A new reconstruction of the Earth's climate history – dating back 11,300 years – found that the planet has rarely been warmer than it is today during that time, and that temperatures are likely to climb into unprecedented territory by 2100, due to increasing amounts of planet warming greenhouse gases in the air.
The study, published Thursday in the journal Science, adds weight to the now famous ‘hockey stick’ graph that Michael Mann published more than a decade ago. That study showed a sharp upward temperature trend over the past century after more than a thousand years of relatively flat temperatures.
But the new report extends that research back much further, using evidence from the seafloor and from lake sediments to gauge past temperatures, not the tree rings previous researchers have used.
“What’s striking,” said lead author Shaun Marcott of Oregon State University in an interview, “is that the records we use are completely independent, and produce the same result.”
The study is also truly global, based on records from 73 different locations around the world, not just regional.
“As far as we know, this is the first time this has been done for the entire Holocene,” Marcott said. That’s the name of the period since the last of the great Ice Age glaciers melted back, which coincides with the rise of civilisation.
The research aimed to understand whether the current warming is unprecedented in the Holocene period, or whether the same thing might have happened before, because of purely natural causes.
Based on the evidence, it has not. Since the ice sheets departed, the warming trend was found based on the chemical composition of ancient shellfish called foraminifera, variation in types of pollen extracted from lake sediments and other temperature-dependent measures. The data shows there was a long, gradual warm-up for about 5,000 years, then a plateau of warm temperatures, and then an equally gradual cooling trend until about 200 years ago.
Marcott said that is in line with the gradual changes known as Milankovitch Cycles, caused by the Earth’s tilt and its orbit around the Sun. Based on where we are in those cycles, Marcott said, the planet should still be cooling. Not only is the Earth warming, it is warming much faster than a Milankovitch Cycle could possibly explain.
“The temperature change is both too abrupt, and going in the wrong direction, to be natural,” Marcott said.
This does not vary from the conclusions of other major reports, like the one issued by the Intergovernmental Panel on Climate Change (IPCC) in 2007, which stated, “There is very high confidence that the net effect of human activities since 1750 has been one of warming.”
While that was enough to convince most scientists of the need for swift and drastic action in cutting back greenhouse-gas emissions, the new report adds a deeper layer of evidence that could help convince policymakers.