The Guardian says this about this paper just out in Nature: The planet is hotter now than it has been for at least 12,000 years, a period spanning the entire development of human civilisation, according to research.
Analysis of ocean surface temperatures shows human-driven climate change has put the world in “uncharted territory”, the scientists say. The planet may even be at its warmest for 125,000 years, although data on that far back is less certain.
The research reached these conclusions by solving a longstanding puzzle known as the “Holocene temperature conundrum”. Climate models have indicated continuous warming since the last ice age ended 12,000 years ago and the Holocene period began. But temperature estimates derived from fossil shells showed a peak of warming 6,000 years ago and then a cooling, until the industrial revolution sent carbon emissions soaring.
"The background here is that the northern hemisphere is more important for global mean 2-m temperature then the southern hemisphere. This is because of the large land mass, while the ocean is thermally more inert. As a result, the position of the ellipse in which the earth orbits around the sun has an effect on global mean temperature. In the holocene, actually more 8000 then 6000 years ago if I remember correctly, the earth was nearest to the sun at the height of the northern hemisphere summer. That made arctic summers warmer and allowed for example neolithic hunters to thrive in places like northern Norway. Another effect was that the Sahara was rather a dry savanna, a bit like Australia nowadays, as opposed to the extreme desert it was now. The question was how much that effect global mean temperature. But: all these things are relatively well understood insofar as they do not concern any changes in the greenhouse gas load of the atmosphere and are entirely orbital driven. (If you ignore Ruddiman's hypothesis that early humans already increased atmospheric CO2, but that would not explain cooling.)"
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