Earlier snowmelt periods associated with a warming climate may hinder subalpine forest regulation of atmospheric carbon dioxide (CO2), according to the results of a new University of Colorado Boulder study. The findings, which were recently published in the journal Geophysical Research Letters, predict that this shift in the timing of the snowmelt could result in a 45 percent reduction of snowmelt period forest carbon uptake by mid-century. 

A separate study, also published in Geophysical Research Letters, found that earlier, slower snowmelt reduces the amount of streamflow, a phenomenon with potentially drastic consequences for agriculture, municipal water supplies and recreational opportunities in Colorado and other areas of the western U.S.

Forests located in seasonally snow-covered areas represent a key terrestrial CO2 sink thanks to the natural photosynthetic processes by which trees uptake carbon. The trees' carbon uptake is restrained during winter, but increases to peak capacity in spring when snowmelt provides sustained water input.

Working at the Niwot Ridge Ameriflux site in Colorado's Rocky Mountains, CU Boulder researchers studied 15 years' worth of snowmelt and atmospheric CO2 data to study the effects of snowmelt periods. The research found that earlier snowmelt periods triggered by climate change align with colder air temperatures, reducing the forests' ability to take CO2 out of the atmosphere.

"This study shows us that, counterintuitively, warming generally causes snow to melt during colder periods of the seasonal temperature cycle due to the effects that warming has on reducing the depth of snowpacks, which causes melt to begin earlier in the year," said Taylor Winchell, a graduate researcher in the Institute for Arctic and Alpine Research (INSTAAR) and lead author of the study. "The colder temperatures associated with early melt reduce the trees' ability to uptake carbon during the snowmelt period, a key period for seasonal carbon uptake." 

"The implications of this research are quite profound as mountains in the western U.S. are an important part of the regional cycling of carbon and water," said Noah Molotch, the director of the Center for Water Earth Science & Technology (CWEST) and a co-author of both new studies. "In this regard, earlier snowmelt will reduce carbon uptake in mountain forests, weakening the ability of forests to offset increases in CO2 associated with human burning of fossil fuels."