Why Arctic ice is disappearing more rapidly than expected

In a new study, researchers have discovered unexpected climate-driven changes in the mighty Mackenzie River's ice breakup. This discovery may help resolve the complex puzzle underlying why Arctic ice is disappearing more rapidly than expected.

The study, led by Simon Fraser University geographer and Faculty of Environment professor Lance Lesack, has been co-authored at Wilfrid Laurier University, the University of Alberta and Memorial University.

Its goal was to understand how warming global temperatures and the intensifying Arctic hydrological cycle associated with them may be driving increasing water discharges and more rapid ice breakup in the Arctic's great rivers.

However, the researchers stumbled upon an unexpected phenomenon while trying to figure out why the Mackenzie River's annual ice breakup has been shortening even though its water discharge isn't increasing, as in Russian rivers.

Just slightly warmer springs with unexpected snowfall declines - rather than warmer winters or increasing river discharge, as previously suspected - can drive earlier-than-expected ice breakup in great Arctic rivers.

The Mackenzie exemplifies this unexpected phenomenon. The researchers discovered this by accessing records dating back to 1958 of the river's water levels, snow depths, air temperatures and times of ice breakup.

This finding is significant, as Arctic snow and ice systems are important climate-system components that affect the Earth's ability to reflect solar radiation.

"Our surprising finding was that spring temperatures, the period when river-ice melt occurs, had warmed by only 3.2 degrees Celsius. Yet this small change was responsible for more than 80 per cent of the variation in the earlier ice breakups, whereas winter temperatures had warmed by 5.3 degrees but explained little of this variation," Lesack said.

"This is a strong response in ice breakup for a relatively modest degree of warming, but further investigation showed that by winter's end snow depths had also declined by one third over this period. The lesser snow depths mean less solar energy is needed to drive ice breakup.

"The polar regions have a disproportionate effect on planetary reflectivity because so much of these regions consist of ice and snow. Most of the planetary sea ice is in the Arctic and the Arctic landmass is also seasonally covered by extensive snow. If such ice and snow change significantly, this will affect the global climate system and would be something to worry about," Lesack added.

The study has been published in Geophysical Research Letters.