Perturbations in the carbon budget of the tropics

Abstract

The carbon budget of the tropics has been perturbed as a result of human influences. Here, we attempt to construct a 'bottom-up' analysis of the biological components of the budget as they are affected by human activities. There are major uncertainties in the extent and carbon content of different vegetation types, the rates of land-use change and forest degradation, but recent developments in satellite remote sensing have gone far towards reducing these uncertainties. Stocks of carbon as biomass in tropical forests and woodlands add up to 271 ± 16 Pg with an even greater quantity of carbon as soil organic matter. Carbon loss from deforestation, degradation, harvesting and peat fires is estimated as 2.01 ± 1.1 Pg annum(-1); while carbon gain from forest and woodland growth is 1.85 ± 0.09 Pg annum(-1). We conclude that tropical lands are on average a small carbon source to the atmosphere, a result that is consistent with the 'top-down' result from measurements in the atmosphere. If they were to be conserved, they would be a substantial carbon sink. Release of carbon as carbon dioxide from fossil fuel burning in the tropics is 0.74 Pg annum(-1) or 0.57 MgC person(-1) annum(-1) , much lower than the corresponding figures from developed regions of the world.

Keywords: carbon emissions; deforestation; forest degradation; human development index; land-use change.

Figure 1

Statistical spread of above-ground Biomass for 11 types of land use in the tropics, plotted from the data of Ziegler et al. (2012). Box plots represent medians and quartiles, SD and outliers.

Figure 2

Typical carbon fluxes for temperate and tropical forests, and tropical savannah, showing Gross Primary Productivity (GPP), Net Primary Productivity (NPP), R_a (autotrophic respiration), Rh (heterotrophic respiration) and the overall carbon balance Net Ecosystem Production (NEP). Units are MgC ha⁻¹ annum⁻¹ for fluxes and MgC ha⁻¹ for biomass stocks (shown in the central box). Based on data from Luyssaert et al. (2007).

Figure 3

Estimates of the carbon flux from deforestation and degradation in the tropics. The solid black line is redrawn from Houghton et al. (2012). From the current analysis: black dashed line from the data of Hansen et al., 2013; the dark blue lines from calculations using the FAO data. The remaining lines are redrawn from the synthesis by Van der Werf et al. (2009) as follows: purple, DeFries et al., 2002; black dots, IPCC working group III (Barker et al., 2007; Nabuurs et al., 2007); pale blue, Achard et al., 2002; brown, IPCC working group I (Denman & Brasseur, 2007).

Figure 4

Components of human-induced change in the tropical biological carbon balance, with uncertainties. Negative denotes uptake from the atmosphere to the land surface, positive denotes loss of carbon to the atmosphere. The data are applicable to the period 2005–2010.