The level of uncertainty in this supposedly “settled” science never fails to amaze. But I observe that it is beginning to be politically acceptable to talk about the uncertainties and even – as in this paper – to begin to question the significance of human activities on climate.
“Climate Effects of Aerosols-Cloud Interactions.“ Daniel Rosenfeld, Steven Sherwood, Robert Wood, Leo Donner. Science VOL 343, 24 JANUARY 2014
Abstract: Aerosols counteract part of the warming effects of greenhouse gases, mostly by increasing the amount of sunlight reflected back to space. However, the ways in which aerosols affect climate through their interaction with clouds are complex and incompletely captured by climate models. As a result, the radiative forcing (that is, the perturbation to Earth’s energy budget) caused by human activities is highly uncertain, making it difficult to predict the extent of global warming (1, 2). Recent advances have led to a more detailed understanding of aerosol-cloud interactions and their effects on climate, but further progress is hampered by limited observational capabilities and coarse-resolution climate models.
The paper is behind a pay-wall but the accompanying press release begins “The warming effect of human-induced greenhouse gases is a given, but to what extent can we predict its future influence?”. I have no doubt that “greenhouse gases” in the atmosphere have a small warming effect but evidence is still lacking even for this simple statement because the carbon balance of the earth is still far from certain and the extent to which fossil fuel combustion contributes to the carbon dioxide concentration is still not certain. So while the warming effect of greenhouse gases is established, its magnitude is not and the impact of humans on the concentration is also not yet certainly established. In fact, the primary contributors to the “greenhouse effect” are still water vapour and clouds but clouds also cause significant cooling by blocking insolation. Carbon dioxide by itself is almost of minor consequence and the weakness of climate models has always been that they make unjustified assumptions for the forcing effects of carbon dioxide. The pause in warming over the last 17-18 years and the slight decline in global temperatures for the last decade – while carbon dioxide concentrations have been steadily increasing – is a further indicator that the warming effect of carbon dioxide has been grossly exaggerated.
The Press Release goes on:
…… Indeed, one could say that the picture is a “cloudy” one, since the determination of the greenhouse gas effect involves multifaceted interactions with cloud cover.
To some extent, aerosols –- particles that float in the air caused by dust or pollution, including greenhouse gases – counteract part of the harming effects of climate warming by increasing the amount of sunlight reflected from clouds back into space. However, the ways in which these aerosols affect climate through their interaction with clouds are complex and incompletely captured by climate models, say the researchers. As a result, the radiative forcing (that is, the disturbance to the earth’s “energy budget” from the sun) caused by human activities is highly uncertain, making it difficult to predict the extent of global warming.
And while advances have led to a more detailed understanding of aerosol-cloud interactions and their effects on climate, further progress is hampered by limited observational capabilities and coarse climate models, says Prof. Daniel Rosenfeld of the Fredy and Nadine Herrmann Institute of Earth Sciences at the Hebrew University of Jerusalem, author of the article in Science. …..
Their recent studies have revealed a much more complicated picture of aerosol-cloud interactions than considered previously. Depending on the meteorological circumstances, aerosols can have dramatic effects of either increasing or decreasing the cloud sun-deflecting effect, the researchers say. Furthermore, little is known about the unperturbed aerosol level that existed in the preindustrial era. This reference level is very important for estimating the radiative forcing from aerosols.
Also needing further clarification is the response of the cloud cover and organization to the loss of water by rainfall. Understanding of the formation of ice and its interactions with liquid droplets is even more limited, mainly due to poor ability to measure the ice-nucleating activity of aerosols and the subsequent ice-forming processes in clouds.
Needless to say they end up asking for more funds:
While it is unfortunate that further progress on understanding aerosol-cloud interactions and their effects on climate is limited by inadequate observational tools and models, achieving the required improvement in observations and simulations is within technological reach, the researchers emphasize, provided that the financial resources are invested.
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