Is Global CO2 Fertilisation Cooling The ‘Greenhouse Effect’?

  • Date: 15/05/20
  • Patrick J Michaels, Climate Etc.

A new paper finds higher than expected CO2 fertilization inferred from leaf to global observations.  The paper predicts that the Earth is going to gain nearly three times as much green matter as was predicted by the IPCC AR5.

Figure 1. Historical annual GPP according Haverd et al. Plotted here are the effects of leaf-level physiological changes directly stimulated by carbon dioxide (dark green), the overall increase in leaf matter i(light green), and the effects of climate change (tan)—i.e. the increase in temperature since 1900.

Earlier this month, I posted a short piece about an explosive paper on planetary greening that appeared in the journal Global Change Biology. I’ve since mused that it deserves a considerably longer, more contextual post.

The innocuously titled paper, “Higher than expected CO2 fertilization inferred from leaf to global observations”, by Vanessa Haverd (of Australia’s CSIRO) and eight coauthors uses a biophysical model and observed climate to back-calculate global primary productivity (GPP; the net change in standing vegetation per year), and to forward-calculate it using climate model forecasts.

Abstract.  “Several lines of evidence point to an increase in the activity of the terrestrial biosphere over recent decades, impacting the global net land carbon sink (NLS) and its control on the growth of atmospheric carbon dioxide (ca). Global terrestrial gross primary production (GPP)—the rate of carbon fixation by photosynthesis—is estimated to have risen by (31 ± 5)% since 1900, but the relative contributions of different putative drivers to this increase are not well known. Here we identify the rising atmospheric CO2 concentration as the dominant driver. We reconcile leaf‐level and global atmospheric constraints on trends in modeled biospheric activity to reveal a global CO2 fertilization effect on photosynthesis of 30% since 1900, or 47% for a doubling of ca above the pre‐industrial level. Our historic value is nearly twice as high as current estimates (17 ± 4)% that do not use the full range of available constraints. Consequently, under a future low‐emission scenario, we project a land carbon sink (174 PgC, 2006–2099) that is 57 PgC larger than if a lower CO2 fertilization effect comparable with current estimates is assumed. These findings suggest a larger beneficial role of the land carbon sink in modulating future excess anthropogenic CO2 consistent with the target of the Paris Agreement to stay below 2°C warming, and underscore the importance of preserving terrestrial carbon sinks.”

The paper predicted that the earth is going to gain nearly three times as much green matter as was forecast in the last (2013) IPCC report. It is noteworthy that Haverd’s model very faithfully reproduced the satellite-sensed changes in leaf area index shown by Zhu et al. (2016), which found the greatest greenings to be in the world’s semiarid tropics, tropical forests, and a smaller (but significant) increase in temperate latitudes. (I noted that paper here in 2018).

It’s very reassuring when two radically different methods—satellite sensing (Zhu) and a biophysical model (Haverd) come up with pretty much the same answer: we are greening up the earth fast, especially in critical tropical ecosystems.

Under a plausible emissions pathway, this will pull so much carbon dioxide out of the air that we could meet the Paris Accord of keeping surface warming below 2⁰C. Specifically, the authors wrote,

[t]hese findings suggest a larger beneficial role of the land carbon sink in modulating future excess anthropogenic CO2 consistent with the target of the Paris Agreement to stay below 2°C warming…”

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