Climate science interview

Deutsche Welle: Scientists have been taking stock of what we know about climate change and its impacts ahead of December's UN meeting to come up with a new world climate agreement. Where do we stand?

Stefan Rahmstorf: Scientific evidence shows us that we need to turn around the tide of rising emissions within the next few years - by 2020 at the latest - so that we actually stay below 2-degree Celsius [3.6 degrees Fahrenheit] warming. This is a real danger level, where global warming may well become unmanageable, and the risks simply too high.

The latest scientific consensus seems to tell us even the international goal of limiting global warming to 2 degrees would be too high, and 1.5 would be more appropriate. Is there any likelihood of us achieving that?

It is indeed true that given the latest knowledge on "tipping points" in the climate system - and the kind of surprises that can come from destabilizing ice sheets or changes in the ocean currents - this really calls for limiting warming to 1.5 degrees, which is still feasible. But that requires much stronger political will than we currently have.

It's almost two years since the last IPCC report. Since then, have there been new developments relating to those ice sheets or ocean currents?

Yes. New results have come out showing that the West Antarctic ice sheet is in some areas already past its tipping point. There's also been a very interesting analysis that shows what happens when you take this into account. This means the "cost-effective emissions pathway" involves limiting warming to just 1.5 degrees. Previous estimates said we could go to 3 degrees, and then just spend money on adaptation. But that doesn't seem to be the best option any more.

So will this information find its way into the UN climate conference in Paris at the end of the year?

It will probably not find its way into any official documents, but of course the governments and delegations there are aware of the science as it evolves. I know for sure that the experts in the German government who go to these meetings are aware of the new scientific findings.

You and some colleagues have just published a new study on sea level rise. What does that tell us?

We we find that invariably, during warmer times the sea level was much higher. It was at least about 6 meters higher than today - even though temperatures were only a little bit higher, maybe 1 to 3 degrees warmer compared to the pre-industrial climate.

So basically the message is: the kind of climate we are moving toward now - even if we limit warming to 2 degrees - has in the past always been associated with a sea level several meters higher, which would of course have catastrophic consequences for many coastal cities and small island nations.

Do you think this knowledge will influence what happens at the Paris UN meeting - COP21 - in December?

The good news is that in terms of the solutions available to us to stop global warming, things are looking much more optimistic - for instance energy solutions. The development of renewable energies - wind, solar and others - has really surpassed the most optimistic expectations. And so there is hope that if we really put our will to it, we will be able to stop global warming below 2 degrees, and hopefully closer to 1.5 degrees.

What would governments have to decide in Paris to make that happen?

Most governments have already put forward their commitments for reducing their own emissions ahead of the Paris summit - and these are not quite enough to limit warming to 2 degrees. But they are enough to really make a difference and set us on the right track of falling emissions. And that's what I really expect from Paris.

The hope is rising that Paris in December will be seen as a turning point, after which emissions start to drop globally, relatively soon. And of course targets will have to be revised at regular intervals, and gradually strengthened again, so that they are sufficient to stay below those 2 degrees. So I think reaching that turning point is important now, even if what is agreed in Paris will fall short of completely solving the issue.

We are experiencing an extreme heat wave here in Europe at the moment. Is this an effect of climate change?

Yes, absolutely. We have done an analysis for the whole globe, looking monthly at record-breaking heat. This kind of heat now occurs five times as often as you would expect in an unchanged, stable climate. So in that sense, most of these heat waves now would not have occurred without the global warming caused by us.

Do you think the extreme heat has affected people's awareness of climate change?

I think so. These increasing numbers and intensity of extreme weather events also apply to extreme rainfall events, and droughts and floods, that get worse due to sea level rise. My hope is that people really start to notice not only that climate is changing, but also how it is really having a negative impact on people.

And that's what it's all about. Climate protection is not about some nature protection issue. It is about protecting people from the massive risks we are facing, if the climate changes further.

Stefan Rahmstorf is a professor of oceans physics at Potsdam University, and head of earth systems analysis at the Potsdam Institute for Climate Impact Research in Germany.