The greenhouse effect of CO2 seems to become stronger with increasing global temperatures. Thus, global warming may have a previously overlooked cumulative effect, which may make the globe warmer than assumed.
By Jens Olaf Pepke Pedersen and Morten Garly Andersen
The global warming resulting from man-made greenhouse gases such as CO2 will not only depend on the scope of future greenhouse gas emissions, because their effect is also reinforced by water vapour, clouds, and other components in the Earth’s atmosphere. This phenomenon is called climate sensitivity and is usually defined as the warming that occurs when the amount of CO2 in the atmosphere doubles.
New research shows that climate sensitivity itself depends on global temperatures and that the effect of CO2 increases with increasing global temperatures. New analyses show that climate sensitivity was significantly higher during past warm climate periods. While climate sensitivity today is approx. 3.0 degrees, it was 5.1 degrees during past warmer periods. Combined with recent assessments, which show that climate sensitivity was only about 2 degrees during the last ice age, the study provides strong evidence that climate sensitivity increases with the Earth’s temperature and thus reinforces the global warming.
"If we want to limit temperature increases by limiting CO2 emissions, it makes a big difference whether a doubling of the CO2 concentration in the atmosphere results in a temperature increase of just 1.5 degree, and thus provides space for future emissions, or whether it increases the temperature by 4.5 degrees."
Senior Scientist Jens Olaf Pepke Pedersen, DTU Space
Warming may be reinforced
Researchers from the Niels Bohr Institute, DTU Space, and universities in the USA and Chile have recently published the results of these analyses in an article in the scientific journal Geophysical Research Letters. According to Professor Gary Shaffer from the University of Copenhagen and University of Magallanes in Chile, who was heading up the study, the result of the research is bad news for humanity, as “higher climate sensitivity to warming may make the Earth even warmer and thus to further reinforce the warming”.
Twenty-five years ago, researchers estimated climate sensitivity to be somewhere between 1.5 and 4.5 degrees—with 3 degrees being the most likely value—and that figure has not changed since then.
“If we want to limit temperature increases by limiting CO2 emissions, it makes a big difference whether a doubling of the CO2 concentration in the atmosphere results in a temperature increase of just 1.5 degree, and thus provides space for future emissions, or whether it increases the temperature by 4.5 degrees, in which case there is an urgent need to stop the emissions,” says Senior Scientist Jens Olaf Pepke Pedersen, DTU Space, one of your co-authors of the new scientific work.
Help from past climate
Climate sensitivity is difficult to measure, because it depends on a number of properties of the Earth's climate system, about which there is great uncertainty, among other things about the extent of the cloud cover and the composition of clouds.
The researchers have therefore reconstructed a known episode of global warming 56 million years ago by means of the Danish Center for Earth System Science climate model. The episode—known as the Paleocene–Eocene Thermal Maximum (PETM)—was triggered by massive emissions of carbon to the atmosphere, and has long been singled out as a possible analogy to today’s global warming.
The reconstruction of past temperatures shows that the Earth—already before PETM—was around 10 degrees warmer than today, and that temperatures rose by an additional 5 degrees during the PETM. The study combined a number of mineralogical and other data from PETM with model runs to calculate the atmospheric concentration of CO2 before as well as during the PETM episode; and it was also possible to assess the source of the CO2 emissions.
Based on these data, it was possible to calculate climate sensitivity. The result showed that while it is approximately 3.0 degrees today, it was 4.5 degrees in the period before PETM and no less than 5.1 degrees during PETM.
Stronger effect of CO2 from now on
The study also shows that the amount of carbon driving the warming during the PETM period, was of the same size as today’s reserves of fossil fuels, i.e. around 4,000 billion tonnes.
“When this amount of carbon after all only resulted in a 5-degree rise in temperatures, this is due to the fact that there was already large amounts of CO2 in the atmosphere of the past—around 1,000 ppm (parts per million),” says Jens Olaf Pepke Pedersen.
“Today, where the atmosphere contains far less CO2—approx. 400 ppm—an additional amount of carbon of similar size will have a much larger effect, because it will multiply the CO2 concentration".
The picture above the article shows Bispehuen on the Limfjord island of Fur, which has tall moler cliffs with numerous layers of ash from the enormous volcanic eruptions that took place in the transition from Paleocene to Eocene 56 million years ago, where the Earth was much warmer than today. Bispehuen was created in connection with heavy volcanic eruptions around current day Iceland. (Photo: K.P. Pedersen).