Segments of the Amazon rainforest now emit more carbon dioxide than they can absorb because of human-caused disturbances, according to a study published in the journal Nature on Wednesday.

Why it matters: The Amazon region hosts the world's largest tropical rainforests and stores vast quantities of CO2, the primary long-lived greenhouse gas. Accelerating rates of deforestation and climate shifts due to human-caused global warming have damaged the forest's effectiveness as a climate change buffer.

The big picture: The researchers performed approximately 600 flyovers above the Amazon region between 2010 and 2018 to measure concentrations of CO2 and carbon monoxide at four sites.

• The aerial measurements revealed that total carbon emissions in eastern portions of Amazonia, which have been subjected to more deforestation and warming, were greater than those in the west.
• Specific regions in southeastern Amazonia experienced the strongest trends and switched from being carbon sinks to emitting more carbon than they could absorb during the study period.
• This marks a tipping point that scientists have foreshadowed in recent years.

How it works: Forests act as carbon sinks by capturing the gas through photosynthesis and storing it in biomass — plants and animals — dead, organic matter and soils.

• When the storage sources are destroyed in fires, most of which are intentionally set to clear land for agricultural purposes as well as through dry conditions, the forest's overall ability to sequester greenhouse gasses from the atmosphere is damaged.
• The region's water cycle also changes, potentially leading the rainforest to transition to a savannah.

What they're saying: Scott Denning, a professor at Colorado State University,wrote in an accompanying but unaffiliated article in Nature that the researchers "have documented the accelerating transition of forests from carbon sinks to sources."

• "The overall pattern of deforestation, warmer and drier dry seasons, drought stress, fire and carbon release in eastern Amazonia seriously threatens the Amazon carbon sink," Denning added.
• "Indeed, the results cast doubt on the ability of tropical forests to sequester large amounts of fossil-fuel-derived CO2 in the future."

Our thought bubble, via Axios' Andrew Freedman: As deforestation in the Amazon has increased in recent years and climate change has altered rainfall and temperature patterns, there's been increasing concern in the scientific and environmental communities that the Amazon could go from a net absorber of carbon dioxide to a source.

• This would make it even harder for the world to limit climate change to the goals set out in the Paris climate agreement.
• This study shows that for at least a portion of the Amazon, that tipping point from sink to source has been crossed.

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