Science1 publisher2 min readPublished
Snow-covered northern lakes carry 1.8 times the cooling effect of nearby land per square meter
Duke's Sarah Cooley and colleagues measured a snow-and-ice cooling effect of -14.4 W/m2 over Northern Hemisphere lakes, against -8.2 over nearby land. Climate models often do not separate lakes from land, though the study as reported does not say how much that misstates the seasonal energy balance.
The Scientist · Science desk
What happened
- The team compared seasonal snow cover and reflected sunlight over lakes and nearby land using 22 years of satellite observations across the Northern Hemisphere.
- In boreal forests, the coniferous belt across northern North America, Europe and Asia, lakes can account for more than a quarter of the cooling from snow and ice.
- The lake contribution matters most in late spring, when snow and ice are starting to disappear from the landscape.
- The authors attribute the gap to two factors: snow melts earlier on land, and snow-covered lakes reflect a larger share of sunlight than snow-covered land.
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Why it matters
- decision Modelling groups that fold lakes into surrounding land now have a 22-year observed target, per square meter and by season, against which to test a separate lake surface.
- constraint The 1.8-times figure is per square meter, so it supports regional and seasonal corrections in lake-dense boreal areas more than any claim of a large hemispheric cooling.
- exposure Because the lake advantage rests partly on lakes staying frozen after the land melts, the effect is exposed to any change in when lake ice goes out in late spring.
Both figures are per square meter of surface, so the first question is the denominator. The cryosphere radiative effect measures how much less solar energy the planet absorbs because snow and ice reflect sunlight [10]. Over lakes that effect is about 1.76 times the land value [1], a difference of 6.2 W/m2 [2]. Lakes cover only a small fraction of Earth's surface [9]. A large effect per square meter can still be a modest part of the hemispheric total. The only share reported is regional: more than a quarter of the snow-and-ice cooling in boreal forests [4].
The two factors behind the gap act at different times of year [6]. The albedo difference applies whenever lake and land are both snow-covered. Earlier melt on land applies only in spring, when the ground has started absorbing sunlight and the lake beside it is still white. That second factor fits the late-spring peak in the lake contribution [5]. The attribution is observational, drawn from 22 years of satellite records [1].
Climate models already include snow and ice. According to the phys.org account, they often do not separate the lake contribution from that of the surrounding land [7]. In effect, a lake folded into its surroundings takes on land melt timing and land albedo, the two properties the satellites show differ. Cooley and colleagues, writing in Geophysical Research Letters [3], argue that lakes should be better accounted for in models [12].
I think the observations make the direction of the model problem clear, and its size is still open. The study, as reported, does not estimate the error in any particular model or project the lake effect into warmer decades. The explanation does indicate where the sensitivity sits. The extra cooling depends partly on lakes staying frozen after the land has melted [6]. Anything that moved lake ice-off closer to land melt would shrink that timing part of the advantage. The albedo part would remain for as long as the lake stays snow-covered.
The authors also set a limit on the finding: it does not mean lakes can offset the effects of global warming [8].
What to watch
- A model run that represents lakes separately from land and reports how much the late-spring energy balance changes in boreal regions.
- Follow-up work projecting the lake share of the cryosphere radiative effect under shorter lake ice seasons.
- An estimate of lakes' share of the total Northern Hemisphere terrestrial cryosphere radiative effect, beyond the boreal quarter.