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Differing Net Zero definitions must ‘mesh’ say Met Office scientists

A new paper published in Nature has highlighted a fundamental mismatch in the way greenhouse gas emissions are measured which could mean that Net Zero could be met in one definition up to five years ahead of the other.

Decorate image of golden wheat harvested at sunset with combine harvester generated by artificial intelligence

Golden wheat harvested at sunset with combine harvester generated by artificial intelligence

The IPCC report shows that global temperature will stop increasing when we reach ‘net zero’ emissions of CO2. To achieve this, human activity cannot put more CO2 into the atmosphere than it removes – we need to massively reduce our emissions, with some removal of CO2 to help areas which are really hard to decarbonise. This sits behind the principle of Net Zero, which countries including the UK hope will be reached by 2050.

The paper by Gidden et al, highlights the differences between the two key greenhouse gas accounting systems: one which underlies the IPCC statement that net zero will halt global warming; and the other from how nations produce a National Greenhouse Gas Inventory (NGHGI) to report to the UNFCCC (United Nations Framework Convention on Climate Change).

Professor Chris Jones, who is the Met Office’s lead on carbon budgets, was asked to write a discussion document alongside the paper.


Chris Jones said: “Both of the key systems for measuring greenhouse gas emissions work well on their own. But there is a challenge when they have to work together in one view. It’s a little like having a bank account and spending in one currency, while trying to save in another. Both currencies work fine on their own, but merging the two together requires an exchange rate mechanism otherwise you risk financial jeopardy by spending beyond your means.”

The mismatch surrounds the different treatments of flows of carbon on so-called managed lands, such as croplands or plantations; these flows can be reported either as natural or human-induced depending on the definition being used. Under the IPCC definition, carbon cycle processes happening on managed lands (such as plantations) are considered natural (and therefore don’t contribute towards progress to net-zero), whereas under the UNFCCC definition the same processes are considered human-caused.

Chris added:

“Although these differences may sound negligible, the result is that the mismatch could add as much to five years to the difference between Net Zero being declared in one system and confirmed by the other. This matters hugely because Net Zero could be being celebrated for several years while billions of tonnes of emissions are still being released to the atmosphere which would cause further warming. The two processes must mesh – or we will miss our climate targets.”

The comment piece and original paper appear in the journal Nature.


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