Media outlets around the world have recently been highlighting favourable environmental impacts of the coronavirus COVID-19 pandemic. They have focused on reduced levels of pollutants, including greenhouse gases, resulting from the forced reduction in economic activity.
As an example of such reductions, Figure 1 shows the density of airborne nitrogen dioxide over China prior to and during coronavirus-related restrictions. 
Figure 1(a): Nitrogen Dioxide over China 28 December 2019 – 7 January 2020
Figure 1(b): Nitrogen Dioxide over China 10 – 20 February 2020
Figure 2 represents daily coal consumption at six major power companies in China for the years 2014 – 2020.  Chinese New Year invariably results in reduced economic activity but in 2020 the return to normal levels has been delayed due to the virus, as the Chinese government enabled a measured increase in activity. (From a public health viewpoint, any return to normal levels of activity risks a second wave of infections at this point in time.)
Figure 2: Daily coal consumption at six major Chinese power companies
Year on year comparisons for many countries confirm the dramatic reduction in pollution levels. 
So, what’s the downside?
Although lower pollution levels are generally welcome, they can represent a double-edged sword in terms of global warming and climate change.
The reason is that the burning of fossil fuels that contributes to nitrogen dioxide and carbon dioxide emissions also releases aerosols, which are airborne particulates including (but not limited to) black carbon, sulphates and nitrates.
Figure 3: Aerosol pollution over Hong Kong
Although some types of aerosol can have a warming impact, the overall global impact is one of cooling, sometimes referred to as “global dimming”. This has offset some of the warming effects of greenhouse gases, keeping warming to lower levels than we would otherwise have experienced.
Aerosols only remain in the atmosphere for days or weeks, so their cooling impact will be short-lived in any transition away from fossil fuels to less carbon-intensive energy sources.
This dilemma has been referred to by leading climate scientist Dr James Hansen as a “Faustian bargain”, alluding to Doctor Faustus of folklore and legend, who sold his soul to the devil in exchange for knowledge and power. 
The dilemma exists not only in relation to coronavirus-induced reductions in pollution levels but in any attempts to reduce our reliance on fossil fuels.
Based on research from organisations such as NASA and Australia’s Commonwealth Scientific and Research Organisation (CSIRO), climate change researcher David Spratt has suggested that aerosol cooling (for reductions in fossil fuel burning well beyond what we have witnessed to date) is in the range of 0.5-1.2°C. 
Does the dilemma apply to all responses to climate change?
The dilemma is virtually non-existent in relation to animal agriculture. This feature would generally enable us to favourably influence global warming by reducing our reliance on animals as a food source, without the negative consequences referred to in this article. [Footnote]
Such a measure will be essential in dealing with the climate emergency, which requires urgent action utilising all valid means at our disposal.
Burning of forests and savannas to create or maintain land for grazing or feed crop production is a key source of black carbon, which has a significant warming impact while airborne and after landing on glaciers and ice sheets.  
 European Space Agency, “COVID-19: nitrogen dioxide over China”, 19 March 2020, https://www.esa.int/Applications/Observing_the_Earth/Copernicus/Sentinel-5P/COVID-19_nitrogen_dioxide_over_China and https://www.esa.int/ESA_Multimedia/Videos/2020/03/Nitrogen_dioxide_emissions_over_China
 Carbon Brief, “Analysis: Coronavirus has temporarily reduced China’s CO2 emissions by a quarter”, 19 Feb 2020, updated 30 Mar 2020, https://tinyurl.com/rc9va5g, Attribution-NonCommercial-NoDerivatives 4.0 International, https://tinyurl.com/nepb6fj
 Watts, J. and Kommenda, N., “Coronavirus pandemic leading to huge drop in air pollution”, The Guardian, 23 March 2020, https://www.theguardian.com/environment/2020/mar/23/coronavirus-pandemic-leading-to-huge-drop-in-air-pollution
 Hansen, J, “Storms of my Grandchildren”, Bloomsbury, 2009, pp. 97-98
 Spratt, D., “Two degrees of warming closer than you may think”, Climate Code Red, 6th February, 2015, http://www.climatecodered.org/2015/02/two-degrees-of-warming-closer-than-you.html#more
 World Preservation Foundation, “Reducing Shorter-Lived Climate Forcers through Dietary Change: Our best chance for preserving global food security and protecting nations vulnerable to climate change” (2010)
 Gerard Wedderburn-Bisshop, World Preservation Foundation, Presentation to Cancun Climate Summit, Dec, 2010 “Shorter lived climate forcers: Agriculture Sector and Land Clearing for Livestock” (co-authors Lefkothea Pavlidis and Dr Hsien Hui Khoo), https://www.youtube.com/watch?v=HFkwdNxiQWU
Featured image: peterschreiber.media, Shutterstock ID 1641583738
Figures 1(a) and (b): See European Space Agency reference above.
Figure 2: See Carbon Brief reference above.
Figure 3: Juhku, Hong Kong tall buildings in haze at day, Shutterstock, ID 536684773
This article has been prepared for the Climate Save Movement in addition to this site.