Large-scale tree planting removes carbon dioxide from the atmosphere, but it also sets off warming effects that take centuries to appear, according to a study using an Earth system model at Simon Fraser University.
The paper, "Ocean dynamics amplify remote warming effects of reforestation", appears in Earth System Dynamics. Its lead author is Pierre Etienne Banville, a master's student in the university's Geography programme and a member of its Climate Research Lab. It was reported by Phys.org.
The first mechanism is a matter of colour. Forests are darker than grasslands and absorb more sunlight. Snow-covered grassland reflects a great deal of it and stays cooler, so planting trees across snowy grassy regions can add warming rather than remove it.
That warming does not stay where the trees are. It spreads through atmospheric circulation, producing climate effects a long way from the planting site.
The oceans then amplify it. Warmer air over the sea raises ocean temperatures, which increases evaporation, which puts more water vapour into the atmosphere. Water vapour is itself a greenhouse gas, so the effect feeds back on itself.
The timings do not match. The carbon removal benefit shows up relatively quickly as the trees grow. The ocean responds slowly, and the model suggests centuries before the full remote warming appears.
Banville's conclusion is a warning to anyone setting a dated target: "you need to account for all those other effects that will happen."
The scenario modelled is a large one. The study's "Reforested State" is reached after reforestation of half the model's grid cells, which is a planting programme on a planetary scale rather than a national one.
The study names no specific regions and the report of it gives no temperature figures, so the finding is about the shape of the effect over time rather than its size in any one place.
That shape is the interesting part, because it describes two clocks running at different speeds. The benefit is inside the current reporting period. The cost is not.
Climate targets carry dates, and a date is an accounting boundary as much as a deadline. A gain that lands within decades and a loss that lands within centuries are, for the purposes of a target set for the middle of this century, a gain.
The oceans have been described as responding slowly. In accounting terms they have been described as responding later, which is a different and much more convenient property.
Our analysts note that a liability maturing some centuries from now appears on no plan that anyone has yet been asked to sign, and that no auditor currently working is in a position to qualify the accounts.
The trees will be counted this century. The ocean will answer in a later one, in a reporting period for which nobody has been appointed.