What GHG Concentration Targets are Reachable in this Century?

Sergey Paltsev, John Reilly and Andrei Sokolov

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Method: We analyze five emissions scenarios. The first reflects commitments made at the Copenhagen conference through 2020, and extended to 2100. The second and third scenarios use the Copenhagen commitments through 2035 and 2015, respectively, and assume an immediate drop to zero emissions after those years. The fourth scenario uses the International Energy Agency's (IEA) proposal to constrain emissions to 450ppm through 2035, with a subsequent slow drop to zero. The last scenario is an alternative to the IEA scenario with a goal to reduce emissions to 350ppm and gradually constrain emissions to zero after 2035.

Key Results:

  • Reaching the 350ppm target is nearly impossible without the development of technology to remove CO2 from the atmosphere – such as using biomass for energy and capturing and storing the carbon when the biomass is burned. Absent this technology, the world community would not be able to constrain carbon emissions to 350ppm, even if CO2 emissions immediately dropped to zero.
  • It would be possible to reduce greenhouse gas concentrations to 350ppm in CO2-equivalence if other gases – like methane – are considered, but only if methane concentrations fell to pre-industrial levels. This would take eliminating basic human practices that are fundamental to our way of life, like growing rice and raising livestock. So in addition to transforming the world's energy sector, the world's agricultural sector would also need to be transformed.
  • The world could still slow warming to 2° Celsius – the target proposed as necessary to prevent serious irreversible consequences – if carbon and other greenhouse gas emissions were contained to 415-450 ppm through 2035 and subsequently were slowly reduced to zero. But, temperatures would peak before they fell because of ocean heat in-take that could take hundreds, or even thousands of years to balance out.