Research ArticleENVIRONMENTAL STUDIES

How close are we to the temperature tipping point of the terrestrial biosphere?

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Science Advances  13 Jan 2021:
Vol. 7, no. 3, eaay1052
DOI: 10.1126/sciadv.aay1052
  • Fig. 1 Temperature dependence of global carbon fluxes.

    The normalized global temperature response of C3 photosynthesis (green), which exhibits TPmax of 18°C, C4 photosynthesis (yellow) that exhibits TPmax at 28°C, and total ecosystem respiration (brown) derived from the FLUXNET 2015 synthesis dataset. The minor thermal optima observed in C4 classified sites validate the mixed C3/C4 nature of some ecosystems and were well explained by the sum of two Gaussian curves (see Materials and Methods). All fluxes were normalized and fit over ambient temperatures observed by FLUXNET (up to 38°C), where the mean across each curve sums to zero. Shaded areas represent the 90% confidence interval of projections.

  • Fig. 2 Temperature dependence of the terrestrial carbon sink.

    Integrated global temperature response curves for normalized photosynthesis (green dashed line), respiration (red dashed dotted line), and a mass balance estimate of the land sink (blue solid line) in relation to current climate (gray bar), where the mean across each curve sums to zero. Photosynthesis represents the integration of C3 and C4 curves (Fig. 1) weighted by global fraction of C3/C4 photosynthesis (37). The gray shaded bar represents observed mean annual temperature range from 1991 to 2015 (9, 22), and vertical dashed line indicates current annual mean temperature at FLUXNET tower sites.

  • Fig. 3 Spatial patterns of temperature tipping points for the biosphere.

    (A) Current cumulative monthly dose of temperature above TPmax by biome (see Materials and Methods) based on 1950–2010 WorldClim data (22). (B) Cumulative temperature dose above TPmax under Representative Concentration Pathway 8.5 (RCP8.5) by 2040–2060 based on WorldClim data (22). (C) Cumulative fraction of terrestrial biosphere in exceedance of TPmax by RCP scenario based on Coupled Model Intercomparison Project Phase 5 (CMIP5) multimodel mean monthly data. Vertical bars represent an integration of the uncertainty across CMIP5 ensemble member projections for changes in temperature, translated into a range of exceedance of TPmax for the vegetated surface. (D) Current mean gridded gross photosynthesis (2003–2013) (38) along with reductions in biosphere productivity due to exceedance of Tmax for 2040–2060 (44% reduction) and 2070–2090 (49% reduction) based on Worldclim CMIP5 downscaled data (22).

  • Definitions:
    Temperature signal: A change in photosynthesis or respiration solely
    attributable to changes in temperature, i.e., the signal excludes other
    climatic effects such as water availability and sunlight.
    Tipping point: This denotes the temperature threshold at which the
    global land surface will switch from a carbon sink to a carbon source
    when—due solely to temperature—photosynthesis is in exponential
    decline while respiration is in exponential increase.
    Thermal maxima: The thermal maximum (Tmax) represents the peak of
    the temperature dependence curve where any additional increase in
    temperature will decrease the metabolic rate of either photosynthesis
    or respiration.
    Warmest Quarter: Mean temperature of warmest quarter (3-month
    period)

Supplementary Materials

  • Supplementary Materials

    How close are we to the temperature tipping point of the terrestrial biosphere?

    Katharyn A. Duffy, Christopher R. Schwalm, Vickery L. Arcus, George W. Koch, Liyin L. Liang, Louis A. Schipper

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    • Figs. S1 to S6
    • Tables S1 to S3

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