For decades, the United Nations' Intergovernmental Panel on Climate Change (IPCC) has offered a snapshot of the planet's changing climate - but University of Toronto researchers have found that some of the underlying data underrepresents a key driver of Arctic warming.
The IPCC reports rely on a wealth of climate data, including observations from the U.S.'s National Oceanic and Atmospheric Administration (NOAA) of autumn snow cover - the extent to which the land is covered by snow - in the Northern Hemisphere. The observations have been made annually since the 1960s.
This metric is critical because, among other roles it plays in the Earth's climate, snow reflects energy from the planet's surface back into space. While land and vegetation reflect less than 50 per cent of the energy reaching the surface, snow reflects about 80 per cent.
"Snow cover is important because it's a positive climate feedback mechanism," explains Aleksandra Elias Chereque, a PhD student in the department of physics in U of T's Faculty of Arts & Science.
"This is referred to as the snow-albedo effect - albedo meaning reflectivity. Snow loss leads to a decrease in albedo, which leads to higher energy absorption, which, in turn, leads to enhanced snow loss. This is a contributing factor to a phenomenon known as 'Arctic amplification,' and it's why we observe a disproportionate amount of heating in the Arctic."
However, climate scientists have long questioned the reliability of the NOAA data, noting that the snow cover trends suggested by the data were dramatically inconsistent with other observations and argue they should be treated with caution.
Now, Elias Chereque and her collaborators have validated these concerns through a comprehensive new analysis of the NOAA data.
The NOAA observations showed increases in Northern Hemisphere snow cover of about 1.5 million square kilometres per decade. That's about 1.5 times the size of the province of Ontario. But the new analysis by Chereque and her colleagues shows snow cover actually decreases by half a million square kilometres per decade, or half the size of Canada's most populous province.
Elias Chereque and her collaborators show that changes over the years in instrumentation and data collection methods in the NOAA data resulted in an increased sensitivity to thin snow cover and, thus, the erroneous observations that snow cover had increased.
"It's as if the satellite's 'eyeglasses' got better and better over that period," says Elias Chereque. "It looks like there's more snow now than there used to be - but that's only because the satellite kept getting better 'prescriptions for its glasses.' It looked like there was more snow but that's not what was happening."
The study, published in the journal Science Advances , was co-authored by atmospheric physicist Paul Kushner, professor and chair in the department of physics and collaborators from the climate research division of Environment and Climate Change Canada. It adds evidence to the finding that snow cover is decreasing throughout the year and increases confidence in that result.
"We know snow loss is influenced by anthropogenic warming and snow loss also creates more potential for warming through the snow-albedo feedback, so we've gained a better understanding of this important mechanism of Arctic amplification," Elias Chereque says.
"Showing how and why the snow cover trend was wrong helps us learn how to use this data set properly when we're estimating past conditions and future trends. And that helps in understanding whether climate models are accurate.
"Developing tools like this help us better understand climate and make better predictions about the future."
