Science never quits. New findings by German researchers challenge IPCC claim that greenhouse gases have driven recent warming. Data point to low cloud cover changes.
By Fritz Vahrenholt and Hans-Rolf Dübal
(Translated, edited and subheadings by P. Gosselin)
Hans-Rolf Dübal and Fritz Vahrenholt have investigated the Earth’s radiation balance over the past 20 years in a peer-reviewed publication appearing in ” Atmosphere”.
The net radiative flux, i.e. the difference between solar irradiance and longwave and shortwave radiation, determines the change in the energy content of the climate system. If it is positive, the Earth is warming; if it is negative, it means cooling. The NASA-operated satellite-based CERES project has now been providing such radiation data, as well as data on the evolution of cloud cover in temporal and spatial resolution for two decades. These data are determined both in relation to an altitude of approx. 20 km (TOA = “Top of Atmosphere”), and also in relation to the Earth’s surface.
This publication: “Radiative Energy flux variation from 2001 – 2020“, Dübal, H.-R.; Vahrenholt, F., appearing this week in Atmosphere 2021, 12, 1297 has brought to light a surprising result for climate science: the warming of the Earth in the last 20 years is mainly due to a higher permeability of clouds for shortwave solar radiation. During the period, shortwave radiation has strongly decreased (see figure) and this equally for the northern and southern hemisphere (NH and SH). With solar irradiance remaining nearly constant, this means that more shortwave radiation reached the Earth’s surface, contributing to warming.
The long-wave back radiation (the so-called greenhouse effect) contributed only to a lesser extent to the warming. It was even compensated to a large extent by the also increasing permeability of the clouds for long-wave radiation emanating from the earth. The authors came to this clear conclusion after evaluating the CERES radiation data.
Net positive energy influx due to less low clouds
NASA researchers led by Norman Loeb, along with Finnish researcher Antero Ollila, had already recently pointed out that shortwave solar radiation increased from 2005 to 2019 due to the decrease in low clouds. Dübal and Vahrenholt now have examined TOA and ground-level radiation fluxes for the entire period and related them to changes in cloud cover. The net energy influx was positive throughout the period, increasing from 0.6 W/m² to 0.75 W/m² from 2001 to 2020.
The 20-year average was 0.8 W/m². The next chart shows the drivers behind this change and these are clearly in the area of shortwave radiation in the cloudy areas, which account for about 2/3 of the Earth’s surface (SW Cloudy Area, +1.27 W/m²). This is in contrast to the assumption presented by the IPCC in its latest report that the warming due to the increase of longwave back radiation was solely due to the anthropogenic greenhouse effect.
Radiation flux differences modulated by cloud cover changes
The IPCC attributes 100% of the warming to this effect and justifies it with model calculations. However, the analysis of the measured data by Dübal and Vahrenholt show that the warming due to the decreased shortwave radiation (1.4 W/m²) and the increased longwave radiation (1.1 W/m²) is mainly due to the cloud effect.
Climate system enthalpy
The authors also considered the effect of this radiative excess on the heat content of the climate system for a longer period since 1750, where “enthalpy” means the sum of heat, work, and the latent heat, i.e., heat of evaporation of water, heat of melting of ice, energetic change of the biosphere (plant growth), and so on. Since about 90% of this enthalpy remains as heat in the oceans, conclusions about enthalpy evolution can also be drawn by looking at long-term ocean heat content (OHC).
Good agreement was found between these two independent datasets for the period 2001-2020. Moreover, existing OHC data were evaluated for earlier, longer periods to provide an overall picture. This shows that warming since 1750 has not been continuous, but has occurred in heating spurts, designated A, B, and C, during each of which a high net radiative flux (0.7 to 0.8 W/m²) acted for 20-30 years, interspersed with milder periods.
The onset of these heating episodes coincided with the change in sign of another known natural climate factor, the AMO (Atlantic Multidecadal Oscillation). The crucial question, whether the present heating phase C will soon come to an end as in cases A and B, or whether it will continue, can only be decided on the basis of longer observations and must therefore remain open.
Decreasing cloudiness since 2000
To investigate the beginning of Phase C around the year 2000, further datasets were used, especially the cloudiness measurements of EUMETSAT, a European satellite project. Here it can be seen that the beginning of phase C is accompanied by a decrease in cloudiness, coinciding with the above-mentioned change in sign of the AMO. From the radiation measurements it can be deduced that 2% less cloud cover means about 0.5 W/m² more net radiation flux.
This could explain most of the 0.8 W/m² mentioned above.
Cloud changes stronger than greenhouse effect
This is also corroborated by the analysis of the near-surface radiation balance. Here an increase of the greenhouse effect is found, which correlates well with the increase of the greenhouse gases water vapor and CO2, but only for the “clear sky” areas. This correlation does not apply to the cloud-covered areas, which make up about 2/3 of the earth. Interesting is the statement on the greenhouse effect.
“We could prove the increased greenhouse effect of the sum of all greenhouse gases (water vapor, CO2 etc.) under ‘clear sky’ conditions with 1.2 W/m² increase in the last 20 years,” says Hans-Rolf Dübal. “However, on an area-weighted basis, this increase is overcompensated by the increasing radiation of longwave radiation in the cloudy area’s amounting to -1.48 W/m².”
The time span of 20 years is still too short to decide conclusively whether the current heating phase is a temporary or permanent development. In the former case, climate projections need to be fundamentally revised. The physical mechanism that led to the cloud thinning is discussed differently in the literature.
“The cloud changes can be triggered by a decrease in aerosols, by atmospheric warming due to natural causes (e.g., the AMO or the PDO), by anthropogenic warming due to CO2, or by a combination of these individual factors,” says Fritz Vahrenholt. “However, one thing can already be stated: the warming of the last 20 years has been caused more by change in clouds than by the classical greenhouse effect.”