For decades, reports from the United Nations’ Intergovernmental Panel on Climate Change (IPCC) have provided a snapshot of the planet’s changing climate and helped guide public policies designed to fight global warming.
The 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 in the northern hemisphere — observations made annually since the 1960s.
Snow cover refers to the extent to which the Earth’s land surface is covered by snow. 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. And while land and vegetation reflect less than 50 per cent of the energy reaching the surface, snow reflects some 80 per cent.
“Snow cover is important because it’s a positive climate feedback mechanism,” explains Aleksandra Elias Chereque, a PhD student in the Faculty of Arts & Science’s Department of Physics.
“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 for decades voiced concerns regarding the reliability of the NOAA data. They point to the fact 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 conducted a new analysis of the NOAA data and concluded that those concerns were warranted.
The NOAA observations showed increases in northern hemisphere snow cover of some 1.5 million square kilometres per decade, or an area equal to one and a half Ontarios. But the new analysis by Chereque and her colleagues shows snow cover actually decreases by half a million square kilometres per decade, or half an Ontario.
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 increased.
"It’s as if the satellite’s ‘eye glasses’ 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.”
Elias Chereque is lead author of a paper, ‘Determining the cause of inconsistent onset-season trends in the Northern Hemisphere snow cover extent record’, published in Science Advances, that describes the analysis. Her U of T co-author is atmospheric physicist Paul Kushner, chair of the Department of Physics. Additional co-authors are from the Climate Research Division of Environment and Climate Change Canada.
The study adds evidence to the finding that snow cover is decreasing throughout the year and increases confidence in that result. According to Elias Chereque, “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.”
She adds that, “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 helps us better understand climate and helps us make better predictions about the future.”