The signs are everywhere. In some places, huge stones are found lying on the land, in a place where no rock exists. In other places, deep scratches in the stony surface, all pointing in one direction. U-shaped valleys are found in hills, a shape which rivers don’t do. To the readers of the landscape, it was obvious that once there had been glaciers, in places where they had no reason to be. Drifting icebergs carried by huge floods were proposed, and this worked in some places but could only explain the erratic stones – not the scratches or the ice erosion. No, the evidence was clear. Where now there was land, once there had been glaciers. Our lands were made by defrosting.
This post will try to explain why ice ages occured – and why they stopped. It grew out of a discussion on this blog about CO2 and global warming. Let’s be clear: it is real, and the physics leaves little doubt. But not every climatic event is CO2 related. The ice age was out of its control. There is little volcanic activity in this post – my apologies. If physics is not your forte, forgive me. Perhaps you’ll still find the explanations interesting.
Let’s go back in time. 50 million year ago the world was considerably warmer than it is now, by as much as 8C. Antartica was fringed by a subtropical forest. There was no ice, and even the deep oceans were a balmy 15C rather than the 4C they are now. CO2 levels were high, 800ppm, but with excursions as high as 2000ppm. But things began to change. From 40 million year ago, temperatures dropped and CO2 levels declined. Several explanations have been proposed. I consider the most likely the growth of the Himalayas. This caused massive erosion, and the weathering removed CO2 from the air. By 30 million year ago, ice began to form at the poles. The slow cooling continued until 1 million year. Now, temperatures began to plummet. The ice came down from the mountains and the poles. The ice age had begun.
The effects were worldwide, not only from the ice. The climate changed everywhere, with many places becoming severely dry. Savannahs grew and deserts expanded.
The temperature record shows four major ice ages, separated by warmer interglacials. We are currently in such an interglacial. But in fact the interglacial interruptions are brief. The ice lasts 100,000 year – the interglacial little more than 10,000 year. And the current one is 12,000 year old.. It is also clear that ice ages start slowly, but end suddenly. The warming is abrupt. Why? It has to do with the thickness of the ice. The ice cap may 2 km thick. Temperatures decline with height, so the top is 20C colder than the ice-free land around. The ice doesn’t melt until the top gets above freezing (in summer). Once the melt begins, the height grows less and temperatures get warmer in response, by 1C per 100 meter. The lower it gets, the faster it melts. The ice can go very quickly! On the other hand, the ice grows slowly: it can grow no faster than the amount of snow fall per year. That explains the saw-tooth pattern in the temperatures. It is also notable that interglacials begin warm and slowly cool afterwards: the warmest climate is immediately after the ice has gone.
Sea levels responded. During the Eocene, sea levels were perhaps 100 meter above current levels, partly because of the lack of ice and partly because warm water expands. (Interestingly, perhaps a bit ironic, this was discovered by the Exxon Production Research Company, off the coast of New Jersey, and initially considered controversial because their raw data was commercially sensitive and not released.) Sea level dropped by 50 meter 30 million year ago, but overall remained well above current levels.
After the depth of the most recent ice age, sea level rose rapidly in two melt water pulses, 13.8 and 11.3 thousand year ago. The rising continued until 6000 year ago, at an average rate of 1 meter per century, for a total rise of 120 meter. After this, it remained largely stable, until around 1900. From 1900, sea level rise resumed, at approximately 1.7 mm/year, and since 1993 it has increased further to 2.8 mm/year. Most of this comes from the warming oceans which causes the water to expand.
The graph shows sea level changes as determined from seismological studies, Haq et al. 1987. The smooth curve shows a long-term average, the other curve shows a multitude of faster fluctuations. Time runs from 70 million year ago to present, sea level is from +250 to -100 meter. Later studies have suggested that the amplitudes were a bit overestimated.
So what caused the ice ages? It was not CO2. Even though CO2 did decline during the ice age, and recover rapidly afterwards, the changes were between 240 and 280 ppm. This is far less than the current changes where we have exceeded 400 ppm. A quick calculation on the global effect of the ice age change is easy to do. The CO2 absorption spectrum is shown here. It absorbs radiation in the range 14-18 micron. This takes out (very roughly) 5-10% of the radiation the Earth tries to get rid of. This 5-10% changed by 15%, i.e. the change in CO2 changed the Earth’s radiation by 1%. But the Earth has to be in balance: what comes in from the Sun, has to go out again. The extra 1% has to be gotten rid off some other way. That is done by changing the temperature: changing the temperature by 0.25% changes the outgoing radiation by the required 1%. 0.25% of 300 Kelvin
is roughly 1 Kelvin. 1K change is the same as 1C, so the Earth’s temperature changes by 1C.
Another way to look at it (bear with me!) is by considering the thickness of the thermally insulating atmosphere layer. Radiation from the Earth escapes from 7km altitude. The layer increases by 1%, or 70 m, by a 40ppm change in CO2. The temperature gradient in the atmosphere at that height is 1C per 100 meter, so a 70 meter increase gives a 0.7C warming at the surface. The two estimates are in good agreement, although I should say that it is only an estimate. Modeling is needed to get the accurate numbers – but they will not be wildly different from this calculation.
In reality, temperatures during the ice age changed by an incredible 10C. CO2 played a minor part, but what was the major player? It shouldn’t surprise you to hear that ice ages are governed by ice.
Ice reflects. It is blindingly white. Water and rock absorbs, but snow and ice reflect. So during an ice age, the Earth keeps hold of less of the Sun’s energy. The ice itself keeps the Earth cold. And removing it makes the Earth warmer. There are two stable possibilities where the Earth is in equilibrium (with current or pre-industrial CO2 levels, I should say): one where the ice covers much of the north, and a much warmer world where the ice is only at polar latitudes. This explains the rapid alternation between ice age and interglacial. As in other bi-stable systems, things can change fast from one to the other.
But what triggers the change from one to the other? Who decides when an ice age should start and end?
The answer was found a century ago by several people, of whom the Serbian Milankovitch gave the most comprehensive answer. It is a slow cycle in the Earth’s orbit.
Did you ever wonder what happened to the missing two days of February? Why does winter have fewer days than summer? It is because the Earth’s orbit around the Sun isn’t a perfect circle. It is an ellipse, which is about 3.5% closer to the Sun at the closest approach. And this happens in January. Being closer to the Sun, the Earth travels a bit faster, and therefore the seasons pass a little quicker. By 2 days, to be precise. We also receive a bit more energy because we are closer the Sun. Sadly that doesn’t help the UK very much: it is winter and the Sun is very low in the sky. There isn’t much solar energy to begin with, and a few per cent extra makes little difference. It is different in the southern hemisphere: here it is summer and they get a lot more energy from the Sun. The northern summer is when we are furthest from the Sun, and we are not as warm as we should be. As usual, the north loses out.
But this is not for ever. The point of closest approach moves slowly through the seasons, making a complete cycle every 26,000 year. In 13,000 year, we will be closest to the Sun in July, and the north will have the balmy summers. And 13,000 year ago, this balmy weather ended the ice age. Summers became warm enough to begin the ice melt. Once it began, it went fast and within a few thousand years, the warm interglacial had started. But ever since, the cycle has become less favourable and the climate slowly cooled again. By 1900, it was 1C cooler than it was 8000 year ago. At the coldest point (in the north!) of the Milankovitch cycle, ice begins to grow, sunlight is reflected, and cooling gets stronger. The ice is coming.
Ice ages begin when the Milankovitch cycle cools the north, and it ends during the warmest point of the cycle. The agreement between the cycles and the ice ages is excellent. Who is to blame for the ice age? It is the Earth.
Why does the cycle not work in the south? It is because there is not enough land there. Only Chile is in the danger zone, and it isn’t large enough to drive an ice age. Canada is.
Volcanoes are active participants in the cycle of the ice ages. It has been known for a long time that there were considerable fewer major eruptions during the ice age, and a spike in eruptions soon after. Why? Part of the reason is in the ice: the weight of kilometers of ice prevents melt in the magma. Once the ice goes, the reduction in weight allows for decompression melt. And the world erupts. The second reason is erosion: the melt water causes extreme erosion, as much as 1 meter per year. The loss of rock has the same effect as the loss of ice, but rock weighs a lot more. This is the second reason for decompression melt. Volcanoes like the end of the ice age.
They have also been implicated in starting one. This is controversial, but there were several large eruptions in (fairly) quick succession at the start of the Little Ice Age. Some models suggest their combined effect allowed enough sea ice to build up to kick start the Little Ice Age. I have some doubts. The Little Ice Age would have come anyway. It was time.
The Litte Ice Age
The Liitle Ice Age is well known from paintings of happy winterscapes, with frost fairs and ice skating on rivers. It was real enough, with some dramatically cold winters and mixed summers. But when was it? Some say the 16th and 17th centuries. Others give earlier times. In America it is typically said to have lasted until 1850. Looking at the temperature records, it started as early as 1275, and indeed lasted well unto the late 19th century. This was not a brief excursion. Glaciers expanded worldwide, and in fact reached their longest length since the ice age. Extreme winters include that of 1709 (the ‘great frost’) but there were many others. Perhaps even the famous winters of 1947 and 1963 were part of it. Perhaps it ended only with the German snow catastrophe of 1979. Greenland died. Iceland became surrounded by ice.
How significant the Little Ice Age was has only slowly become apparent. On Baffin Island, in the north of Canada, the rocky and barren landscape is covered with lichens. There is ice on the hills. But in a large area around the hills, at low levels, old lichens are absent and even older ones are dead. Lichen can survive many things, but not year-round snow cover. Apparently, this area had permanent snow cover. And dating the lichens show this happened only a few hundred years ago.
Why is this important? This is the area where ice ages start. The glaciers first begin to grow here, in north Canada. And here, snow became permanent during the Little Ice Age. We were at the brink. The ice was on the move.
The Little Ice Age was not little at all. It was the beginning of the real ice age: the big one had begun, sneakingly. where no one was looking. The growing glaciers in the mountains should have been a warning sign.
Saving the Earth
So what stopped the ice age? It seems, it was us, and it was the industrial revolution. Our emissions turned around the global cooling. We were still close to the critical climate point, and only a little warming -at first- was enough. CO2, a minor player during the ice age, took control. It warmed the world by just enough to stop the ice.
I do not subscribe to the opinion that all CO2 is bad. We have warmed the Earth back to temperatures that existed 8000 year ago. That is not a bad thing! Some places are negatively affected, but the effects of a not-so-little ice age would have been much worse.
However, and something I feel strongly about, this would be a good place to stop. Further warming would be detrimental. The Greenland and West Antarctica are beginning to destabilise. Each will cause sea level rise of 5-7 meter. Up to 1 meter of sea level rise (probably a bit less – perhaps 80 cm) is already inevitable. The UK had sea flooding a few weeks ago, at a time the tides weren’t that extreme. The relatively small rise we have had so far is beginning to bite. It is time to cash in our bets, now that we have won the Earth. And without volcanic help.