The last great global warming

As world leaders gather in the Danish capital Copenhagen to negotiate a successor to the Kyoto Agreement, it’s probably worth pondering on what happened last time temperatures were at levels predicted for the end of this century.  

In 1874, a scientist called Pieter Harting took a number of cores that had been taken around the Dutch city of Amersfoort. The sediments they contained provided a treasure trove of information on an ancient climate.  Not only did the ocean muds Harting find show this part of the Netherlands was once under the sea but the seashells contained therein belonged to species that did not live in the 19th century North Sea. Crucially, some of his fossil species were identical to those living in the Mediterranean, suggesting that the North Sea had been warmer than today. He named the layers ‘Système Eémien’ after the River Eem that flowed alongside Amersfoort. We now know these layers date back to between 130,000 and 116,000 years ago. Although this sounds like an interminably long time ago, it was yesterday as far as our planet is concerned and was a period of low ice volume; a time when ice sheets were smaller than today. Could this be a vision of the future?

Hippos were once native to Britain
There’s no doubt it was warm at this time. Eemian Britain, for instance, had a menagerie of hippos, elephants, rhinos and hyenas that roamed the island. It looked more like the African savannah than the rain-sodden landscape we’d recognize today. Fossil bones of these creatures have been found as far north as Yorkshire, with some of the best evidence discovered under London’s Trafalgar Square. Their presence might suggest that it was blisteringly hot, but it’s not as clear-cut as that. Although we might associate this wildlife with today’s Africa, these beasties have often happily inhabited Britain during interglacials. The Eemian was no exception. There is no evidence for people at the same time, suggesting that our ancestors’ absence from Britain allowed these exotic animals to prosper. As a result, their presence is not a great indicator of temperature. It tells us it was warm, but by how much?

Fortunately we have an Arctic goldmine of climate data in the Greenland Ice Sheet. Even today, the dimensions of this icy preserve are staggering. Outside of Antarctica, half of all the freshwater in the world is locked up in Greenland. Of the 2.2 million square kilometres that make up this island, 85% is covered in ice. The highest point – known as Summit – is nearly 3.3 kilometres above sea level, while the sheet itself measures 2500 kilometres north to south.  Ice cores taken from here can be more than 3 kilometres long, stretch back 123,000 years and allow exquisitely detailed reconstructions year by year. It’s probably the closest thing climate researchers have to holy ground.

Appropriately for the climate negotiations, much of the key findings on Greenland has been led by University of Copenhagen researchers working on kilometres of ice stored within huge walk-in freezers set to a frigid -20˚C. Early work recognised that in the central and north Greenland cores, the Eemian ice layers had a heavier isotopic makeup than snow falling in the same areas today. The size of this difference meant that if the signal was converted to temperature the Eemian was some 5 °C warmer than now. Not only this, but the southern part of Greenland appears to have had little or no ice at this time. It all points to the Arctic being a lot warmer during the Eemian than in the late 20th century. 

We know that because of the Earth’s changing orbit, the Arctic received a lot more heat from the Sun during spring and summer. As the surface of the ice melted, the overall height of the ice sheet would have dropped, exposing it to the warmer temperatures of lower altitudes, and exaggerating the melting further. If this wasn’t enough, as temperatures increased around the world, natural processes released ancient carbon stored in the soils and oceans of the world, warming the planet further by enhancing the greenhouse effect. This rise in temperature left an indelible mark on the surface of our planet, most notably along many of today’s coastlines. By investigating ancient beaches and corals dating back to this time we can see the world’s sea level was some 4 to 6 metres higher than present day.  This is an ominous sign for the future. 

The Eemian shows that if Arctic temperatures are allowed to rise above and beyond the much quoted global 2˚C target we could be in for a challenging time. Unlike warming, where the average hides a range of different trends, sea level is global and the changes will happen virtually instantaneously. Anyone living on the coast will see the effects: rich folks with their trendy beach huts and poor people living in their fishing villages.  Sea level change will be the ultimate leveller.

The key question is just how long high temperatures need to be sustained over the poles before we start seeing a comparable rise in sea level.  We’re still not sure but it’s clear from the Eemian that we can’t allow high temperatures to be the norm over the Arctic. 

It’s going to be a challenging week for our leaders in Copenhagen. Good luck to them.

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