Abstract
At biome-scale, terrestrial carbon uptake is controlled mainly by weather variability. Observational data from a global monitoring network indicate that the sensitivity of terrestrial carbon sequestration to mean annual temperature (T) breaks down at a threshold value of 16°C, above which terrestrial CO2 fluxes are controlled by dryness rather than temperature. Here we show that since 1948 warming climate has moved the 16°C T latitudinal belt poleward. Land surface area with T > 16°C and now subject to dryness control rather than temperature as the regulator of carbon uptake has increased by 6% and is expected to increase by at least another 8% by 2050. Most of the land area subjected to this warming is arid or semiarid with ecosystems that are highly vulnerable to drought and land degradation. In areas now dryness-controlled, net carbon uptake is ~27% lower than in areas in which both temperature and dryness (T < 16°C) regulate plant productivity. This warming-induced extension of dryness-controlled areas may be triggering a positive feedback accelerating global warming. Continued increases in land area with T > 16°C has implications not only for positive feedback on climate change, but also for ecosystem integrity and land cover, particularly for pastoral populations in marginal lands.
Highlights
Correspondence and requests for materials should be addressed to Warming climate extends dryness-controlled areas of terrestrial carbon sequestration
Observational data from a global monitoring network indicate that the sensitivity of terrestrial carbon sequestration to mean annual temperature (T) breaks down at a threshold value of 166C, above which terrestrial CO2 fluxes are controlled by dryness rather than temperature
We assume that net ecosystem-atmosphere exchanges of CO2 (NEE) in the areas close to the cold side (T, 16uC) of the shifted boundary are controlled by both temperature and dryness[5], written as NEEB that is predicted by equation (1)
Summary
Warming climate extends dryness-controlled areas of terrestrial carbon sequestration. The frontal boundary (or the shifted area) of the WPL has been transformed by global warming into more vunerable regions where weather gradients are stronger (Fig. 2), ecosystems are more sensitive to even slight increases in water deficit (Fig. 3)[25], crop yield is reduced by extreme heat waves[29], and vegitated land cover and pastroral population are reduced. The present work raises the following two questions: (1) what atmospheric circulation mechanisms support the hypothesis of a year time lag between the WPL temperature response and the ENSO water phases; and (2) is the synergistic poleward expansion of the frontal boundary of the WPL with the HC a long-term or a short-term behavior and what are the consequences of this synergy for global NEE and for the rate of change in atmospheric CO2?
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