Global dimming

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Global dimming is a decline in the amount of sunlight reaching the Earth's surface, a measure also known as global direct solar irradiance.[2][3] It was observed soon after the first systematic measurements of solar irradiance began in the 1950s, and continued until 1980s, with an observed reduction of 4–5% per decade,[1] even though solar activity did not vary more than the usual at the time.[4][2] Instead, global dimming had been attributed to an increase in atmospheric particulate matter, predominantly sulfate aerosols, as the result of rapidly growing air pollution due to post-war industrialization. After 1980s, reductions in particulate emissions have also caused a "partial" reversal of the dimming trend, which has sometimes been described as a global brightening.[1] This reversal is not yet complete, and it has also been globally uneven, as some of the brightening over the developed countries in the 1980s and 1990s had been counteracted by the increased dimming from the industrialization of the developing countries and the expansion of the global shipping industry,[5] although they have also been making rapid progress in cleaning up air pollution in the recent years.[6][7]

Global dimming has interfered with the hydrological cycle by lowering evaporation, which is likely to have reduced rainfall in certain areas,[1] and may have caused the observed southwards shift of the entire tropical rain belt between 1950 and 1985, with a limited recovery afterwards.[8] Since high evaporation at the tropics is needed to drive the wet season, cooling caused by particulate pollution appears to weaken the Monsoon of South Asia, while reductions in pollution strengthen it.[9][10] Multiple studies have also connected record levels of particulate pollution in the Northern Hemisphere to the monsoon failure behind the 1984 Ethiopian famine,[11][12][13] although the full extent of anthropogenic vs. natural influences on that event is still disputed.[14][15] On the other hand, global dimming has also counteracted some of the greenhouse gas emissions, effectively "masking" the total extent of global warming experienced to date, with the most-polluted regions even experiencing cooling in the 1970s. Conversely, global brightening contributed to the acceleration of global warming which began in the 1990s.[1][16]

In the near future, global brightening is expected to continue, as nations act to reduce the toll of air pollution on the health of their citizens. This also means that less of global warming would be masked in the future. Climate models are broadly capable of simulating the impact of aerosols like sulfates, and in the IPCC Sixth Assessment Report, they are believed to offset around 0.5 °C (0.9 °F) of warming. Likewise, climate change scenarios incorporate reductions in particulates and the cooling they offered into their projections, and this includes the scenarios for climate action required to meet 1.5 °C (2.7 °F) and 2 °C (3.6 °F) targets.[17] It is generally believed that the cooling provided by global dimming is similar to the warming derived from atmospheric methane, meaning that simultaneous reductions in both would effectively cancel each other out.[18] However, uncertainties remain about the models' representation of aerosol impacts on weather systems, especially over the regions with a poorer historical record of atmospheric observations.[19][20][21][22]

The processes behind global dimming are similar to those which drive reductions in direct sunlight after volcanic eruptions. In fact, the eruption of Mount Pinatubo in 1991 had temporarily reversed the brightening trend.[23] Both are considered an analogue for stratospheric aerosol injection, a solar geoengineering intervention which aims to counteract global warming through intentional releases of reflective aerosols, albeit at much higher altitudes, where lower quantities would be needed and the polluting effects would be minimized.[24] That intervention may be very effective at stopping or reversing warming and its main consequences, yet it would also have substantial effects on the global hydrological cycle, as well as regional weather and ecosystems. Further, it would have to be carried out over centuries until the greenhouse gas concentrations are normalized to avoid aerosols leaving the atmosphere too early. Otherwise, a rapid and violent return of the warming, sometimes known as termination shock, would occur.[25]

History[edit]

The observed trends of global dimming and brightening in four major geopolitical regions. The dimming was greater on the average cloud-free days (red line) than on the average of all days (purple line), strongly suggesting that sulfate aerosols were the cause.[20]

In the 1970s, numerous studies have shown that the atmospheric aerosols could affect the propagation of sunlight through the atmosphere.[26][27] One of them had shown 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. [28] Another suggested that sulfate pollution or a volcano eruption could provoke the onset of an ice age.[29][30] In the 1980s, research in Israel and the Netherlands revealed an apparent reduction in the amount of sunlight,[31] and 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.[32] In the 1990s, this was followed by the papers describing multi-decade declines in Estonia,[33] Germany,[34] Israel[35] and across the former Soviet Union.[36]

Subsequent research estimated an average reduction in sunlight striking the terrestrial surface of around 4–5% per decade over late 1950s–1980s, and 2–3% per decade when 1990s were included.[35][37][38][39] 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.[4][2] Additionally, only visible light and infrared radiation were dimmed, rather than the ultraviolet part of the spectrum.[40] 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.[41][2][20]

Causes[edit]

Anthropogenic sulfates[edit]

Snapshot of atmospheric sulfur dioxide on April 15, 2017. As it moves through the atmosphere with prevailing winds, weather patterns and seasonality alter these distributions from day-to-day. Sulfur dioxide forms highly reflective sulfates, which are considered the main cause of global dimming.[42]

Global dimming had been widely attributed to the increased presence of sulfate particles which hang in the Earth's atmosphere as aerosols.[43] These aerosols have both a direct contribution to dimming, as they reflect sunlight like tiny mirrors.[44] and an indirect effect where 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.[42] In models, these smaller droplets also decrease rainfall.[45]

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.[46] These annual levels have been largely stable for a long time. On the other hand, the global human-caused emissions of sulfur into the atmosphere were less than 3 million tons per year in 1860, yet they increased 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.[46] 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".[46]

Black carbon[edit]

If smoke from wildfires mixes into clouds, it darkens them, decreasing their albedo. If there are no clouds, then smoke can increase albedo, particularly over oceans.[47]

Another important type of aerosol is black carbon (predominantly soot), which is formed due to incomplete combustion of fossil fuels (such as diesel), as well as of wood and other plant matter. Once black carbon particles are in the air, they absorb solar energy and heat up. This reduces the overall amount of sunlight received on the surface and so contributes to dimming, but it also contributes to warming.[48]

Aircraft contrails[edit]

Aircraft contrails both reflect incoming solar radiation and trap outgoing longwave radiation that is emitted by the Earth. Their heat-trapping effect is larger than their dimming effect, which results in a net increase in radiative forcing. In 1992, the overall warming effect of contrails was estimated to be between 3.5 mW/m2 and 17 mW/m2.[49]

Aircraft contrails (white lines) and natural clouds.

Certain real-world events have been studied for their potential to provide short-term demonstrations of global dimming and the associated effects.[3] For instance, aircraft leave behind visible contrails (also known as vapor trails) as they travel. In the 1990s, it was suggested that these trails have a strong cooling effect,[50] and when no commercial aircraft flew across the USA following the September 11 attacks, the diurnal temperature variation (the difference in the day's highs and lows at a fixed station) was widened by 1.1 °C (2.0 °F).[51]

Measured across 4,000 weather stations in the continental United States, this increase was the largest recorded in 30 years.[51] Without contrails, the local diurnal temperature range was 1 °C (1.8 °F) higher than immediately before.[52] In the southern US, the difference was diminished by about 3.3 °C (6 °F), and by 2.8 °C (5 °F) in the US midwest.[53][54] However, follow-up studies found that a natural change in cloud cover can more than explain these findings.[55][56] When the global response to the 2020 coronavirus pandemic led to a reduction in global air traffic of nearly 70% relative to 2019, multiple studies found "no significant response of diurnal surface air temperature range" as the result of contrail changes, and either "no net significant global ERF" (effective radiative forcing) or a very small warming effect.[57][58][59]

Reversal[edit]

Sun-blocking aerosols around the world steadily declined (red line) since the 1991 eruption of Mount Pinatubo, according to satellite estimates.

After 1990, the global dimming trend had clearly switched to global brightening.[31][60][61][62][63] 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.[64][65] 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.[66] By 2010, this reduction in sulfate pollution led to estimated healthcare cost savings valued at $50 billion annually.[67] Similar measures were taken in Europe,[66] 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.[68]

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 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 continued dimming during that period, albeit at a lower confidence level due to the lower number of observations.[5][69] Later research found that over China, the dimming trend continued at a slower rate after 1990,[70] and did not begin to reverse until around 2005.[71] Due to these contrasting trends, no statistically significant change had occurred on a global scale from 2001 to 2012.[1] 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.[7] 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.[72]

Relationship to climate change[edit]

Past and present[edit]

Air pollution, including from large-scale land clearing, has substantially increased the presence of aerosols in the atmosphere when compared to the preindustrial background levels. Different types of particles have different effects, and there is a variety of interactions in different atmospheric layers. Overall, they provide cooling, but complexity makes the exact strength of cooling very difficult to estimate.[47]

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.[63] 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.[46] By 1995, the IPCC Second Assessment Report had confidently assessed the overall impact of aerosols as negative (cooling);[73] however, aerosols were recognized as the largest source of uncertainty in future projections in that report and the subsequent ones.[1]

At the peak of global dimming, it was able to counteract the warming trend completely, but by 1975, the continually increasing concentrations of greenhouse gases have overcome the masking effect and dominated ever since.[66] Even then, regions with high concentrations of sulfate aerosols due to air pollution had initially experienced cooling, in contradiction to the overall warming trend.[74] The eastern United States was a prominent example: the temperatures there declined by 0.7 °C (1.3 °F) between 1970 and 1980, and by up to 1 °C (1.8 °F) in the Arkansas and Missouri. As the sulfate pollution was reduced, the central and eastern United States had experienced warming of 0.3 °C (0.54 °F) between 1980 and 2010,[75] even as sulfate particles still accounted for around 25% of all particulates.[67] By 2021, the northeastern coast of the United States was instead one of the fastest-warming regions of North America, as the slowdown of the Atlantic Meridional Overturning Circulation increased temperatures in that part of the North Atlantic Ocean.[76][77]

The extent to which physical factors in the atmosphere or on land affect climate change, including the cooling provided by sulfate aerosols and the dimming they cause. The large error bar shows that there are still substantial unresolved uncertainties.

Globally, the emergence of extreme heat beyond the preindustrial records was delayed by aerosol cooling, and hot extremes accelerated as global dimming abated: it has been estimated that since the mid-1990s, peak daily temperatures in northeast Asia and hottest days of the year in Western Europe would have been substantially less hot if aerosol concentrations had stayed the same as before.[1] In Europe, the declines in aerosol concentrations since the 1980s had also reduced the associated fog, mist and haze: altogether, it was responsible for about 10–20% of daytime warming across Europe, and about 50% of the warming over the more polluted Eastern Europe.[78] Because aerosol cooling depends on reflecting sunlight, air quality improvements had a negligible impact on wintertime temperatures,[79] but had increased temperatures from April to September by around 1 °C (1.8 °F) in Central and Eastern Europe.[80] Some of the acceleration of sea level rise, as well as Arctic amplification and the associated Arctic sea ice decline, was also attributed to the reduction in aerosol masking.[81][82][83]

In addition to revealing the limited effect of contrails, COVID-19 lockdowns provided another "natural experiment", as there had been a marked decline in sulfate emissions caused by the curtailed road traffic and industrial output. That decline did have a detectable warming impact: it was estimated to have increased global temperatures by 0.01–0.02 °C (0.018–0.036 °F) initially and up to 0.03 °C (0.054 °F) by 2023, before disappearing. Regionally, the lockdowns were estimated to increase temperatures by 0.05–0.15 °C (0.090–0.270 °F) in eastern China over January–March, and then by 0.04–0.07 °C (0.072–0.126 °F) over Europe, eastern United States, and South Asia in March–May, with the peak impact of 0.3 °C (0.54 °F) in some regions of the United States and Russia.[84][85] In the city of Wuhan, the urban heat island effect was found to have decreased by 0.24 °C (0.43 °F) at night and by 0.12 °C (0.22 °F) overall during the strictest lockdowns.[86]

Pollution from black carbon, mostly represented by soot, also contributes to global dimming. However, because it absorbs heat instead of reflecting it, it warms the planet instead of cooling it like sulfates. This warming is much weaker than that of greenhouse gases, but it can be regionally significant when black carbon is deposited over ice masses like mountain glaciers and the Greenland ice sheet, where it reduces their albedo and increases their absorption of solar radiation.[87] Even the indirect effect of soot particles acting as cloud nuclei is not strong enough to provide cooling: the "brown clouds" formed around soot particles were known to have a net warming effect since the 2000s.[88] Black carbon pollution is particularly strong over India, and as the result, it is considered to be one of the few regions where cleaning up air pollution would reduce, rather than increase, warming.[89]

Future[edit]

Early 2010s estimates of past and future anthropogenic global sulfur dioxide emissions, including the Representative Concentration Pathways. While no climate change scenario may reach Maximum Feasible Reductions (MFRs), all assume steep declines from today's levels. By 2019, sulfate emission reductions were confirmed to proceed at a very fast rate.[6]

Since changes in aerosol concentrations already have an impact on the global climate, they would necessarily influence future projections as well. In fact, it is impossible to fully estimate the warming impact of all greenhouse gases without accounting for the counteracting cooling from aerosols. Climate models started to account for the effects of sulfate aerosols around the IPCC Second Assessment Report; when the IPCC Fourth Assessment Report was published in 2007, every climate model had integrated sulfates, but only 5 were able to account for less impactful particulates like black carbon.[44] By 2021, CMIP6 models estimated total aerosol cooling in the range from 0.1 °C (0.18 °F) to 0.7 °C (1.3 °F);[90] The IPCC Sixth Assessment Report selected the best estimate of a 0.5 °C (0.90 °F) cooling provided by sulfate aerosols, while black carbon amounts to about 0.1 °C (0.18 °F) of warming.[17] While these values are based on combining model estimates with observational constraints, including those on ocean heat content,[7] the matter is not yet fully settled. The difference between model estimates mainly stems from disagreements over the indirect effects of aerosols on clouds.[91][92]

Regardless of the current strength of aerosol cooling, all future climate change scenarios project decreases in particulates and this includes the scenarios where 1.5 °C (2.7 °F) and 2 °C (3.6 °F) targets are met: their specific emission reduction targets assume the need to make up for lower dimming.[17] Since models estimate that the cooling caused by sulfates is largely equivalent to the warming caused by atmospheric methane (and since methane is a relatively short-lived greenhouse gas), it is believed that simultaneous reductions in both would effectively cancel each other out.[18] Yet, in the recent years, methane concentrations had been increasing at rates exceeding their previous period of peak growth in the 1980s,[93][94] with wetland methane emissions driving much of the recent growth,[95][96] while air pollution is getting cleaned up aggressively.[7] These trends are some of the main reasons why 1.5 °C (2.7 °F) warming is now expected around 2030, as opposed to the mid-2010s estimates where it would not occur until 2040.[6]

Between 1980s and 2010s, the reduction in aerosol density over Europe had lowered the difference between thermal energy entering the atmosphere from the sun and between thermal energy radiating off the ground (top left, blue), which increased the net amount of heating and contributed to increase in extreme temperatures (other images, red).[80]

It has also been suggested that aerosols are not given sufficient attention in regional risk assessments, in spite of being more influential on a regional scale than globally.[22] For instance, a climate change scenario with high greenhouse gas emissions but strong reductions in air pollution would see 0.2 °C (0.36 °F) more global warming by 2050 than the same scenario with little improvement in air quality, but regionally, the difference would add 5 more tropical nights per year in northern China and substantially increase precipitation in northern China and northern India.[97] Likewise, a paper comparing current level of clean air policies with a hypothetical maximum technically feasible action under otherwise the same climate change scenario found that the latter would increase the risk of temperature extremes by 30–50% in China and in Europe.[98] Unfortunately, because historical records of aerosols are sparser in some regions than in others, accurate regional projections of aerosol impacts are difficult. Even the latest CMIP6 climate models can only accurately represent aerosol trends over Europe,[20] but struggle with representing North America and Asia, meaning that their near-future projections of regional impacts are likely to contain errors as well.[19][20][21]

Relationship to hydrological cycle[edit]

Sulfate aerosols have decreased precipitation over most of Asia (red), but increased it over some parts of Central Asia (blue).[99]

On regional and global scale, air pollution can affect the water cycle, in a manner similar to some natural processes. One example is the impact of Sahara dust on hurricane formation: air laden with sand and mineral particles moves over the Atlantic Ocean, where they block some of the sunlight from reaching the water surface, slightly cooling it and dampening the development of hurricanes.[100] Likewise, it has been suggested since the early 2000s that since aerosols decrease solar radiation over the ocean and hence reduce evaporation from it, they would be "spinning down the hydrological cycle of the planet."[101][102] In 2011, it was found that anthropogenic aerosols had been the predominant factor behind 20th century changes in rainfall over the Atlantic Ocean sector,[103] when the entire tropical rain belt shifted southwards between 1950 and 1985, with a limited northwards shift afterwards.[8] Future reductions in aerosol emissions are expected to result in a more rapid northwards shift, with limited impact in the Atlantic but a substantially greater impact in the Pacific.[104]

Most notably, multiple studies connect aerosols from the Northern Hemisphere to the failed monsoon in sub-Saharan Africa during the 1970s and 1980s, which then led to the Sahel drought and the associated famine.[11][13][12] However, model simulations of Sahel climate are very inconsistent,[105] so it's difficult to prove that the drought would not have occurred without aerosol pollution, although it would have clearly been less severe.[14][15] Some research indicates that those models which demonstrate warming alone driving strong precipitation increases in the Sahel are the most accurate, making it more likely that sulfate pollution was to blame for overpowering this response and sending the region into drought.[106]

Another dramatic finding had connected the impact of aerosols with the weakening of the Monsoon of South Asia. It was first advanced in 2006,[9] yet it also remained difficult to prove.[107] In particular, some research suggested that warming itself increases the risk of monsoon failure, potentially pushing it past a tipping point.[108][109] By 2021, however, it was concluded that global warming consistently strengthened the monsoon,[110] and some strengthening was already observed in the aftermath of lockdown-caused aerosol reductions.[10]

In 2009, an analysis of 50 years of data found that light rains had decreased over eastern China, even though there was no significant change in the amount of water held by the atmosphere. This was attributed to aerosols reducing droplet size within clouds, which led to those clouds retaining water for a longer time without raining.[45] The phenomenon of aerosols suppressing rainfall through reducing cloud droplet size has been confirmed by subsequent studies.[111] Later research found that aerosol pollution over South and East Asia didn't just suppress rainfall there, but also resulted in more moisture transferred to Central Asia, where summer rainfall had increased as the result.[99] IPCC Sixth Assessment Report had also linked changes in aerosol concentrations to altered precipitation in the Mediterranean region.[1]

Solar geoengineering[edit]

This graph shows baseline radiative forcing under three different Representative Concentration Pathway scenarios, and how it would be affected by the deployment of SAI, starting from 2034, to either halve the speed of warming by 2100, to halt the warming, or to reverse it entirely.[112]

Global dimming is also a relevant phenomenon for certain proposals about slowing, halting or reversing global warming.[113] An increase in planetary albedo of 1% would eliminate most of radiative forcing from anthropogenic greenhouse gas emissions and thereby global warming, while a 2% albedo increase would negate the warming effect of doubling the atmospheric carbon dioxide concentration.[114] This is the theory behind solar geoengineering, and the high reflective potential of sulfate aerosols means that they were considered in this capacity for a long time. In 1974, Mikhail Budyko suggested that if global warming became a problem, the planet could be cooled by burning sulfur in the stratosphere, which would create a haze.[115] This approach would simply send the sulfates to the troposphere – the lowest part of the atmosphere. Using it today would be equivalent to more than reversing the decades of air quality improvements, and the world would face the same issues which prompted the introduction of those regulations in the first place, such as acid rain.[116] The suggestion of relying on tropospheric global dimming to curb warming has been described as a "Faustian bargain" and is not seriously considered by modern research.[14]

Instead, starting with the seminal 2006 paper by Paul Crutzen, the solution advocated is known as stratospheric aerosol injection, or SAI. It would transport sulfates into the next higher layer of the atmosphere – stratosphere, where they would last for years instead of weeks, so far less sulfur would have to be emitted.[117][118] It has been estimated that the amount of sulfur needed to offset a warming of around 4 °C (7.2 °F) relative to now (and 5 °C (9.0 °F) relative to the preindustrial), under the highest-emission scenario RCP 8.5 would be less than what is already emitted through air pollution today, and that reductions in sulfur pollution from future air quality improvements already expected under that scenario would offset the sulfur used for geoengineering.[24] The trade-off is increased cost. Although there's a popular narrative that stratospheric aerosol injection can be carried out by individuals, small states, or other non-state rogue actors, scientific estimates suggest that cooling the atmosphere by 1 °C (1.8 °F) through stratospheric aerosol injection would cost at least $18 billion annually (at 2020 USD value), meaning that only the largest economies or economic blocs could afford this intervention.[112][119] Even so, these approaches would still be "orders of magnitude" cheaper than greenhouse gas mitigation,[120] let alone the costs of unmitigated effects of climate change.[114]

The main downside to SAI is that any such cooling would still cease 1–3 years after the last aerosol injection, while the warming from CO2 emissions lasts for hundreds to thousands of years. This means that neither stratospheric aerosol injection nor other forms of solar geoengineering can be used as a substitute for reducing greenhouse gas emissions, because if solar geoengineering were to cease while greenhouse gas levels remained high, it would lead to "large and extremely rapid" warming and similarly abrupt changes to the water cycle. Many thousands of species would likely go extinct as the result. Instead, any solar geoengineering would act as a temporary measure to limit warming while emissions of greenhouse gases are reduced and carbon dioxide is removed, which may well take hundreds of years.[25]

While stratospheric aerosol injection may temporarily halt or outright reverse the warming, there would still be significant changes in weather patterns in many areas, affecting the ecosystems. Climate change can affect the distribution of infectious diseases, and these changes would also shift the habitat of mosquitoes and other disease vectors, with currently unclear impacts. There have also been early concerns about the impacts on crop yields and carbon sinks,[113] but most recent science suggests that globally, they would be largely unaffected or may even increase slightly relative to the early 21st century. This is because reduced photosynthesis due to lower sunlight would be offset by CO2 fertilization effect and the reduction in thermal stress.[25]

See also[edit]

References[edit]

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