Learning from past climate events

Changes in the climate affect the seas and, in turn, changes in the oceans influence the climate. But have oceans circulated differently in the past? And how would we know? Kat Arney spoke to David Thornalley from University College London...

David - It hasn't always been the same circulation as we see today. As we've talked about earlier, there's been these swings in climate between warm interglacial climates and cold glacial climates. We think that these swings in climate are associated with different styles of ocean circulation as well.

Kat - So, can you give me some examples of what we know has changed in the oceans over - I guess let's start with a really big timescale?

David - So, if we start with that big timescale - this glacial, interglacial hundred thousand-year timescale of climate change - one good example to start with is if we looked at the southern ocean.

Kat - So, what kind of area are we talking about here?

David - So the southern ocean, it's the bit at the bottom of the globe, the ocean surrounding Antarctica. This is a really important part of the ocean today because it's where this deep water that's rich in dissolved carbon dioxide can upwell and come to the surface. And there, it releases its carbon dioxide to the atmosphere.

Kat - What do we know about the way that that's changed then over this kind of hundred thousand-year timescale?

David - So, we think that in the past, the amount of upwelling of this carbon dioxide-rich gas has changed. We've lost some of this leakage of carbon dioxide in the past. So, in the glacial climate, one example we think has happened is it was a lot colder and we had an expansion of sea ice around Antarctica. That acted as a cap to keep carbon dioxide trapped within the ocean. It's almost, you have this feedback between the ocean and climate whereby, climate is colder, you grow sea ice. That means you cap more carbon dioxide in the ocean. That in turn means the climate gets colder and we grow more sea ice, draw down more carbon dioxide. You can get into quite an extreme state of a very cold glacial climate.

Kat - But we're not like that now. Obviously, we have a much smaller ice cap down in the south. What do we know about what happened to tip that change and how have things change since then?

David - There's natural drivers of climate change - change in the way the Earth's orbit that can help start off these feedback processes. And so, we're able to get out of the glacial by the sea ice, gradually melting back. We just run that feedback process in reverse. The sea ice melts that allows some carbon dioxide to come out and we feedback on that process until we get to a current warm climate like the current interglacials that we're in today. So, we have the see-sawing back between cold and warm glacial climates.

Kat - So, that's on the scale of hundreds of thousands of years. What about wobbles, changes in the climate on maybe a scale of a thousand or even hundreds of years?

David - So, another interesting area we can look at and how the ocean influences climate is if we switch to the other area of the globe. If we go all the way now to the northern north Atlantic, this is an important area where we have warm surface waters, flow northwards and they sink and flow south. This is like a big ocean conveyor belt that sends heat northwards and helps keep Britain and northern Europe warm. We think that the strength of that conveyor belt may have changed in the past. What we know that on the big scale, it has weakened in the past. But then the big question is, on these shorter timescales - these century timescales - have wobbles in the strength of this conveyor belt influenced our climate?

Kat - Because I remember seeing pictures in medieval times, they were skating on the Thames and all this kind of thing. Was that linked to these smaller changes?

David - Potentially and a lot of people are trying to research that question. But it could be that in these colder climates, there was a slightly weaker conveyor belt that was bringing less heat northwards and it could've led to these colder climates that we associated with this interval called the little ice age. 

Kat - So, that's hundreds of thousands of years, hundreds of years. Are these changes happening maybe in terms of decades? What kind of short scale can we see these changes on and why are these important?

David - When we look at these short timescales, we can think about changes in the water masses we see at the surface. So, in the north Atlantic, we have cold arctic water and then we have warmer tropical waters making their way northwards. That balance of what type of water mass you have matters. Different types of fish and different types of plankton live in those different water masses. And so, this changing water masses can affect where fish stocks may be. We have records whereby you get incursions of warm water moving up and down the Norwegian coast. But they're associated with historical documents of different fish yields up and down the Norwegian coast. And so, we're obviously interested in those natural variability that we can see today. How is that going to impact where the fish are today?

Kat - That's fish. Obviously, very important but when we look at these wobbles on the short term, the mid-term, the long term, what do they tell us because there's a lot in the media about, "This climate change. Maybe it's just a wobble." How do we know what's a wobble and what's actually worrying?

David - So, this is one of the important reasons why we should look at the past because it allows us to see what kind of variability can we expect. We obviously have to take into account what's driving that variability. Today, we're worried about the human influence on the climate. But we want to look at what that natural variability is and monitoring the climate system today, and are we seeing anything unexpected outside the range of what we might expect from natural variability. And that's when we start to see something unexpected. We think, "Hang on! Maybe we need to be aware and maybe we do need to be somewhat concerned." It's not about just these regional changes, maybe Britain being a little bit warmer which would be nice. It's about these global impacts the ocean circulation can have.