Rising temperatures could trigger Earth's terrestrial ecosystems to transition from carbon sinks to carbon sources in the next 20 to 30 years, according to data from the world's largest continuous carbon monitoring network. The researchers suggest that up to half of land ecosystems could reach this tipping point - when plants begin to release carbon into the atmosphere faster than they sequester it - by 2100 under a business-as-usual emissions scenario. However, biomes that store the most carbon, including rainforests and Taiga forests, may lose more than 45% of their carbon sink capabilities by midcentury. Research suggests that climate change may play out through a series of tipping points - thresholds beyond which the climate system undergoes dangerous and irreversible shifts. To avoid these tipping points, the Paris Agreement established a goal of keeping global temperature increases below 2°C above preindustrial levels. But while scientists know that temperature influences the rates of photosynthesis and respiration in terrestrial ecosystems, which currently absorb about 30% of carbon emissions from human activities each year, it has remained uncertain how these processes will be altered on a global scale as the climate warms. To investigate when global and regional temperatures might reach the critical threshold at which the land's carbon sink declines, Katharyn Duffy and colleagues analyzed records spanning from 1991 to 2015 from the global network FLUXNET, which accounts for the movement of carbon dioxide between ecosystems and the atmosphere. The researchers determined photosynthesis and respiration changes attributed solely to changes in temperature at each flux tower site, then aggregated these temperatures at the biome and global levels. While the data suggests global photosynthesis reaches a peak rate at temperatures of 18° Celsius for C3 plants and 28° Celsius for C4 plants, and declines at higher temperatures, respiration rates increased across the full range of observed temperatures without appearing to reach a maximum threshold. While less than 10% of land ecosystems currently experience temperatures beyond these photosynthesis thresholds (and only for a small fraction of the year), Duffy et al. caution that failure to implement agreements that meet or surpass the Paris Accord goals could dramatically alter carbon storage in terrestrial biomes around the world.
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