The Economist USA - 28.03.2020

68 The EconomistMarch 28th 2020

1

I

n venice, waterin the canals is running

clear, offering glimpses of fish swim-

ming against the current. As human activi-

ty grinds to a halt, natural rhythms resume.

A similar, less visible story is being played

out in the skies. Around the world, levels of

toxic air pollutants are dropping as places

go into lockdown in an attempt to curb the

spread of sars-cov- 2 , the virus causing a

pandemic of a new disease called covid-19.

Emissions of greenhouse gases are follow-

ing a similar pattern.

One example of pollution falling is that

satellites looking down on China’s large

cities have witnessed a dramatic drop since

January in levels of nitrogen dioxide, a gas

generated by machinery such as internal-

combustion engines. This fall coincides

with the imposition of a countrywide quar-

antine, travel restrictions, and the shutting

down of power stations and factories. Ni-

trogen dioxide causes respiratory pro-

blems. A drop in its levels therefore brings

benefits. The concentration of another pul-

monary irritant—fine soot particles—was

also lower in those cities, by 20-30%, in

February of this year compared with levels

in the previous three years.

Clearing the air

Similar patterns have shown up elsewhere,

as the virus spreads and lockdowns follow.

Satellite data from Italy reveal a marked de-

cline in nitrogen-dioxide concentrations,

particularly in the Po valley, the original fo-

cus (see box on next page) of the country’s

epidemic, and where Italy’s shelter-at-

home rules were first imposed. South Ko-

rea also saw a drop, starting in mid-Febru-

ary. And in New York City, data collected by

TomTom, a gps-navigation firm, show

peak-hour traffic down between 13.5% and

26%. Not surprisingly, carbon-monoxide

levels in the city are half those during the

corresponding period last year, according

to researchers at Columbia University.

Drops like these, in pollutants that are

directly harmful to human health, would

be expected to be matched by falling emis-

sions of those more-subtly harmful pollut-

ants, the greenhouse gases produced by

human activity. And the team at Columbia

did indeed find that carbon-dioxide con-

centrations over New York have fallen.

They dropped by 8-10% this month com-

pared with March 2019. In China, mean-

while, industrial shutdowns are estimated

to have caused a 25% drop in emissions of

CO 2 in February, compared with the same

month in 2019.

An optimist might see these changes as

a silver lining to what is an extremely dark

cloud. But that would depend on their be-

ing sustained when things return to nor-

mal. As François Gemenne of the Universi-

ty of Liège, in Belgium, puts it, “the climate

needs a sustained drop in greenhouse-gas

emissions, not a year off”. Unfortunately,

not only is that unlikely to happen, but the

response to the crisis could easily make

things worse.

The short-term amelioration of CO 2

emission is likely to be real. On March 16th

Covid-19 and climate change

Clear thinking required

The coronavirus pandemic is causing a short-term improvement in air quality,

but the long-term outlook is grim

Science & technology

69 Covid-19 and pollution

70 Detecting elevated body heat

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