LAGOS, Nigeria — Rapidly growing countries generally see sharp increases in air pollution as their populations and economies expand. But a new study of air quality in Africa published on Monday has found the opposite: One of the continent’s most vibrant regions is becoming less polluted.
The study, published in Proceedings of the National Academy of Sciences, found that levels of dangerous nitrogen oxides, a byproduct of combustion, in the northern part of sub-Saharan Africa have declined sharply as wealth and population in the area have increased.
“The traditional paradigm is that as middle and low-income countries grow you often see more emissions, and to see a different kind of trajectory is very interesting,” said Jonathan Hickman, a researcher at the NASA Goddard Institute for Space Studies who was the lead author on the study. “It’s nice to see a decline occurring when you’d expect to see pollution increasing.”
The reason, according to researchers, is that an increase in pollution from industry and transportation in the area studied — from Senegal and Ivory Coast in the west to South Sudan, Uganda and Kenya in the east — appears to have been offset by a decline in the number of fires set by farmers.
While not a big industrial polluter like Asia and North America, Africa has long been the site of widespread biomass burning during the dry season.
Burning vegetation is considered a cheap and efficient method of clearing land in preparation for planting season, and burning has the advantage of retaining mineral nutrients in the soil. But the consequences for human health and global warming are potentially grave. Fires for land management can combine with urban pollution to produce toxic air. And fires emit planet-warming carbon dioxide into the atmosphere.
Brush fires tend to conjure images of out-of-control blazes in places like Australia or the Western United States, but north Equatorial Africa is the region with the world’s largest area of biomass fires, according to researchers, with roughly 70 percent of the world’s burned land.
The new study used NASA satellite data and imagery to measure dangerous gases present in the region’s air and determine fire trends between 2005, when NASA’s records started, and 2017. At the peak of fire seasons, levels of nitrogen dioxide, or NO2, a harmful gas produced by road traffic and other fossil fuel combustion and linked to respiratory problems like asthma, declined by 4.5 percent in the lower atmosphere.
That drop was so significant, Dr. Hickman said, that it resulted in a net decrease of the pollutant in the region.
The finding is important because Africa’s growing population, currently at 1.2 billion but expected to top two billion by 2040, is urbanizing fast. Pollution has surpassed AIDS as the leading cause of death on the continent. But governments often prioritize economic growth over the environment, meaning there’s little emphasis on gathering air quality data or putting clean-air policies in place.
The new study “provides an important tool for filling some of these data gaps in Africa where there is a dearth of air pollution studies at multiple levels,” said Andriannah Mbandi, an environmental researcher based in Kenya and affiliated with the Stockholm Environment Institute. “It would be great if follow-up work from this paper would quantify these levels to health and economic metrics, which is useful to policymakers.”
While fires may be in decline, though, pollution is still growing.
Emissions from the burning of fossil fuels are projected to rise considerably in Africa. Despite a 2015 African Union commitment to green energy, 80 percent of the power generated on the continent is from coal or other fossil fuels. More and more used cars are being imported, which drives up emissions from transportation.
That could trigger a reversal of the positive trend identified in the study on Monday, particularly in populous, richer countries like Nigeria.
“As you increase G.D.P., you see a decrease in the amount of NO2 but it only followed that pattern to a certain point,” Dr. Hickman said, describing the analysis the team carried out, tinkering with wealth and pollution levels within the model.
“At the highest levels of this G.D.P. metric, air pollution levels were almost back to the levels that they were when we started out,” he said. “What it suggests is this decline we are seeing is probably going to slow down and may reverse as the consequences of increased use of fossils.”