finding 19.3 : key-message-19-3

Human-driven changes in land cover and land use will continue to be key contributors to carbon cycle changes into the future, both globally and in North America. Globally, land-use change is projected to contribute 10 to 100 Pg C to the atmosphere by 2050 and between 19 and 205 Pg C by 2100. Conversely, in the United States, land use and land-use change activities are projected to increase carbon stocks in terrestrial ecosystems by about 4 Pg C from 2015 to 2030. This projected increase is primarily driven by the growth of existing forests and management activities that promote ecosystem carbon uptake, often in response to changes in market, policy, and climate (high confidence).

This finding is from chapter 19 of Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report.

Description of evidence base:

Global estimates are based on Brovkin et al. (2013), who examined the difference in land carbon storage between the ensemble averages of simulations with and without land-use changes using RCP2.6 and RCP8.5. The RCP2.6 scenario assumes that climate change mitigation is partially achieved by increasing the use of bioenergy crops. Under this scenario, the global land area used for pastures is more or less constant over the simulation period, and increases in production (animal-based products) are achieved through changes in approaches to animal husbandry (Brovkin et al., 2013). In the RCP8.5 scenario, food demands and increasing population drive the expansion of croplands and pastures (and the loss of forested lands). The model ensemble includes six CMIP5 models for the projections: CanESM2, EC-Earth, HadGEM2-ES, IPSL-CM5A-LR, MIROC-ESM, and MPI-ESM-LR. Across all models, Brovkin et al. (2013) found a robust signal showing a loss of global land carbon storage because of projected land-use and land-cover change activities.

There is a lack of projections of emissions and sink trends for land use, land-use change, and forestry (LULUCF) activities specific to North America as a whole. U.S. estimates are based on the Second Biennial Report of the United States of America (U.S. Department of State 2016). That report presents a range in carbon sequestration estimates (689 to 1,118 teragrams [Tg] of CO2 equivalent [CO2e] per year by 2030) associated with U.S. land-use change and forestry activities. Also estimated is that emissions from forestry and land use will be 28 Tg CO2e in 2030.

To project cumulative carbon uptake from 2015 to 2030, the emissions estimate associated with forestry and land use (28 Tg CO2e) is subtracted from the low and high estimates of sequestration associated with forestry and land use (689 to 1,118 Tg CO2e). These values are then combined and divided by 2 to arrive at an average projected net uptake per year in 2030 of 875.5 Tg CO2e per year. This value is converted to teragrams of carbon (239 Tg C per year) and multiplied by 15 to arrive at a cumulative uptake of 3.6 Pg C from 2015 to 2030.

New information and remaining uncertainties:

Uncertainties arise from how land use and land-use change information is implemented into the carbon cycle representation of ecosystem models (i.e., the inclusion or exclusion of specific land-use processes such as wood harvest; Brovkin et al., 2013). In global projections, uncertainty also arises from the lack of coupled carbon-nitrogen (and phosphorus) dynamics in models. The models in this study do not account for the effect of nitrogen or phosphorus limitation on land ecosystems or CO2 fertilization.

For both the global and North American projections, there is also uncertainty in estimates of population growth and its potential impact on forest and agricultural land area. Moreover, there is general uncertainty in the potential future magnitude and timing of land-use change impacts on the land carbon cycle because of the difficulty in projecting the outcome of complex and interacting environmental, climate, and socioeconomic systems.

Assessment of confidence based on evidence: Several studies generally agree with high confidence that direct human influence on land use and land-cover change is a large driver of future potential carbon cycle changes. Model projections for North America agree that U.S. LULUCF activities will continue to result in net carbon uptake (i.e., carbon sequestration) to 2030. However, uncertainty in population growth and its impact on forests and agricultural land leads to considerable uncertainty in carbon uptake projections beyond 2030 associated with land-use change and forestry activities.

This finding was derived from figure P.2: P.2. Likelihood and Confidence Evaluation

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