The glacial cycles that have dominated the Earth's climate for millions of years are driven by changes in the Earth's orbit, reinforced by greenhouse gasses. But these changes occur over the course of tens of thousands of years, and we only have good global temperature records for the last 1,500 or so. This leaves questions about how the orbital cycles have interacted with shorter-term changes, like volcanic activity and changes in solar activity.

To provide a broader perspective on our climate, a team of researchers has reconstructed its history for the entire Holocene, the period that started with the end of the last ice age. The record shows that the Holocene temperatures largely followed the orbital forcings, peaking over 6,000 years ago and then gradually falling until roughly 1900. That's when the temperatures experienced a sudden reverse, going from among the coldest of the entire period to the warmest in less than a century.

The Earth's orbit and axis undergo cyclical changes, called Milankovitch cycles after the astronomer who first recognized them. These cycles cause changes to the amount and distribution of sunlight that strikes the Earth, changes that can raise or lower the average temperature of the planet. The result is what's called an "orbital forcing," which can drive long-term climate changes.

In recent history, orbital forcings have controlled the entry and exit to glacial periods. Although the amount of energy from the forcing itself is relatively small, it sets off a variety of feedbacks. Retreating ice sheets give way to open water and vegetation, which can absorb more sunlight and allow carbon dioxide to escape the deep ocean, ultimately causing a rise in greenhouse gasses. As the heating from orbital forcings slowly declines, these processes begin to reverse themselves. A recent study indicates that we were only about 1,500 years away from the onset of the next glacial period.

Coring into the past

But those are just the rough outlines of what happens. Within both glacial and interglacial periods, there are changes in volcanic activity, aerosols, and ocean currents that can alter the timing and extent of any temperature changes. To better understand our current interglacial period and whether recent temperature changes are truly anomalous, it would be helpful to have a complete picture of what the globe's been up to. That's precisely what the authors have done, using dozens of records spread around the globe that record the temperature history for thousands of years.

All told, the authors had access to 73 proxy records of temperature, mostly from marine sediment cores, each of which spanned at least 6,500 years of the Holocene. These provided records of temperature through things like isotope ratios and pollen records. The authors combined them to create a single global temperature record that spans roughly 12,000 years and thus extends back to the warming that ended the last ice age. Because of the resolution of things like sediment and ice cores, short-term variability of less than 300 years can't be resolved with this data, but the authors show that it seems to capture trends spanning 2,000 years and more very well.

That effectively means all the sorts of short-term events that have the impacts we're most familiar with—volcanic eruptions, changes in ocean currents, etc.—are averaged out. What remains is primarily the influence of the orbital changes.

The results make a number of things clear. To begin with, they overlap all the reconstructions of the last 1,500 years, at least within experimental error. These climate reconstructions, commonly known for showing a hockey stick pattern of temperature change (and most closely associated with the work of climatologist Michael Mann), have been the subject of a lot of controversy. But the new record joins a lot of other work indicating that a hockey stick pattern—with a sudden upswing of temperatures in the last century or so—is simply a feature of our planet's history. This won't make the controversy go away overnight, but it has become increasingly clear that there are more important things to talk about.

Another very obvious thing in the record is that our planet has (up until recently) been in a 5,000-year-old cooling trend. You will sometimes see people arguing that we've been warming since the end of the last ice age, but this simply isn't an accurate depiction of the data. Orbital forcings and temperatures did rise sharply at the end of the ice age, but they then remained relatively stable for about 5,000 years at about 0.6°C above the temperature of the last 1,500 years. Orbital forcings would have peaked about 9,000 years ago, and temperatures seem to have had a small peak about seven thousand years ago. But a steady decline started about 5,000 years ago, and it accelerated within the last thousand years, with a sharper drop associated with the period we call the Little Ice age. As a whole, this decline took the Earth down by about 0.7°C, dropping it below the average temperature of the last 1,500 years.

But within those gradual changes, there are some dramatic regional differences. By the same measure, the North Atlantic dropped roughly 2°C during the decline, suggesting it was especially sensitive to the decline in orbital forcings. Given that many of our historic accounts of the climate are biased towards the same region, this may give us a bit of a stronger reason to focus less on history and more on generating a truly global picture through the sort of temperature proxies that are used here.

A sudden reversal

The picture up to 1900 is consistent with the estimates that the best of the Holocene was behind us and we were cooling towards an inevitable re-glaciation. The authors calculate that the decade from 1900-1910 was cooler than more than 95 percent of all the other decades in the Holocene. But things pretty much ended there. As in the hockey stick reconstructions of the recent climate, this one shows a dramatic upswing in the century just past.

Although the most recent decade (2000-2009) isn't the warmest of the Holocene, it's not too far off. The authors estimate that it was warmer than 82 percent of the decades of the last 12,000 years. "Global temperature, therefore, has risen from near the coldest to the warmest levels of the Holocene within the past century, reversing the long-term cooling trend," the authors conclude. And based on records of things like solar output, ocean currents, and volcanic eruptions, there's little indication of anything other than greenhouse gasses that could have caused this sort of reversal.

Given the greenhouse emissions we've already produced, the authors also conclude that we're certain to exceed the warmest decades of the past sometime this century. The only scenario that would keep us from doing so is if we froze emissions around a decade ago. The real question seems to be how much we'll exceed these temperatures by. Continuing along an emissions trajectory similar to the one we're currently on, they suggest, means "by 2100, global average temperatures will probably be five to 12 standard deviations above the Holocene temperature mean."

In other words, it will be dramatically warmer than any point of the entire 12,000 year interglacial period, and no amount of statistical noise could account for the difference.

Science, 2013. DOI: 10.1126/science.1228026 (About DOIs).