Professor Dorthe Dahl-Jensen receives the Frontiers of Knowledge Award

Understanding Greenland's past to understand the Earth’s future climate

Dorthe Dahl-Jensen's contributions lie primarily in the reconstruction of past climate from the study of Greenland ice cores, as written up in a 1998 paper published in Science. “The knowledge from the past preserved in ice cores is important for understanding what can happen in the future,” she explains, “because most modeling work is based on records from weather stations that only reach 150 years back in time, a very stable climate period. And we know from the past that this is not the full story.”

“The ice gives us both temperature, through stable water isotopes, and greenhouse gas levels, through the air trapped inside,” Dahl-Jensen relates. Her research has found that past temperatures rose during periods of increased solar energy influx, which in turn increased CO₂ in a positive feedback loop: “We know that when temperature rises, it warms the ocean, which releases CO₂ into the atmosphere. These higher levels of CO₂ warm the atmosphere and ocean even more, and then CO₂ starts to rise again. So we get the warming we have observed in the climate changes of the past.”

But what Dahl-Jensen’s research has revealed is that today’s greenhouse gas concentrations are unmatched in the record of the past 800,000 years. “Even though there have been previous warm periods, like we had 115,000 years ago, where the temperature was actually four degrees warmer, we never saw CO₂ values higher than 300 parts per million, while the average today is 420 ppm. So it is us who have brought about these heightened values of CO₂ that are driving up temperatures.”

This is why she is so concerned about the potential impacts of human-induced global warming, some of which are probably already unstoppable: “We know that CO₂ stays in the atmosphere for more than 100 years, so it's going to be very hard to stop temperatures rising with the greenhouse gas concentrations we have right now.” In fact, the Danish scientist believes some tipping points have already been passed, like the inevitable disintegration of the large Antarctic ice shelves that is visibly underway.

Dahl-Jensen also points out that, based on the results of her research on abrupt changes in past climate, there is a risk that the pouring of fresh water into the ocean, due to the ice melt, could disrupt the ocean currents that keep Europe temperate. “The Gulf Stream is important because it warms northern Europe, Denmark and Spain as well. And if it shuts down due to global warming, it will strongly impact climate in our countries, though ironically enough, it will cool the temperature.”