Global Greening Is Firm, Drivers Are Mixed

Evidence for global greening is converging, asserting an increase in CO2 uptake and biomass of the terrestrial biosphere. Global greening refers to global net increases in the area of green canopy, stocks of carbon, and the duration of the growing season. The growing seasons in general have prolonged while the stock of biomass carbon has increased and the rate of deforestation has decelerated, although these trends are mixed in the Tropics. Evidence for these trends comes from firm empirical data obtained through atmospheric CO2 observations, remote sensing, forest inventories and land use statistics. The drivers of global greening cannot be assessed based only on unambiguous empirical measurements. They include spatially and temporally heterogeneous combinations of changing land use and management - including green revolution and increasing yields, afforestation, forest protection and management, and abandonment of agricultural land -, changes in the global environment (increased CO2, warmer temperatures and longer growing seasons in the northern latitudes, acceleration of the global nitrogen cycle), and shifts in demand for forest and farm products. The global trade of biomass-derived commodities affects the link between consumption patterns and the land cover impact. Global greening confirms the immediacy of global change and may be associated with more or less biodiversity and diverse environmental and human consequences depending on local circumstances. Understanding causes, mechanisms, and implications of global greening requires integrated analyses spanning land use and management, demand for products of the terrestrial biosphere, and the atmosphere and climate. Understanding the pace and drivers of global greening matters crucially for assessing the future of the terrestrial C sink; ecological, economic, social, and cultural assessments of the bio-economy; and the preservation of ecosystems.