Climate Sensitivity by Energy Balance with Urban and Natural Warming

Contributed By Ken Gregory, P.Eng. 2020-06-14

The sensitivity of the Earth’s climate to increasing concentrations of greenhouse gases (GHG) is the most important parameter in climate science. Climatologists Nicholas Lewis and Dr. Judith Curry published a paper in the Journal of Climate in 2018 (LC2018) that used the observationally-based energy balance method to estimate the Equilibrium Climate Sensitivity (ECS) and the Transient Climate Response (TCR). The ECS is the global average surface temperature change due to a doubling of CO2 after allowing the oceans to reach temperature equilibrium, which takes about 1500 years for the upper 3 km of the ocean. The TCR is more relevant to climate policy as it is the global surface temperature change at the time of the CO2 doubling assuming that the change in forcing takes place gradually over at least 70 years, which it does for the base and final periods used. A doubling of CO2 at the current exponential growth rate of 0.60%/year would take 116 years.

The LC2018 paper states “The energy budget framework provides an extremely simple physically-based climate model that, given the assumptions made, follows directly from energy conservation.” The energy balance method relates the ECS and TCR to changes in the global mean surface temperature (GMST), the effective radiative forcing and the planetary radiative imbalance between a base and final period. The radiative imbalance is the downward solar radiation net of albedo (reflection) less the upward longwave radiation from the surface and the atmosphere at the top of the atmosphere.

The surface temperature change is based on the HadCRUT4.5 data set. LC2018 uses the GHG forcings as estimated in the fifth assessment report (AR5) of the Intergovernmental Panel on Climate Change (IPCC), except for an upward revision of the methane forcing, an upward revision of the lower uncertainty bound of the aerosol forcing and updating the forcings to 2016. Earth’s top-of-atmosphere radiative imbalance is necessarily equal to the total heat uptake by the climate system, which is over 90% by the oceans. The preferred base (1869-1882) and final periods (2007-2016) were chosen to avoid the period of sparse temperature data before 1869, avoid significant volcanism and to have the largest change in forcing so to give the narrowest uncertainty ranges. The long time between these periods averages out the temperature effects of short-term ocean oscillations such as the ENSO, the Atlantic Multi-decadal Oscillation and the Pacific Decadal Oscillation.

The energy balance method employed by LC2018 is deficient for two reasons;
• it falsely assumes that all of the temperature change from the base period was due to anthropogenic causes (other than a small solar irradiance forcing) and,
• it doesn’t account for the urban warming contamination of the surface temperature record which exaggerates post 1970 warming.

A paper by Gebbie and Huybers (GH2019) used an ocean circulation model and modern and paleoceanographic observations from both the end of the 19th century and the end of the 20th century to show that the deep Pacific Ocean is still cooling. The paper says “historical model simulations are biased toward overestimating ocean heat uptake when initialized at equilibrium during the Little Ice Age”. The ongoing deep ocean cooling revises Earth’s overall heat budget since 1750 downward by 35%. Taking this into account would revise downward the ECS calculated by energy balance.

Solar forcing may be several times larger than just that caused by the change in the total solar irradiance (TSI) as interpreted by the IPCC. A paper Scafetta et al 2019 shows that the TSI increased from the 1986 minimum to the 1996 minimum by about 0.45 W/m2 and that 2000–2002 was likely a grand solar maximum. This implies that the TSI forcing was greater than that used in LC2018. There are several estimates of TSI. Two historical TSI reconstructions are presented as figures 1 and 2. They are quite different but what is clear is that generally the TSI has been increasing through the 20th century and is significantly higher than that in 1600-1700 and 1800-1820.

Read the full paper here.