Principal Investigator
At a Glance
Studies of ice cores from Greenland show that the Greenland ice sheet has persisted for at least 1 million years. This result puts limits on the sensitivity of the Greenland ice sheet to climate change, and provides a test for models of the ice sheet.
Research Highlight
In 2015, the Bender group completed a study of properties of the Greenland ice sheet during the last interglacial period (about 120,000–130,000 years ago) and earlier times. The ultimate motivation is to understand how much Greenland ice will melt, and thus cause sea level to rise, due to global warming.
The study included ice from the last interglacial period preserved in the GISP2 ice core drilled through the center of the Greenland ice sheet around 1995. This core contains a continuous climate record extending back to about 105,000 years ago. Below this continuous archive, there are about 200 meters of clean glacial ice with an intact climate record, but out of stratigraphic order due to the flow of the glacier. Some of this ice formed at temperatures (inferred from the isotopic composition of the ice) as least as warm as today, and is understood to have originated during the last interglacial period.
The Bender group dated all such samples by measuring the concentration of methane and the isotopic composition of oxygen in fossil air trapped in the ice. Antarctic ice core records extend back 800,000 years, and reveal how these two properties have changed with time, allowing researchers to date each ice sample. Plotting climate properties against time allowed reconstruction of the climate record at the center of Greenland during the last interglacial period.
This work shows that temperature at the summit of Greenland, in the center of the landmass, rose to its present value (-31˚C mean annual temperature) about 127,000 years ago, and continued rising until the temperature was about 5˚C warmer than during the current interglacial period. Temperatures began to fall around 120,000 years ago.
According to a related modeling study, this warming would have melted about half of the Greenland ice sheet, and meltwater flowing into the oceans would have raised sea level by about 4 meters. This rise would have occurred toward the end of the warmest period in our record, around 120,000 years ago, and such a sea level maximum has been observed in ancient beaches at several locations around the world.
At the very bottom of two other Greenland ice cores, there is about 5 meters of ice that was deposited under warm temperatures at some unknown time in the past. We have developed a method to date this ice based on the isotopic composition of argon in the fossil air. This property has continuously changed with time at a known constant rate. Argon dating of ice from the Dome C core, from Southern Greenland, shows that ice was present in this region during the last interglacial period. In other words, the melting of Greenland did not progress to the point where the vulnerable southern region was deglaciated.
In an ice core taken from the summit, in central Greenland, ice at the base of the core dates to at least 1 million years of age. This great antiquity is further evidence that Greenland was not completely deglaciated during the last interglacial period, during a previous, longer interglacial period around 400,000 years ago, or during the shifts in climate patterns (for example, to somewhat higher carbon dioxide (CO2) concentrations and less cold glacial periods) observed between 800,000 and 1 million years ago.
This work shows that, with the level of warming possible in the coming centuries, melting of the Greenland ice sheet may contribute several meters to sea level rise over several thousand years. The results also present a target for ice sheet and climate models that can be tested against these observations. The evidence is encouraging, in that it demonstrates that no climate change over the past million years was sufficient to melt the ice sheet entirely.
Figure 1.3. On July 12, 1992, Sigfus Johnsen of the University of Copenhagen triumphantly holds up the deepest section of the GRIP core, at 3,029 meters depth, drilled through the ice at the summit of the Greenland ice sheet. The brown color is due to dirt in the ice. The dirt originates from soil, lake water, bogs, and mud, and dates roughly to the time when the ice sheet formed. For 20 years, the age of this ice remained unknown for lack of an accurate dating method. Michael Bender recently invented a method based on the isotopic composition of argon in trapped air, applied it to this 24-year-old ice archived in freezers in Copenhagen, and found that this ice is at least 1 million years old.