Environmental Engineering Reference
In-Depth Information
could release deep-sea carbon stored in the form of methane clathrates 7
in marine sediments. Other contributions could come from the substantial
reservoir of near-surface organic carbon in soils and permafrost, whose
stability is poorly understood. For example, a release rate of a half GtC per
year from such sources would add 2,500 Gt of carbon over 5,000 years to
the carbon emitted directly by humans. For reference, paleoclimate studies
suggest that during the Paleocene-Eocene Thermal Maximum (about 55
million years ago), similar amounts of carbon were released in less than
10,000 years. A number of recent studies show that large methane releases
from particular local sites have been observed, but these are too limited to
imply that globally significant changes are already occurring or will occur
for warming levels in the near term. {6.1}
2. Stabilization and Climate Change of the Next
Few Decades and Next Several Centuries
Because the global anthropogenic source of carbon dioxide greatly exceeds
the net global sink (through removal mechanisms in the ocean, land, and
biosphere), stabilization of carbon dioxide concentrations at any selected
target level would require reductions in total emissions of at least 80 per-
cent (relative to any peak emission level).
Unless the source matches the sink, concentrations of carbon dioxide
(and resulting warming influences) will continue to rise, much like the water
in a bathtub when water is coming in faster than it is going out. Because cur-
rent carbon dioxide emissions exceed removal rates, stabilization of carbon
dioxide emissions at current rates will not lead to stabilization of carbon
dioxide concentrations (see Figure S.4). A robust consequence of the stock
and flow nature of atmospheric carbon and the physics of the carbon cycle
is that emissions reductions larger than about 80% (relative to whatever
peak emission level occurs) are required to approximately stabilize carbon
dioxide concentrations for a century or so and even greater reductions in
emissions would be required in the longer term; this applies for any chosen
stabilization target.
Observed climate responses in coming decades will be smaller than the
longer-term temperature response to any given stabilization level. If car-
bon dioxide equivalent concentrations were to be stabilized at some point
7 Methane clathrates, also called methane hydrates, are material in which methane is trapped
inside a larger crystalline water chemical structure.
Search WWH ::




Custom Search