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In Venice, the often murky canals recently began to get clearer, with fish visible in the water below. Italy's efforts to limit the coronavirus meant an absence of boat traffic on the city's famous waterways. And the changes happened quickly.

Countries that have been under stringent lockdowns to stop the spread of the coronavirus have experienced an unintended benefit. The outbreak has, at least in part, contributed to a noticeable drop in pollution and greenhouse gas emissions in some countries.

Although grim, it's something scientists said could offer tough lessons for how to prepare — and ideally avoid — the most destructive impacts of climate change.

Water in Venice's canals appeared to run clearer in the absence of boat traffic in early March. Marco Capovilla / Venezia Pulita

"If we can think about how to prepare for climate change like a pandemic, maybe there will be a positive outcome to all of this," said Christopher Jones, lead developer of the CoolClimate Network, an applied research consortium at the University of California, Berkeley. "We can help prevent crises in the future if we are prepared. I think there are some big-picture lessons here that could be very useful."

The coronavirus pandemic has sickened more than 180,000 people and killed more than 7,100 worldwide since early January. Some countries, most notably China and Italy, have been forced to seal their borders and restrict residents' movements to control the rates of infection.

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Satellite observations have shown that the temporary measures have also driven significant decreases in harmful emissions.

"Carbon dioxide is tied to industrial activity, electricity production and transportation, so anything that affects those sectors will impact greenhouse gases, as well," Jones said.

The coronavirus first emerged in late December in Wuhan, China. As it rapidly spilled into neighboring regions, the Chinese government locked down the city, quarantining 11 million people in Wuhan. Eventually, the lockdown would include almost 60 million people in the province of Hubei.

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Industrial operations in the coronavirus hot spot ground to a halt, and travel restrictions within China meant that air, rail and road traffic were paused or scaled back across some regions.

According to Lauri Myllyvirta, an analyst at the Centre for Research on Energy and Clean Air in Finland, the restrictions contributed to a 25 percent drop in China's carbon dioxide emissions over four weeks beginning in late January, compared to the same time last year.

Myllyvirta's analysis also found that industrial operations were reduced by 15 percent to 40 percent in some sectors and that coal consumption at power plants fell by 36 percent.

Pollution-monitoring satellites operated by NASA and the European Space Agency observed drastic decreases in air pollution over China over two weeks in February when the quarantine was in effect. The satellites measured concentrations of nitrogen dioxide, which is released by cars, power plants and industrial facilities, from Jan. 1 to Jan. 20 and again from Feb. 10 to Feb. 25. The difference was unmistakable.

Clearer waters in Venice in early March were an unintended climate benefit of the coronavirus lockdowns in Italy. Marco Capovilla / Venezia Pulita

The cloud of nitrogen dioxide that was parked over China in January seemed to evaporate in February. NASA scientists said that similar emissions reductions have been observed in other countries during economic disruptions but that the sharp decrease in air pollution in China during the quarantine period was especially rapid.

"This is the first time I have seen such a dramatic dropoff over such a wide area for a specific event," Fei Liu, an air quality researcher at NASA's Goddard Space Flight Center, said in a statement this month.

Pollution levels have similarly decreased over Italy, which has become the center of the coronavirus pandemic outside China. On March 8, as cases spiked, Italy locked down its northern Lombardy region. Two days later, the prime minister expanded the quarantine to include the entire country.

Concentrations of nitrogen dioxide in the atmosphere over Italy also fell precipitously, as they did in China. An analysis by The Washington Post found that the most dramatic drop was observed over northern Italy.

Nitrogen dioxide can irritate the lungs, and inhaling the pollutant can increase the risk of asthma and inflammation of the lungs. Although the noxious gas isn't thought to be a major contributor to climate change, studying its concentration in the atmosphere can help scientists understand other heat-trapping greenhouse gases that do drive global warming.

Jacqueline Klopp, co-director of the Center for Sustainable Urban Development at Columbia University in New York City, said she expects to see greenhouse gas emissions plummet across the board because of the quarantine measures.

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"People were in their homes and really stopped a lot of the activities that lead to greenhouse gas emissions and other pollution," she said.

Early observations have shown that extreme social-distancing measures are likely also having an effect on air pollution at the city level in the U.S.

Jordan Wildish, a project director at Earth Economics, an environmental non-profit organization based in Tacoma, Washington, developed an online dashboard to track air quality in San Francisco, New York City and the Seattle area, comparing the measurements with figures from the same time last year.

In San Francisco, which is under shelter-in-place orders to control the spread of the coronavirus, the average concentration of fine particulate matter — tiny particles in the air that are dangerous because they can be breathed deeply into the lungs — over the past five days was almost 40 percent lower than the previous year.

In New York City, there was a 28 percent drop over the same period of time, and the Seattle-Tacoma-Bellevue saw a 32 percent decrease.

But experts warned that observed reductions are temporary and that as cities, countries and economies bounce back, so, too, will emissions — unless major infrastructure or societal changes are adopted.

Klopp said the pandemic could make companies and governments realize that other threats to humanity, including climate change, could be just as devastating and that it's imperative to develop protective measures.

"As we move to restart these economies, we need to use this moment to think about what we value," she said. "Do we want to go back to the status quo, or do we want to tackle these big structural problems and restructure our economy and reduce emissions and pollution?"