For about 85% of the time that Earth has existed the temperature has been high enough that there have been no polar icecaps – a “greenhouse Earth”. The remaining 15% of the time is referred to as “icehouse Earth”, and during these longer cooler periods are glacial periods (like 20,000 years ago when the ice sheets reached as far as Germany) and inter-glacial periods like the current time where the ice is just at the poles. The experts discussing ice ages on In Our Time were Jane Francis (University of Leeds), Richard Corfield (Oxford University) and Carrie Lear (Cardiff University).
I looked at Bragg’s blog post on the Radio 4 blog for the episode, as I often do before I start writing one up, and was surprised that several people had commented complaining about how the discussion was minimising the impact that climate change and rising temperatures would have on our civilisation. Surprised because J and I came away from the programme with the distinct impression that all three experts thought the planet would be just fine with higher temperatures, and that life would survive as it has done before. But our civilisation? Well, that would be in more trouble.
However that was not the focus of the programme, as In Our Time is not a current affairs programme. Instead the programme was about what an ice age is and how we know about them. My first paragraph is a good summary of what they told us about what an ice age is. Continental drift plays a part in producing the conditions that lead to an icehouse Earth – all 5 that have occurred are correlated with the presence of land at one or both of the poles. When there is open water at both poles then the currents moving the water between the poles and the equator counteract any cooling of polar region – obviously when there’s land there this can’t be true. I’m not sure if every time there is land at the poles then there is an icehouse Earth, or if the correlation is only the other way round (every icehouse has a landlocked pole). I don’t think they said. But this thermal isolation of one of the poles seems to be a requirement to get the process going.
The change from a greenhouse Earth to an icehouse Earth is a slowish process (from a geological point of view) but once it starts there are positive feedback loops that mean the Earth continues to cool. One of these feedback loops is because ice & snow are white and reflect back more of the sun’s energy so the land doesn’t warm up as much as it would if snow were black. Another is that CO2 gets frozen in the ice caps and so the atmospheric CO2 concentration goes down – and low temperatures, and icehouse Earths, are correlated with low CO2 concentrations. They were mostly just saying things were correlated rather than speculating on causes – but I think Lear said that CO2 levels are a driver of temperature change.
Once in an icehouse Earth there are these oscillations between cold-cold-cold and not-quite-so-cold. These are due in part to Milankovitch cycles – cyclical changes in the Earth’s orbit which (effectively) change how cold winter gets compared to summer and how long winter lasts. So when the Milankovitch cycles are in a cold-winter phase then you get a glacial era, and when they’re not you get an inter-glacial such as our current climate. I guess in a greenhouse Earth you get tropical and not-so-tropical eras similarly.
The five icehouse epochs have not been identical. One of them only had ice across the southern pole which was where the continent Gondwana was positioned. This comprised of most of the southern landmasses – India, South American, South Africa, Australia etc. The rest of the land on the planet was situated around the equator and had a tropical climate. Another of the icehouse epochs is what was known as Snowball Earth because the icesheets covered the whole of the planet. Bragg was curious as to how the planet had got out of that, the only answer was that it must’ve involved rising CO2 levels but no clear ideas as to what would’ve kicked off the rise.
The evidence for these climate changes come from a variety of places. Francis told us about the physical evidence you can see in the geological record, for instance particular rock types that’re formed from the bits & pieces that a glacier grinds up and carries with it. There are also distinctive scratches that can be seen where a glacier has been. The problem with this sort of evidence is that it’s incomplete. A far more complete picture has been built up using the sediment in the oceans and the ice sheet on Antarctica.
Corfield told us that the old-fashioned way of using sedimentary cores to look at what the climate used to be was to look at the various species of small fossils and see how many were warm water species & how many cold. Lear told us about the more sophisticated techniques that are used now. The first of these is to look at the ratio of 16O and 18O isotopes in the fossils. This reflects the ratio in the water in which they lived, which is dependent on the temperature of the water and the sea levels. As water evaporates from the sea the molecules containing 16O preferentially evaporate. If there is no ice then once the water rains it ends up back in the sea so the ratio stays the same, but if there are ice caps then some of the rain ends up locked up in the icesheets and the ratio in the water is changed. There is also another way of looking at the temperature using magnesium & calcium, but Lear didn’t explain what that was. Cores from the icesheets can be used to look at the atmospheric conditions during the current icehouse epoch. As the ice forms there are small bubbles in it, and it’s possible to extract these & look at the CO2 levels. For most of the glacial period the CO2 level was around 280ppm, which is pretty low compared to today’s 390ppm. In a greenhouse Earth the CO2 levels might be several times that.
Another indicator of ice levels in the past is fossilised coral. Coral always grows pretty close to the surface of the ocean, so where you find the fossils shows you where the coastline was in the past. At the glacial maximum the sea level was a lot lower than now (by about 70m I think they said), during a greenhouse Earth the sea level is a lot higher. Which is where the problems come in for us humans – think of how many important cities are on the coast … But as I said, the programme didn’t dwell on that or spell it out explicitly.