Current atmospheric CO2 levels would be significantly higher were it not for the uptake and storage of CO2 by the Earth’s biosphere. Land ecosystems all over the globe provide important ecosystem services by removing about 30% of the CO2 emitted through burning fossil fuels and human changes in land use and land cover (see Figure 1). Land-use change (LUC) refers to human activities carried out on land and the ways land is used, while land cover is the physical cover of the Earth’s surface. This ecosystem service, commonly referred to as the natural land sink, slows down the growth rate of atmospheric CO2, hence reducing the rate of climate change. However, the natural land sink is not constant as it directly responds to environmental changes, both of natural and anthropogenic origins, which influence its capacity to absorb CO2 from the atmosphere. The amount of CO2 absorbed by the land has almost doubled since 1960, mainly because of a phenomenon known as CO2 fertilization. This is when the increase of CO2 in the atmosphere enhances plant photosynthesis and resource-use efficiencies, resulting in plants taking up and storing more carbon.
Decreasing land sink in the tropics
The increased natural land sink has so far occurred despite increased large-scale human disruptions to ecosystems, such as deforestation and degradation of natural areas, but it cannot be taken for granted in the future. There is now evidence that some of the largest carbon sinks of the planet have already saturated, particularly in tropical ecosystems, due to different reasons. First, there are processes that could eventually limit the sink. In particular, low availability of certain nutrients such as nitrogen and phosphorus, reduce the ability of global ecosystems to translate the increased photosynthesis into increased biomass and thus carbon storage. Recent studies highlight how CO2 fertilization effects on vegetation photosynthesis are globally declining as a result of these and other offsetting factors such as water limitations. Second, there are regionally specific processes that determine the net balance of the natural land sink and the net land-use change flux. While certain tropical regions appear to be at or near sink saturation, other regions such as boreal and temperate zones continue to see their sink capacity increasing. The decrease of the net sink in the tropics is mainly due to human LUC such as deforestation, while several factors drive increase in boreal forests, such as growing season extension and regrowth of forests from past disturbances. In some regions there is also an increase in forest mortality due to changes in the frequency of extreme weather events.
Wildfires and permafrost thaw add to emissions
Unprecedented carbon emissions have occurred due to wildfires in Australia, California, the Amazon, and the Arctic. Wildfires in 2020 are estimated to have caused global carbon losses of 244 megatonnes of carbon (corresponding to 895 megatonnes of CO2) and their impacts are predicted to worsen as a result of anthropogenic climate change. The ability of land to take up and store carbon is also negatively impacted by the warming of the soil (which increases decomposition rates) and thawing of permafrost. As was explained in Insight 2, the carbon release by thawing permafrost could be worse than previously expected. This “bad news” in cold regions is to some extent balanced by “good news” in warmer regions: recent studies suggest that previous models used to estimate long-term aridity changes have overestimated dryland aridification as they did not account for the watersaving effect of CO2 on plants. This means that carbon losses in some dry areas may not be as bad as feared.
The future will depend on how we
Several knowledge gaps exist regarding the future potential of the natural land sink and although it is now widely acknowledged that CO2 affects the productivity of global ecosystems, it is still unclear exactly to what extent this occurs. Better quantification of land-use change fluxes is thus key for a better understanding of the natural land sink. Land management is still an important unknown, but it is clear that practices that focus on decarbonization and simultaneously address food security, land-degradation and desertification are urgently needed. Different climate strategies based on nature-based solutions – such as the protection and sustainable management of ecosystems, the application of ecosystem-based approaches and of soil carbon sequestration (SCS) – currently exist. If well implemented, these strategies could potentially contribute to the goal of staying well below 2°C. However, approaches based on global afforestation need to take into account the potential negative impacts and trade-offs of tree planting. Focused attention on these knowledge gaps can help narrow down projections of the expected trajectory of the land sink under various socio-economic pathways, to better inform effective policy design.