It's global warming, stupid. And how. In an extensive study published in today's issue of Science, researchers report that today's global temperatures are warmer than any point in at least the last 4,000 years, and show no signs of declining: analyses of weather patterns since the end of the last Ice Age presage that the coming century will see Earth's average surface temperatures soar to intensities greater than any point in human history.
The study, led by Oregon State University paleoclimatologist Shaun Marcott, provides us with the most detailed climate reconstruction ever produced for the last 11,300 years, and accounts for all but a sliver of the modern geological era. That era, called the Holocene, kicked off roughly 12,000 years ago, and has been witness to a host of monumental global changes, including the rise of human civilization around 8,000 years ago. Point being: this reconstruction is absolutely unprecedented in scope. Previous studies have rarely examined beyond the last 2,000 years of Earth's climatic history. Marcott and his colleagues have pushed back our knowledge of global climate by thousands and thousands of years.
Climate data can be derived from many sources — ice cores, cave formations, coral reefs, even the shells of marine organisms — but all of them carry chemical and physical signatures that provide researchers with a reliable record of the planet's past climate. Marcott and his team worked by combining climate record data from around the globe into one vast stockpile of information. The patterns that emerged were extremely telling of a modern spike in global temperatures.
Worldwide temperatures are higher today than at any point in the last 4,000 years, and warmer than about three quarters of anything we've seen in the last 11,000. The study reports that, on the heels of the last Ice Age, global temperatures rose gradually until around the middle of the Holocene, at which point a cooling trend (also gradual) dominated the planet for roughly five millenia. But that all ended around two-hundred years ago. Ever since, temperatures have risen. Steadily at first, then rapidly over the course of the last century or so.
Climate scientists have been communicating the last century's global temperature spike with variations of the so-called "hockey stick" graph for around a decade now (imagine a line graph, fluctuating gradually over the course of many years, only to rise sharply in the shape of a hockey stick's blade — see below). The results of Marcott's team corroborate the shape of the hockey stick, while placing it in a much more extensive historical context. In doing so, they've demonstrated that the warming we're curently experiencing is, in fact, unique — and over an even longer period that previously believed. [Below: Comparison of several different methods and reconstructions of global and hemispheric temperature anomalies, via Marcott et al.. Notice a pattern? Spoiler: it's the vertical spike at the right hand side of each graph.]
That context demonstrates two things pretty unambiguously. One: it shows that there are, in fact, points in the Holocene when humans endured temperatures warmer than they are today; taking the flipside of the 75% statistic cited earlier, roughly 25% of the last 11,300 years (all of them earlier than 4,000 years ago) have been warmer than they are today.
Two: By the end of the century, that will no longer be the case. If the planet continues to warm in the fashion that Marcott's team's data suggests, surface temperatures will almost certainly surpass any and all record-highs from the last 11,300 years. "Intergovernmental Panel on Climate Change model projections for 2100 exceed the full distribution of Holocene temperature under all plausible greenhouse gas emission scenarios," write the researchers. Translation: even if our planet's temperature increases wind up being on the low end of estimates, Marcott and his colleagues say the planet will be at least as warm as the Holocene's absolute toastiest periods. (In an interview with the NYT, climate expert Michael E. Mann, who was not involved in the research, said Marcott's team "made conservative data choices in [its] analysis.")
The models presented by Marcott's team suggest that, in the absence of human-induced climate change, the Northern Hemisphere would likely freeze over in a few thousand years. But he and other climatologists believe that humanity's continued impact on global temperatures will almost surely prevent that from happening.
As for the temperature increases projected to occur in the coming centuries, it's important to note that as we approach, and exceed, climate records set all the way back at the beginning of the Holocene, the temperatures themselves are not nearly as problematic as the rate at which we are reaching them — a point aptly summarized by Mann in his interview with the Times:
"We and other living things can adapt to slower changes," he said. "It's the unprecedented speed with which we're changing the climate that is so worrisome."
The researchers' findings are published in today's issue of Science.