Global Dimming

Global dimming is a term describing the gradual reduction in the amount of global hemispherical irradiance (or total solar irradiance) at the Earth's surface since the 1950s. The effect varies by location but worldwide it is of the order of a 5% reduction over the three decades 1960-1990; the trend has reversed during the past decade. Global dimming creates a cooling effect that has led scientists to underestimate the effect of greenhouse gases on global warming.

Cause and effects

It is now thought that the effect is probably due to the increased presence of aerosols particles in the atmosphere. Aerosol particles absorb solar energy and reflect sunlight back to space. The pollutants can also become nuclei for cloud droplets. It is thought that the water droplets in clouds coalesce around the particles, and more aerosol particles result in the clouds consisting of a greater number of smaller droplets, which in turn makes them more reflective: bouncing more sunlight back into space.

Clouds intercept both heat from the sun and heat radiated from the Earth. Their effects are complex and vary in time, location and height. Usually, during the day the interception of sunlight predominates, giving a cooling effect; however, at night the re-radiation of heat to the Earth slows the earth's heat loss.


Research

Early reports of "global dimming" attracted little interest, perhaps because the name had not yet been coined. The earliest reports seem to be by M. Budyko "The effect of solar radiation variations on the climate of the Earth" in 1969 published in Tellus. From late 1980s on, scientists independently started working on solar radiation datasets and discovered declining trends worldwide; Atsumo Ohmura Secular variation of global radiation in Europe in 1989; Vivii Russak in 1990 "Trends of solar radiation, cloudiness and atmospheric transparency during recent decades in Estonia", and Beate Liepert in 1994 "Solar radiation in Germany - Observed trends and an assessment of their causes". Gerry Stanhill who studied these declines worldwide in many papers (see references) coined the term "dimming". G. Abakumova et al. published evidences in 1996 that dimming exists in sites all over the Former Soviet Union.

Independent research in Israel and Netherlands in the late 1980s showed an apparent reduction in the amount of sunlight despite wide spread evidence that the climate was actually getting hotter (see global warming). The rate of dimming varies around the world but is on average estimated at around 2–3% per decade, with a possibility that the trend reversed in the early 1990s. It is difficult to make an exact measurement because of the difficulty in accurately calibrating the instruments and the problem of spatial coverage. Nonetheless the effect is almost certainly real.

Note that the effect (2-3%, as above) is due to changes within the Earth's atmosphere; the value of the solar radiation at the top of the atmosphere has not changed by more than a fraction of this amount.

The effect varies greatly over the globe, but estimates of the global average value are:
5.3% (9 W/m²) over 1958-85 (Stanhill and Moreshet, 1992)
2%/decade over 1964–93 (Gilgen et al, 1998)
2.7%/decade (total 20 W/m²) up to 2000 (Stanhill and Cohen, 2001)
4% over 1961-1990 (Liepert 2002)

The largest reductions are found in the northern hemisphere mid-latitudes.

Experiments in the Maldives (comparing the atmosphere over the northern and southern islands) in the 1990s showed that the effect of macroscopic pollutants in the atmosphere at that time (blown south from India) caused about a 10% reduction in sunlight reaching the surface in the area under the pollution cloud – a much greater reduction than expected from the presence of the particles themselves.

Prior to the research being undertaken, predictions were of a 0.5% to 1% effect from particulate matter; the variation from prediction may be explained by cloud formation with the particles acting as the focus for droplet creation. Clouds are very effective at reflecting light back out into space.

Some climate scientists have theorised that aircraft contrails are implicated in global dimming, but the constant flow of air traffic meant that this could not be tested. The near-total shutdown of civil air traffic during the three days following the September 11, 2001 attacks afforded a rare opportunity in which to observe the climate of the USA absent from the effect of contrails. During this period an increase in diurnal temperature variation of over 1 °C was observed in some parts of the US, i.e. aircraft contrails may have been raising nighttime temperatures and/or lowering daytime temperatures by much more than previously thought.

Recent reversal

In 2005 Wild et al. and Pinker et al. found that the "dimming" trend had reversed since about 1990 [8]. It is likely that at least some of this change, particularly over Europe, is due to decreases in pollution; most governments have done more to reduce aerosols released into the atmosphere that help global dimming instead of reducing CO2 emissions.

The Baseline Surface Radiation Network (BSRN) has been collecting surface measurements. BSRN didn't get started until the early 1990s and updated the archives. Analysis of recent data reveals the planet's surface has brightened by about 4 % the past decade. The brightening trend is corroborated by other data, including satellite analyses.


Effects

Global dimming may have caused large scale changes in weather patterns. Climate models speculatively suggest that this reduction in sunshine at the surface may have led to the failure of the monsoon in sub-Saharan Africa during the 1970s and 1980s, together with the associated famines such as the Sahel drought, caused by Northern hemisphere pollution cooling the Atlantic. This claim is not universally accepted and is very difficult to test.


Relationship to global warming

Some scientists now consider that the effects of global dimming have masked the effect of global warming to some extent and that resolving global dimming may therefore lead to increases in predictions of future temperature rise.

The phenomenon underlying global dimming may also have regional effects. While most of the earth has warmed, the regions that are downwind from major sources of air pollution (specifically sulfur dioxide emissions) have generally cooled. This may explain the cooling of the eastern United States relative to the warming western part.

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