Extreme heat, driven by climate change, sapped the ability of land ecosystems to absorb carbon dioxide (CO2) last year, leading to a record increase in atmospheric CO2 concentrations, according to new research published in the journal National Science Review.

The Earth’s land-based carbon sinks absorbed far less CO2 from the atmosphere in 2023 than usual, due in part to massive wildfires that ravaged Canadian forests and a powerful drought that dried out parts of the Amazon rainforest.

The research comes from members of the Global Carbon Project (GCP),  a global research project of Future Earth.

In an average year, land ecosystems suck up roughly every third molecule of CO2 emitted by human activities. However, in 2023, this capacity dropped to a fifth of its usual level, reaching the lowest carbon sink in two decades.

The study attributes nearly 30% of this decrease to extreme temperatures, with climate-enhanced disturbances like wildfires and droughts contributing to significant carbon losses in key regions, particularly the Amazon basin and Canada’s  boreal forests.

Pierre Friedlingstein, one of the paper’s  authors and lead author of GCP’s Global Carbon Budget based at the University of Exeter in the UK, said 2023 was “quite unique.” 

“Not only was it a strong El Niño year, which always leads to a reduced land sink in the tropics, but it also recorded very warm temperatures in the northern hemisphere, leading to large fires in Canada,” he told Future Earth. “Altogether, that led to a much lower-than-average global land carbon sink.”

While fossil fuel emissions only increased slightly, the diminished land sink alone was enough to push CO2 levels to new highs in 2023, as measured at the Mauna Loa Observatory, a US-run laboratory that holds the longest record of direct measurements of CO2 in the atmosphere. 

Impacts on the Amazon and Beyond

The droughts in the Amazon and widespread wildfires in Canada in 2023 released roughly the same amount of carbon as the entire fossil CO2 emissions from North America, illustrating the severity of climate change’s impact on natural ecosystems. The Amazon, one of the world’s most important carbon sinks, is now showing longer-term signs of strain, with some areas transitioning from sinks to sources of carbon emissions. 

David Lapola, a researcher not involved with the study but who is part of the Analysis, Integration, and Modeling of the Earth System (AIMES) research network, based at UNICAMP, Brazil, explained the complex interplay of factors affecting the Amazon’s resilience.

“Although we had a moderate El Niño in 2023, the whole Atlantic Ocean had way higher-than-normal surface temperatures. The role of the Atlantic Ocean in generating droughts in the Amazon is not mentioned in the study, but it plays a vital role, just like ENSO,” he said. “And now, in 2024, we should see the same fading carbon sink or an even worse pattern.”

Forests across the northern hemisphere have already been declining as a carbon sink in response to recurrent hot conditions since 2015, reaching half their former level by 2023.

The findings underscore the need for urgent and comprehensive global climate action. With natural carbon sinks under threat from climate change, humanity can no longer rely solely on ecosystems to absorb excess CO2. The focus must shift to reducing emissions at a faster pace while, at the same time, protecting and restoring vital ecosystems.

As Lapola cautioned, “On the one hand this reveals that there is clearly a limit to how much these forests can help us abate carbon emissions to the atmosphere. And on the other hand it also reveals that the Amazon is also vulnerable to climate change, which highlights that the responsibility over the world’s largest tropical forest is global, and not only of the Amazonian countries.”

A Call for Deeper Emission Cuts

Researchers say the weaker land carbon sink suggests the Earth’s ecosystems may be reaching their limits in terms of absorbing CO2, even before humanity passes the 1.5C warming threshold laid out in the Paris Agreement. The study emphasizes that climate models may underestimate the speed and scale at which extreme events, like droughts and fires, are eroding natural carbon sinks. 

As a result, emissions reductions will need to be even more aggressive than previously thought  to keep global warming within safe limits, explained Philippe Ciais, co-author of the study based at LSCE at the University Paris Saclay, France, and GCP member. 

“Earth system models lack processes for rapid carbon losses, such as extreme fires and climate-induced tree mortality. This implies that current estimates of remaining carbon budgets may be too optimistic,” he said. “We need faster and deeper cuts to anthropogenic greenhouse gas emissions if we are to have a chance of staying within the Paris Agreement’s warming thresholds.”

Beyond cutting emissions, the study calls for better management of forests to keep ecosystems resilient to climate change. Protecting old-growth forests and reducing deforestation would help mitigate some of the damage caused by extreme weather events.

The study illustrates the benefit of integrating real-time data and observations into the global carbon budget. Julia Pomgratz, a researcher at the Analysis, Integration, and Modeling of the Earth System project (AIMES) and GCP, based at LMU Munich, noted that rapid assessments of carbon cycle disruptions provide key insights for understanding the scale and impact of extreme events sooner after they occurred.

“Extreme events happen more and more frequently, like the massive Canadian forest fires, the extreme heat in the Mediterranean, or the drought in South America just in the last two years, and gain a lot of public attention,” said Pomgratz, who was not involved with the study. “The near-real time approach connects these events with detailed scientific insights into the impacts on the carbon cycle. This provides added value for communicating the urgency of climate change to the public.”