The carbon cycle
in the Earth system
T
he global carbon cycle is
the pathway by which carbon
moves through the Earth system, including the land, oceans,
atmosphere and biosphere.
Some components of the Earth system,
such as the oceans and land, at times act
as reservoirs of carbon by storing it for
long periods, and at other times act as
carbon sources by releasing it back into
the atmosphere (Fig. 1). Human emissions
of greenhouse gases such as carbon dioxide (CO2) and methane (CH4) are interfering with — and altering — this pathway.
Now, more than ever, understanding the
global carbon cycle in all its complexity is
a pressing research issue.
The climaTe connecTion
Since the onset of the industrial revolution,
more than two hundred years ago, greenhouse gases have been released into the atmosphere through human activities such as
burning fossil fuels and deforestation. Atmospheric concentrations of carbon-based
compounds, such as CO2 and CH4, are now
far higher than they have been at
any stage during the past several thousand
years. However, of the carbon released
in greenhouse-gas emissions, only about
40% remains in the atmosphere. The rest
is absorbed by the oceans and the land
biosphere.
On time scales of up to a few thousand
years, the atmosphere, oceans, vegetation
and soils rapidly exchange carbon in vast
amounts through a multitude of physical,
chemical and biological processes. Many of
these processes will either slow or accelerate
the growth of greenhouse-gas levels in
response to warming, and thus represent a
positive or negative feedback, respectively,
between the global carbon cycle and the
»
The disruption of the global carbon cycle is tightly linked to
human development, and to the need for energy and food
resources on land and in the seas.
climate. On land, for instance, warmer
temperatures can lead to enhanced soil
respiration, thereby increasing the release of
CO2 back into the atmosphere. Conversely,
in northern latitudes, warmer temperatures
can increase the length of the growing
season and foster enhanced CO2 uptake by
the vegetation1.
From sinks To sources
How can scientists keep an eye on the
cycling of carbon through such a complex
system? Some regions are especially
important in maintaining the global carbon
cycle and can provide vital clues to the
overall health of the Earth system.
On land, the most important regions for
sucking up carbon from the atmosphere are
the tropical rainforests of Amazonia, the
Congo basin and Southeastern Asia, and the
boreal forests and Arctic tundra.2 Collectively
known as the ‘green lungs’ of the planet,
these regions have vast quantities of carbon
locked up in vegetation and soil.3 Sizeable
fractions of boreal forest and tundra regions
have an added store of carbon in their
underlying permafrost layer. In a warming
climate, thawing of permafrost could thus
release large amounts of carbon as CO2 or,
in swamps and bogs, as CH4, which would
further amplify climate change4.
In the oceans, two important carbon
‘hot-spots’ exist in the North Atlantic
Ocean and the Southern Ocean around
Antarctica. Here, excess carbon moves
from the surface into deep waters where
it is stored over timescales of centuries
T
o assess which anthropogenic emissions of carbon dioxide are
compatible with the goal of limiting global warming to 2 °c,
our climate model must include changes in the carbon cycle.
earth-system simulations by the max Planck institute for meteorology
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Research Perspectives of the Max Planck Society | 2010+
to millennia. Changes in the oceanic
circulation in these areas, which might
happen as temperatures rise, could decrease
the oceans’ capacity to store carbon.
Initial studies suggest that this is already
happening in the Southern Ocean, which
calls into question whether, in the future,
carbon sinks will continue to operate or will
saturate and perhaps even become carbon
sources5.
carbon cycle challenges
The challenges for research are clear, starting
with understanding the carbon cycle as an
integral component of the global climate
system. For example, further studies are
needed to elucidate the key processes that
transform carbon in terrestrial and marine
ecosystems, and to understand how the
carbon cycle is coupled both to the cycles of
nutrients, such as nitrogen and phosphorus,
and to the hydrological cycle.
Also warranting further attention is
the multitude of climate system–carbon
cycle feedbacks that operate on timescales
ranging from days to to geological epochs.
This cannot be achieved without improving
our modelling tools. Here, the international
research community has a long-term
commitment to the improvement of
Earth-system models — in other words,
global climate models with a closed carbon
cycle3,6.
Complex models must be constrained
with real-world observations. Long-term
observations of key carbon hot-spots are
thus imperative. One operational example
demonstrate substantially reduced ‘permissible’ carbon dioxide
emissions during the twenty-first century when a coupled carbon cycle
is included (Roeckner, E. et al. Clim. Change doi:10.1007/s10584-010-9886-6,
2010).
Chemistry, PhysiCs and teChnology
The element carbon is a fundamental constituent of life. its global cycle is
tightly connected to the habitability of our planet.
human activities such as the use of fossil fuels, deforestation and biomass
burning are altering the global carbon cycle.
understanding the global carbon cycle and how it interacts with the climate
is a key research challenge; it is essential for managing future climate
change so that it remains within acceptable bounds.
Fig. 1 | The carbon cycle. Storage and movement of carbon in
land- and water-based systems.
is the Zotino Tall Tower Observatory facility
in the Siberian taiga forest, which includes
a 300-m-tall mast for measuring regional
atmospheric greenhouse gases, reactive
chemistry, aerosols and meteorology.7 A
similar observatory will be established in
the Amazon forest in the short term.
These ground-based measurements should
be complemented with repeated air- and
space-borne remote-sensing systems.
Images courtesy of Michael Hielscher, MPI for Biogeochemistry.
a sTable sTaTe
Perhaps most challenging of all will be
managing the carbon cycle such that it
continues to keep the planet in a stable
climatic state. From a natural-science
perspective, this will involve developing
non-fossil-fuel-based energy sources such
as biofuels, as well as finding ways of sequestering carbon such as by afforestation
or air capture and storage.
Technologies that might have multiple
benefits, such as using biomass taken from
the farming cycle, are especially worthy of
investigation. Biomass can potentially be
used either as a low-carbon fuel or as a
means of storing carbon. The latter case has
the added benefit of generating products
that could be traded for credits on the carbon market. Biomass that is transformed
into long-lasting carbon materials can effectively remove atmospheric CO2, at least
for the lifetime of the products, and as such
is termed ‘carbon negative’8.
The global carbon cycle and its management cannot be studied from only a
natural-science perspective. The disruption of the global carbon cycle is tightly
linked to human development, and to the
need for energy and food resources on
land and in the seas. Scientific assessment
of any management options thus clearly
needs to accommodate the multitude of
socioeconomic drivers in the modern
world. Addressing this in a rational, scientific way poses a huge challenge that must
be met in order to steer the Earth systems
within acceptable bounds over the next
100 years and beyond.
➟ For references see pages 70 and 71
(GtC, gigatonnes of carbon)
left and
below
The Zotino Tall Tower in
the Siberian taiga forest
is 300 m tall. It measures
regional atmospheric
greenhouse gases, reactive
chemistry, aerosols and
meterology.
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