Thawing permafrost in high-altitude mountain ecosystems may be a stealthy, underexplored contributor to atmospheric carbon dioxide emissions, new CU Boulder research shows.
The new findings, published today in the journal听Nature Communications, show that alpine tundra in Colorado鈥檚 Front Range emits more CO2听than it captures annually, potentially creating a feedback loop that could increase climate warming and lead to even more CO2听emissions in the future.
A similar phenomenon exists in the Arctic, where research in recent decades has shown that melting permafrost is unearthing long-frozen tundra soil and releasing CO2听reserves that had been buried for centuries.
鈥淲e wondered if the same thing could be happening in alpine terrain,鈥澨齭aid John Knowles, lead author of the new study and a former doctoral student in CU Boulder鈥檚听Department of Geography听and a researcher at the听. 鈥淭his study is a strong indication that that is indeed the case.鈥�
Forests have long been considered vital carbon 鈥榮inks,鈥� sequestering more carbon than they produce and helping to mitigate global CO2听levels. As part of the Earth鈥檚 carbon cycle, trees and other vegetation absorb CO2听via photosynthesis while microbes (which decompose soil nutrients and organic material) emit it back to the atmosphere via respiration, just as humans release CO2听with every breath.听
Melting permafrost, however, changes that equation. As previously frozen tundra soil thaws and becomes exposed for the first time in years, its nutrients become freshly available for microbes to consume. And unlike plants, which go dormant in winter, microscopic organisms can feast all year long if environmental conditions are right.
To study this effect in alpine conditions, researchers measured the surface-to-air CO2听transfer over seven consecutive years (2008-2014) at the听听site in Colorado, a high-altitude research project funded by the National Science Foundation that has been in continuous operation for over 35 years. The team also collected samples of soil CO2听and used radiocarbon dating to estimate how long the carbon forming that CO2听had been present in the landscape.
The study showed, somewhat surprisingly, that barren, wind-scoured tundra landscapes above 11,000 feet emitted more CO2听than they captured each year, and that a fraction of that CO2听was relatively old during the winter, the first such finding of its kind in temperate latitudes. The findings suggest higher-than-expected year-round microbial activity, even in the absence of a deep insulating snowpack.
鈥淢icrobes need it to be not too cold and not too dry, they need liquid water,鈥澨齭aid Knowles, now a researcher at the University of Arizona. 鈥淭he surprise here is that we show winter microbial activity persisting in permafrost areas that don鈥檛 collect much insulating snowpack due to wind stripping it away.鈥�
While the alpine tundra鈥檚 net CO2听contributions are small compared to a forest鈥檚 sequestration capability, the newly-documented effect may act as something of a counterweight, hampering atmospheric CO2听reductions from mountain ecosystems in general. The findings will need to be factored in to future projections of global warming, Knowles said.
鈥淯ntil now, little was known about how alpine tundra behaved with regard to this balance, and especially how it could continue emitting CO2听year after year鈥� Knowles said. 鈥淏ut now, we have evidence that climate change or another disturbance may be liberating decades-to-centuries-old carbon from this landscape.鈥�
Additional co-authors of the study include Peter Blanken of CU Boulder鈥檚 Department of Geography; Mark Williams of CU Boulder and INSTAAR; and Corey Lawrence of the U.S. Geological Survey. The National Science Foundation provided funding for the research.