Global dimming

Global dimming is a decline in the amount of sunlight reaching the Earth's surface. It is caused by atmospheric particulate matter, predominantly sulfate aerosols, which are components of air pollution. Global dimming was observed soon after the first systematic measurements of solar irradiance began in the 1950s. This weakening of visible sunlight proceeded at the rate of 4–5% per decade until the 1980s. During these years, air pollution increased due to post-war industrialization. Solar activity did not vary more than the usual during this period. Aerosols have a cooling effect on the earth's atmosphere, and global dimming has masked the extent of global warming experienced to date, with the most polluted regions even experiencing cooling in the 1970s. The opposite effects it has on global warming appears to be beneficial to the environment. Global dimming has interfered with the water cycle by lowering evaporation, and thus has probably reduced rainfall in certain areas. It may have weakened the Monsoon of South Asia and caused the entire tropical rain belt to shift southwards between 1950 and 1985, with a limited recovery afterwards. Record levels of particulate pollution in the Northern Hemisphere caused or at least exacerbated the monsoon failure behind the 1984 Ethiopian famine. Since the 1980s, a decrease in air pollution has led to a partial reversal of the dimming trend, sometimes referred to as global brightening. This global brightening had contributed to the acceleration of global warming, which began in the 1990s. According to climate models, the dimming effect of aerosols most likely offsets around 0.5 °C (0.9 °F) of warming as of 2021. As nations act to reduce the toll of air pollution on the health of their citizens, the masking effect on global warming is expected to decline further. The scenarios for climate action required to meet 1.5 °C (2.7 °F) and 2 °C (3.6 °F) targets incorporate the predicted decrease in aerosol levels. However, model simulations of the effects of aerosols on weather systems remain uncertain. The processes behind global dimming are similar to stratospheric aerosol injection. This is a proposed solar geoengineering intervention which aims to counteract global warming through intentional releases of reflective aerosols. Stratospheric aerosol injection could be very effective at stopping or reversing warming but it would also have substantial effects on the global water cycle, regional weather, and ecosystems. Furthermore, it would have to be carried out over centuries to prevent a rapid and violent return of the warming.

History

In the 1970s, numerous studies showed that atmospheric aerosols could affect the propagation of sunlight through the atmosphere, a measure also known as direct solar irradiance. One study showed that less sunlight was filtering through at the height of 1.7 km (1.1 mi) above Los Angeles, even on those days when there was no visible smog. Another suggested that sulfate pollution or a volcano eruption could provoke the onset of an ice age. In the 1980s, Atsumu Ohmura, a geography researcher at the Swiss Federal Institute of Technology, found that solar radiation striking the Earth's surface had declined by more than 10% over the three previous decades, even as the global temperature had been generally rising since the 1970s. In the 1990s, this was followed by the papers describing multi-decade declines in Estonia, Germany, Israel and across the former Soviet Union. Subsequent research estimated an average reduction in sunlight striking the terrestrial surface of around 4–5% per decade over the late 1950s–1980s, and 2–3% per decade when 1990s were included. Notably, solar radiation at the top of the atmosphere did not vary by more than 0.1–0.3% in all that time, strongly suggesting that the reasons for the dimming were on Earth. Additionally, only visible light and infrared radiation were dimmed, rather than the ultraviolet part of the spectrum. Further, the dimming had occurred even when the skies were clear, and it was in fact stronger than during the cloudy days, proving that it was not caused by changes in cloud cover alone.

Causes

Anthropogenic sulfates

Global dimming is primarily caused by the presence of sulfate particles which hang in the Earth's atmosphere as aerosols. These aerosols have both a direct contribution to dimming, as they reflect sunlight like tiny mirrors. They also have an indirect effect as nuclei, meaning that water droplets in clouds coalesce around the particles. Increased pollution causes more particulates and thereby creates clouds consisting of a greater number of smaller droplets (that is, the same amount of water is spread over more droplets). The smaller droplets make clouds more reflective, so that more incoming sunlight is reflected back into space and less reaches the Earth's surface. In models, these smaller droplets also decrease rainfall. Before the Industrial Revolution, the main source of sulfate aerosols was dimethyl sulfide produced by some types of oceanic plankton. Emissions from volcano activity were the second largest source, although large volcanic eruptions, such as the 1991 eruption of Mount Pinatubo, dominate in the years when they occur. In 1990, the IPCC First Assessment Report estimated dimethyl sulfide emissions at 40 million tons per year, while volcano emissions were estimated at 10 million tons. These annual levels have been largely stable for a long time. On the other hand, global human-caused emissions of sulfur into the atmosphere increased from less than 3 million tons per year in 1860 to 15 million tonnes in 1900, 40 million tonnes in 1940 and about 80 million tonnes in 1980. This meant that by 1980, the human-caused emissions from the burning of sulfur-containing fuels (mostly coal and bunker fuel) became at least as large as all natural emissions of sulfur-containing compounds. The report also concluded that "in the industrialized regions of Europe and North America, anthropogenic emissions dominate over natural emissions by about a factor of ten or even more".

Black carbon

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