Over the past century, global average temperatures appear to have risen faster than at any time since the end of the last ice age 11,300 years ago, and perhaps longer. Meanwhile, the magnitude of the increase has been unmatched in at least the past 4,000 years.
Researchers say those are the implications of a new study that uses natural stand-ins for thermometers to trace temperature trends back to the beginning of the current warm, interglacial period. Significantly, the study’s findings suggest the current warming trend cannot be explained by some forms of naturally occurring temperature variability, a lingering issue in the debate over the impact of human activity on global warming. [Editor's note: The reference to temperature variability has been revised.]
The main trigger for the current warming trend, especially since the middle of the last century, has been rising emissions of heat-trapping carbon dioxide as people burn fossil fuels and change land-use patterns, researchers say.
Although other so-called paleoclimate records reach farther back into geological time, the team focused on the Holocene epoch, in which human civilizations emerged and evolved.
"To our knowledge, based on this reconstruction, the rate of change today is unprecedented" in the Holocene, says Shaun Marcott, an atmospheric scientist at Oregon State University who led a team formally reporting the results in Friday's issue of the journal Science. Indeed, it may be unprecedented in the past 22,000 years, he adds, when previous paleoclimate research he and his colleagues have conducted is taken into account.
Other researchers have focused on the Holocene as well, notably Michael Mann, a Penn State University climatologist, and his colleagues. But their reconstructions have taken the record back only about 1,500 years.
The new work, using different thermometer stand-ins, or proxies, not only reaches results similar to these previous efforts covering the recent past. It also accounts for natural variations in climate over longer time scales in ways that suggest rising temperatures will exceed the range of natural fluctuations. The long-term variations would include changes in Earth’s orbit, for instance.
Based on the reconstructed temperatures records, natural variability over the study's time span accounts for roughly 1 degree C from coldest to warmest compared with the current climate, observes David Anderson, branch chief for the Paleoclimate Program in at the National Oceanic and Atmospheric Administration's National Climatic Data Center office in Boulder, Colo.
"If you go grab the mount of warming expected just within the next 80 years, that's more like 3 degrees," says Dr. Anderson, who was not a member of the study team – three times the change one would expect from natural variability alone, and all in the warm direction.
According to the reconstruction, global average temperatures increased by about 0.6 degrees Celsius (1 degree Fahrenheit) from 11,300 to 9,500 years ago. Temperatures remained relatively constant for about 4,000 years. From about 4,500 years ago to roughly 100 years ago, global average temperatures cooled by 0.7 degrees C.
But over just the past century, the climate recouped the lost warmth – driven to an increasing degree by rising carbon-dioxide emissions from burning fossil fuel and from land-use changes.
The team estimates that now, even if CO2 emissions follow the most optimistic path envisioned by the UN's Intergovernmental Panel on Climate Change, by 2100 global average temperatures will set a Holocene record. To date, only about 25 percent of the Holocene period has seen higher average temperatures than exist on Earth today, the researchers calculate.
The new study is important, according to Michael Mann, a climate scientist at Penn State University in University Park, Pa., who was not part of the team but who has used proxy records for more than a decade to reconstruct Holocene temperature records.
"The real issue, from a climate-change impacts point of view, is the rate of change, because that's what challenges our adaptive capacity," he writes in an e-mail exchange. "This paper suggests that the current rate has no precedent, as far back as we can go with any confidence."
Beyond the broad temperatures trends, the study also shows how warming affects key features of the climate system that have a more direct influence on people than global averages, adds NOAA's Anderson.
For instance, the data suggest that for a temperature increase of about 0.5 degrees C., the average amount of rainfall during the Asian monsoons increases, he says. The paper also notes the effect temperature changes have on displacing a zone of heavy tropical rainfall that runs like a belt around the world, currently hovering around 10 degrees north of the equator.
These regional aspects of the paper are less-well quantified than the overall temperature trends, notes Anderson. But they still serve to illustrate "that as climate changes, significant aspects that affect people and regions and economic sectors like agriculture are going to change too," he says.
The team looked at a range of records involving thermometer stand-ins from 73 locations around the globe. Those stand-ins ranged from pollen samples and ice cores to microfossils and growth rings in stalactites and stalagmites in caves.
Some indicators recorded average temperatures at roughly 20-year intervals, while other records could provide a reading that amounted to a 500-year average. The average interval over all the records was around 120 years.
By using so may different forms of proxies from so many areas, the team not only paints a global picture of temperature changes during the past 11,300 years, it also does so in a way that reduces the uncertainties that would come from using a smaller set of proxies, Anderson says.
Still, he says, proxy records carry uncertainties tied to the uncertainties in dates for them, so it can be hard to identify rapid changes in temperatures across a broad geographic range.
Dr. Mann identifies another issue: a set of records that are more numerous for the northern hemisphere summer than for other seasons and the southern hemisphere – essentially because the northern hemisphere hosts most of the planet's land mass and people to explore it.
Changes in Earth's orbit over time have subjected the northern hemisphere summer to a substantial long-term cooling trend, he says. It could well be that after accounting for that bias, the warming the climate is experiencing globally today may be even more pronounced.
Researchers may have to look as far back as the last interglacial period, the Eamian period 125,000 years ago, to find comparable absolute warmth, he says.