Speaking at the European Geosciences Union Assembly in Vienna, Allen detailed his SAFE – or Sequestered Adequate Fraction Extracted – carbon scheme for keeping total cumulative emissions of carbon below one trillion tonnes. Touted as an alternative to emissions permits or carbon taxes, SAFE carbon calls for fossil fuel firms to sequester a proportion of the carbon they extract.

That proportion would depend on the remaining atmospheric capacity for "safe" carbon emissions. This time last year, Allen and colleagues released work indicating that for a most likely peak warming of 2°C above pre-industrial temperatures, the world must emit a total of no more than one trillion tonnes of carbon (or 3.67 trillion tonnes of carbon dioxide). Two degrees of warming has been chosen by many parties, including the European Union, as a target for avoiding dangerous levels of climate change.

To date, we have emitted about half a trillion tonnes of carbon, leaving us with an atmospheric capacity of around a further half-trillion if we are to have an even chance of avoiding 2 degrees, said Allen. So the sequestered fraction under the SAFE scheme would have to increase, on average, by 1% for each 5 Gigatonnes of carbon released into the atmosphere. Crucially, the figure must reach 100% when the trillionth tonne of carbon is emitted, so that no further carbon is released to the atmosphere. The idea was published earlier this year.

"The only people who can afford to do this are the fossil fuel companies," said Allen, who reckons that they would pass this cost on to fossil fuel consumers.

The SAFE approach decouples carbon policy from consumption and may be easier to achieve politically, he believes. For example, if the world discovered that sequestration was cheap it may decide to follow an A1 scenario (i.e. high) emissions path. That said, when Allen raised the idea with politicians before the Copenhagen climate negotiations meeting in December they effectively told him to "shut up".

To those who suggest that we could emit more carbon now if we planned to pump it out of the atmosphere later, Allen points out that rapid removal of carbon dioxide can cause disruption to the hydrological cycle. "People tend to think of sucking carbon dioxide from the air as good geoengineering and solar radiation management as bad geoengineering," he said, "but it’s a bit more ambiguous than that." Such schemes in any case would require removal of several gigatonnes of carbon each year, which may not even be feasible.