Scientists have a good sense of what the Earth’s climate has been like over the past handful of centuries. But what about many thousands of years back?
In a recent article in the journal Science, researchers at Oregon State University and Harvard explained how they used marine fossils to piece together a rough temperature record going back 11,300 years to the most recent ice age. That record indicates that the Earth warmed as it emerged from the ice age, followed by a long-term cooling trend. The cooling continued until the industrial revolution, when humans began burning fossil fuels in earnest — and pumping heat-trapping greenhouse gases such as carbon dioxide into the atmosphere. Then the temperature spiked. With the amount of carbon dioxide already in the atmosphere, models suggest, the Earth’s average temperature in 2100 will surpass that detectable in any of the millennia studied.
The most dramatic implication of the study is not the magnitude of the current warming but its extremely rapid pace. The authors note that the Earth has warmed over the past century as much as it cooled over several millennia before that, with no similar spike detectable in the data. That finding would be yet more strong evidence against climate skeptics who cling to the wishful thinking that global warming is an entirely natural phenomenon.
The scientists have more to do. The recent warming has been so quick that it’s still hard to tell whether there were any similarly brief and sharp temperature spikes many thousands of years ago that quickly abated. Michael Mann, a prominent climate scientist, suggested to the New York Times’s Andrew Revkin that the temperature estimates match more robust records of recent temperatures so well that their conclusions should be taken seriously. They should also spur more research.
As scientists continue to refine their sense of how the climate behaved since the most recent ice age, Mr. Revkin points out that perhaps the biggest unknown that will affect the future of the climate is human behaviour. Debates will continue about exactly how sensitive the climate is to greenhouse emissions from the burning of coal, oil and natural gas. But the evidence does not allow policy-makers to ignore scenarios that predict very serious warming over the next century if world leaders — including policymakers in the United States — continue to do too little.