The world is headed for average surface temperatures warmer than it has seen in at least 11,300 years. That’s one conclusion US researchers have reached after bringing together 73 studies of ancient climate from across the world into a single global record. Their work supports previous records for the past 2,000 years built mainly from tree ring data, explained Shaun Marcott from Oregon State University, and gives a much broader view.
“We can put today’s global temperature into context against the entire Holocene period,” Shaun told me. “That’s when human civilisation was born, developed and progressed to today.” Modern temperatures are higher than in around three-quarters of that period, which reaches back to the end of the last ice age. And their comparison against forecasts for 2100 made in models used by the Intergovernmental Panel on Climate Change (IPCC) is even starker. “If those scenarios come to fruition, we’ll be well outside anything human civilisation has seen,” Shaun warned. “We won’t have even have been close.”
Knowing climate’s history helps understand its present, and so researchers have puzzled out temperatures on the Earth’s surface from proxy, or indirect, records for the last 2,000 years. In particular, bringing together measurements from tree rings, ice and coral has showed a sharp recent temperature rise often referred to as the ‘hockey stick’. Meanwhile, studies scattered across the world had reached back across the 11,300 years since the beginning of the Holocene. But they can be influenced by regional effects, and no one had pieced them into a global view that would overcome that.
Shaun therefore made that challenge the focus of his PhD research. But to reach further into history he couldn’t use tree rings. While they can provide relatively high-resolution and precise year- or decade-average temperature data, trees simply don’t live long enough. Instead, he called on data other scientists had collected from proxy records trapped in cylinders, or cores, drilled from ice and sediment layers formed over thousands of years.
That switch brought problems: These records can only provide lower-resolution temperature data, averaged over periods of hundreds or thousands of years. They are also harder to pin down to precise dates and temperatures. And because of their comparatively low resolution they’re hard to link to modern directly-measured global temperature records, which have been collected for less than two centuries.
The scientists therefore had to proceed with great care. “We tried to follow very robust criteria to decide which records we used,” Shaun said. “We wanted very high resolution records, greater than 300 years, well dated, from established temperature proxies. In our field there’s very little disagreement over whether these proxies are recording temperatures.” The team also had to work out just how uncertain bringing the different proxies together made their temperature history.
They had experience of the caution needed though, having used similar proxy records to study links between historic levels of CO2 in the air and temperature last year. There the researchers used 80 sets of proxies in ice from the Antarctic and Greenland, fossils of tiny shells from the seabed and pollen from lake bottoms. In their latest study, just published in top research journal Science, they use a different group of proxy sets out of hundreds that Shaun collected during his PhD studies.
Long view, worrying prospect
Although the tree-ring record has been attacked by climate skeptics, Shaun’s team’s combined record is a very close match with it. That answers a technical criticism, relating to the fact that trees grow fast when young, and then slow down when they get older. Some critics say the calculations needed to take this into account mean tree rings can’t show longer-term century and millennium-scale temperature patterns, only year and decade-scale ones. That might take out longer-term natural patterns that could cause warming like the world is now seeing.
“Our dataset is mostly marine fossils, whereas the records for the last 2,000 years are nearly all from land,” Shaun said. “They’re two totally different archives, and yet they’re giving the exact same answer for the period that they overlap. In our minds that puts to rest arguments that the warming we’re seeing now is part of some long term natural variability. We don’t see that in our reconstruction.”
That agreement gave Shaun’s team a way to link their record to modern temperatures. “We can reference our reconstruction to the tree ring temperatures, which are in turn referenced to 1961-1990 instrumental records,” he explained. For most of the record, that shows long-term temperature changes that are expected from Earth’s oval orbit around the sun and its spinning on a slightly tilted axis. For these reasons, a long-term 0.7°C cooling began around 5000 years ago, leading to the decade from 1900-1909 being cooler than 95% of all decades in the whole 11,300 year record.
This cooling trend has been interrupted at the very end, Shaun noted, thanks to human-caused CO2 emissions. Since 1909 they find 0.7°C warming, meaning that the decade from 2000-2009 is warmer than at least 70% of their record. And from here the IPCC model outcomes forecast further warming of another 1-6°C, which would push the Earth be beyond anything seen in the Holocene. “I think people knew this, but our study quantifies it,” Shaun said. “The idea was to really get those numbers pinned down.”
Shaun A. Marcott, Jeremy D. Shakun, Peter U. Clark, Alan C. Mix (2013). A Reconstruction of Regional and Global Temperature for the Past 11,300 Years Science, 339, 1198-1201