CO2 fertilisation and the greening of the Sahara

Not surprisingly, increasing CO2 in the air is greening the earth due to enhancement of photosynthesis. It’s self-evidently obvious as CO2 is the foundation of plant-based ecosystems. (So yes – if you’re a denizen of an ocean floor hydrothermal vent community, this won’t apply to you, but otherwise – pay attention!) Our current glacial period, the Pleistocene, is CO2 starved, and the current pronounced CO2 greening is proof of that. Let’s get to the literature.

One of the most quoted publications on recent CO2 greening is the Nature paper by Zaichun Chu and colleagues:

This influential paper made it no longer possible for the alarmist narrative to dismiss CO2 fertilisation by falsely attributing recent enhancement of plant growth to warming alone. Quoting from Chu’s abstract:

Here we use three long-term satellite leaf area index (LAI) records and ten global ecosystem models to investigate four key drivers of LAI trends during 1982–2009. We show a persistent and widespread increase of growing season integrated LAI (greening) over 25% to 50% of the global vegetated area, whereas less than 4% of the globe shows decreasing LAI (browning). Factorial simulations with multiple global ecosystem models suggest that CO2 fertilization effects explain 70% of the observed greening trend, followed by nitrogen deposition (9%), climate change (8%) and land cover change (LCC) (4%). CO2 fertilization effects explain most of the greening trends in the tropics, whereas climate change resulted in greening of the high latitudes and the Tibetan Plateau. LCC contributed most to the regional greening observed in southeast China and the eastern United States.”

Taub (2010) reviewed experiments of CO2 enrichment of mixed plant ecosystems and showed significant gains especially for C3 plants and legumes, while acknowledging compositional changes in mixed plant communities:

This year – 2020 – Vanessa Haverd and colleagues clearly demonstrated that increased CO2 in air, not warming, is the “dominant driver” of the plant growth enhancement that is happening worldwide, to the tune of 30% since 1900, and 47% per doubling of CO2. Now that’s a carbon sensitivity that can be believed:

McElwain et al looked at the plant palaeo record and showed that plant speciation rates were higher during periods of earth’s history when CO2 was higher – not too surprising either:

[this paper can be found on ResearchGate]

J. C. McElwain et al 2011. Long-term fluctuations in atmospheric CO2 concentration influence plant speciation rates. In: Climate Change, Ecology and Systematics, ed. Trevor R. Hodkinson, Michael B. Jones, Stephen Waldren and John A. N. Parnell. Published by Cambridge University Press.

© The Systematics Association 2011.

Venter et al. 2018 showed that trees are regrowing into subSaharan regions where they were formerly absent. Amusingly, while recognising the role of CO2 in this recovery of the trees, the authors felt the need to consider the proliferation of trees as a “problem” – something bad, such that they discussed the need to “mitigate” it. Trying to avoid acknowledging anything good from CO2 leads to contortions of logic and semantics. Trees growing in subSaharan Africa where they weren’t growing before should be celebrated, not “mitigated”!

Bond et al 2003 had shown that CO2 in air  determines the to and fro between forest and grassland, via the agency of fire. In low CO2 conditions, trees grow too slowly to become large enough to survive periodic Savannah fires. But as CO2 rises tree saplings can grow faster to reach a safe size before the next fire comes along. The result is re-forestation.

So greening of the Sahara desert and the repopulation of savanna with trees is the expected consequence of atmospheric enrichment with CO2. What’s not to like about that?

Finally, Rebecca Thomas and colleagues (2016) explored the mechanism for CO2 greening, finding it to be attributable to increased light use efficiency by photosynthesising leaves:

Just imagine – a green and verdant Sahara, covered in farms and fruit trees. That would shake up the world’s economic and political landscape.

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