Economic variations in dating
Nicholas Lewis writes, “In this context, what is IMO a compelling new paper by Bjorn Stevens estimating aerosol forcing using multiple physically-based, observationally-constrained approaches is a game changer.” Stevens is an expert on cloud-aerosol processes.
He derived a new lower estimate of aerosol forcing of -1.0 W/m.
Summary High estimates of climate sensitivity to greenhouse gases assume aerosols caused a large cooling effect, which canceled some of the previous warming effect, and little or no natural climate change.
A more policy-relevant parameter is the Transient Climate Response (TCR) which is the global temperature change at the time of the CO at the current growth rate of 0.55%/year would take 126 years.
The transient climate response (TCR) to greenhouse gas emissions, the warming when carbon dioxide (CO) doubles in about 125 years, was estimated by climatologist Nicholas Lewis at 1.2 °C using an energy balance approach and the new aerosol estimates but assuming that there was no natural long-term warming nor any urban heat contamination of the temperature record.
However, proxy records demonstrate that there are millennium scale natural climate cycles and numerous studies indicate that the major temperature indexes are contaminated by the urban heat island effect (UHIE) of urban development.
The IPCC did not provide a 95% upper estimate of ECS, but estimates the 90% upper limit at 6 °C.
The upper 95% limit dropped dramatically from 4.05 °C using AR5 forcing to only 2.2 °C when using the new Stevens aerosol forcing estimates.
The AR5 has substantially reduced this uncertainty compared to AR4, but this reduced uncertainty was not available soon enough to be incorporated into the climate models used in AR5.