labs_title

The terrestrial biosphere as a net source of greenhouse gases to the atmosphere

H. Tian, C. Lu, P. Ciais, A.M. Michalak, J.G. Canadell, E. Saikawa, D.N. Huntzinger, K.R. Gurney, S. Sitch, B. Zhang, J. Yang, P. Bousquet, L. Bruhwiler, G. Chen, E. Dlugokencky, P. Friedlingstein, J. Melillo, S. Pan, B. Poulter, R. Prinn, M. Saunois, C.R. Schwalm and S.C. Wofsy

Carbon dioxide, methane, and nitrous oxide are the three most important anthropogenic greenhouse gases in terms of their overall impact on the Earth’s radiative budget. This study explores not the direct emissions of these gases from human activity, but rather the emissions from global plants, animals, and microbes (a.k.a. biogenic emissions) that result from human actions. These fluxes include not only the global carbon “sink,” but also emissions from agriculture, waste management, and biomass burning, among others. The study finds that human actions have led to the land biosphere becoming a net emitter of greenhouse gases, further contributing to global climate change.


Figure: The overall biogenic GHG balance of the terrestrial biosphere in the 2000s. Top-down (TD) and bottom-up (BU) approaches are used to estimate land CO2 sink, CH4 and N2O fluxes for four major categories merged from 14 sectors. Global warming potential (GWP100) is calculated after removing pre-industrial biogenic emissions of CH4 (125 ± 14 Tg C yr−1)
 and N2O (7.4 ± 1.3 Tg N yr−1). Negative values indicate GHG sinks and positive values indicate GHG sources. TD* indicates estimates of agricultural CH4 and N2O emissions that include CH4 sources from landfill and waste, and an N2O source from human sewage, respectively.

Abstract

The terrestrial biosphere can release or absorb the greenhouse gases, carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), and therefore has an important role in regulating atmospheric composition and climate. Anthropogenic activities such as land-use change, agriculture and waste management have altered terrestrial biogenic greenhouse gas fluxes, and the resulting increases in methane and nitrous oxide emissions in particular can contribute to climate change. The terrestrial biogenic fluxes of individual greenhouse gases have been studied extensively, but the net biogenic greenhouse gas balance resulting from anthropogenic activities and its effect on the climate system remains uncertain. Here we use bottom-up (inventory, statistical extrapolation of local flux measurements, and process-based modelling) and top-down (atmospheric inversions) approaches to quantify the global net biogenic greenhouse gas balance between 1981 and 2010 resulting from anthropogenic activities and its effect on the climate system. We find that the cumulative warming capacity of concurrent biogenic methane and nitrous oxide emissions is a factor of about two larger than the cooling effect resulting from the global land carbon dioxide uptake from 2001 to 2010. This results in a net positive cumulative impact of the three greenhouse gases on the planetary energy budget, with a best estimate (in petagrams of CO2 equivalent per year) of 3.9 ± 3.8 (top down) and 5.4 ± 4.8 (bottom up) based on the GWP100 metric (global warming potential on a 100-year time horizon). Our findings suggest that a reduction in agricultural methane and nitrous oxide emissions, particularly in Southern Asia, may help mitigate climate change.

Tian, H., C. Lu, P. Ciais, A.M. Michalak, J.G. Canadell, E. Saikawa, D.N. Huntzinger, K.R. Gurney, S. Sitch, B. Zhang, J. Yang, P. Bousquet, L. Bruhwiler, G. Chen, E. Dlugokencky, P. Friedlingstein, J. Melillo, S. Pan, B. Poulter, R. Prinn, M. Saunois, C.R. Schwalm, S.C. Wofsy (2016) "The terrestrial biosphere as a net source of greenhouse gases to the atmosphere", Nature, 531 (7593), 225–228, doi:10.1038/nature16946.