THIS year, the news was full of reports of a huge “brown haze” of pollution, mostly from burning coal and biomass, that was blamed for blocking out up to 15 per cent of the Sun’s radiation over southern Asia. Since then scientists have built up an even starker picture of how pollution affects sunlight in different regions of the globe.
Airborne pollution from burning fossil fuels and vegetation is plunging vast regions of the planet into a chilly darkness. The knock-on effects are likely to put a strain on already poor water supplies and damage crop yields.
Studies of sunlight in China show vast polluted regions that have experienced a steady decline in sunshine alongside steadily dropping temperatures. Meanwhile, in Zambia and the Brazilian Amazon, pollution blots out around a fifth of the Sun’s radiation at certain times of year.
Advertisement
Yun Qian and Dale Kaiser at the US government’s Pacific Northwest National Laboratory in Richland, Washington, investigated the impact of coal burning in China over the past 50 years, to see what effect the aerosols produced have had on the amount of sunshine. In recent decades, China’s pollution has soared. Average emissions of sulphur dioxide, mostly from burning coal, have risen fivefold since the 1960s. The country now emits as much SO2 as the whole of North America.
Qian and Kaiser examined data from standard sunshine recorders that are powered by solar radiation to monitor the amount of sunlight reaching different locations in China. When there is insufficient sunlight, such as at dusk or dawn, or when a cloud obscures the Sun, the instruments stop recording. Atmospheric pollution can also block the Sun’s radiation, having the same effect.
The team found a significant decline in the amount of sunshine reaching most of the country. The timing and extent of the downward trend matches the increased use of fossil fuels. In the heavily polluted east, the duration of sunshine has fallen the most – by between 2 and 3 per cent a decade. In the same region, maximum summer temperatures have also fallen by around 0.6 °C a decade (Geophysical Research Letters, vol 29, p 2042).
Aerosols do more than just block sunlight. They “affect human health and acidify the land”, says Qian. Blotting out sunlight will impact heavily on agriculture as less energy from the Sun may lead to reduced plant and crop growth. Reduced sunlight could also lead to a drop in evaporation, cutting rainfall and putting further strain on struggling water supplies.
And it’s not just fossil fuels that are blocking out sunlight. Soot and ash from forest and grass fires lit by farmers to clear land in the Brazilian Amazon and the savannah grasslands of Zambia have also dramatically cut the amount of sunlight reaching the land, says NASA physicist Joel Schafer. He, along with colleagues at the University of Maryland in Baltimore and the University of São Paolo, Brazil, found that during the Amazonian burning season, the amount of sunlight reaching the ground drops by 16 per cent. In Zambia, burning vegetation cuts sunlight by about 22 per cent (Geophysical Research Letters, vol 29, p 1823).
Smoke from fossil fuels and burning biomass contains fine aerosols of sulphur dioxide, ash and black soot, that can cool the ground in a variety of ways. Dense clouds of aerosol particles act as nuclei for water droplets to grow, typically resulting in clouds with smaller water droplets than normal. These clouds reflect more sunlight back into space, so the ground beneath them receives less radiation. Clouds of particles themselves can directly shade the ground, and black particles will absorb solar radiation, leading to local heating of the air, with unpredictable effects on the climate.
The cooling effect of pollution over the Amazon, Zambia and China masks these regions from global warming to some extent. But this is no excuse to stop cutting down on emissions of greenhouse gases such as carbon dioxide, says Qian. Aerosols typically stay in the atmosphere for weeks, while CO2 levels remain stable for years.
