The global ocean is experiencing unprecedented stresses due to human activities.
• increasing discharges of macro- and micronutrients caused by land use changes
• rapid changes in marine biodiversity and marine ecosystem structure due to heavy fishing pressure and other human activities
• rising levels of CO2 in the surface ocean
• rising temperature
These changes have direct impacts on marine physics, chemistry and biology, and direct consequences for society.
A planetary-scale ocean circulation, thermohaline circulation (THC) or ocean conveyor belt, plays a significant role in Earth’s climate.
The conveyor is driven by temperature changes and freshwater inputs into the ocean. These cause a global-scale deep overturning of water masses.
A prominent feature of the conveyor is the sinking water in the North Atlantic Ocean. Water that has travelled up from the Gulf of Mexico cools and sinks, releasing vast amounts of heat to the atmosphere. This makes northern Europe significantly warmer than other regions of the Earth at that latitude.
Research shows the conveyor is a fragile system that can respond in a highly non-linear fashion to changes in surface climate.
There is strong evidence that this has repeatedly occurred in the past, and reason for concern that it might happen again in the future.
The best evidence for major past changes comes from the last glacial period (ice age) 120,000 to 10,000 years before today.
Two main types of abrupt and large climate shifts occurred in this time: Dansgaard-Oeschger events and Heinrich events.
These event typically started with an abrupt warming (by up to 10°C in Greenland) within a few decades or less, followed by gradual cooling over several hundred or thousand years.
Heinrich events were caused by a massive influx of freshwater from melting North American ice sheets. The result was either a complete shut down or drastically reduced overturning of water masses in the North Atlantic. This led to a strong cooling in the North Atlantic region.
Climate change presents a number of new possibilities to alter substantially the freshwater balance of the North Atlantic. This could trigger THC variability or even collapse, causing strong regional cooling in the midst of global warming.
When air temperature rises, surface waters tend to warm too. The warmer atmosphere may also cause the hydrological cycle to accelerate. The observed increase in river runoff in the high latitudes may be due to this phenomenon.
These effects tend to reduce the THC because heating and freshening both decrease surface water density.
Large uncertainties remain as to the future of the conveyor belt, particularly in the North Atlantic. In 2004, scientists led by the UK deployed a pilot observation system across the Atlantic ocean to continuously monitor the circulation. They have found that the circulation varies considerably month by month and year by year.
Model simulations indicate that the threshold may be crossed if the forcing is strong enough and applied for long enough, though recent research plays down the risk of a shutdown this century.
The risk of major ocean circulation changes becomes significant for the more pessimistic warming scenarios, but can be greatly reduced if global warming is limited.
The rate of increase in CO2 matters: the ocean-atmosphere system appears less stable under faster perturbations.
This is a rapidly developing area of research. Based on present knowledge of the climate system, the following results appear to be robust:
• The Atlantic THC can have several stable states, which implies thresholds
• Reorganisations of the THC can be triggered
by changes in the surface heat and freshwater fluxes
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