A reconstruction of 2,000 years of global temperatures shows that a long-term decline in Earth's temperatures ended abruptly about 1900, replaced by a warming trend that has continued despite the persistence into the 20th century of the factors driving the cooling, according to a new study.
Indeed, for several continents, the increase in global average temperatures from the 19th century to the 20th was the highest century-to-century increase during the 2,000-year span, the study indicates. It's the first study to attempt building a millennial-scale climate history, continent by continent.
The research wasn't designed to identify the cause of the warming trend, which climate researchers say has been triggered by a buildup of greenhouse gases – mainly carbon dioxide – as humans burned increasing amounts of fossil fuel and altered the landscape in ways that released CO2.
Still, it's hard to explain 20th-century warming without including the influence of rising CO2 levels, because the factors driving the cooling were still present, notes Darrell Kaufman, a researcher at Northern Arizona University and one of the lead authors on the paper formally reporting the results in the journal Nature Geoscience.
The study, five years in the making, drew on the work of 87 scientists in 24 countries as part of the International Geosphere-Biosphere Program. One goal of the 27-year-old program is to gain a deeper understanding of Earth's climate history and the factors that contribute to climate variability.
The study used nature's proxies for thermometers – tree rings, pollen, and other natural temperature indicators – to build continent by continent a coordinated record of temperature changes during the past two millenniums.
Scientists use this proxy approach to reach further into the climate's temperature history than the relatively short thermometer record allows. Such efforts aim to put today's climate into a deeper historical context as well as to identify the duration and possible triggers for natural swings that the climate undergoes over a variety of time scales.
Last March, for instance, a team led by Shaun Marcott at Oregon State University used climate proxies to build a global temperature record reaching back 1,200 years – one that also noted the pre-1900 cooling trend.
Until now, however, the proxy approach has been used to reconstruct changes in global-average and hemisphere-wide temperatures, Dr. Kaufman explains.
"There was very little information about past climate variability at the regional scale," he says. Yet the team notes that no one lives in a global-average world. People live in specific regions where geography plays a vital role in shaping the climate patterns they experience.
The team omitted Africa from its analysis because the proxy records are too few to be useful. Instead, the researchers looked at seven continent-scale regions in South America, the southwestern Pacific (including Australia), as well as North America, Europe, Asia, Antarctica, and the Arctic.
This approach reveals a more complex picture over various intervals than a record of global average or hemispheric temperatures would present, the researchers find.
The team also notes that their work likely understates the overall intensity of warming because the reconstruction record stops short of the record-setting warm years and the warmest decade on record that have occurred since 1990.
On millennial time scales, the cooling trend was statistically significant on all continents except North America, where the cooling was weaker, according to the study.
Over the period the study covers, the cooling was driven variously by volcanic activity, which pumps light-reflecting aerosols high into the atmosphere, subtle changes in Earth's orbit, subtle changes in the amount of light the sun was emitting at the time, and land-use changes, the researchers say. The relative influence of each factor varied, depending on location.
The Medieval Warming Period, which the team pegs at between 830 and 1110 AD, encompassed the northern hemisphere at that time, while in South America and the southwestern Pacific, a period of relatively sustained warmth ran from 1160 to 1370.
The cooling trend first appeared to take hold in the Arctic, Europe, and Asia between 1200 and 1500 AD. North America felt it too, but less intensely, the study suggests. By the late 1500s, the cooling had expanded to include Antarctica.
That came as a surprise, Kaufman says. The progression of cooling from north to south is a pattern one would expect to see from the combined effects of subtle changes in Earth's orbit and changes in its tilt, he explains. That cooling in the north came, in part, as the Arctic received less sunlight over a given surface area in the summertime. But that also should lead to more sunlight per square meter in Antarctica.
In addition, changes in the sun's output and other sources of natural "forcings" on climate appear to have been far less influential at the time in Antarctica than over the northern hemisphere's continents.
Cooling was not as pronounced there as it was at the top of the world, but the evidence that cooling happened at all represents a new puzzle for researchers to solve.
The team also notes that its reconstruction indicates that the Medieval Warming Period and later the Little Ice Age were not global events, as some have claimed, but climate excursions whose timing and intensity varied by region.
After 1900, warming hit all continents except Antarctica. The bottom of the world continued to cool. It's a trend other researchers attribute to the slow pace of temperature changes in the frigid moat of a Southern Ocean that surrounds the ice-encrusted continent. And high-altitude winds that encircle the continent have intensified, keeping cold air trapped at high southern latitudes.