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Vegetation like that in the Chapada das Mesas National Park, Brazil, is no longer removing as much CO\u2082 from the atmosphere<\/p>\n
Pulsar Imagens\/Alamy<\/p>\n<\/p>\n
Hot and wet weather in 2024 \u2013 the hottest, wettest year on record<\/a> \u2013 caused ecosystems on land to emit nearly as much carbon dioxide as they took out of the atmosphere, according to a preliminary analysis. This is the second year in a row in which the land carbon sink has nearly vanished due to climate-related stressors, and would explain why 2024 saw a record jump in the concentration of CO\u2082 in the atmosphere.<\/p>\n The findings could also mean that the land carbon sink \u2013 which normally removes billions of tonnes of CO\u2082 from the atmosphere each year and is essential for meeting climate targets \u2013 is weakening decades earlier than expected. It remains unclear, however, if the past two years represent a lasting trend.<\/p>\n \u201cEverybody I\u2019ve talked with who is working on this subject is very surprised,\u201d says Guido van der Werf<\/a> at Wageningen University in the Netherlands. \u201cIt was projected to be a weak sink, but now there\u2019s hardly any sink.\u201d<\/p>\n Forests, grasslands and other ecosystems on land take up CO\u2082 from the atmosphere as they grow. When they decompose or burn, this carbon is returned to the atmosphere. The uptake and release should generally be in balance. But over the past half century or so, these ecosystems have tended to take up more CO\u2082 than they have released, creating a carbon sink that varies in strength from year to year.<\/p>\n This tilted balance is thought to be due mainly to increased concentrations of CO\u2082 in the atmosphere fertilising plants, plus other factors like nutrient pollution and reforestation. But it isn\u2019t expected to last forever because the climate consequences of rising CO\u2082 are catching up with the fertilisation effect. \u201cWe know the land carbon sink will diminish, but we don\u2019t know how fast it will diminish,\u201d says Van der Werf.<\/p>\n In 2023, researchers were concerned when the land carbon sink nearly disappeared<\/a> due to extreme wildfires, heat and drought making ecosystems less productive. These conditions were driven mainly by rising concentrations of greenhouse gases<\/a>, as well as the emergence of the El Ni\u00f1o pattern in the Pacific ocean, which is generally associated with a weaker sink.<\/p>\n In 2024, the sink was expected to strengthen as El Ni\u00f1o faded and there were fewer wildfires. But an international research team that includes Van der Werf has found that the sink was extremely weak again.<\/p>\n To estimate carbon cycling, the researchers used satellite data on the greenness of the land\u2019s surface \u2013 which generally corresponds to plant growth \u2013 to calculate the productivity of the planet\u2019s terrestrial ecosystems, and therefore the amount of CO\u2082 they took up. They then subtracted from this the amount of CO\u2082 released in wildfires and by decomposition, using measurements of CO\u2082 in the atmosphere from all over the world to estimate how much was released.<\/p>\n They found the land carbon sink in 2024 nearly disappeared, removing around 2.6 billion tonnes less CO\u2082 than usual. This was even less than the struggling sink removed during the 2015-2016 El Ni\u00f1o, making it the weakest land carbon sink in over a decade.<\/p>\n Unlike in 2023, however, the researchers found this weakening wasn\u2019t mainly driven by fires and dry weather. Instead, their analysis found hot and wet conditions sped up the rate at which organic matter was decomposing. Ecosystems also saw an increase in productivity \u2013 greenness reached record highs \u2013 but this was outpaced by the overall amount of CO2 released, a measure called total ecosystem respiration.<\/p>\n \u201cThe large decline in land sink is caused by the respiration,\u201d says Guanyu Dong<\/a> at Nanjing University in China, who led the analysis. \u201cIt\u2019s two quite different underlying mechanisms.\u201d While most regions saw a weaker sink, this effect was especially notable in grasslands and shrublands in the tropics, he says.<\/p>\n Van der Werf says these numbers are a first estimate, and other teams teasing apart what happened with the carbon sink could get different results. But this weakening would explain most of the record jump in the concentration of CO\u2082 in 2024, which was too large to be explained by fossil fuel emissions alone.<\/p>\n The disappearance of the sink for the second year running could also be a sign it is diminishing earlier than expected. \u201cThis to me looks like the more pessimistic models may be right,\u201d says Van der Werf. The early loss of the sink would mean concentrations of CO\u2082 in the air would rise faster than anticipated, and and could contribute to temperatures continuing to rise even after reaching net zero emissions<\/a>.<\/p>\n