Arctic warm periods

DW: What exactly did the expedition reveal?

Martin Melles: First it confirmed what we expected to find - the Arctic reacted to the warm and cold cycles of the last few million years. But the big surprise was that against the background of these cycles, at irregular intervals there were times when the warm spells were extremely hot: four or five degrees hotter than the warm period in which we are living today.

That would indicate that the Greenland ice shield either didn't exist at all at those times or must have been much smaller. And the sea ice we have on the Arctic ocean today probably didn't exist in that form either. Sea level must have been far higher. If all the Greenland ice melted, we would be talking about around seven meters (23 feet) of global sea-level rise - and that's the kind of level that must have existed around those times.

Around the crater lake where we drilled, we have tundra today, where the highest bushes are around 20 centimeters (8 inches). During those super-warm periods there were thick forests, taiga, which we only find say 100 kilometers (62 miles) further south today.

What could explain these "super-warm" periods?

First we used a climate model to calculate whether these warm temperatures could have been caused by the mechanisms which we know to result in warm or cold periods. That includes the earth's position with regard to the sun or natural climate variations caused by greenhouse gases, which also occured in the earth's history without human intervention. But neither of these factors can explain it.

That means there must have been another impulse that led to this unusual warming in the Arctic. One possibility would be that it comes from the Antarctic. A few years ago, a sediment core was drilled in the Antarctic which shows that over the same time scale we have been looking at, during several individual warm periods, the glacial ice had disappeared in a large part of the Antarctic. Those periods when the West Antarctic ice shield had melted correspond to the times when we reconstructed these extremely warm periods in the Arctic.

How were you able to detect this?

This lake in the far north east of Siberia is unusually old. It dates back 3.6 million years. It was formed by a meteorite crashing into the earth and it is in an area where no glaciers formed during the cold periods of our recent earth history. That means the sediment layers lay undisdurbed. When you drill into them, you can read the sediment cores like a book and use them to reconstruct the history of the environment and the climate. One example would be the pollen and spores you find in the mud. They tell us what the vegetation was like around the lake. Since we know exactly how tolerant different plants are to temperature and rainfall, we can calculate how warm it was and how much rain or snow fell at different times.

What do these findings mean for climate research and modelling future climate change?

It tells us the situation in the northern hemisphere is probably not as independent of what's happening down in the Antarctic as we previously believed. For the first time, we have found this interaction between the north and the south pole. And that interaction has to be taken into account when it comes to forecasting the future climate. Especially since there are some clear indications that the West Antarctic inland ice is again retreating very rapidly. This process might speed up further in the coming years or decades. Then the geological history which we have unearthed could serve as a model for what we might possibly be seeing in the future.

So in the Arctic, it was once warmer than it is today even without our human-made emissions of greenhouse gases. Some might wonder if that means humans are not responsible for global warming after all?

I 'd say there is no doubt amongst respectable scientists today that human influence - especially emissions of greenhouse gases - are warming the planet. But as a geologist, it is also clear to me that the conditions on our planet are not static. Changes are happening all the time and happened all the time in the past. We have to know about those so that we can compare these natural variations with the changes being caused by humankind. The sum of the two will tell us what we have to expect in the future.

If the Arctic was able to survive warm periods in the past, some people could draw the conclusion that it's not so urgently necessary to avoid more global warming?

In many areas of the Arctic, climate change would not necessarily be bad for all those affected. You would be able to use the resources in the area, for instance have forestry. But that does not mean that climate change is fundamentally positive. Many parts of the earth would experience very negative impacts. I am thinking in particular of sea level rise. If the West Antarctic ice shield and the Greenland ice were to melt completely, we would be talking about a total of 12 meters (39 feet) of possible sea rise. Not even our sea walls in Hamburg would be able to cope with that.

Martin Melles is professor of geology and mineralogy at the University of Cologne in Germany. The drilling operation was carried out in 2009. Following detailed analysis of the sediment cores, the results have been published in the June edition of the magazine "Science".