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A wildfire burning in the Amazon rainforest<\/p>\n
Fernando Lessa\/Alamy<\/p>\n<\/p>\n
Many of the forest-based carbon-offset schemes certified by the world\u2019s largest carbon registry, Verra, may be at risk of becoming useless due to wildfires or other disturbances releasing the carbon they store back into the atmosphere.<\/p>\n
A forest-based carbon-offset project involves protecting or replanting forests to generate carbon credits, which can then be sold to companies or individuals to offset their greenhouse gas emissions. Such projects are supposed to set aside \u201cbuffer pools\u201d of unsold carbon credits as a form of insurance against future carbon losses, such as when trees are destroyed by wildfires, insects or storms. But an analysis of these buffer pools shows carbon-offset schemes probably don\u2019t set aside enough.<\/p>\n
\u201cThe numbers aren\u2019t actually based on any science, as far as anyone can tell,\u201d says William Anderegg<\/a> at the University of Utah. A Verra spokesperson defended the organisation\u2019s approach, saying decisions about the size of buffer pools are \u201cgrounded in a robust science-based risk assessment\u201d and that projects that do dip into the buffer must replenish what they take.<\/p>\n Buffer pools are especially an issue when forest-based carbon credits are sold to offset carbon emissions from burning fossil fuels<\/a>, which remain in the atmosphere for centuries to millennia. \u201cIf you\u2019re going to try to lock up that carbon in a bunch of trees, you\u2019re going to have to guarantee that the carbon stays there for a very long time,\u201d says Anderegg.<\/p>\n In theory, a sufficiently large buffer pool offers that guarantee by protecting enough carbon to make up for any that might be lost during decades of disturbances. But earlier work<\/a> by Anderegg and his colleagues found that forest carbon projects certified by Verra have set aside just 2 per cent of credits, on average, as insurance against natural risks.<\/p>\n To determine whether this is a sufficient amount, Anderegg and his colleagues used an ecological model to estimate the size of the buffer that would be required to adequately balance out the risk of natural disturbances in various types of tropical forest. They compared their results against the buffer pools now required for Verra\u2019s certification.<\/p>\n They found that Verra\u2019s requirements are far too small to guarantee permanent carbon storage in almost all scenarios. In some cases, they are more than 11 times smaller than what would be needed. \u201cFor these natural risks, [the buffer] needs to be at least double, maybe more than double, to be adequate,\u201d says Anderegg.<\/p>\n The Verra spokesperson says few of the 76 million carbon credits currently in the buffer pool have been used, which indicates \u201cthe buffer has not been \u2018swamped\u2019 by reversals to date\u201d. \u201cIts effectiveness is demonstrated by the way the buffer pool has been maintained over time, even amid the risk of reversals,\u201d says the spokesperson.<\/p>\n Buffer pools aren\u2019t just a consideration for Verra. For instance, a carbon-offset programme managed by the state of California has seen wildfires over just the past few years eat up large chunks<\/a> of its buffer pool, which was meant to last for a century.<\/p>\n