Amplifying
Feedbacks — Warming Tropics Found to Now Release 2 Gigatons More
Carbon Each Year
3
February, 2014
In
looking at the problem of human-caused climate change, we often miss
a critical point. Human greenhouse gas emissions warm the atmosphere,
melt ice, and heat the ocean, yes. But a warming world also releases
its own additional volumes of carbon, often in the form of methane
and CO2. In the Arctic, we have seen massive releases from natural
stores of CO2 and methane. Releases that, on their own, are enough to
amplify the already significant warming caused by human fossil fuel
emissions. Now, according
to a new study,
the tropics are also contributing an amazing volume of carbon as they
continue to endure the insults of warming.
In
essence, when humans warm the globe, they are kicking nature. And an
insulted nature then releases increasing volumes of her own carbon
stores in an ever-worsening cycle until humans stop and the severely
riled natural cycle runs its course. Such an agitated carbon cycle
can swiftly trip into a phase that accelerates atmospheric heating to
rates far more rapid than initially feared. And, as research
progresses, it appears the worst fears about nature’s sensitivity
to our insults are steadily being realized.
Chasing
down the feedbacks
Climate
researchers must have felt an ominous sinking feeling when they
discovered this year that, as the globe heated up, changes to the
world’s clouds resulted in an amplifying feedback to human-caused
warming. Many researchers had previously clouds out of climate models
and, as such, the models showed lower rates of warming than what we
would find from comparable measures in paleoclimate. For example,
many climate models only showed about 1 to 1.5 C warming as a result
of atmospheric CO2 levels at 400 parts per million (the level seen
today), while paleoclimate showed long term warming twice that, in
the range of 2-3 C. This disparity created a bit of cognitive
dissonance in the science between those who modeled climate and those
who studied past climates. But, slowly, the gap is being bridged and
the new research in clouds makes up a portion of this difference. But
it was not the only missing feedback.
Another
measure some climate models leave out is the complex carbon cycle
response from the Earth System as it warms. These include the carbon
feedbacks in the Arctic we’ve explored so much on this blog but
they also include large systems around the globe that are likely to
experience profound change as they warm. (It’s worth noting that
some models do include growing portions of these measures, such
as the NCAR global climate models which show up to 7 C warming with
each doubling of CO2).
Tropics
carbon out-gassing found to equal almost 20% of human emissions
(Forest
Fires in the Tropics, like these Amazon fires, are one of the many
human-driven changes to the region resulting in a 2 gigaton per year
annual carbon emission increase over the past 50 years. Image source:
Earth
Observatory.)
Now,
new
research from the University of Essex
has discovered that the tropics are also beginning to contribute
massive volumes of carbon to the global climate system. In total, the
study found that carbon emissions from the tropics had increased by 2
gigatons over the past 50 years in response to human-caused warming.
This 2 gigaton tropical emission is just a little less than 20% of
the carbon emission coming from the human use of fossil fuels. This
amount is a staggering total, roughly equivalent to total US carbon
emissions from all CO2 related sources. When combined with the
emission already ramping up in the Arctic, this new source is now a
very powerful contributor to warming the planet.
The
study showed that just one degree of global temperature increase was
enough to result in the added flood of this extraordinary volume of
carbon. The extra heat particularly impacted rain forest systems
which saw the increased insults of added heat, drought and fire.
Paper
authors, including Professor Friedlingstein, who is an expert in
global carbon cycle studies, attributed a good share of this change
to the increasing prevalence of drought which results in both more
rapid decomposition and more widespread forest fires.
It
is worth noting that tropical systems have been plagued with
increasing instances of fire and drought. As an example, the massive
forests of the Amazon are under the combined assault of drier
conditions, large forest fires, as well as smaller understory fires
that eat away at large sections of the Amazon. In a typical year, an
area of the Amazon the size of South Carolina is destroyed by small
fires alone. And it is mechanisms such as these that were found to
increase the rate at which tropical carbon is returned to the
atmosphere in a warming climate. Dr. Friedlingstein noted: “Current
land carbon cycle models do not show this increase over the last 50
years, perhaps because these models underestimate emerging drought
effects on tropical ecosystems.”
The
tropical forests are one of the largest carbon sinks on the planet.
And climate scientists have long warned that this carbon could be
returned to the atmosphere as the Earth was forced to rapidly warm by
human emissions during the 21rst Century. Overall, what we are now
witnessing is just such a rapid intensification of that transfer. It
is one that probably approaches the significance of another major
carbon stores transfer emerging in the Arctic. When combined with the
massive and still growing human fossil fuel emission, these carbon
flows are in the process of tipping the planet into a very hostile
and unstable new climate state.
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