The Greenhouse Effect
Empirical estimates of equilibrium climate sensitivity (ECS) can be obtained by comparing measurements of short-term radiation changes at the top of the atmosphere during the satellite era to corresponding changes in surface temperatures. Dr. Ray Bates from University College Dublin estimates ECS using a two zone energy balance model, where the radiative responses in the tropics (30 N to 30 S) and extratropics are estimated separately, and the dynamic heat transport from the tropics to the extratropics are explicitly estimated. Using likely ranges of the parameters from observations, he finds that the ECS is tightly constrained with a best estimate of 1.02 °C and a likely range of 0.85 °C to 1.28 °C.
Dr. Roy Spencer analysis 15 years of CERES satellite net top-of-atmosphere radiative measurements verses HadCRUT4.3 surface temperature, utilizing annual data with a four month time lag to maximize the correlations. Spencer writes, "Time-varying radiative forcing in the climate system (e.g. due to increasing CO2, volcanic eruptions, and natural cloud variations) corrupt the determination of radiative feedback. ... diagnosing feedback by comparing observed radiative flux variations to observed surface temperature variations is error-prone…and usually in the direction of high climate sensitivity." Using annual data reduces the noise caused by El Nino and La Nina. The results of the analysis shows a climate sensitivity of 1.3 deg. C. This analysis still includes non-forcing radiative noise, so the actual climate sensitivity is likely less than 1.3 deg. C for double CO2.
Dr. Richard Lindzen of the Massachusetts Institute of Technology explains how the greenhouse effect works in this paper published in 2007. Lindzen explains that the incoming energy to the earth from the sun is nearly balanced by the outgoing longwave radiation. He shows that increases in greenhouse gas concentrations causes the "characteristic emission level", which is about 7 to 8 km altitude in the tropics, to rise to a higher altitude where it is cooler. Emission from the cooler level is less, so the temperature rises a little to re-establish the radiative balanced. He also shows that the climate models predict that the warming above the tropics should be 2 to 3 times more than the warming at the surface, but the observations do not show this enhanced warming trend, implying that the positive feedbacks in the models are wrong. He concludes,"Using basic theory, modeling results and observations, we can reasonably bound the anthropogenic contributions to surface warming since 1979 to a third of the observed warming".
Climate models predict that back-radiation will increase with increasing CO2, but an analysis of accurately measured back-radiation in the southern USA shows a declining trend over the period from 1996 to 2010. This summary of the paper from "The Hockey Schtick" also shows that precipitable water vapor has declined over the period, which is the opposite of the model forecasts. The declining back-radiation trend is due to changes in cloudiness and water vapor.
The Transient Climate Response due to double CO2 is calculated using the CERES satellite outgoing longwave radiation measurements and HadCRUT surface temperatures. This analysis by FoS Director Ken Gregory suggests that the temperature change from June 2013 to January 2100 due to increasing CO2 would be 0.20 C (from HadCRUT3) or 0.39 C (from HadCRUT4), assuming the CO2 continues to increase along the recent linear trend. The transient climate response to doubled CO2 is 0.38 +/- 0.54 C using hadCRUT3, and 0.74 +/- 0.54 C using hadCRUT4 data at 95% confidence. These values are much less than the multi-model mean estimate of 1.8 C for TCR given in the IPCC 5th assessment report. Revised Feb. 23, 2014.