by Fritz Vahrenholt and Rolf Dubal
The warming of the last 20 years has its essential cause in the change of the clouds.
We have investigated the Earth’s radiation balance over the last 20 years in a peer – reviewed publication in ” Atmosphere”. The net radiation flux, i.e. the difference between solar irradiation and long- and short-wave radiation, determines the change in the energy content of the climate system. If it is positive, the Earth is heating up; if it is negative, it means cooling. The NASA-operated satellite-based CERES project has been providing such radiation data for two decades now, 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.
Our new publication “Radiative Energy flux variation from 2001 – 2020″ 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 short-wave solar radiation. Short-wave radiation has decreased sharply over this period (see figure), equally in the northern and southern hemispheres (NH and SH). With solar radiation remaining nearly constant, this means that more shortwave radiation has 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 largely compensated for by the likewise increasing permeability of the clouds to long-wave radiation emanating from the Earth. The authors come to this clear conclusion after evaluating the CERES radiation data.
NASA researcher Norman Loeb and collaborators [link], as well as the Finnish researcher Antero Ollila [link], had already recently pointed out that the short-wave solar radiation increased from 2005 to 2019 due to the decrease in low clouds. Our latest publication has 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 bridge chart shows the drivers of this change and these are clearly in the area of shortwave radiation in the cloudy areas, which make up about 2/3 of the Earth’s surface (SW Cloudy Area, +1.27 W/m²).
This contrasts with the assumption made by the IPCC in its most recent report that the warming caused by the increase in long-wave back radiation was due solely to the anthropogenic greenhouse effect. The IPCC attributes 100% of the warming to this effect and justifies this with model calculations. However, the analysis of the measured data by Dübal and Vahrenholt shows that the warming due to the decrease of 1.4 W/m² short-wave radiation and the – 1.1 W/m² increase in long-wave radiation is mainly attributable to the cloud effect.
We 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), etc. Since about 90% of this enthalpy remains as heat in the oceans, conclusions about enthalpy development can also be drawn by looking at long-term ocean heat content (OHC). Good agreement was found between these two independent data sets for the period 2001-2020, and 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 episodes, 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 phases. The onset of these heating episodes coincided with the change of sign of another known natural climate factor, the AMO (Atlantic Multidecadal Oscillation). The crucial question of 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.
In order to investigate the beginning of phase C around the year 2000, further data sets were used, including the cloudiness measurements of EUMETSAT, a European satellite project. Here it can be seen that the onset 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, which could explain most of the 0.8 W/m² mentioned above.
This result 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 vapour and CO2, but only for the cloudless areas (“clear sky”). This correlation, however, does not apply to the cloud-covered areas, which make up about 2/3 of the earth.
We could prove the increased greenhouse effect of the sum of all greenhouse gases (water vapour, CO2 etc.) under “Clear Sky” conditions with 1.2 W/m² increase in the last 20 years. However, this increase is overcompensated on an area-weighted basis by the increasing radiation of long-wave radiation in the cloudy zones (“Cloudy Areas”) amounting to -1.48 W/m².
The time span of 20 years is still too short to be able to decide conclusively whether the current heating phase is a temporary or permanent development. In the former case, climate forecasts will 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 caused 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. However, one thing can already be stated: the warming of the last 20 years has been caused more by change in the clouds than by the classical greenhouse effect.