n 1896 Svante Arrhenius wrote that human influence on the climate system might
become noticeable over the course of the next millennium (Box 2.1). In less than
120 years, human activities—mostly fossil fuel burning and deforestation—resulted
in the release of nearly two trillion tons of carbon dioxide (IPCC, 2013a), significantly
increasing concentrations in the atmosphere (Figure 1.3) and generating urgent concern
about climate change. Today, scientists, engineers, and policy makers are working
together to discover, validate, and implement strategies to reduce CO2 emissions as
well as other greenhouse gases. As such, efforts to reduce anthropogenic CO2 emissions
to the atmosphere are likely to be a primary component within the portfolio of
solutions to reduce climate change impacts (Figure 1.4). In addition, further mitigation
options involving the removal of CO2 from the atmosphere may provide cost-effective
means to stabilize atmospheric CO2 at concentrations that would limit adverse effects
of global warming (IPCC, 2014a).
In the sections that follow, the committee discusses various potential methods for
removing CO2 from the atmosphere, together with estimates about possible rates of
removal and total amounts that might be removed via these methods. To put these
rates and totals in context, Table 2.1 summarizes human emissions of CO2 and the associated
increase of CO2 in the atmosphere and CO2 sinks since 1750 and in a recent
10-year period. Over the past decade, human activities have produce approximately
34 GtCO2/yr annually with about 16 GtCO2/yr, or about 2 ppm/yr, accumulating in the
atmosphere (more recent estimates of annual emissions sources are ~39 GtCO2/yr:
36 GtCO2 from fossil fuel combustion and cement production and ~3 GtCO2 from
land use changes [Global Carbon Project, 2014]). Note that less than half of current
and historical anthropogenic CO2 emissions remain in the atmosphere; the remainder
(18 GtCO2/yr) has been taken up by the ocean and the terrestrial biosphere. This
existing uptake and removal of CO2 from air, natural “carbon dioxide removal” (CDR),
already moderates the impacts of human emissions on atmospheric CO2 levels and
global climate. Indeed this uptake is seasonally so great that atmospheric CO2 concentrations
intra-annually decline (Figure 1.3). Nevertheless, substantially increasing
existing CDR by natural or unnatural means such that the average annual growth rate
of atmospheric CO2 is reduced or reversed poses a significant challenge. One reason
is that if enough CO2 were removed from the atmosphere to cause a decline in overall
atmospheric concentrations, CO2 would “outgas” from the ocean into the atmosphere