Republican administrations in the US have generally dismissed the danger of global warming, saying that any fluctuations in temperatures are “natural.” Last October, President Donald Trump – who still favors the use of petroleum to make things go and has called global warming “a hoax” – cast doubt on a report by his own government that cautioned about the calamitous effects of climate change.
A report by the Intergovernmental Panel on Climate Change – the main international body evaluating climate change – maintained that the damage could be stopped only if the world implemented major and costly changes – in the way we treat the environment. Despite this, most other governments and leading environmental specialists declare that climate change is caused by mankind and warn that natural fluctuations in temperature are being made worse by human activity.
The Paris climate change agreement signed last fall requires 187 other countries, except the US, to minimize the rise in global temperatures.
But now an esteemed Hebrew University of Jerusalem expert on climate has declared that “we need to rethink everything we know about global warming,” as new calculations show that scientists have grossly underestimated the effects of air pollution. Prof. Daniel Rosenfeld, an expert in the university’s Institute of Earth Sciences, writes in the journal Science that aerosol-driven droplet concentrations dominate the coverage and water of oceanic low-level clouds.
For some time, he wrote, the scientific community has known that global warming is caused by manmade emissions in the form of greenhouse gases and global cooling by air pollution in the form of aerosols. These are tiny particles that float in the air and can form naturally (as with desert dust) or artificially (from smoke from coal and car exhaust). Aerosols cool our environment by enhancing cloud cover that reflect the sunlight (heat) back to space.
But the Hebrew University-led research shows that the degree to which aerosols cool the earth has been grossly underestimated, necessitating a recalculation of climate change models to more accurately predict the pace of global warming.
Rosenfeld and his colleague Yannian Zhu from the Meteorological Institute of Shaanxi Province in China developed a new method that uses satellite images to separately calculate the effect of vertical winds and aerosol cloud droplet numbers. They applied this methodology to low-lying cloud cover above the world’s oceans between the equator and the 40th parallel south.
With this new method, Rosenfeld and his colleagues were able to more accurately calculate aerosols’ cooling effects on the Earth’s energy budget. And, they discovered that aerosols’ cooling effect is nearly two times higher than previously thought.
Clouds form when wind rises and cools. However, cloud composition is largely determined by aerosols. The more aerosol particles a shallow cloud contains, the more small the water droplets that it will hold. Rain forms when these droplets bind together. Since it takes longer for small droplets to bind together than it does for large droplets, aerosol-filled or “polluted” clouds contain more water, remain in the sky longer (while they wait for droplets to bind and rain to fall, after which the clouds will dissipate) and cover a greater area. All this time, the aerosol-laden clouds reflect more solar energy back into space, thereby cooling the Earth’s overall temperature.
Looking into how much aerosols cool down our environment, the authors noted that all estimates have been unreliable because it was impossible to separate the effects of rising winds which create the clouds, from the effects of aerosols which determine their composition. That is, until now.
But if this is true, why is the Earth getting warmer, not colder? For all of the global attention on climate warming, aerosol pollution rates from vehicles, agriculture and power plants is still very high, they wrote. For Rosenfeld, this discrepancy might point to an ever deeper and more troubling reality. “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 and points to a greater amount of global warming than we previously thought,” he explained.
The fact that our planet is getting warmer even though aerosols are cooling it down at higher rates than previously thought brings us to a Catch-22 situation: Global efforts to improve air quality by developing cleaner fuels and burning less coal could end up harming our planet by reducing the number of aerosols in the atmosphere, and by doing so, diminishing aerosols’ cooling ability to offset global warming.
According to the Hebrew University and Chinese researchers, 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 enter deep clouds, meaning those 10 kilometers or more above the Earth.
Man-made emissions of particulate air pollution can offset part of the warming induced by emissions of greenhouse gases, by enhancing low-level clouds that reflect more solar radiation back to space, they wrote.
“The aerosol particles do that because cloud droplets must condense on pre-existing tiny particles in the same way as dew forms on cold objects and more aerosol particles from man-made emissions lead to larger number of smaller cloud droplets. One major pathway for low-level cloud enhancement is through the suppression of rain by reducing cloud droplet sizes. This leaves more water in the cloud for a longer time, thus increasing the cloud cover and water content, which reflect more solar heat to space.”
This effect, they note, is strongest over the oceans, where moisture for sustaining low-level clouds over vast areas is abundant. Predicting global warming requires a quantitative understanding of how cloud cover and water content are affected by man-made aerosols.
The Israel Space Agency and France’s National Center for Space Studies (CNES) have teamed up to develop new satellites that will be able to investigate this deep cloud phenomenon, with Rosenfeld as its principal investigator.
Either way, the conclusion is the same. Our current global climate predictions do not correctly take into account the significant effects of aerosols on clouds on Earth’s overall energy balance. Further, Rosenfeld’s recalculations mean his scientific colleagues will have to rethink their global warming predictions, which currently predict a temperature increase of 1.5° to 4.5° degree Celsius by the end of the 21st century to provide us a more accurate diagnosis and prognosis of the Earth’s climate.