Revised estimates of the annual net flux of carbon to the atmosphere from changes in land use and land management 1850–2000

Abstract

Recent analyses of land-use change in the US and China, together with the latest estimates of tropical deforestation and afforestation from the FAO, were used to calculate a portion of the annual flux of carbon between terrestrial ecosystems and the atmosphere. The calculated flux includes only that portion of the flux resulting from direct human activity. In most regions, activities included the conversion of natural ecosystems to cultivated lands and pastures, including shifting cultivation, harvest of wood (for timber and fuel) and the establishment of tree plantations. In the US, woody encroachment and woodland thickening as a result of fire suppression were also included. The calculated flux of carbon does not include increases or decreases in carbon storage as a result of environmental changes (e.g., increasing concentrations of CO 2 , N deposition, climatic change or pollution). Globally, the long-term (1850-2000) flux of carbon from changes in land use and management released 156 PgC to the atmosphere, about 60% of it from the tropics. Average annual fluxes during the 1980s and 1990s were 2.0 and 2.2 PgC yr −1 , respectively, dominated by releases of carbon from the tropics. Outside the tropics, the average net flux of carbon attributable to land-use change and management decreased from a source of 0.06 PgC yr −1 during the 1980s to a sink of 0.02 PgC yr −1 during the 1990s. According to the analyses summarized here, changes in land use were responsible for sinks in North America and Europe and for small sources in other non-tropical regions. The revisions were as large as 0.3 PgC yr −1 in individual regions but were largely offsetting, so that the global estimate for the 1980s was changed little from an earlier estimate. Uncertainties and recent improvements in the data used to calculate the flux of carbon from land-use change are reviewed, and the results are compared to other estimates of flux to evaluate the extent to which processes other than land-use change and management are important in explaining changes in terrestrial carbon storage.

Amplifying effects of land‐use change on future atmospheric CO2 levels

“…Similarly, variability or shifts in the ocean circulation, not accounted for in our model, could be responsible for the mismatch. It remains also possible that both Houghton's and our calculation of the land-use source is underestimated over 1800 -1970, as recent new estimations made by Houghton [2002] and House et al [2001] suggest.…”

Interactions between land use change and carbon cycle feedbacks

“…We consider the direct effect of land use conversion and harvesting on carbon, as well as a potential indirect impact, which is an interaction of changes in LULCC with climate, rising CO 2 , and nitrogen deposition (see Figure 1; we use some of the nomenclature from Gitz and Ciais [2003] and Pongatz et al [2009], but the details of our calculation are different). Note that there are many different approaches used in the literature for reporting land carbon stock changes, and thus we try to be clear here which processes we are including [e.g., Houghton, 2003;Gasser and Ciais, 2013;Lawrence et al, 2012b;Hansis et al, 2015]. The direct fluxes are by definition the change in carbon stocks that occur during 1 year from the conversion of natural lands to managed lands.…”

“…Components of vegetation that are left on site and that decompose more slowly, including wood, over subsequent years to decades are referred to as the QuasiDirect Carbon stock change (ΔQDC). ΔQDC is not archived to the output file but is important over the timescales investigated in this study (decades to centuries) [Houghton, 2003;Lawrence et al, 2012b]. The ΔQDC carbon changes are not usually included in the calculations for direct or indirect changes in carbon [e.g., Lawrence et al, 2012b], although in some approaches they are considered [Houghton, 2003;Gasser and Ciais, 2013].…”

Carbon stock changes in a peaty gley soil profile after afforestation with Sitka spruce (Picea sitchensis)

“…Substantial losses of C from vegetation and soils can also be caused by harvesting [10]. Soil carbon storage is likely to initially decline after clear cutting, because C inputs from plant production are too low to counteract losses by soil respiration.…”