere is changing towards conditions that have not been experienced for
millions of years, so we are headed for unknown territory, and uncertainty is large. The climate system
involves many competing processes that could switch the climate into a different state once a threshold
has been exceeded.
A well-known example is the south-north ocean overturning circulation, which is maintained by cold salty
water sinking in the North Atlantic and which involves the transport of extra heat to the North Atlantic via
the Gulf Stream. During the last ice age, pulses of freshwater from the ice sheet over North America led to
slowing down of this overturning circulation and to widespread changes in climate around the Northern
Hemisphere. Freshening of the North Atlantic from the melting of the Greenland ice sheet is however,
much less intense and hence is not expected to cause abrupt changes. As another example, Arctic
warming could destabilise methane (a greenhouse gas) trapped in ocean sediments and permafrost,
potentially leading to a rapid release of a large amount of methane. If such a rapid release occurred, then
major, fast climate changes would ensue.
Such high-risk changes are considered unlikely in this century, but are by definition hard to predict.
Scientists are therefore continuing to study the possibility of such tipping points beyond which we risk
large and abrupt changes. |