Current Shifts in Abrupt Climate Change: The Stability of the North Atlantic Conveyor and its Influence on Future Climate
Greg O’Hare, Andy Johnson and Richard Pope
When climate shifts abruptly, changes occur so rapidly and unexpectedly that human and natural systems have difficulty adapting to, or coping with, them. By using palaeoclimatic indicators such as tree and coral growth rings, ice cores and deep sea sediments, climate scientists have uncovered many instances of abrupt and relatively short-lived climate changes in the past. These abrupt (some taking place within several -50 years) climate changes are especially evident over the last 100,000 years when the planet, under longer-term Milankovitch and other forcings was slipping into and then out of an ice age. Relatively short-term thermohaline (temperature and salt) driven changes in the vigour and distribution of ocean currents are believed to lie at the heart of the abrupt climate shifts. In particular, thermohaline driven changes between three ‘modes’ of the North Atlantic Conveyor (incorporating the Gulf Stream and North Atlantic Drift) are held to be the key factor explaining abrupt climate shifts over the North Atlantic and adjacent regions. By switching from an ‘on’ mode (maximum heat advection) to a ‘half-on’ mode (moderate heat advection) to an ‘off’ mode (little heat advection) and back again, the northern Conveyor is thought to drive the main pattern in abrupt climate change in the North Atlantic region during the last ice age. This pattern involves multiple sequences of rapid switching from relatively warm periods (interstadials), to cold periods (Dansgaard-Oeschger events) to very cold episodes (Heinrich events) and then back to warm interstadial periods. By currently releasing massive quantities of fresh water into the North Atlantic through ice melt, global warming today is seen as a powerful mechanism able to switch the present ‘on’ state of the northern conveyor to the ‘half-on’ and even the ‘off’ mode condition. Global warming paradoxically has the potential to plunge the northern Atlantic region into new glacial conditions. Because some scientists and others in the media believe in the return or near return of an ice age, this possibility is addressed in our article. Using modern high quality palaeoclimatic data (ice cores and deep sea sediments) a comparison of the last four interglacials, including MIS 11 (430,000 years ago), shows us that natural factors alone are unlikely to cause a quick return to ice age conditions. In addition, theoretical and empirical findings together with modelling studies of the northern Conveyor reveal that although there is likely to be a significant weakening in the Gulf Stream/North Atlantic Drift by the end of the present century, a total collapse of the system is not expected. In terms of our future climate, therefore, we should expect continued warming as a result of anthropogenic release of greenhouse gases rather than cooling over the next 50-80 years. One favourite climate scenario suggests that cooling by a shut-down of the Gulf Stream at the end of the present century is more than likely to be balanced by global warming. In the final analysis, however, there are too many uncertainties in the science of climate change for us to be confident of what the climate will be like in the future. What we do know with a better level of authority is that abrupt climate change is a feature of the past, and it could well be one of the future.
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Date: Autumn 2005