The warming of the last 20 years is mainly due to changes in clouds - KS

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The main cause of the warming of the last 20 years is the change in the clouds October 9, 2021

by Kalte Sonne by Fritz Vahrenholt and Hans-Rolf Dübal In a peer-reviewed publication in “Atmosphere”, Hans-Rolf Dübal and Fritz Vahrenholt examined the earth's radiation balance over the past 20 years. The net radiation flux, i.e. the difference between solar radiation and long and short-wave radiation, determines the change in the energy content of the climate system. If it is positive, the earth heats up; if it is negative, it means cooling. The satellite-based CERES project operated by NASA has been providing such radiation data for two decades, as well as data on the development of cloud cover in temporal and spatial resolution. 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. The publication “Radiative Energy flux variation from 2001 - 2020” (https://www.mdpi.com/20734433/12/10/1297; Dübal, H. ‐ R .; Vahrenholt, F. Radiative Energy Flux Variation from 2001–2020. Atmosphere 2021, 12, 1297. https://doi.org/10.3390/ atmos12101297) has 1

brought to light a result that is surprising for climate science: the warming of the earth in the last 20 years is essentially due to a higher permeability of the clouds for the shortwave solar radiation. The short-wave radiation has decreased significantly during this period (see figure), and this applies equally to the northern and southern hemispheres (NH and SH). With almost constant solar radiation, this means that more short-wave radiation has reached the earth's surface and thus contributed to the warming. The long-wave reflection (the so-called greenhouse effect) only contributed to a minor part of the warming. It was even largely compensated for by the increasing permeability of the clouds for long-wave radiation emanating from the earth. The authors come to this unequivocal result after evaluating the CERES radiation data.

The NASA researchers led by Norman Loeb (https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021GL093047) and the Finnish researcher Antero Ollila (https://www.journalpsij.com/index.php/PSIJ / article / view / 30174) recently pointed out that short-wave solar radiation increased from 2005 to 2019 due to the decrease in low clouds. Dübal and Vahrenholt now examined TOA and ground-level radiation fluxes for the entire period and related these to changes in cloud cover. The net energy inflow was positive throughout the period and increased from 0.6 W / m² to 0.75 W / m² between 2001 and 2020. In the 20-year average it was 0.8 W / m². In the bridge chart, the driving forces of this change are shown and these 2

are clearly in the range of short-wave radiation in the cloudy areas, which make up about 2/3 of the earth's surface (SW Cloudy Area, +1.27 W / m²). This is in contrast to the assumption made by the IPCc in its most recent report that the warming caused by the increase in long-wave reflections was solely due to the anthropogenic greenhouse effect. The IPCC ascribes 100% of the warming to this effect and justifies this with model calculations. The analysis of the measurement data by Dübal and Vahrenholt show, however, that the warming due to the decrease of 1.4 W / m² of short-wave radiation and the 1.1 W / m² increased long-wave radiation is mainly due to the cloud effect.

The authors also considered the effect of this excess radiation on the heat content of the climate system for a longer period of time since 1750. The "enthalpy" means the sum of heat, work and latent heat, i.e. the heat of evaporation of water, heat of fusion from ice, energetic change in the biosphere (Plant growth) etc. Since about 90% of this enthalpy remains as heat in the oceans, conclusions can also be drawn about the enthalpy development by looking at the long-term oceanic heat absorption (Ocean Heat Content, OHC). A good agreement between these two independent data sets was established for the period 2001-2020 and the existing OHC data were evaluated for earlier, longer periods in order to obtain an overall picture. This shows that the warming has not occurred continuously since 1750, but rather in bursts of heating, designated as A, B and C, in each of which a high net radiation flux (0.7 to 0.8 W / m²) acted for 20-30 years, interrupted by milder phases. The beginning of these heating surges coincided with the change in sign of another well-known natural climate factor, the AMO (Atlantic Multidecadal Oscillation). The decisive question of whether the current heating phase C 3

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.

To investigate the beginning of phase C around the year 2000, additional data sets were used, especially the cloud measurements from EUMETSAT, a European satellite project.

It can be seen here that the beginning of phase C is accompanied by a decrease in cloud cover, at the same time as 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, which could explain the largest proportion of the abovementioned 0.8 W / m².

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This is also corroborated by the analysis of the near-surface radiation balance. An increase in the greenhouse effect is found here, which correlates well with the increase in the greenhouse gases water vapor and CO2, but only for the cloudless areas ("Clear Sky") Make up earth.

The observation about the greenhouse effect is interesting. Hans-Rolf Dübal: “We were able to demonstrate the increased greenhouse effect of the sum of all greenhouse gases (water vapor, CO2, etc.) under“ Clear Sky ”conditions with an increase of 1.2 W / m² in the last 20 years. However, this increase is overcompensated for by the area-weighted increase in radiation of long-wave radiation in the cloudy areas ("Cloudy Areas") amounting to -1.48 W / m². " The time span of 20 years is still too short to finally be able to decide whether the current heating phase is a temporary or permanent development. In the first case, the climate forecasts have to be fundamentally revised. The physical mechanism that led to the cloud thinning is discussed differently in the literature. Vahrenholt: “The cloud changes can be triggered by a decrease in aerosols, by warming of the atmosphere due to natural causes (e.g. the AMO or the PDO), by anthropogenic warming by CO2 or a combination of these individual factors. However, one thing can already be said: the warming of the last 20 years was caused more by changes in the clouds than by the classic greenhouse effect. ”. - Revised version, October 9, 2021.

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