Assessing the potential of soil carbonation and enhanced weathering to
sequester atmospheric CO2, through Life Cycle Assessment: a case study
for Sao Paulo State, Brazil
Abstract
Scientists generally agree that, by 2100, the annual extraction of an
average of 3.3 Gt of carbon equivalent from the atmosphere will be
necessary to limit the increase in global average temperature to 2°C
relative to pre-industrial levels. A greenhouse gas (GHG) removal
technology (GGRT) is one that can remove a GHG from the atmosphere.
Enhanced silicate rock weathering for long-term CO2 sequestration has
considerable potential as a GGRT, but depends on the availability of
suitable rocks coupled with proximity to locations where it can be
applied. Extracting, processing, transporting and applying ground rock
to land consume energy and thus generate GHG emissions, which must be
balanced against the sequestration potential. In this study, we i)
assess the environmental impacts of the practice, using existing basalt
rock quarrying in Sao Paulo (SP) state by considering the emissions
associated with the rock extraction, comminution, transport and
application through a Life Cycle Assessment (LCA) approach; ii) estimate
the contribution of the different processes involved with soil
carbonation and enhanced weathering and iii) assess the potential net
CO2 removal of Sao Paulo agricultural land through soil carbonation and
weathering of basalt rock. Our results show that enhanced weathering and
carbonation respectively emit around 75 and 135 kg CO2eq per tonne of
CO2eq removed (considering an average quarry to field distance of 65
km.) We underline transportation as the principal process negatively
affecting the practice and establish a limiting road travel distance
from quarry to field of 540 ± 65 km for carbonation and 990 ± 116 km for
enhanced weathering. Above these distances, the GHG emissions exceed the
potential sequestration. The application of crushed basalt at 1 t/ha to
all of Sao Paulo State’s 12 million hectares of agricultural land could
capture around 1.3 to 2.4 Mt CO2eq through carbonation and enhanced
weathering, respectively. If 50 t/ha is applied, the sequestration
potential could reach up to 40 to 76 % of the total Sao Paulo State
emissions in 2016 through carbonation and enhanced weathering
respectively, assuming no loss in the effectiveness of basalt at high
application rates. This study lowers previous net sequestration
estimates from Kantola et al. (2017) by around 50% and, as various
techniques are increasingly promoted as efficient solutions to remove or
sequester CO2, it emphasizes the need to consider the practice in its
entirety to show the limitations and potential caveats. The holistic
approach of LCA helps identify processes with the most impact and their
significance. However, its accuracy and reliability is inevitably
limited by the amount, quality and representativeness of the input data.
Therefore, detailed studies on each process stage should be conducted to
increase accuracy, focusing on local practices especially transport.