A possible lifeline

I’ve just finished reading a fascinating research paper in this weeks issue of Nature.  Researchers in Germany, headed up by Noel Keenlyside of the Leibniz Institute of Marine Sciences, have looked at the importance of circulating water in the North Atlantic on global temperatures.  By running a climate model that included the natural rhythm of the ocean they’ve investigated what might happen to future temperatures. What they found demonstrates once again just how important natural climate variability is in the debate about global warming. 

Before we look at the new results we need to understand how ocean circulation has an influence on the world’s temperature.

The global conveyor belt

When travelling around Britain you’ll often read tourist brochures waxing lyrically about the virtues of the Gulf Stream and why it makes the western coastal resorts so unusually warm for their latitude. Usually there’s a picture of something that’s mistakenly described as palm tree with a happy couple in various stages of disrobement and wild abandon. Although chucking your clothes off in the heat of the day across Britain isn’t always possible, it’s hard to deny some sympathy with the sentiment. You only have to look at the world climate maps to see that New York is often below freezing during winter while London is generally warmer. A major reason for this unseasonable warmth is the tropical water bought up from the Gulf of Mexico into the North Atlantic.

As the Gulf Stream heads north, it splits into two currents: the Canary Current which shoots off to the southeast, and the North Atlantic Drift which continues on up to the northeast. The result is warm water that’s pushed up past the British Isles. As the North Atlantic Drift heads north, water from the surface evaporates, delivering the newly released heat downwind, courtesy of the winds blowing from the west; the amount of heat given off is equivalent to something like a million power stations. Continued evaporation causes an increase in the amount of sea salt within the surface water. As the warm waters head north, they keep the Arctic winter sea ice at bay but eventually get cold and salty enough to sink in the Greenland-Norwegian and Labrador seas, forming North Atlantic Deepwater. This deepwater returns south at depths of between 2 and 5 kilometres, which thanks to winds and tides eventually well back up to the surface.  

Although this all sounds hunky dory the ocean doesn’t carry warm waters north at the same rate.  The circulation system changes strength on the timescale of decades and centuries. When the ocean circulation is strong, it brings heat to the high latitudes, helping to keep the northern hemisphere relatively warm. Of importance to the immediate future, the German team factored in the influence of a natural 70-80 year cycle.  The key point is the ocean circulation now looks like it’s about to start weakening and return to a long-term average. So, as the circulation slows down, less heat should head north, offsetting the warming caused by rising greenhouse gases.   So, instead of a ‘nice’ simple trend of increasing temperature over time, we may have the perverse situation that we will see little warming for a decade as the ocean helps to take the heat out of the situation. Once the natural cycle switches back the other way, warming will come back with a vengeance.

At one level this could just be the lifeline we need.  If cooling does take place in the North Atlantic it might just give us the time we so desperately need to get on top of the problem.  A decade of intensive cuts in greenhouse gas emissions would help get us get back into the game of putting the planet in a more habitable state.  But this does all raise an interesting issue.  With a slowdown in warming, there is a danger that the perceived risk will lessen. There may even be calls that the worst has passed. 

It will be a tricky path for policy makers to steer.