Earth Radiance Trends

October 1, 2021

Introduction

The basis of the theory of global warming is that greenhouse gasses change terms of the energy balance of the earth atmosphere system. At its simplest, earth energy balance represents the balance between incoming and outgoing energy. Incoming energy is determined by absorbed sunshine. Outgoing energy is determined by infrared energy emitted by the earth and earth atmosphere. Of course, this excludes negligible terms, including the very slowly cooling earth interior.

Data

The National Aeronautics and Space Agency (NASA) includes a program called Clouds and the Earth’s Radiant Energy System (CERES) which consolidates cloud and radiation data from numerous satellites to estimate the earth radiation budget. In the words of the CERES web site:

“CERES is the only project worldwide whose prime objective is to produce global climate data records of ERB from instruments designed to observe the ERB.”

CERES provides estimates of incoming solar radiation, the amount absorbed, and of outgoing infrared radiation all depicted below.

Also considered are the radiation estimates from the ECMWF ReAnalysis (ERA5), which provides estimates of the past based on physically consistent solutions to the atmosphere including for areas which may not otherwise be measured.

Also below are solar radiation data from professor Greg Kopp, data from NASA’s climate models, and estimates of greenhouse gas forcing from the National Oceanic and Atmospheric Administration (NOAA).

Uncertainty

It is important to state that most of the estimates below contain uncertainty of an unknown extent. Past satellite estimates of solar radiation have a wide range of means. The reconstruction uses 'Community-Consensus TSI Composite'. Solar radiation reflected from earth is reflected largely by clouds, but cloud reflection is ‘anisotropic’, meaning sunshine may reflect differently depending on the angle of the sun and of the observer. Satellites have only a single angle of observation at any instant from a moving platform. These and other difficulties cast doubt on the data and trends below.

Incoming Solar Radiation

Figure 1 Estimates of incoming solar radiation from ERA5 reanalysis, from CERES satellite measurements, from the Kopp reconstruction based on sunspot data and other historical factors, and as used in NASA GISS climate models.

Figure 1 above depicts estimates of solar radiation reaching earth at the so called ‘top of the atmosphere’. This value represents the total amount of sunshine, before any reflection occurs. The long running series in blue is the reconstruction which is based on satellite observations for the most recent decades, but is based on a model using sunspot and isotope data for the pre-satellite era.

Features of note are the close correspondence of the ERA5 and CERES estimates with those of the reconstruction. The relatively low solar radiation of the 1600s represents the so called ‘Maunder Minimum’ when sunspots were not observed and solar radiation was thought to be low. Also of note, the NASS GISS models use solar radiation values that are roughly 1 W/m2 higher than those of other estimates.

Absorbed Solar Radiation

Figure 2 Estimates of solar radiation incident on earth that is absorbed from ERA5 reanalysis, CERES satellite estimates and NASA GISS model runs.

Figure 2 depicts the amount of solar radiation that is absorbed by earth and earth atmosphere. This value represents the incoming solar radiation minus the amount of solar energy reflected by earth.

Most of the reflection of solar energy occurs from clouds in the atmosphere. For the ERA5 estimates, the two downward spikes of solar radiation around 1983 and around 1991 are from the increased reflection imposed by the El Chichon and Mount Pinatubo volcanic eruptions. The NASA models and CERES satellite estimates of absorbed solar radiation have values that are relatively similar, while the ERA5 estimates are nearly 2 W/m2 higher.

The trend of the CERES data is positive and relatively steep at 6.7 W/m2/century. The trend of the ERA5 data is also positive, but not nearly so steep at 0.7 W/m2/century.

Outgoing Infrared Radiation

Figure 3 Estimates of outgoing infrared radiation from from ERA5 reanalysis, CERES satellite estimates and NASA GISS model runs, and NOAA estimated greenhouse gas forcing rates.

Figure 3 contains plots of the estimates of the outgoing infrared radiation from earth and earth atmosphere. The relatively smooth green segment from 1979 through 2020 represents the estimated radiative forcing from the increased of greenhouse gasses in that time span. The scale for these values is indicated on the right hand side. This estimate represents the theoretical reduction of infrared emission to space were warming not to have occurred.

The ERA5 estimates for outgoing infrared radiation are again nearly 2 W/m2 higher than those for CERES or the NASA models. For the years 2000 through 2020, the ERA5 trend is slightly negative ( decreasing amounts of infrared energy going to space ) while the CERES trend is positive ( increasing amounts of infrared energy going to space ).

Discussion

To reiterate, there is uncertainty with all the estimates. It may be that none of the above estimates are satisfactorily accurate. This is further exemplified by the differences between the estimates.

Of note, the trend of CERES absorbed solar radiation at 6.7 W/m2 per century is nearly twice as large as the NOAA greenhouse gas radiative forcing trend of 3.5 W/m2 per century. To the extent that the CERES absorbed solar radiation estimate is accurate and to the extent that this effect on the radiation budget is reflected in temperature trends, most of the warming for the period 2000 through 2020 is due not to greenhouse gas forcing, but rather increased absorption of solar radiation.

References

Copernicus Climate Change Service (C3S) (2017): ERA5: Fifth generation of ECMWF atmospheric reanalyses of the global climate, Copernicus Climate Change Service Climate Data Store (CDS), accessed January 6, 2021, https://cds.climate.copernicus.eu/cdsapp#!/home

NASA, (Earth Observing System, Langley Research Center), "Clouds and the Earth's Radiant Energy System (CERES)," accessed April 22, 2021, https://ceres-tool.larc.nasa.gov/ord-tool/jsp/EBAFTOA41Selection.jsp.

Kopp TSI reconstruction, based on 'Community-Consensus TSI Composite' (Dudok de Wit et al., GRL, 2017) and SATIRE-T model (Wu et al., A&A, 2018) (with modifications to fix spurious values prior to 1650) Model is scaled by 0.999996 (-0.0061 W/m^2) to match TSI data record (see Kopp & Lean, GRL, 2011) Extended using Community-Consensus TSI Composite annual averages from 1978 onward. Computed by Greg Kopp using 'Community-Consensus TSI Composite' on Thu Jul 18 14:31:53 2019. Data from: https://spot.colorado.edu/~koppg/TSI/Historical_TSI_Reconstruction.txt

J. Hansen, M. Sato, R. Ruedy, P. Kharecha, A. Lacis, R. Miller, L. Nazarenko, K. Lo, G. A. Schmidt, G. Russell, I. Aleinov, S. Bauer, E. Baum, B. Cairns, V. Canuto, M. Chandler, Y. Cheng, A. Cohen, A. Del Genio, G. Faluvegi, E. Fleming, A. Friend, T. Hall, C. Jackman, J. Jonas, M. Kelley, N. Y. Kiang, D. Koch, G. Labow, J. Lerner, S. Menon, T. Novakov, V. Oinas, Ja. Perlwitz, Ju. Perlwitz, D. Rind, A. Romanou, R. Schmunk, D. Shindell, P. Stone, S. Sun, D. Streets, N. Tausnev, D. Thresher, N. Unger, M. Yao, and S. Zhang, "Dangerous human-made interference with climate: A GISS modelE study", Atmos. Chem. Phys. 7,9 (07 May 2007): 2287-2312 https://doi.org/10.5194/acp-7-2287-2007, global annual NASA GISS data are from https://data.giss.nasa.gov/modelE/ar5plots/lptim.html

James H. Butler and Stephen A. Montzka, "NOAA Annual Greenhouse Gas Index(AGGI)," Spring 2021, https://www.esrl.noaa.gov/gmd/aggi/aggi.html.