Prof Anders Levermann, Potsdam Institute for Climate Research
In the past, changes in ocean circulation may have amplified changes in climate. But what does this mean for us today, and if our climate continues to warm what will happen in the future? Anders Levermann is a climate scientist at Potsdam University in Germany. He spoke to Chris Smith...
Anders - If you heat up the atmosphere, you also heat up the ocean. Simply because they are strongly coupled and heat is going all the way from the atmosphere into the ocean, and also back. So, there's a strong link and we see this link already. We see the temperature increasing in the atmosphere already 400 years and the ocean is also taking up part of the sea that is expanding which is why sea level is rising. So, there's actually no doubt that the oceans are already influenced by human activity.
Chris - That being the case, what would be the long term consequences?
Anders - Sea level rise will continue for hundreds of years actually. But also, ocean currents are influenced by the warming, simply because ocean currents are driven partially by winds but also, because of the density differences in the ocean. The north Atlantic current which is the Gulf Stream, once it has parted from the coast of North America is what we are, in Europe, particularly interested in because it is putting a lot of heat to northern Europe and to the UK particularly. This north Atlantic current is then density driven and can be changed if the temperature of the planet is changing.
Chris - So, what would be the consequence then if these ocean currents like this one stop working?
Anders - The Atlantic overturning is quite unique. If it stops, this heat source is stopped and you can already see what is going to happen if you go at the same latitude from the UK towards northern Canada where you have a completely different agriculture. You have actually almost no agriculture there while you're able to do agriculture in Great Britain. So, this is what's going to happen. Itís going to get much colder in northern Europe.
Chris - If you're not cutting the heat off to the northern hemisphere and high latitudes like where we are, where does the heat go?
Anders - It stays in the south. So, the north gets colder, the south gets warmer, and the sea level changes because at the moment, the sea level in the north Atlantic is a bit lower than it should be because of the Atlantic overturning. Once you stop the circulation, the sea level comes up quite rapidly in the north. Itíll be up to 1 metre of sea level rise.
Chris - This is theoretical obviously, isn't it? Is there any evidence to support this actually happening at the moment?
Anders - Well at the moment, we see a mild decline of the Atlantic overturning circulation. We see this from observations. There have been recent publications showing this again. For the future, we can expect further reduction in the Atlantic overturning, simply because we understand the physics. So, if we trust in physics then we can trust in the fact that the Atlantic overturning is soon going to reduce if the planet is warming.
Chris - What about also the loss of ice because weíre adding, when you melt ice, a lot of freshwater to the existing salty water? That must also change the density. So, does that have an additional effect on currents like the overturning circulation?
Anders - If you add melt water for example from Greenland then you freshen the water and you make it less dense in the north. What you can get is actually an abrupt change of the circulation and thereby, an abrupt change in the temperatures of the north.
Chris - If we extrapolate this globally, what will be the implications for us as humankind, our ability to feed ourselves, our ability to catch fish and so on?
Anders - The Atlantic overturning is really quite a unique circulation on the planet. We have other big ocean currents but none of them is driven by this kind of extraordinary density difference that we find in the Atlantic. Thatís why we donít have to fear abrupt changes in ocean currents. Apart from the north Atlantic which however would be a dramatic change, not just for the United Kingdom or for northern Europe, but for the entire planet because the monsoon systems far away, for example, in India and China are linked to this temperature difference in the north Atlantic. And this will actually affect our ability to feed the population of the earth. For example in India, if you reduced the monsoon system by just 10 per cent as weíve seen this in the past, you'll already get famine and a lot of problems economically. So changing something as big as the ocean currents is changing a lot and humans have to adapt to change, and if itís too quick, it might be difficult.
Chris - Is this an inevitability? Is there anything we can do about this?
Anders - The warmer we make our planet, the higher the risk of an Atlantic overturning collapse. What we can do is obvious. We can reduce the emission of carbon dioxide into the atmosphere. There are other methods around that are called geo-engineering, but they donít reverse what weíve done with our climate. They just add another complication to the problem.