Reviews and syntheses: One forest carbon model to rule them all? Utilizing ensembles of forest cover and biomass datasets to determine carbon budgets of the world’s forest ecosystems

Abstract. Understanding global forest carbon stocks is necessary to assess the world’s global carbon budget, with land cover change being estimated to contribute roughly 20 % of the emissions of greenhouse gases to the atmosphere. In the last decade or so, remote sensing has contributed estimates of above ground stocks of biomass – a key part of forest carbon stocks – with over twenty biomass maps available at pan-tropical and global scales. To further the understanding of forest carbon stocks, this research seeks to synthesize the findings of disparate data sources on: (i) forest cover, (ii) forest cover change, (iii) above ground biomass (AGB) / above ground carbon (AGC) stocks in forests. Satellite-derived forest cover and AGB estimates have substantial variability. In 2020, forests were estimated to cover between 22.6 million and 49.7 million km² of the Earth’s land surface, thus ranging from 17.1 % to 37.6 % of total land cover. Likewise, examining forest cover change from available datasets, the estimated change in global forest cover between 2000 and 2020 was loss of approximately 88,734 to 124,184 km² per year, combined with regrowth of forest cover of approximately 58,628 to 169,912 km² per year. Combining that forest cover data with remotely sensed AGB estimates, total stocks of AGB for the year 2000 were estimated to be 325–697 Gt, while for the year 2020, the range was 401–580 Gt. The equivalent quantity of CO₂ (i.e., CO₂e) of that stock of forest biomass was therefore estimated to be 560 to 1,200 Gt for the year 2000, and 692–999 Gt for the year 2020. Our analysis found that the forest cover loss in tropics was the largest, at the rate of 1.4 % to 3.5 % net reduction between 2000 and 2020, whereas for the same time period, the temperate and boreal zones showed substantially lower forest cover loss (-2.5 % to 0.5 % and 1 % to 5.3 % respectively). This synthesis paper demonstrates that there is a fairly wide range of variability in estimates related to forest cover, forest cover change, and above ground biomass stocks, which are the main inputs for estimating forest carbon stocks and greenhouse gas emissions from land cover change.