Unpicking ancient climates

Our planet's climate has been changing for millions of years. So should we worry now? To answer this question we need to understand what causes these natural variations in atmospheric carbon dioxide concentration, and how this, in turn, influences the climate. To do that, we need a way to look back into the past and work out what the climate and carbon dioxide levels were doing, which is what Cambridge University's Julia Gottschalk works on, as she explained to Chris Smith... 

Julia - We have a variety of climate archives that we can study in detail and that record the past climate. So for instance, there are these beautiful ice cores from Greenland and Antarctica. These ice cores basically comprise of snow that has fallen in the past and that accumulated over time. For instance, the isotopic composition of that snow can tell us something about air temperature in the past.

Chris - So, there's a sort of chemical fingerprint written into there that tells you at a certain time back in history, this is what the temperature of the Earth must be when the water that led to that snow was around.

Julia - Exactly, yes.

Chris - What else is in the snow? Are there any other things trapped in there that we can measure?

Julia - Yeah. So luckily, the snow has enclosed air from the past.

Chris - What like little bubbles?

Julia - Yeah, we can actually measure the chemical composition of that air. So, we exactly know what the carbon dioxide concentration was in the past.

Chris - How do we know that it hasn't changed after the bubble gets trapped in the ice?

Julia - The way it can actually change is by diffusion through the ice. It's the only way because the ice encapsules, they are bubble, right? But we know that diffusion is very low and very little.

Chris - So, it should be really trustworthy.

Julia - It is very trustworthy, yes.

Chris - And how far can you go back with an ice core?

Julia - Our longest ice core goes back to about 1 million years ago.

Chris - Not very long. Can we go back further in any way with any other markers or measures?

Julia - Yes. So, we have sediment cores from the seafloor, and these comprise of particles that have accumulated in the past ocean, for instance, but they also save or capture creatures that lived in the past ocean, and I have been working on so-called foraminifera that are tiny creatures that are comprised of calcium carbonate.

Chris - They have a little shell.

Julia - Yeah. They have very beautiful fossils basically, they have very delicate shells.

Chris - Again, you can look at the chemistry of that shell to work out what must be happening to the carbon dioxide level in the atmosphere. It's like a proxy measure.

Julia - Not for the carbon dioxide but you must imagine - so, the foraminifera lived in the surface ocean. They basically record the properties of the ambient water they were living in. So for instance, the temperature, possibly the salinity as well, the pH of the seawater. So, what we can do is measure different proxies to reconstruct these properties.

Chris - Now, the cynic in me is saying, fine, so you look at these ice cores, you look at these bubbles of trapped gas in the ice cores, you look at these foraminifera and look at the chemistry of their shells and you say, "I think this is what's going on in the climate when they were alive." How do you validate that? How do you say, "Right. Independently, I can confirm that is the case"?

Julia - This is a very important task that we have to do as scientists. One approach that we usually adopt is to look at different locations on the globe but also to use different approaches, different proxies. If very different proxies show the same trend then we can actually be very confident in saying that the trend is actually robust.

Chris - One is corroborating another I suppose.

Julia - Exactly.

Chris - When you look at these independent measures over long geological timescales, what do they show that the climate has done and the ocean chemistry has done over these long periods of time?

Julia - For instance, let's say, we go back to a hundred million years ago, for instance during the time when the dinosaurs lived, so we know that the Jurassic and Cretaceous atmosphere was much higher in carbon dioxide, the global temperature was much higher. But we also know that the climate since then followed a global cooling trend.

Chris - But what would have driven that? Why would the carbon dioxide level have been extremely high and then gone down and temperatures with it?

Julia - Tectonic activity was much higher. So seafloor spreading was much more active so the carbon dioxide that was outgassing from the mantle at that time was evading into the atmosphere and was...

Chris - So, there was more production carbon dioxide. The other thing that can pull carbon dioxide out of the atmosphere though is mountains, isn't it? If you get weathering of mountains, this can chemically react. So, was that also responsible for a drawdown of carbon dioxide?

Julia - Yeah, absolutely. For instance, the formation of the Himalayan plateau - India was colliding with Asia and the uplift of a lot of rocks at that time and the formation of a lot of rocks at that time interfered with the atmosphere. As you said, the silicate weathering pulled a lot of carbon dioxide out of the atmosphere and that helped to bring global temperatures down over time.

Chris - And if you look at what your markers and proxies have shown the atmosphere and the oceans have done chemically and thermally over time. And you ask, how does the climate today compare with what your proxies suggest should be happening?

Julia - Let's consider for instance the past million years. We know that the climate has shifted very frequently from a cold climate state to a warm climate state. So, sea level has changed in time with it and also, carbon dioxide concentration has changed in time with it. Considering that this sort of rhythm would happen in the future, we would expect that soon, we will go back to a cold climate state again. But with the changes in anthropogenic carbon dioxide emissions, this might be very different now.

Chris - Are you saying the world should be getting colder based on past measures and past cycle behaviour? In fact, it's not. It's actually getting warmer and also, the carbon dioxide in the atmosphere should be going down, but it's going up.

Julia - That's correct, yes. So, what is actually quite worrying is the rate of the climate change. So, we have never seen the climate system change that rapidly as we observe it today so that's quite worrying. We know that an interglacial climate state has actually lasted for a certain amount of time. After that amount of time, we would expect that the climate system would go back to glacial. But I would not be so sure that this will happen now with changes in the chemistry of the atmosphere that we might have caused.