Photo of Hebrew University of Jerusalem Professor Daniel Rosenfeld. In a study published in the journal Science, Prof. Rosenfeld argues that the degree to which aerosol particles cool the earth has been greatly underestimated. Photo: Hebrew University of Jerusalem

20 January 2019 (ISRAEL21c) – The world’s scientific community has known for a long time that global warming is caused by manmade emissions in the form of greenhouse gases, while global cooling is caused by air pollution in the form of aerosols.

In a new study published in the journal Science, Hebrew University of Jerusalem Prof. Daniel Rosenfeld argues that the degree to which aerosol particles cool the earth has been grossly underestimated.

Aerosols are tiny particles of many different materials that get into the air, like dust and vehicle exhaust. They cool our environment by enhancing the cloud cover that reflects the sun’s heat back to space.

Aerosols are tiny particles of many different materials that get into the air, like dust and vehicle exhaust. They cool our environment by enhancing the cloud cover that reflects the sun’s heat back to space.

Rosenfeld says his findings necessitate a recalculation of climate-change models to more accurately predict the pace of global warming.

He and his colleague Yannian Zhu from the Meteorological Institute of Shaanxi Province in China developed a new method that uses satellite images to calculate the effect of vertical winds as well as aerosol cloud droplet numbers. Until now, it was impossible to separate the effects of rising winds, which create the clouds, from the effects of aerosols, which determine clouds’ composition.

Using this new methodology, Rosenfeld and his colleagues were able to more accurately calculate aerosols’ cooling effects on the Earth. They discovered this effect is nearly two times higher than previously thought.
But this finding does not necessarily mean we can stop worrying about global warming. Rosenfeld has several theories about why temperatures are rising despite the aerosol effect.

“If the aerosols indeed cause a greater cooling effect than previously estimated, then the warming effect of the greenhouse gases has also been larger than we thought, enabling greenhouse-gas emissions to overcome the cooling effect of aerosols,” Rosenfeld said.

Another hypothesis to explain why Earth is getting warmer even though aerosols have been cooling it down at an even a greater rate is a possible warming effect of aerosols when they lodge in “deep clouds” located 10 kilometers or more above the Earth.  [more]

We need to rethink everything we know about global warming

ABSTRACT: Lack of reliable estimates of cloud condensation nuclei (CCN) aerosols over oceans has severely limited our ability to quantify their effects on cloud properties and extent of cooling by reflecting solar radiation – a key uncertainty in anthropogenic climate forcing. Here we introduce a methodology for ascribing cloud properties to CCN and isolating the aerosol effects from meteorological effects. Its application showed that, for a given meteorology, CCN explains 3/4 of the variability in clouds radiative cooling effect, mainly through affecting shallow cloud cover and water path. This reveals a much greater sensitivity of cloud radiative forcing to CCN than previously reported, which means too much cooling if incorporated in present climate models. This hints to yet unknown compensating aerosol warming effects, possibly through deep clouds.

Aerosol-driven droplet concentrations dominate coverage and water of oceanic low level clouds