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Contrasting climate and carbon-cycle consequences of fossil-fuel use versus deforestation disturbance

K. U. Jayakrishnan, Govindasamy Bala, Long Cao, and Ken Caldeira · Environmental Research Letters 17, 064020 · 2022

Key finding. A thousand years after an abrupt release of about 600 petagrams of carbon, fossil-fuel emissions leave roughly 20% of the initial atmospheric carbon dioxide perturbation in place and the climate about 1 degree Celsius warmer than preindustrial — whereas the same quantity released by deforestation with vegetation regrowth returns atmospheric carbon dioxide close to its preindustrial value.

Three stacked time-series panels over 1,000 years, comparing a red FOSSIL-FUEL curve with an orange DEFOREST curve against dashed preindustrial reference lines. Panel a is atmospheric carbon dioxide concentration, where both spike above 500 parts per million and then fall, the fossil curve levelling near 350 and the deforestation curve returning to the dashed preindustrial line. Panel b is change in surface air temperature, with the fossil curve holding near 1 degree Celsius and the deforestation curve returning to zero and slightly below. Panel c is change in surface ocean pH, with the fossil curve recovering only to about minus 0.05 while the deforestation curve returns above zero.
The same 600 petagrams of carbon, released two ways. The orange curves come home — the forest regrows and takes its carbon back — while the red curves do not, leaving about a degree of warming and a persistent acidification a thousand years later. Figure 5 from Jayakrishnan et al. (2022), Environmental Research Letters 17, 064020. Reproduced under CC BY 4.0. Extracted from the published PDF and resized for web display.

What question did this research address?

Carbon dioxide from clear-cutting or a forest fire is measured in the same units as carbon dioxide from burning coal, and inventories add them together. But a forest that is allowed to regrow takes its carbon back, and a coal seam does not.

This paper asked how different the two really are once the regrowth is accounted for, over the centuries and millennia that matter for a carbon budget.

What did we find?

Two 1,000-year simulations in a coupled climate-carbon model add the same abrupt emission, about 600 petagrams of carbon, to a preindustrial state: once as fossil fuel use, once as deforestation disturbance followed by regrowth.

The fossil case leaves about 20% of the atmospheric concentration perturbation after a millennium, with about 1 degree Celsius of residual warming.

The deforestation case does not, because the deforested land recovers its carbon over decades to centuries, pulling atmospheric carbon dioxide back down toward where it started.

The divergence is a matter of where the carbon can go back to. Fossil carbon has no reservoir waiting to reabsorb it on that timescale; forest carbon does, and the forest itself is the reservoir.

Why does it matter?

Emissions inventories that add land-use carbon to fossil carbon are summing quantities with very different futures. On a century-to-millennium view the two are not interchangeable, and a tonne is not simply a tonne.

The asymmetry cuts in a specific direction: it makes fossil emissions worse relative to deforestation, not deforestation harmless. Regrowth is doing the work, so the conclusion holds only where the forest is actually allowed to return.

That makes permanence the operative question for land-based carbon, which is the same issue that governs whether temporary storage counts — the value depends entirely on whether the carbon stays put.

Citation

K. U. Jayakrishnan, Govindasamy Bala, Long Cao, and Ken Caldeira (2022). Contrasting climate and carbon-cycle consequences of fossil-fuel use versus deforestation disturbance. Environmental Research Letters 17, 064020.

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