Some climate models say greenhouse gases cause a lot of warming and others don’t. Yet they all fit 20th century temperature data equally well. How is that possible? Simple: They all have an offsetting fudge factor based on the cooling effects of tiny pollution particles called aerosols. Not the observed effects, you understand. The high-sensitivity models simply assume aerosols do a lot of cooling by promoting extra cloud formation, and the low sensitivity models assume they don’t. So the key question is how much cooling aerosols actually do. If it’s not much, then greenhouse gases can only cause a little warming, given how little warming happened over the 20th century. And now we know it’s not much.

New evidence published in July is truly devastating. It doesn’t just say the cloud response to aerosols is very weak, though it does say so. It says aerosols do the opposite of what has been assumed and what they must do to make the models work. Instead of bundling liquid water up inside the clouds, which would lead to strong cooling of the Earth's surface that might counteract a hypothetical strong impact of warming from GHGs, extra pollution aerosols cause clouds to expel water, weakening their overall cooling properties. Here is the authors' exact wording (emphasis added).

The cooling of the Earth’s climate through the effects of anthropogenic aerosols on clouds offsets an unknown fraction of greenhouse gas warming. An increase in the amount of water inside liquid-phase clouds induced by aerosols, through the suppression of rain formation, has been postulated to lead to substantial cooling, which would imply that the Earth’s surface temperature is highly sensitive to anthropogenic forcing. Here we provide direct observational evidence that, instead of a strong increase, aerosols cause a relatively weak average decrease in the amount of water in liquid-phase clouds compared with unpolluted clouds. Measurements of polluted clouds downwind of various anthropogenic sources—such as oil refineries, smelters, coal-fired power plants, cities, wildfires and ships—reveal that aerosol-induced cloud-water increases, caused by suppressed rain formation, and decreases, caused by enhanced evaporation of cloud water, partially cancel each other out. We estimate that the observed decrease in cloud water offsets 23% of the global climate-cooling effect caused by aerosol-induced increases in the concentration of cloud droplets. These findings invalidate the hypothesis that increases in cloud water cause a substantial climate cooling effect and translate into reduced uncertainty in projections of future climate.

If this study is correct, and science we remind everyone proceeds skeptically by challenging findings including congenial ones, it is one more piece of evidence that most climate models predict too much warming in response to greenhouse gas emissions. The models with high sensitivity only get to stay in the game by having a big cooling offset in the form of aerosol-induced cloud formation, otherwise they'd have predicted so much warming over the 20th century we'd be fried to a crisp by now.

Up to now no one has been sure how much cooling happens from aerosol pollution because it is hard to measure (although there has previously been research showing that aerosol cooling is weaker than modelers usually assume). This new study knocks the legs out from under the modelers who want to keep a high climate sensitivity in their models.

And since the paper was published in Nature magazine, it will be hard for the IPCC folks to dismiss, though we don't doubt they'll try to find a way.