Last year, within a matter of weeks, hundreds of millions of people shuttered down at home and many of the biggest corporations on Earth halted operations at their facilities during the early days of the COVID-19 pandemic. This resulted in a staggering decline in greenhouse gas emissions, which caused months of below-average pollution levels.
Yet, despite this temporary decline in emissions, researchers say aerosols such as soot, black carbon, and sulfate, caused a counterintuitive effect that deserves some explaining: The planet actually warmed up.
This observation comes from a study by the National Center for Atmospheric Research (NCAR), which found that reduction of aerosols during the COVID-19 lockdowns “caused a small net warming during the spring months in 2020.” The study says that these changes in aerosol levels had a bigger impact on temperatures than the effects from varying levels of carbon dioxide, ozone, and contrail cooling effects. However, the researchers predict the peak impact on global temperatures from the aerosol decline will not be felt until 2022.
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While it might be confusing that lower greenhouse gas emissions had a temporary warming effect, some aerosols naturally occur in the atmosphere from forest fire smoke and volcanic ash and play a significant role in shaping the climate because these gases actually have a cooling effect. Aerosols reflect sunlight away from the Earth’s surface and make clouds brighter and more reflective, so heat that would otherwise get trapped in our atmosphere travels out to space.
Daily greenhouse gas emissions had the most severe decline in March 2020 when many lockdowns began. Data revealed that clouds became dimmer between March and June, which caused more energy from the Sun to be absorbed by the Earth instead of being reflected out to space. The study reports that temperatures over some land areas were between 0.1-0.3°C warmer than usual and the largest warming of 0.37°C occurred over the United States and Russia, which are countries that release some of the highest levels of aerosols.
Despite this temporary warming, the researchers say that the long term impact from the COVID-19 lockdowns could “slightly slow climate change because of reduced emissions of carbon dioxide, which lingers in the atmosphere for decades and has a more gradual influence on climate,” as stated in NCAR’s press release. Unlike carbon dioxide, which lingers in the atmosphere anywhere between 300 to 1,000 years after being released, the impact that aerosols have on the Earth’s temperature diminishes within a few years.
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Even though aerosols are capable of creating a cooling effect, researchers say that they are dangerous to human health and not a viable solution that could combat global warming.
A study from the Carnegie Institution for Science investigated the potential of aerosol-induced cooling to improve conditions for agricultural productivity and human labour around the world. The researchers found that the public health benefits of removing aerosols from the atmosphere significantly outweighed the economic benefits of releasing them because of their risk to human health—aerosols are tiny particles that can be inhaled or absorbed by the skin that cause damage to respiratory and cardiovascular organs.
"Estimates indicate that aerosol pollution emitted by humans is offsetting about 0.7°C, or about 1.3°F, of the warming due to greenhouse gas emissions," said Yixuan Zheng, the lead author in a press release from Carnegie Institution for Science. "This translates to a 40-year delay in the effects of climate change. Without cooling caused by aerosol emissions, we would have achieved 2010-level global mean temperatures in 1970."
Even though aerosols have offset some of the warming caused by greenhouse gases, Zheng says it is clear that aerosols have “profound harm far outweighs their meager benefits” and reducing greenhouse gas emissions is essential for improved public health and addressing climate change.
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