Simulating effects of land use changes on carbon fluxes: past contributions to atmospheric CO₂ increases and future commitments due to losses of terrestrial sink capacity

Abstract

The impact of land use on the global carbon cycle and climate is assessed. The Bern carbon cycle‐climate model was used with land use maps from HYDE3.0 for 1700 to 2000 A.D. and from post‐SRES scenarios for this century. Cropland and pasture expansion each cause about half of the simulated net carbon emissions of 188 Gt C over the industrial period and 1.1 Gt C yr−1 in the 1990s, implying a residual terrestrial sink of 113 Gt C and of 1.8 Gt C yr−1, respectively. Direct CO2 emissions due to land conversion as simulated in book‐keeping models dominate carbon fluxes due to land use in the past. They are, however, mitigated by 25% through the feedback of increased atmospheric CO2 stimulating uptake. CO2 stimulated sinks are largely lost when natural lands are converted. Past land use change has eliminated potential future carbon sinks equivalent to emissions of 80–150 Gt C over this century. They represent a commitment of past land use change, which accounts for 70% of the future land use flux in the scenarios considered. Pre‐industrial land use emissions are estimated to 45 Gt C at most, implying a maximum change in Holocene atmospheric CO2 of 3 ppm. This is not compatible with the hypothesis that early anthropogenic CO2 emissions prevented a new glacial period.

Potential climate forcing of land use and land cover change

“…As noted in previous studies (e.g., Strassmann et al, 2008;Arora and Boer, 2010;Pongratz et al, 2009Pongratz et al, , 2014, this methodology does not account for the CO 2 -fertilization feedback in which the CO 2 attributed to LULCC leads to greater fertilization of natural and managed vegetation and an enhanced terrestrial carbon sink. Arora and Boer (2010) show that excluding the CO 2 -fertilization feedback leads to a form of "double-counting" land carbon storage and can cause overestimates of 20th century LULCC net carbon flux by about 50 %.…”

“…Land carbon uptake in coupled models using the CN version of CLM was only 40 % as sensitive to changes in CO 2 concentration and surface temperature increases (known as the climate change feedback) compared to the model used by Arora and Boer (2010). Therefore we adjusted the yearly LULCC net carbon flux downward by 20 % to account for the CO 2 fertilization feedback and make our calculations of CO 2 concentration increases attributed to LULCC more consistent with the "E2" group of studies as defined by Pongratz et al (2014), including Arora and Boer (2010), Strassmann et al (2008), and Pongratz et al (2009).…”

“…Lal (2004) estimates that cultivation has caused the loss of 78 ± 12 PgC from soils since 1850. Modeling studies suggest that LULCC can contribute a net loss of soil carbon globally, from ∼ 13 % of total LULCC carbon emitted (Strassmann et al, 2008) to ∼ 37 % (Shevliakova et al, 2009), or a net gain as in Arora and Boer (2010). Recently, Levis et al (2014) implemented a cultivation parameterization that includes impacts on soil carbon and found an additional global flux of 0.4 PgC yr −1 from soils due to crop management in recent decades.…”

“…Conversion of land from natural vegetation to agriculture or pasturage releases carbon from vegetation and soils into the atmosphere (Houghton et al, 1983), often quickly through fires, which emit carbon dioxide (CO 2 ), methane (CH 4 ), ozone (O 3 )-producing compounds, and aerosols (Randerson et al, 2006). Deforested areas have a diminished capacity to act as a CO 2 sink as atmospheric CO 2 concentrations increase (Arora and Boer, 2010;Strassmann et al, 2008). Furthermore, agriculture and pasturage emits CH 4 and nitrous oxide (N 2 O), accelerates soil carbon loss (Lal, 2004), and changes aerosol emissions (Foley et al, 2011).…”

Present state of global wetland extent and wetland methane modelling: methodology of a model inter-comparison project (WETCHIMP)

“…LPJ-Bern is a subsequent development of the Lund-PotsdamJena dynamic global vegetation model Joos et al, 2004;Gerber et al, 2003) that combines processbased, large-scale representations of terrestrial vegetation dynamics, soil hydrology (Gerten et al, 2004;Wania et al, 2009a), human induced land use changes (Strassmann et al, 2008;Stocker et al, 2011), permafrost and peatland establishment (Wania et al, 2009a,b) and simulation of biogeochemical trace gas emissions, such as CH 4 (Wania et al, 2010;Spahni et al, 2011;Zürcher et al, 2013). LPJ-Bern uses a different ebullition mechanism for CH 4 emissions from peatlands, which includes variations in partial pressure of CO 2 .…”