Prize recipient 2022 | Dorthe Dahl-Jensen

What is your research field?

I look at what the climate was like in the past, and what we can learn from that. This could help us understand what will happen in the future with global warming. I’ve helped drill most of the deep ice cores from the Greenland ice sheet. These enable us to follow numerous parameters that tell us what the ancient climate was like year by year as we get deeper and deeper down into the ice. 

The Greenland ice cores contain layers up to 250,000 years old. They also contain bubbles of atmospheric air from the time the snow fell. By testing this air, we can see how levels of greenhouse gases have changed over the years. The ice sheet is currently losing mass, and I’m particularly interested in how the different ice streams contribute to rising sea levels, and how the fresh water from increased ice melt changes conditions in the ocean. This is interesting because, among other things, it affects the lives of the people living on the coasts of Greenland and Canada.

What are the challenges and prospects for your research?

The challenges in my research come from drilling deep ice cores in the Greenland ice sheet. Each of the six ice core projects I’ve been involved with there have taken four to seven years and been carried out by international research teams. They’ve always been led by the Ice and Climate group at Copenhagen University, and we’ve been responsible for building and running ice camps for around 30 researchers. We’ve also developed the ice core drills that are used. We’ve drilled the actual ice cores, and we’ve developed many of the test methods that we use.

The research prospects are considerable, as there is still much we don’t understand about the climate system. The climate models normally used to predict tomorrow’s climate struggle to model the huge climate shifts we’ve seen in the past. Will the models be able to tell us whether there’s a risk of a similar shift in the future now that our climate is changing as much as it is? Will the huge inflow of fresh water into the ocean lead to major climate shifts? Will the coming warming cause large parts of the Antarctic to melt? And how will the rise in greenhouse gas concentrations affect the climate system?

How did you become interested in this research field?

I’ve always loved being out in the wilds, and I’ve enjoyed mountain and glacier hiking since I was young. When I finished school, I saw in the prospectus that studying Ice and Climate would require fieldwork. I was sold – and have been ever since. I love that this research field is still so small that I can take part in everything from planning and leading the projects, helping with the actual drilling, analysing the samples, interpreting the results and publishing in the best journals. 

I find it fascinating that you can squeeze so much information about the climate system out of such clear and seemingly pure chunks of ice – that we can reconstruct temperatures and precipitation levels in prehistory, that we can see dust from China, how the sea ice has changed, what levels of solar radiation and greenhouse gases were like back in time, and much more. The ice sheet is the perfect place to preserve climate information – it’s cold, and the ice is so pure that the few impurities there are remain unchanged for thousands of years.

What are the greatest insights or discoveries you have made?

The greatest insight has been that the climate was so unstable during the last ice age, with 25 events lasting 1,000 to 3,000 years where there was a sharp increase in temperature (up to 15°C in Greenland) followed by gradual cooling. By drawing comparisons with ice cores from the Antarctic, we can see differences between what happens at the two poles. When there’s cooling in the north, there’s warming in the south. The planet’s warmth (energy) flips between north and south, and these events are associated with big changes in inflows of fresh water from icebergs and ice melt, ocean currents that start and stop, and substantial increases in sea levels. We’re still working on fully understanding these flips and whether there’s a risk of them starting again with the climate changes we’re now seeing.

Another major discovery we’ve made is that in the last interglacial period, the Eemian, from 130,000 to 115,000 years ago, Greenland was 5°C warmer than it is now. During this warm period, the ice sheet did lose mass, but "only" 25 per cent, equivalent to a 2 metre rise in global sea levels. So the ice sheet can withstand a fair amount of warmth without disappearing, which has to be seen as good news in the current climate.

What does it mean to you to win the Carlsberg Foundation Research Prize?

It means an awful lot! It’s a huge honour of which I’m truly proud. But all of the insights and discoveries I’ve made have been a result of teamwork, so I want to extend my thanks to all who’ve contributed – from the logistical and drilling people to the scientists and students. My Danish research group are a highly dedicated bunch, and this prize shows that our research is important and makes a difference. 

The prize money will help support the Danish side of our upcoming project to drill an ice core through the Müller ice cap on Axel Heiberg Island in Arctic Canada. This is a joint project between Canada and Denmark looking at how the sea ice in the Arctic Ocean has changed over the past 10,000 years.

Tell us about your background, family and leisure interests

I come from a family of four children. We lived in Geneva during my childhood, because my father worked at CERN. I went to an international school and spoke English more than Danish until I was 12. I’m married to Jørgen Peder Steffensen, who is also a professor at the Niels Bohr Institute and works on the ice core projects. So we’re a pretty nerdy couple who talk a lot of shop at home! 

We have four children, three of whom have studied physics at the Niels Bohr Institute, while the youngest has just finished studying at the Animation Workshop in Viborg. They’re all doing really well and have been with us to Greenland many times, especially Kangerlussuaq. Our spare time is probably mostly taken up with ice cores, but we also enjoy walking at our farm in Sweden, books, movies and good food. During lockdown, I designed and built a treehouse in our apple tree.