Global dimming

Global dimming is a decline in the amount of sunlight reaching the Earth's surface, a measure also known as global direct solar irradiance. 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, even though solar activity did not vary more than the usual at the time. 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. 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, although they have also been making rapid progress in cleaning up air pollution in the recent years.

Global dimming has interfered with the hydrological cycle by lowering evaporation, which is likely to have reduced rainfall in certain areas, and may have caused the observed southwards shift of the entire tropical rain belt between 1950 and 1985, with a limited recovery afterwards. Since high evaporation at the tropics is needed to drive the wet season, cooling caused by particulate pollution appears to weaken Monsoon of South Asia, while reductions in pollution strengthen it. Multiple studies have also connected record levels of particulate pollution in the Northern Hemisphere to the monsoon failure behind the 1984 Ethiopian famine, although the full extent of anthropogenic vs. natural influences on that event is still disputed. 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.

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. 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. 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.

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. 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. 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.