PARIS – The Earth was cooling until the end of the 19th century, yet 100 years later the planet’s surface was on average warmer than at any time in the previous 1,400 years, according to climate records presented Sunday.
In a study spanning two millenniums published in Nature Geoscience, scientists said a “long-term cooling trend” around the world swung into reverse in the late 19th century.
In the 20th century, the average global temperature was 0.4 degrees Celsius higher than that of the previous 500 years, with only Antarctica bucking the trend.
From 1971 to 2000, the planet was warmer than at any other time in nearly 1,400 years.
This measure is a global average, and some regions did experience warmer periods than that — but only for a time. Europe, for instance, was probably warmer in the first century A.D. than at the end of the 20th century.
The investigation is the first attempt to reconstruct temperatures over the past 2,000 years for individual continents.
It seeks to shed light on a fiercely contested aspect in the global-warming debate.
Skeptics have claimed bouts of cooling or warming before the Industrial Revolution — including two episodes in Europe called the Medieval Warm Period and the Little Ice Age — are proof that climate variations are natural, not man-made.
The new study does not wade into the debate about greenhouse gases, but points to two planetary trends.
The first is a clear, prolonged period of cooling. It may have been caused by a combination of factors, including an increase in volcanic activity, with stratospheric ashes reflecting sunlight, or a decrease in solar activity or tiny changes in Earth’s orbit, both of which would diminish sunlight falling on the planet.
The cooling — between 0.1 and 0.3 degree per 1,000 years, depending on the region — went into reverse toward the end of the 19th century, and was followed by an intensifying period of warming in the next century, the paper said.
Beneath this global trend over 2,000 years were episodes of continental cooling or warming, some of which were quite long.
And some continents lagged the overall planetary trend, but with the exception of Antarctica, all followed it.
“Distinctive periods, such as the Medieval Warm Period or the Little Ice Age stand out, but do not show a globally uniform pattern on multidecadal time scales,” said Heinz Wanner of the University of Bern in Switzerland and one of 78 researchers from 24 countries who took part in the project.
“There are things that are common to all the regions of the planet — long-term cooling, until the 19th century, followed by warming on all continents, except for Antarctica, where it is less clear, but also strong variations from one region to another,” said Hugues Goosse, a climatologist at Belgium’s Catholic University of Leuven.
Previous research into climate change has pointed to a warming spurt in the 20th century and attributed it to the rise of heat-trapping carbon gases emitted by the burning of coal, oil and gas.
The warming trend hit high gear in the mid-1970s, in line with record-breaking levels of carbon dioxide, according to this past research.
Last year was the 36th in a row that global temperatures were above average since 1880, when scientifically acceptable records were first kept, and was the ninth or 10th warmest on record, U.S. scientists said in January.
The temperature reconstruction published Sunday was coordinated by a scientific initiative called the Past Global Changes (PAGES) 2K Network.
It brings together weather data as well as signs of temperature variation from tree rings, pollen, coral, lake and marine sediments, ice cores and stalagmites garnered at 511 locations across seven continental-scale regions.
Climate records for Africa, though, were sparse, the researchers cautioned.