Declining emissions and rising atmospheric concentrations point to a stubborn fact about carbon dioxide: once it’s in the air, it stays there. How long, exactly, is a complicated question; for all intents and purposes, though, CO2 emissions are cumulative. The comparison that’s often made is to a bathtub. So long as the tap is running, a stoppered

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For most air pollutants, once the emissions stop, the pollutant will disappear and the impacts will go away. However, this is not the case for CO2, which stays in the atmosphere for centuries.

Recent good news is that the world’s forests have been a large and persistent carbon sink (storing more than they emit), locking away about 2.4 billion tons of carbon every year between 1990 and 2007, and satellite data for 2000–2017 indicate that one-third of the world’s vegetated area has been greening (showing a significant increase in the avera

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First, it takes decades before the trees grow enough to capture the maximum of CO2 they are supposed to. So the impact of such programs would (theoretically) be felt in decades while companies book the results of their offsets immediately.