After Earth's mean surface temperature climbed sharply through the 1990s, the increase has levelled off nearly completely at its 2000 level. Ocean warming also appears to have stabilised somewhat, despite the fact that CO2 emissions and other anthropogenic factors thought to contribute to global warming are still on the rise.

Uncertainties about the overall results of feedback mechanisms make it very difficult to predict just how much of the rise in Earth's mean surface temperature is due to manmade emissions. According to the Intergovernmental Panel on Climate Change (IPCC) the climate sensitivity to doubled atmospheric CO2 levels is probably between 2°C and 4.5°C, with the most probable being 3°C of warming.

In the Norwegian project, however, researchers have arrived at an estimate of 1.9°C as the most likely level of warming.

"The Earth's mean temperature rose sharply during the 1990s. This may have caused us to overestimate climate sensitivity.

"We are most likely witnessing natural fluctuations in the climate system changes that can occur over several decades and which are coming on top of a long-term warming. The natural changes resulted in a rapid global temperature rise in the 1990s, whereas the natural variations between 2000 and 2010 may have resulted in the levelling off we are observing now."