Around 20,000 years ago, large parts of the northern hemisphere were covered by glaciers – it was the climax of the last ice age. But how cold it was on earth at that time is still fraught with great uncertainty. Now researchers have narrowed down the temperatures at that time with the help of geochemical indicators and climate models. According to this, the global mean temperature at the height of the last Ice Age was around 6.1 degrees lower than in the current Holocene. On the basis of this data, the scientists have also recalculated the sensitivity of the earth’s climate – the climate’s reaction to changes in greenhouse gases. For every doubling of the CO2 value, the temperatures rise by 3.4 degrees.
The last ice age had a decisive influence on the development of many landscapes and also the living environment of our latitudes. At the same time, it is the youngest of the many glaciations that our planet has experienced in the course of the earth’s history. “The last glacial maximum is one of the best studied paleoclimatic time periods because it offers excellent opportunities to study how the climate system reacts to changes in the cryosphere and greenhouse gases,” say Jessica Tierney of the University of Arizona at Tuscon and her colleagues. At the same time, however, the climate of this phase is difficult to determine more precisely. Scientists can only infer the conditions at that time indirectly, via fossil or geochemical “witnesses”. Marine microfossils such as foraminifera, diatoms or radiolarians, whose species spectrum allows conclusions to be drawn about sea temperatures, provide information. The isotope values of their shells can also allow conclusions to be drawn about the climatic conditions.
On average it was six degrees cooler
But such “contemporary witnesses” are not available in all regions and their evaluation involves great uncertainties, as the researchers explain. As a result, the range of estimated values for Ice Age temperatures is wide: it ranges from just 1.7 degrees less than the mean of the post-Ice Age Holocene to eight degrees less. To narrow this range, Tierney and her team have now also evaluated four different geochemical indicators, including the ratio of magnesium to calcium in fossil plankton, the content of certain hydrocarbons and the oxygen isotope 18-O. They fed a good 1700 such data for different locations into a climate-ocean model, which enabled the detailed reconstruction of sea and land temperatures during the last ice age. Accordingly, the mean temperature of the oceans fell by 2.4 to 2.9 degrees, the mean value was 3.1 degrees lower than in the Holocene. “This range is narrower than that of the earlier models, which ranged from minus 2.7 to minus 4 degrees,” the researchers said.
For the global mean air temperature, Tierney and her team determined that it was 6.1 degrees cooler at the height of the last Ice Age than in most of the Holocene. “It doesn’t sound like much by our everyday standards, but it’s actually a huge change,” says Tierney. If you take into account that the global mean temperature is only 14 to 15 degrees today, six degrees less is a drastic cooling. At the same time, this value agrees well with previous studies, but limits the uncertainties more, according to the researchers. Because the confidence interval determined by them, which comprises 95 percent of the values, is between 5.7 and 6.5 degrees.
Regional differences
But depending on the region, there were clear differences both on land and in the oceans. It was particularly cold in the North Atlantic, North Pacific and parts of the South Atlantic, where the water at the last glacial maximum was more than eight degrees cooler than today. Similar to today, the polar latitudes also reacted more strongly to climate changes than the tropics. Researchers refer to this increased sensitivity as polar amplification. “Today’s climate models predict that high latitudes will warm up faster than low latitudes,” says Tierney. “During the last ice age, polar amplification had the opposite effect – it got much colder there.” On land, the high and temperate latitudes of the northern hemisphere were the coldest – with two interesting exceptions: “We see remarkably little cooling in Alaska and the west of the Bering Land Bridge,” report Tierney and her colleagues. “This agrees well with observations that these areas remain ice-free and only cooled minimally.” The land bridge is also the region where the first people to move from Asia to America might have stayed at that time.
On the basis of their models and the base values for the Ice Age, the researchers also determined how sensitively the climate reacts to changes in greenhouse gas concentrations in the atmosphere. During the last ice age, atmospheric carbon dioxide levels were around 180 parts per million (ppm). Today they are a good 400 ppm. With the help of her model, Tierney and her tram calculated a climate sensitivity of 3.4 degrees. This means: for every doubling of the CO2 values, the global mean temperature rises by around 3.4 degrees. The confidence interval for this value is 2.5 to 4.3 degrees. “This narrowed this range significantly more than in the last world climate report of the IPCC, which assumed one to six degrees per doubling of the CO2 value,” say the researchers. Their results not only provide more insight into the ice age climate, but could also help to more precisely predict future climate developments.
Source: Jessica Tierney (University of Arizona, Tucson) et al., Nature, doi: 10.1038 / s41586-020-2617-x