The Carbon Cycle - Introduction to Carbon Capture and Sequestration - page 125

Section 6
The future carbon cycle
What will the carbon cycle look like in the future? In response to anthro-
pogenic changes in atmospheric CO 2 , we expect the weathering mecha-
nism and the volcanos of the inorganic carbon cycle to eventually bring
the CO 2 levels back to approximately what they were in 1850. Recall
however that the time scale of this process is measured in units of
100,000 years! This is arguably relatively short on a geological time scale,
but what will happen in the meantime on the 100 year time scale associ-
ated with current political, economic, and ecological forces?
Climate model predictions
To address this time scale we employ the climate models that were dis-
cussed in Chapter 2. First we need to assume specifi c scenarios for
future carbon emissions. Climate model predictions have been made
with two general scenarios [3.12]: a “moderate” and a “large” scenario.
The “large” scenario assumes that most of the proven fossil fuel reserves
(mainly coal) will be consumed in the coming 300 years, giving a total
carbon emission of 5,000 Gt. The “moderate” scenario is one in which we
use only 20% of these reserves. To put these scenarios in context, the
“large” scenario assumes slightly lower emissions than the A2 SRES
scenario of the IPCC (see Figure 3.6.1 ). Also, recall that the earth's
natural emission from volcanos is about 0.15 Gt per year, which is a fac-
tor of 160 smaller than the maximum emissions in the “large” scenario
(see Box 2.5.1 ).
Can we predict future CO 2 levels in the atmosphere based upon
these emission scenarios? To answer this question, we examine the time
scales that impact the carbon cycles we have discussed in this chapter;
it is these processes that sequester excess CO 2 from the atmosphere.
These predictions are made with climate models. As we have seen in
Chapter 2, the standard way of testing the sensitivity of the predictions
is to run many different models built by different research groups around
the world. The idea is that each of these models employs different
assumptions, and if these models give very different predictions, then the