We are now halfway to doubling the radiative forcing of CO2 since pre-industrial times, although CO2-concentrations are not halfway to doubling.
It is now 50 years since the publication of Syukuro Manabe and Richard Wetherald’s classic article, which in many ways marked the start of modern era of climate modelling.
Manabe and Wetherald calculated the effect of several possible drivers of climate change, but especially the effect of doubling CO2. The doubling of atmospheric CO2 concentrations has since become a standard experiment in climate research, and a convenient way of comparing the sensitivity of different climate models.
Gunnar Myhre (CiCERO) and colleagues Cathrine Lund Myhre (NILU), Piers Forster (University of Leeds) and Keith Shine (University of Reading) show that we are now halfway to doubling the radiative forcing of CO2 since pre-industrial times, although concentrations are not halfway to doubling.
Cathrine Lund Myhre states that the global average value of CO2 was 400 parts per million (ppm) in 2015 (WMO), while the global mean value in pre-industrial times was about 278 ppm. The combined radiative forcing from all well-mixed greenhouse gases presently, is 84% of the radiative forcing of a CO2 doubling.
– We estimate that total well-mixed greenhouse gases radiative forcing will be equivalent to a doubling of CO2, with present growth rates, by around 2030, says Gunnar Myhre.
– This is earlier than previous estimates, taking into account the revised stronger radiative forcing of methane, says Cathrine Lund Myhre.
– A recent study (Etminan et al., 2016) published in Geophysical Research Letters provides revised forcing estimates for methane that is 25% higher than in the previous IPCC report.