While satellite images of the Arctic clearly show that sea ice in the region has been on a steady decline since those images began in 1979, the relatively short span of that history has been seized on by some climate denialists to discount its significance in concluding humans are warming the planet.

Now, scientists have compiled the most detailed study to date of sea ice records going back more than a century and a half. The data shows that the rapid meltdown that satellites have been documenting since 1979 is unprecedented since at least 1850 and coincides with the buildup of heat-trapping greenhouse gases in the atmosphere from burning fossil fuels.

Arctic sea ice has not been at levels as low as today's for at least 5,000 to 7,000 years, according to Julienne Stroeve, a researcher with the National Snow and Ice Data Center (NSIDC), who was not involved in the study. "It may have been sometime during the mid-Holocene, based on driftwood found in Greenland that came from Siberia," she said. "Some other studies have suggested at least 800,000 years."

"Any reconstruction back before the satellite data record is extremely helpful in putting today's changes in context. None of the models show this decline without including the observed record of greenhouse gases. This helps to confirm that rising levels of GHGs is causing the sea ice to decline," Stroeve said.

The researchers, with the NSIDC, The University of Alaska, Fairbanks and the University of Illinois, compiled and digitalized sea ice measurements from 14 historical sources. Those included hand-drawn ice-boundary maps, and documents like logbooks from the U.S. whaling fleet, records from the Danish Meteorological Institute and reports by U.S. Navy oceanographers, from 1850 through 1978. This analog data was digitized to make it compatible with satellite records starting in 1979. The work yielded a finely gridded dataset that can be used by climate models to project how sea ice will change in the decades ahead.

The corresponding paper was published July 11 in the journal Geographical Review.

"There are ups and downs in the extent, and ups and downs in particular areas, but what has happened in the last two decades is unprecedented," said John Walsh, co-author of the paper and chief scientist with the International Arctic Research Center at the University of Alaska, Fairbanks.

When the sea ice hit an all-time record low in 2012, it was 1.27 million square miles smaller than the 1979-2000 average, the benchmark period against which scientists measure modern-era ice extent.

Credit: "A gridded database of Arctic sea ice extending back to the 1800s"

The future of sea ice has important implications for Earth's climate. Loss of the reflective ice surface will speed up global warming as ocean waters absorb more heat. A number of studies also suggest links between melting Arctic ice and extreme weather events across the Northern Hemisphere, and the meltdown may also disrupt important ocean currents.

The new paper bolsters previous examinations of historic Arctic sea ice trends and fills in gaps for remote parts of the Arctic Ocean and around the northern coast of Scandinavia that had not previously been documented. It also adds information on sea ice in years and months for which data was previously sparse.

The data reinforces the scientific consensus that the rapid decline of sea ice is driven by rising levels of carbon dioxide in the atmosphere. Seasonal and year-to-year fluctuations are still subject to natural variability as well.

"From this work it's clear that recent changes are enormously outside the bounds of natural variability over the historical period," said Gavin Schmidt, a climate scientist and director of the NASA Goddard Institute for Space Studies, who was not involved in the study.

In order to cast doubt on the reality that human activity is heating up the planet, some climate denialists over the years have said that the satellite monitoring record beginning in 1979 is too short to determine whether the current meltdown is caused by man-made greenhouse gases.

"There is tendency for people who don't want to acknowledge the ongoing changes to focus on the shortest records and point to the noise there to imply that we are still uncertain about the long-term changes," Schmidt said. "Suggestions that the Arctic trend from 1979 is somehow just a natural fluctuation is totally rejected by this new data."

Credit: "A gridded database of Arctic sea ice extending back to the 1800s"

While scientists can't predict an exact year when the sea ice will melt altogether, the new data set can help climate models "come up with better probability distribution," narrowing the meltdown timeframe, according to Walsh.

Schmidt said: "It indicates that future changes are very likely, and under continued emissions we are likely to see summer ice in particular effectively disappear over the next few decades."