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 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 and 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 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. 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 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
Another important type of aerosol is black carbon, colloquially known as soot. It is formed due to incomplete combustion of fossil fuels, as well as of wood and other plant matter. Globally, the single largest source of black carbon is from grassland and forest fires, including both wildfires and intentional burning. However, coal use is responsible for the majority of black carbon emissions in Asia and Africa, while diesel combustion produces 70% of black carbon in Europe and The Americas.Black carbon in the lower atmosphere is a major contributor to 7 million premature deaths caused by air pollution every year. Its presence is particularly visible, as the so-called "brown clouds" appear in heavily polluted areas. In fact, it was 1970s research into the Denver brown cloud which had first found that black carbon particles absorb solar energy and so can affect the amount of visible sunlight. Later research found that black carbon is 190 times more effective at absorbing sunlight within clouds than the regular dust from soil particles. At worst, all clouds within an atmospheric layer thick are visibly darkened, and the plume can reach transcontinental scale Even so, the overall dimming from black carbon is much lower than that from the sulfate particles.
Reversal
After 1990, the global dimming trend had clearly switched to global brightening. This followed measures taken to combat air pollution by the developed nations, typically through flue-gas desulfurization installations at thermal power plants, such as wet scrubbers or fluidized bed combustion. In the United States, sulfate aerosols have declined significantly since 1970 with the passage of the Clean Air Act, which was strengthened in 1977 and 1990. According to the EPA, from 1970 to 2005, total emissions of the six principal air pollutants, including sulfates, dropped by 53% in the US. By 2010, this reduction in sulfate pollution led to estimated healthcare cost savings valued at $50 billion annually. Similar measures were taken in Europe, such as the 1985 Helsinki Protocol on the Reduction of Sulfur Emissions under the Convention on Long-Range Transboundary Air Pollution, and with similar improvements.File:Pollution over east China.jpg|thumb|left|Satellite photo showing a thick pall of smoke and haze from forest fires in Eastern China. Such smoke is full of black carbon, which contributes to dimming trends but has an overall warming effect.
On the other hand, a 2009 review found that dimming continued to increase in China after stabilizing in the 1990s and intensified in India, consistent with their continued industrialization, while the US, Europe, and South Korea continued to brighten. Evidence from Zimbabwe, Chile and Venezuela also pointed to increased dimming during that period, albeit at a lower confidence level due to the lower number of observations. Later research found that over China, the dimming trend continued at a slower rate after 1990, and did not begin to reverse until around 2005. Due to these contrasting trends, no statistically significant change had occurred on a global scale from 2001 to 2012. Post-2010 observations indicate that the global decline in aerosol concentrations and global dimming continued, with pollution controls on the global shipping industry playing a substantial role in the recent years. Since nearly 90% of the human population lives in the Northern Hemisphere, clouds there are far more affected by aerosols than in the Southern Hemisphere, but these differences have halved in the two decades since 2000, providing further evidence for the ongoing global brightening.
Relationship to climate change
Cooling from sulfate aerosols
Aerosols have a cooling effect, which has masked the total extent of global warming experienced to date.It has been understood for a long time that any effect on solar irradiance from aerosols would necessarily impact Earth's radiation balance. Reductions in atmospheric temperatures have already been observed after large volcanic eruptions such as the 1963 eruption of Mount Agung in Bali, 1982 El Chichón eruption in Mexico, 1985 Nevado del Ruiz eruption in Colombia and 1991 eruption of Mount Pinatubo in the Philippines. However, even the major eruptions only result in temporary jumps of sulfur particles, unlike the more sustained increases caused by anthropogenic pollution.
In 1990, the IPCC First Assessment Report acknowledged that "Human-made aerosols, from sulphur emitted largely in fossil fuel combustion can modify clouds and this may act to lower temperatures", while "a decrease in emissions of sulphur might be expected to increase global temperatures". However, lack of observational data and difficulties in calculating indirect effects on clouds left the report unable to estimate whether the total impact of all anthropogenic aerosols on the global temperature amounted to cooling or warming. By 1995, the IPCC Second Assessment Report had confidently assessed the overall impact of aerosols as negative ; however, aerosols were recognized as the largest source of uncertainty in future projections in that report and the subsequent ones.