tundra stores carbon during the summer and releases some of it during
the winter. But a
that carbon released during the winter now outweighs the summertime
gains, resulting in a net loss of carbon to the atmosphere.
to the study’s authors, these results suggest the northern tundra
may be shifting from its historical role as a carbon sink to a carbon
source. To date, the Arctic has warmed more
than any other region globally
and researchers expect this warming to continue in the coming
decades. If the tundra becomes a carbon source, it could amplify
global warming and accelerate climate change, according to the
more the Arctic and sub-Arctic warm, the more carbon will be released
to the atmosphere in the form of carbon dioxide and methane, which
are greenhouse gasses and will trap more heat, warming the high
latitudes even more,” said Elizabeth Webb, an ecosystem ecologist
at the University of Florida in Gainesville and lead author of the
study, published in the Journal
of Geophysical Research – Biogeosciences,
a publication of the American Geophysical Union, provides a
comprehensive analysis of carbon flux on the tundra during the winter
motivated my study was this interest in what’s happening in the
winter, because it’s such an important time of year in the northern
latitudes,” Webb said. “The Arctic winter is bearing the brunt of
global warming, and we need to be able to quantify how increases in
temperature will increase the amount of carbon dioxide lost during
massive carbon freezer
the tundra, the net carbon flux – the exchange of carbon between
the ground and the atmosphere – is a balance between plant
photosynthesis and microbial respiration. Plants take in carbon
dioxide from the atmosphere and store it as organic carbon. Soil
microbes decompose organic material and exhale carbon dioxide through
in permafrost, or permanently frozen soils, respiration slows almost
to a halt. As a result, the tundra slowly accumulates organic carbon
– like a giant freezer storing food underground.
research has shown that even as northern latitudes warm, carbon
storage from photosynthesis outweighs losses from respiration.
However, most of this research has been done in the short summer
season when there is plentiful sunlight to drive photosynthesis. The
Arctic is also expected to see greater warming in the winter than in
the summer in the coming decades. As temperatures rise, how much
stored carbon stored is released during the long, dark winter is “the
great unknown,” according to Webb.
wintertime carbon flux
team built snow fences to simulate how warming temperatures would
affect carbon flux in the Alaskan permafrost.Credit:
team measured the net carbon flux of tundra in Alaska’s interior
during the winter of 2008-2009 through the winter of 2012-2013. The
team built snow fences that accumulate snow on one side of a barrier.
On the leeward side of the fence, snow accumulates, insulating the
soil below and simulating what would happen under warmer
the windward side, no additional snow accumulates, showing what would
happen if temperatures remain the same. The soil on the warming side
becomes, on average, 1.5 degrees Celsius (2.7 degrees Fahrenheit)
warmer than on the non-warming side, according to Webb.
team detected a net loss of carbon dioxide from the soil to the
atmosphere over the five winters studied on both sides of the fence.
They found that warmer temperatures increased carbon dioxide loss to
the atmosphere between 9 percent and 36 percent, depending on which
of four different measurement methods were used.
though we have these different methods of measuring it, we’re still
showing the same result, which is that temperature is important,”
Webb said. “We weren’t able to clarify the best method, but we
found that by using different methods, we could converge on what
might be a reasonable estimate of the carbon dioxide that’s lost
during the winter.”
the vast freezer starting to thaw?
with summertime carbon flux data from three of the study years,
Webb’s research showed that during the years analyzed – 2009,
2010 and 2011 – this region of tundra was releasing carbon to the
atmosphere, rather than storing it – even on the non-warming side
of the fence.
to Webb, with only a few years of data, it’s too soon to say
whether this marks an end to carbon storage in northern permafrost.
very difficult to make large conclusions just based on three years –
it could be a total anomaly,” Webb said. But her data does agree
with other field studies and permafrost models that show the tundra
is moving from a carbon sink to a carbon source, she said.
can’t say if that is a tipping point or not, but I can say that in
terms of climate change, it’s not a step in the right direction,”
she said. Because the field site for this study was just south of the
Arctic Circle, the change at this site might be a good indicator of
what is to come farther north, she added.
research has found that even under layers of snow, plants are still
taking in carbon late into the fall and before the snow melts in the
spring, but researchers had not taken that into consideration when
calculating carbon flux during the winter or on an annual basis,
according to Webb. The next step for the team will be to spend more
time understanding the carbon flux during the fall and spring
climate change progresses, we’re going to see longer shoulder
seasons – the fall [and spring] will be lasting longer and the
winter will be shorter,” Webb said. “So we need to be able to
figure out what’s happening during that time.”
Lipuma is a public information specialist and writer at AGU.