The latest preliminary data from Finland's 2023 greenhouse gas inventory reveals that forests have shifted from being a carbon sink to a carbon source. The latest inventory indicates that forests became a source of carbon in 2021. The change is attributed to increased logging, rising emissions in peatland forests and declining carbon sink of mineral soils.
Finland’s forest carbon sink has decreased on a steady trend, and in 2021, they turned from a carbon sink into a source of carbon, reports the Natural Resources Institute Finland (LUKE). In 2023, forests contributed 1.12 Mt CO2 equivalent emissions. Although forest growth continued to exceed removals through felling and natural loss, it was no longer enough to offset the increased emissions from forest soil.
Forests are part of the land use and land use change sector (LULUCF). The Finnish LULUCF sector emitted 11.8 Mt CO2 equivalent in 2023. The decline in the carbon sink of the LULUCF sector began gradually around 2010 and then accelerated rapidly from 2018 onwards. Since then, Finland's land use sector has been a source of emissions.
According to LUKE, the primary reasons for the changes in the forest carbon sink change are:
- A decline in the total biomass growth of trees (leaves, branches, trunk, root system).
- Increased logging. Approximately 90 percent of forest growth is now lost to logging and natural loss, compared to just over 70 percent in the early 2010s.
- Rising soil carbon dioxide emissions and a halt in the growth of carbon stocks in mineral soils. Climate warming accelerating decomposition, resulting in less new litter being produced from living trees.
“This is of course worrying news. Finland has based its climate policy partly on the forest sinks and now it seems this sink has disappeared”, comments Prof. Annalea Lohila, Focal Point of ICOS Finland. “The annual growth of trees, which used to increase since 1970’s until about 2010, is not increasing anymore but has become steady at about 100 Mm3, and can be explained at least partly by the age structure of forests. In addition, harvest levels have stayed high.”
Data from ICOS Finland was not directly used in the inventory. Greenhouse gas inventories, generally, rely on statistical and other spatial data. While the forest sites in Finland cannot be representative of the whole country, there are some ways ICOS Ecosystem data could benefit the inventories in the future.
"One possible way to incorporate ICOS Ecosystem data into the inventory is by including the indirect use of data to verify process models. These models are used to calculate soil carbon emissions or drive atmospheric inversions, which, in turn, utilise ICOS Atmospheric data”, Annalea Lohila says.
Rising temperatures increase emissions from forest soil
The soil plays a major role in the forest carbon sink. It can store three times more carbon than the biomass above ground.
According to LUKE, Finnish mineral soils also turned into an emission source in 2021. The long-term decline of the mineral soils from a sink to a source, which began in the early 2000's, is explained by the decrease in the stony input from trees (both absolutely and relative to the increased carbon stock) and global warming.
Rising temperatures also increase the rate at which peatlands break down and release greenhouse gases into the air. Previous inventory methods have included some uncertainties with emissions from peatland forests.
“The good news is that the inventory methods have developed and now reflect the emissions from the decaying peat in peatland forests, as well as their response to the warming climate, more realistically. Peatlands being more accurately included in the inventory is one - but not the only - reason behind the forests becoming a carbon source”, says Annalea Lohila.
Carbon neutrality will not be possible without strong land carbon sinks
To achieve carbon neutrality by 2035, Finland needs to reduce emissions by 19 million tonnes of CO2 equivalent. This target, and Finnish climate policy in general, has relied heavily on strengthening the carbon sinks of the land use sector. Achieving carbon neutrality will not be possible without strong land carbon sinks.
“We need to take care of the forests and ensure that logging rates do not continue to increase.”, Annalea Lohila emphasises. “One clear message for the research community is that there are still large uncertainties in the peatland forest emissions, and more data is needed to reduce the uncertainties. Also, long-term standardised data series from ICOS Ecosystem sites in upland forests serve as crucial benchmarks for continuously monitoring how exchange fluxes respond to climate variations.”
Finland is not alone in its experience of decline or vanishing land sinks. According to reporting by The Guardian, France, Germany, the Czech Republic, Sweden and Estonia are among those that have seen significant declines in their land sinks.
The challenges facing the Czech forest carbon sink are primarily related to extreme events which, as has been seen, weakens the health of the trees and thus their capacity to absorb carbon.
“The decline in the land sink in the Czech Republic was mainly caused by a spruce bark beetle epidemic that took place between 2017-2019. The outbreak, caused by droughts that weakened the natural defence mechanisms of the trees, damaged 3-5% of the Norway spruce forests, causing the destruction of these forests in some regions”, explains Dr Manuel Acosta, Senior Scientist, Global Change Research Institute CAS - CzechGlobe. “Nowadays the forest sink is affected by wildfires whose frequency has increased due to hot and dry summers. In the summer of 2022, the Bohemian and Saxon Switzerland National Park experienced devastating wildfire damage that threatened its ecosystem as well as nearby communities. Both mentioned events influenced the sink forest budget in the Czech Republic.”
As of early 2025, ICOS has around 45 forest stations around Europe that measure the greenhouse gas fluxes in European forests. This information is vital as countries monitor the state of their forest carbon sinks and work to ensure they remain effective for climate change mitigation.
- Learn more about ICOS Ecosystem measurements.
- Read the news by Natural Resources Institute Finland (LUKE) (in Finnish).
- Read the FLUXES article Forest Carbon Sinks Under Pressure (from 2023).