Thank you
TheVat for the comments. You ask
" ... have other researchers followed up on Ramanathan et al more recently?"That is something that puzzles me. This paper by Ramanathan et al (1989) and three others from 1995, all mention quantifications of cloud forcings, but since then, almost every paper I have come across seem to cautiously state words to the effect that clouds are the unknown quantity in 'climate change'. They also tend to put a spin on their conclusions to the effect that 'global warming' may be having some effect on clouds.
Noting a possible change in attitude by researchers over time, I did a search for the period 1989 to 1996 and found the following.
1989-1996Also in 1989, the same year as Ramanathan's paper, Wigley (1989;
https://www.nature.com/articles/339365a0) Abstract --
"It has been hypothesized that climate may be noticeably affected by changes in cloud condensation nuclei (CCN) concentrations, caused either by changes in the flux of dimethylsulphide (DMS) from the oceans1,2 and/or by man-made increases in the flux of sulphur dioxide (SO2) into the atmosphere3. When oxidized, the sulphur compounds produce non-sea-salt sulphate (n.s.s.-SO2−4,) aerosols, which may act as CCNs. The CCN changes affect climate by altering the number, density and size distribution of droplets in clouds, and hence their albedo. Here I am concerned primarily with the possible effects of SO2. Because the increase in SO2 emissions has been largely in the Northern Hemisphere, this raises the possibility of a cooling of the Northern Hemisphere relative to the Southern3. By comparing observed differences in hemispheric-mean temperatures with results from a simple climate model, one can place limits on the possible magnitude of any SO2-derived forcing. The upper limit is sufficiently large that the effects of SO2 may have significantly offset the temperature changes that have resulted from the greenhouse effect." Remember that Ramanathan suggested that the net negative cloud forcing may exceed the positive effect of doubling the carbon dioxide manifold.
Arking (1991;
https://journals.ametsoc.org/doi/abs/10 ... -0477(1991)072%3C0795:TREOCA%3E2.0.CO;2) conducted a review of the literature at the request of the Radiation Commission of the International Association of Meteorology and Atmospheric Physics (IAMAP) to establish an overview of key results published over the last 15 or 20 years, along with some relevant unpublished model studies. The focus is on 1) the impact of clouds on the incoming and outgoing radiation at the top of the atmosphere, and 2) the two-way interaction of clouds with other variables of the climate system—i.e., the cloud/climate feedback problem—as revealed by climate model simulations. Excerpts from the Abstract include "
There is general agreement that the annual global mean effect of clouds is to cool the climate system, but there is significant disagreement on magnitude, with the two' investigations based on recent satellite data indicating a range from 17 to 27 W/m2.", " Sensitivity of clouds to cloud condensation nuclei raises the issue of a more direct role of clouds in climate change, where aerosols associated with S02 emissions can ultimately lead to brighter clouds and a reduction in solar heating." " On cloud feedback in climate simulations, there are wide discrepancies amongst models." Please note the second excerpt suggesting that sulphur dioxide could have been associated with 'brighter clouds' (more droplets reflecting and deflecting more solar radiation).
Hartmann and Doelling (1991;
https://agupubs.onlinelibrary.wiley.com ... /90JD02065) also investigated the effect of clouds using data from the Earth Radiation Budget Experiment (ERBE). You'll remember that this was the same satellite program and data sets used by Ramanathan. They measured the net radiative effectiveness of clouds using the magnitude of outgoing longwave radiation and on reflected solar radiation to determine the net radiative effectiveness of clouds.
Both estimates of the cloud radiative effectiveness indicate that the global average net radiative effect of today's clouds is cooling, with the decrease in absorbed solar greater than the decrease in outgoing longwave radiation. The ratio of the solar to the longwave effect of cloud is about 1.85 based on the regression method and 1.55 based on the comparison of clear‐sky and average radiation budget climatologies.
The ERBE system of satellites was replaced by a new set referred to as CERES round about 1990.
Cess et al (1995;
https://science.sciencemag.org/content/267/5197/496)
Abstract --
"There has been a long history of unexplained anomalous absorption of solar radiation by clouds. Collocated satellite and surface measurements of solar radiation at five geographically diverse locations showed significant solar absorption by clouds, resulting in about 25 watts per square meter more global-mean absorption by the cloudy atmosphere than predicted by theoretical models. It has often been suggested that tropospheric aerosols could increase cloud absorption. But these aerosols are temporally and spatially heterogeneous, whereas the observed cloud absorption is remarkably invariant with respect to season and location. Although its physical cause is unknown, enhanced cloud absorption substantially alters our understanding of the atmosphere's energy budget."Rather than go through hundreds if not thousands of papers over the next 20 years I jumped to literature dated from 2013.
2013-2019This one seemed out of place with the thrust of the earlier ones and I put it in for the record -- Hartmann (2016;
https://www.pnas.org/content/113/32/8897.short)
" ... An advance was made in the most recent report of the Intergovernmental Panel on Climate Change, which concluded that cloud feedback is likely positive, meaning that the response of clouds to climate change acts to increase the magnitude of the surface temperature change (3). This consensus is based in part on the development of basic physical understanding of why high clouds get higher (4) and low clouds decrease their area coverage in a warmed climate (5, 6). In PNAS, Bony et al. (7) propose a basic thermodynamic mechanism that may cause the temperature profile to become more stable in the upper troposphere when the Earth warms. They expect this stabilization to cause anvil cloud area to decrease in a warmed climate, although they conclude that the effect of this anvil area reduction on cloud feedback is uncertain." It was decided by some sort of consensus; not basic science measurements. I could not find this mention in the IPCC report on clouds, but I will have something to say on the IPCC recommendations later.
The next lot seemed to lack quantification and all seemed a bit wishy-washy in that they all seemed to suggest uncertainty about clouds and climate change.
Stocker et al (2013;
https://agupubs.onlinelibrary.wiley.com ... 18GL081871) --
" ... Unfortunately, the extent to which the myriad of possible climate feedbacks influences Arctic amplification is currently plagued with uncertainty. This is especially the case for cloud feedbacks, which continue to be the leading cause of uncertainty in climate projections. "Ying and Thompson (2019;
https://journals.ametsoc.org/doi/full/1 ... -18-0417.1) --
" ... there is still considerable uncertainty about the underlying mechanisms, whereby CRE (Cloud Radiative Effects) govern the jet response to climate change."Nuijens and Siebesma (2019;
https://link.springer.com/article/10.10 ... 19-00126-x) --
" Clouds in nature are more complex than the idealized cloud types that have informed our understanding of the cloud feedback. Remaining major uncertainties are the coupling of clouds to large-scale circulations and to the ocean, and mesoscale aggregation of clouds."Hentgen et al (2019;
https://agupubs.onlinelibrary.wiley.com ... 18JD030150) --
" Although crucial for the Earth's climate, clouds are poorly represented in current climate models, which operate at too coarse grid resolutions and rely on convection parameterizations."Sancho-Lorenzo et al (2019;
https://www.nature.com/articles/srep41475 --
"Clouds affect the global energy balance as they reflect a large fraction of the Sun’s incoming radiation and at the same time absorb and emit longwave radiation1,2. Despite their relevance, large uncertainties remain with respect to the response and feedbacks of the clouds to anthropogenic forcing. Consequently they are considered as one of the main sources of uncertainty for climate sensitivity and future climate scenarios3,4,5."Those that investigated reduced cloud as a cause of possible increased surface solar radiation seemed more positive.
Reduced cloud literatureSanchez-Lorenzo et al (2017;
https://www.nature.com/articles/srep41475)
Abstract "Here we report a widespread decrease in cloud cover since the 1970 s over the Mediterranean region, in particular during the 1970 s–1980 s, especially in the central and eastern areas and during springtime. Confidence in these findings is high due to the good agreement between the inter-annual variations of cloud cover provided by surface observations and several satellite-derived and reanalysis products, although some discrepancies exist in their trends. Climate model simulations of the historical experiment from the Coupled Model Intercomparison Project Phase 5 (CMIP5) also exhibit a decrease in cloud cover over the Mediterranean since the 1970 s, in agreement with surface observations, although the rate of decrease is slightly lower. The observed northward expansion of the Hadley cell is discussed as a possible cause of detected trends."
Kambezidis et al (2016;
https://www.sciencedirect.com/science/a ... 2616303145)
Partial Abstract; " … Although several studies have examined the solar radiation trends over Europe, North America and Asia, the Mediterranean Basin has not been studied extensively. This work investigates the evolution and trends in the surface net short-wave radiation (NSWR, surface solar radiation - reflected) over the Mediterranean Basin during the period 1979–2012 using monthly re-analysis datasets from the Modern Era Retrospective-Analysis for Research and Applications (MERRA) and aims to shed light on the specific role of clouds on the NSWR trends. The solar dimming/brightening phenomenon is temporally and spatially analyzed over the Mediterranean Basin. The spatially-averaged NSWR over the whole Mediterranean Basin was found to increase in MERRA by +0.36 Wm−2 per decade, with higher rates over the western Mediterranean (+0.82 Wm−2 per decade), and especially during spring (March-April-May; +1.3 Wm−2 per decade). However, statistically significant trends in NSWR either for all-sky or clean-sky conditions are observed only in May.
The increasing trends in NSWR are mostly associated with decreasing ones in cloud optical depth (COD), especially for the low (<700 hPa) clouds. The decreasing COD trends (less opaque clouds and/or decrease in absolute cloudiness) are more pronounced during spring, thus controlling the increasing tendency in NSWR. The NSWR trends for cloudless (clear) skies are influenced by changes in the water-vapor content or even variations in surface albedo to a lesser degree, whereas aerosols are temporally constant in MERRA. The slight negative trend (not statistically significant) in NSWR under clear skies for nearly all months and seasons implies a slight increasing trend in water vapor under a warming and more humid climatic scenario over the Mediterranean.
Pfeifroth et al (2018;
https://www.researchgate.net/publicatio ... _in_Europe)
Abstract --
"Solar radiation is the main driver of the Earth's climate. Measuring solar radiation and analysing its interaction with clouds are essential for the understanding of the climate system. The EUMETSAT Satellite Application Facility on Climate Monitoring (CM SAF) generates satellite-based, high-quality climate data records, with a focus on the energy balance and water cycle. Here, multiple of these data records are analyzed in a common framework to assess the consistency in trends and spatio-temporal variability of surface solar radiation, top-of-atmosphere reflected solar radiation and cloud fraction. This multi-parameter analysis focuses on Europe and covers the time period from 1992 to 2015. A high correlation between these three variables has been found over Europe. An overall consistency of the climate data records reveals an increase of surface solar radiation and a decrease in top-of-atmosphere reflected radiation. In addition, those trends are confirmed by negative trends in cloud cover. This consistency documents the high quality and stability of the CM SAF climate data records, which are mostly derived independently from each other. The results of this study indicate that one of the main reasons for the positive trend in surface solar radiation since the 1990's is a decrease in cloud coverage even if an aerosol contribution cannot be completely ruled out." Note that these figures are only from 1992; if data had been available, the trend may have been much more marked.
A graph (probably at the end of this post; my 'place in line' is not working) shows this inverse relationship between surface solar radiation and cloud fractional coverage.
Paudel et al (2019;
https://www.scirp.org/journal/paperinfo ... erid=90428) concluded that
"Changes in cloud, both in the fraction of sky covered and in their radiative characteristics, played a major role in determining the global radiation measured in Israel during the last 60 years. Highly significant inverse linear relationships between normalized solar energy reaching the surface (Eg↓) and cloud cover indicate that a reduction in cloud transmission occurred in both the central coastal plain and central mountain region with a much smaller change in the transmission of cloudless skies. Analysis by stepwise regression indicated that since 1970 changes in cloud cover accounted for 61% of the changes in Eg↓ while the major increase in local fossil fuel consumption, serving as a proxy for anthropogenic aerosol emissions, only accounted for an additional 2% of the changes. Although the interaction between cloud cover and fossil fuel consumption is not statistically significant the indirect aerosol effect demonstrated in this study suggests that an important microphysical interaction may exist."I think that's enough to make the point. There are mountains of publications available.
What I found interesting was that the IPCC in its 2013 document appears to be making serious recommendations about cloud whitening, using sulphur dioxide at stratospheric level. It would make interesting Sunday reading -- See
https://pure.mpg.de/rest/items/item_200 ... 48/content --
Section 7.7 Solar Radiation Management (SRM)and Related Methods. There are many considerations and the following excerpt is just a very small sample. Remember the suggestions of the five Copenhagen Consensus Centre economists about cloud whitening. Here is one small excerpt
" 7.7.2.1 Stratospheric Aerosols Some SRM methods propose increasing the amount of stratospheric aerosol to produce a cooling effect like that observed after strong explosive volcanic eruptions (Budyko, 1974; Crutzen, 2006). Recent studies have used numerical simulations and/or natural analogues to explore the possibility of forming sulphuric acid aerosols by injecting sulphur-containing gases into the stratosphere (Rasch et al., 2008b). Because aerosols eventually sediment out of the stratosphere (within roughly a year or less), these methods require replenishment to maintain a given level of RF. Research has also begun to explore the efficacy of other types of aerosol particles (Crutzen, 2006; Keith, 2010; Ferraro et al., 2011; Kravitz et al., 2012) but the literature is much more limited and not assessed here. The RF depends on the choice of chemical species (gaseous sulphur dioxide (SO2), sulphuric acid (H2SO4) or sprayed aerosols), location(s), rate and frequency of injection."I think it's ironic that we have been clearing our fossil fuel emissions of SO2 and N2O, and now (well at least 6 years ago) the IPCC was considering injecting SO2 back into the stratosphere to produce more cloud.