Across the world, many countries have enforced drastic ‘lock-down’ measures in recent weeks, in order to reduce the transmission rate of the COVID-19 coronavirus. This generally means that all non-essential travel and interactions between people have been forbidden.
In the most densely populated, urbanised countries, this has led to stunning reductions in air pollution, mainly from reduced ground and air traffic. This Guardian article published March 23rd uses striking visuals to demonstrate the scale of change in China, Italy and the UK.
From the figures given there and in other related articles (e.g. this BBC news release), I get a sense that the average reduction in pollution within major cities surrounds is close to one quarter. Associated countries may be seeing country-averaged reductions of some 10-30%.
So, now we come to the big question and focal point of this blog piece:
Will This Drop in Pollution Noticeably Affect Global Temperatures?
Broadly speaking, pollutants have the capacity to either block or reflect sunlight away from Earth’s surface. Sometimes this is direct, other times via seeding cloud formation, as often occurs with airplane contrails.
However, different pollutants have markedly different capabilities, which makes it difficult to pin down the large-scale atmospheric response to an overall reduction.
Even so, numerous research studies have demonstrated that reduced air pollution can result in increased temperatures, at least regionally. For example, this study establishes a link between reduced pollution across Europe since its 1970s peak and an increased pace of warming in the Arctic region.
“…numerous research studies have demonstrated that reduced air pollution can result in increased temperatures, at least regionally”
From this, it’s reasonable to deduce that an upward spike in temperatures could be an unintentional consequence of the ‘lockdown’ measures, if the reduction in pollution is large enough.
This is where we become mired down in uncertainty.
An Uncertain Side-Effect
Simply put, there hasn’t been enough research into the impacts of a sudden cut in pollution levels occurring near-simultaneously across a great many countries.
Instead, research has rightly focused on the possible consequences of realistic rates of adaptation to mitigate climate change. This publication by Carbon Brief provides an excellent summary of a very recent example, in which even the highest-impact modelled scenarios saw only a short-lived (2-3 year) and relatively minor (0.3°C per decade) acceleration of global mean temperature rise.
That would equate to at most about 0.1°C of additional warming – but this is likely to be for a pace of pollution reduction considerably slower than we’ve seen mid-late March 2020. The models they used account for ‘the time required to transform power generation, industry and transportation leads…’ – which is not relevant to current real-life events.
While this can’t be confirmed without further rigorous study, it seems plausible to this meteorologist that while widespread lock-downs are in effect, global mean lower-atmospheric temperatures may increase by at least a few tenths of a degree Celsius.
“…it seems plausible to this meteorologist that while widespread lock-downs are in effect, global mean lower-atmospheric temperatures may increase by at least a few tenths of a degree Celsius.”
That may not sound much but bear in mind it’s an average across both sunny and cloudy conditions. Most of the contribution to the global mean rise will be where there’s plenty of sun reaching the surface (e.g. India; often hot and dry Apr-May).
More generally in locations with much-reduced pollution levels, this means an increased likelihood of exceptionally high temperatures, whenever it’s sunny with light winds. This will be especially the case for large cities, where extreme temperatures are already at raised probability due to the urban heat island effect.
Slower Is Steadier
Let me be clear: This potential unwanted side-effect is mainly a consequence of how fast pollution levels are dropping, not the act of reducing pollution itself.
There is no question that city-wide reductions in air pollution (let alone country-wide!) bring a great many positive changes to the world around us.
Pollution can often become highly visible after it combines with water droplets to form a ‘dirty fog’ known as smog. For this reason, some large cities have higher incidences of low visibility than their surroundings.
Polluted air can cause all manner of health problems – particularly respiratory ones – for both people and wildlife. As meteorologist Dr Bob Henson wrote in 2017:
“More than 10,000 lives are lost worldwide every 24 hours as a direct or indirect consequence of poor air quality”.
As extraordinary as it sounds, depending how treatment of COVID-19 evolves and how well we slow the spread, this means we could feasibly see a reduction in overall death rates (from all causes) during the coming weeks and months.
In the wake of the major economic disruption and immeasurable personal tragedies inflicted by the coronavirus, will this be remembered and acted upon? Or will restoring the economy be prioritised at the expense of measures to keep pollution levels from returning to, or even exceeding, what they were beforehand?
James Peacock MSc
Head Meteorologist at MetSwift