Abstract
Terrestrial and oceanic carbon sinks together sequester >50% of the anthropogenic emissions, and the major uncertainty in the global carbon budget is related to the terrestrial carbon cycle. Hence, it is important to understand the major drivers of the land carbon uptake to make informed decisions on climate change mitigation policies. In this paper, we assess the major drivers of the land carbon uptake—CO2 fertilization, nitrogen deposition, climate change, and land use/land cover changes (LULCC)—from existing literature for the historical period and future scenarios, focusing on the results from fifth Coupled Models Intercomparison Project (CMIP5). The existing literature shows that the LULCC fluxes have led to a decline in the terrestrial carbon stocks during the historical period, despite positive contributions from CO2 fertilization and nitrogen deposition. However, several studies find increases in the land carbon sink in recent decades and suggest that CO2 fertilization is the primary driver (up to 85%) of this increase followed by nitrogen deposition (∼10%–20%). For the 21st century, terrestrial carbon stocks are projected to increase in the majority of CMIP5 simulations under the representative concentration pathway 2.6 (RCP2.6), RCP4.5, and RCP8.5 scenarios, mainly due to CO2 fertilization. These projections indicate that the effects of nitrogen deposition in future scenarios are small (∼2%–10%), and climate warming would lead to a loss of land carbon. The vast majority of the studies consider the effects of only one or two of the drivers, impairing comprehensive assessments of the relative contributions of the drivers. Further, the broad range in magnitudes and scenario/model dependence of the sensitivity factors pose challenges in unambiguous projections of land carbon uptake. Improved representation of processes such as LULCC, fires, nutrient limitation and permafrost thawing in the models are necessary to constrain the present-day carbon cycle and for more accurate future projections.
Highlights
Fossil fuel combustion in the industrial era has perturbed the global carbon (C) cycle by releasing a significant amount of CO2 to the atmosphere (IPCC 2013)
As the land carbon cycle presents the largest uncertainty in constraining the global carbon budget (IPCC 2013, Jones et al 2013, Friedlingstein et al 2014, Brovkin and Goll 2015, Sitch et al 2015, Wieder et al 2015) it is crucial to assess the drivers of changes in the land carbon sinks in the recent past, recent decades, and in the projections of future climate
The cumulative land carbon uptake estimated by thirteen CMIP5 models shows a multi-model mean of −19 PgC, the values range from a land source of −124 PgC to a land sink of +50 PgC in 2005 (Jones et al 2013)
Summary
Fossil fuel combustion in the industrial era has perturbed the global carbon (C) cycle by releasing a significant amount of CO2 to the atmosphere (IPCC 2013). The terrestrial biosphere is a sink for carbon in recent decades due to mainly the increases in net primary production (NPP = GPP − Rh), driven by the increases in atmospheric CO2 (Friedlingstein et al 2010, Le Quéré et al 2015, 2018, Sitch et al 2015, O’Sullivan et al 2019). As the land carbon cycle presents the largest uncertainty in constraining the global carbon budget (IPCC 2013, Jones et al 2013, Friedlingstein et al 2014, Brovkin and Goll 2015, Sitch et al 2015, Wieder et al 2015) it is crucial to assess the drivers of changes in the land carbon sinks in the recent past (historical period), recent decades, and in the projections of future climate
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