Recent reversal in loss of global terrestrial biomass

Vegetation change plays a critical role in the Earth’s carbon (C) budget and its associated radiative forcing in response to anthropogenic and natural climate change1–4. Existing global estimatesofabovegroundbiomasscarbon(ABC)basedonfield survey data provide brief snapshots that are mainly limited to forest ecosystems5–8. Here we use an entirely new remote sensing approach to derive global ABC estimates for both forest and non-forest biomes during the past two decades from satellite passive microwave observations. We estimate a global average ABC of 362PgC over the period 1998–2002, of which 65% is in forests and 17% in savannahs. Over the period 1993–2012, an estimated −0.07PgC yr−1 ABC was lost globally, mostly resulting from the loss of tropical forests (−0.26PgC yr−1) and net gains in mixed forests over boreal and temperate regions (+0.13 PgC yr−1) and tropical savannahs and shrublands (+0.05PgC yr−1). Interannual ABC patterns are greatly influenced by the strong response of water-limited ecosystems to rainfall variability, particularly savannahs. From 2003 onwards, forest in Russia and China expanded and tropical deforestation declined. IncreasedABC associatedwith wetter conditions in the savannahs of northern Australia and southern Africa reversed global ABC loss, leading to an overall gain, consistent with trends in the global carbon sink reported in recent studies1,3,9. Over the past two decades, the terrestrial biosphere has acted as a sink for atmospheric CO2, removing on average approximately 2.5 petagrams of carbon per year (PgC yr−1): equivalent to 25% of fossil fuel emissions1–4. Additional emissions from land-use change reduce the global net land sink to approximately 1.5 PgC yr−1, with forests playing a dominant role5. Monitoring C stock changes over time can be used to determine which ecosystems and processes drive changes in C fluxes and to develop strategies for climate change mitigation. The existing global ABC estimates based on field survey data, such as the recent global synthesis by Pan et al.5, provide snapshots in time and are limited to forest ecosystems only. Estimating ABC using satellite remote sensing can provide a more consistent methodology and global coverage6. Although offering high spatial resolution, current remote sensing products have limited temporal frequency and record length at the global scale6–8. Here, we derive global ABC estimates for all vegetation types for the past two decades using an entirely new approach that uses satellite-based passive microwave data rather than the optical or radar observations used previously. The intensity of natural microwave radiation from the Earth is a function of its temperature, soil moisture and the shielding effect of water in aboveground vegetation biomass, including canopy and woody components10–12. The biomass signal is captured in the vegetation optical depth (VOD; refs 13,14). A distinct advantage of passive microwavederived VOD is that it remains sensitive to biomass variations at a relatively high biomass density (for example, rainforests), whereas optical-based remotely sensed vegetation products rapidly saturate14,15. ABC estimates can be derived for all vegetation types, not only forests, as a suitable harmonized global VOD record exists from the 1990s onwards14. The main disadvantage of this technique is the relatively coarse spatial resolution (>10 km), which is a consequence of the low energy of the Earth’s natural microwave emissions. This means that individual plot measurements cannot be used directly to establish a relationship between VOD and ABC. Instead, we use an indirect calibration method based on spatial ‘snapshot’ ABC estimates from Saatchi et al.6, who combined three types of satellite observations with plot-based measurements to estimate ABC in tropical regions (see Methods and Supplementary Information).