Arctic permafrost contains enough carbon to raise global mean temperatures by more than 3 degree Celsius Photo Credit: Unsplash/ Hubert Neufeld |
The Earth is approximately 1.1 degree Celsius warmer than it was at the start of the industrial revolution. That warming has not been uniform, with some regions warming at a far greater pace. One such region is the Arctic.
A new study shows that the Arctic has warmed nearly four
times faster than the rest of the world over the past 43 years. This means the
Arctic is on average around 3 degree Celsius warmer than it was in 1980.
This is alarming because the Arctic contains sensitive and
delicately balanced climate components that, if pushed too hard, will respond
with global consequences.
Why is the Arctic warming so much faster? A large part of
the explanation relates to sea ice. This is a thin layer (typically one metre
to five metres thick) of seawater that freezes in winter and partially melts in
the summer.
The sea ice is covered in a bright layer of snow which
reflects around 85 percent of incoming solar radiation back out to space. The
opposite occurs in the open ocean. As the darkest natural surface on the
planet, the ocean absorbs 90 percent of solar radiation.
When covered with sea ice, the Arctic Ocean acts like a
large reflective blanket, reducing the absorption of solar radiation. As the
sea ice melts, absorption rates increase, resulting in a positive feedback loop
where the rapid pace of ocean warming further amplifies sea ice melt,
contributing to even faster ocean warming.
This feedback loop is largely responsible for what is known
as Arctic amplification and is the explanation for why the Arctic is warming so
much more than the rest of the planet.
Is Arctic amplification underestimated? Numerical climate
models have been used to quantify the magnitude of Arctic amplification. They
typically estimate the amplification ratio to be about 2.5, meaning the Arctic
is warming 2.5 times faster than the global average. Based on the observational
record of surface temperatures over the last 43 years, the new study estimates
the Arctic amplification rate to be about four.
Rarely do the climate models obtain values as high as that.
This suggests the models may not fully capture the complete feedback loops
responsible for Arctic amplification and may, as a consequence, underestimate
future Arctic warming and the potential consequences that accompany that.
How concerned should we be? Besides sea ice, the Arctic
contains other climate components that are extremely sensitive to warming. If
pushed too hard, they will also have global consequences.
One of those elements is permafrost, a (now not so)
permanently frozen layer of the Earth's surface. As temperatures rise across
the Arctic, the active layer, the topmost layer of soil that thaws each summer,
deepens. This, in turn, increases biological activity in the active layer
resulting in the release of carbon into the atmosphere.
Arctic permafrost contains enough carbon to raise global
mean temperatures by more than 3 degree Celsius. Should permafrost thawing
accelerate, there is the potential for a runaway positive feedback process,
often referred to as the permafrost carbon time bomb. The release of previously
stored carbon dioxide and methane will contribute to further Arctic warming,
subsequently accelerating future permafrost thaw.
A second Arctic component vulnerable to temperature rise is
the Greenland ice sheet. As the largest ice mass in the northern hemisphere, it
contains enough frozen ice to raise global sea levels by 7.4 metres if melted
completely.
When the amount of melting at the surface of an ice cap
exceeds the rate of winter snow accumulation, it will lose mass faster than it
gains any. When this threshold is exceeded, its surface lowers. This will
quicken the pace of melting, because temperatures are higher at lower
elevations.
This feedback loop is often called the small ice cap
instability. Prior research puts the required temperature rise around Greenland
for this threshold to be passed at around 4.5 degree Celsius above
pre-industrial levels. Given the exceptional pace of Arctic warming, passing
this critical threshold is rapidly becoming likely.
Although there are some regional differences in the
magnitude of Arctic amplification, the observed pace of Arctic warming is far
higher than the models implied. This brings us perilously close to key climate
thresholds that if passed will have global consequences. As anyone who works on
these problems knows, what happens in the Arctic doesn't stay in the Arctic.
0 comments:
Post a Comment