Tree planting schemes to “offset” carbon emissions offers guilt-free travel for those of us (like me!) with itchy feet. Or do they?

The idea is simple: if I’m concerned about the carbon emissions associated with flying or driving, I pay somebody to plant or restore vegetation that theoretically fixes an equivalent quantity of carbon as it grows, thus offsetting increases in atmospheric carbon.

But there are inherent limits on the amount of biomass an ecosystem can support (and thus the quantity of carbon that can be sequestered), set by the availability of key resources like light, water and nutrients. Through competition between species to access these resources and the myriad of ecological niches they create to do so, nature maximises the allocation of carbon to ecosystems for a given set of resource constraints.

Absurdly, only because we have reduced terrestrial carbon storage through land clearance are our ecosystems available to sequester carbon. Tree planting and ecosystem restoration schemes – at best – can never pay back more than this historical land clearing carbon debt. In practice, even this is unlikely, since resource constraints on biological production have intensified – land has been degraded, soil carbon and nutrients lost, and much of the land surface appropriated for other purposes.

Done properly, these schemes may (for now) result in carbon “neutrality” insofar as an amount of carbon equivalent to that emitted through fossil fuel combustion is sequestered in biomass. But it is dangerous to view the two processes as being equivalent.

Since the accumulation of fossil carbon in geological reservoirs is a process that takes millions of years, its transfer to the atmosphere via fossil fuel combustion is effectively permanent. This is a genie that cannot be put back in the bottle over time scales of human relevance.

Imagine that we lived in a world in which ecosystems were intact, but fossil carbon emissions were the same. How would we offset them? For the most part we couldn’t. We could always clear some land, but to then claim that the regrowth was sequestering carbon from fossil fuel emissions would seem like carbon accounting sleight-of-hand.

Tree planting schemes are immensely valuable in that they repay the land clearing carbon debt, and may have additional ecological and economic benefits. But they should be considered an adjunct to cuts in fossil fuel emissions, not an alternative.