Natural Environment Research Council Home
 
Skip to content

European climate could change rapidly, over decades rather than centuries

Scientists know that in the past, climate has changed dramatically and quickly. For example, Britain and the rest of Europe has been plunged into periods of extreme cold in just a few decades because of changes in the ocean's thermohaline circulation.

What is the thermohaline circulation?

The sun heats the Earth's equatorial regions most strongly. This warmth would stay there but for a huge global current - the thermohaline circulation.

Graphic showing the Thermohaline Circulation

Graphic: The Thermohaline Circulation

This circulation transports about half of the sun's energy from the tropics to the poles. Temperature differences between the poles and the tropics drive this circulation like they drive atmospheric circulations. Warm surface currents travel towards the poles, becoming cooler, and therefore denser, as they reach high latitudes. And when seawater evaporates or freezes, it leaves behind its salt, making the remaining water denser still. At certain places in high latitudes, the dense water sinks to near the seafloor, and flows back towards the equator. This continual circular flow is the northern Atlantic's contribution to the thermohaline circulation - also known as the Meridional Overturning Circulation or Atlantic conveyor belt.

How can European climate change so rapidly?

Part of the thermohaline circulation, called the Meridional Overturning Circulation (MOC) or Atlantic conveyor belt, takes warm surface water northwards towards the Arctic Circle. The heat it releases as it travels is equivalent to the power from about one million power stations, and gives western Europe its temperate climate. (Similar latitudes to Europe, for example Newfoundland on the Atlantic coast of North America, suffer more extreme winters than the UK.)

Computer models predict that more freshwater flowing into a few crucial places in the North Atlantic could slow, or even stop, dense water forming and sinking. This could shut down the return flow in this current, effectively switching off Europe's giant central heating system. We know this has happened in the past and we know when it did, it happened very quickly - in a matter of decades.

Is the Atlantic conveyor belt shutting down now?

On 1 December 2005, a team of scientists from NERC collaborative centre, the National Oceanography Centre, Southampton, reported in Nature that the Atlantic Meridional Overturning Circulation, or Atlantic conveyor belt seems to have weakened by about 30% in the last decade or so.

The team analysed measurements taken along a line at about 25°N from the Bahamas to Morocco in 2004, and compared these to measurements taken along approximately this line in 1957, 1981, 1992 and 1998. The results suggest that the Gulf Stream flow across 25°N has remained nearly constant in this period, but the Meridional Overturning Circulation has slowed from about 20Sv in 1957 to 14Sv in 2004.

The evidence for the slow-down can be found both in the increase in southward recirculation of warm water in the surface layer, and in a decrease in southward transport of lower North Atlantic Deep Water between 3000 and 5000m depth.

Some climate models have suggested that we could expect such a slowdown as a result of global warming, and that this could cool northwest Europe by a few °C, with the change happening relatively fast, perhaps over just a decade. These measurements are evidence that the models could be right.

The five sections are snapshots and cannot tell us for sure whether the changes are a result of natural variability. Very little is known about how the Meridional Overturning Circulation varies seasonally or from year to year. To be sure that the observed change is the result of a long-term trend rather than natural fluctuations we will need to monitor flow across 25°N continuously for about a decade.

The MOC monitoring array at 25°N

In April 2004 a team of NERC scientists deployed an array of instruments across the Atlantic at 25° North. The array measures temperature, salinity, currents and pressure. The first full year of measurements were collected in April 2005, and analysis of this data is now underway. The mooring array will continue to give data until the end of the project in 2008. However, the team are hopeful that they will be able to continue the work beyond this to collect a full decade's worth of data.

To build up a picture of the natural variability in the Atlantic overturning circulation the team intends to combine data from the Meridional Overturning Circulation array with measurements from oceanographic cruises, satellite images and data from floats. Armed with this knowledge they should be able to say with greater certainty if changes observed in the 'snapshot' data from 1957 to 2004 represents actual long-term change in the Meridional Overturning Circulation.

Based on their analysis and their experiences with the prototype array the team are also hoping to develop recommendations for a longer term 'early warning system', which could continue to monitor the health of the Atlantic conveyor in a cost-effective way.

Is the Atlantic Conveyor Belt the same as the Gulf Stream?

No, but they both bring warm water north. The Gulf Stream is a warm surface current driven northwards by the wind. The thermohaline circulation is a much larger deeper current driven by heat and salinity.