Anthropogenic Changes in Atmospheric CO2
The natural CO2 cycle is so sluggish that the recent rise in atmospheric CO2 is entirely due to human
emissions. About 56% of the CO2 introduced by humans is removed fairly rapidly, but the remaining 44%
is only very slowly removed. For example, as shown in Table 1, CO 2 emissions over the 261 years
between 1750 and 2011 have introduced about 555 GtC (109 tons of C or 1015 g C as CO2, note 1 PgC = 1
GtC, where PgC is a picogram of carbon) into the atmosphere. About 375 GtC came from the burning of
fossil fuels (with a bit due to manufacture of cement), and 180 GtC from land use changes (mainly
deforestation). At the same time the volumetric concentration of CO 2 rose from 278 to 390.5 ppmv,
representing a mass addition of carbon of 240 GtC. Thus, about 43% of the introduced CO 2 has been
retained. This is exactly as might have been anticipated from the IPCC CO 2 removal formula discussed
here. Over a 100 year fuel transition, somewhat more CO 2 will be retained. A good number turns out to
be ~55%, as is shown here.
Table 1. Anthropogenic emissions to the atmosphere from 1750 to 2011 from IPCC (2013, Table 6.1.)
Land use changes
Fossil fuel combustion and cement
Total
Rise in atmospheric CO2
Percent anthropogenic Carbon retained in atmosphere
180 ±80 GtC
375 ±30 GtC
555
GtC
240 ±10 GtC
43%
Over the industrial era (1750 to 2011) about 1/3rd of anthropogenic CO2 emissions were related to land
use changes (Table 1). In the future land use changes will be much less, and fossil fuel combustion much
more influential, as shown in Figure 1.
For a reasonable business as usual fuel transitions in which oil and gas use continues over the next 50
years until these fuels (but not coal) are on the verge of exhaustion and then a transition is made to zero
carbon energy sources as described here, about 1220 GtC (giga tonnes of carbon CO 2) will be introduced
into the atmosphere. With a 55% retention, the concentration of atmospheric CO 2 will rise by 318 ppmv
(=1220 GtC x 0.55 x 0.470 ppmv/GtC), bringing atmospheric CO2 levels to 708 ppmv from the 2011 levels
of 390.5 ppmv. This 81% increase in atmospheric CO 2 would constitute a greenhouse forcing of 3.2 W
m-2 (see method of calculation here), and, if the forcing is maintained indefinitely, for the mid-range
(and most commonly used) equilibrium climate sensitivity of 0.8 C W-1 m2 (see discussion here), global
temperatures will eventually rise by ~2.56°C .
Ways in which this climate change might be reduced are discussed here.
Figure 1. Annual anthropogenic CO2 emissions from IPCC(2013) Figure 6.8b. Land use change currently constitute ~10% of
the total CO2 emissions, with 90% related to fossil fuel combustion. The increase in CO 2 in the atmosphere currently
accommodates ~50% of these emissions, with the rest taken up by land sinks and ocean dissolution. Fossil fuels will be an
even more dominant source in the near future, and atmospheric retention will grow to ~55%.