GLOBAL DIMMING

Global dimming is the gradual reduction in the amount of global direct irradiance at the Earth’s surface, observed since the beginning of systematic measurements in 1950s. The effect varies by location, but worldwide it is of the order of a 4% reduction over the three decades from 1960–1990. This trend may have reversed during the past decade. Global dimming creates a cooling effect that may have partially masked the effect of greenhouse gases on global warming.

Cause and effects of global dimming

It is currently thought that the effect of global dimming is probably due to the increased presence of aerosol particles in the atmosphere. Aerosol particles and other particulate pollutants absorb solar energy and reflect sunlight back into space. The pollutants can also become nuclei for cloud droplets. It is thought that the water droplets in clouds coalesce around the particles. Increased pollution, resulting in more particulates, creates clouds consisting of a greater number of smaller droplets, which in turn makes them more reflective, therefore 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 altitude. Usually during the daytime 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.

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. It may be suggested that increasing aerosol might be a possible solution to global warming. However, aerosol has negative effects (acid rain) which is why nations have made efforts to reduce it. Also, the aerosol has a very short lifetime (weeks).

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 US relative to the warming western part.

Research into global dimming

Early reports of “global dimming” attracted little interest, perhaps because the term itself 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 the late 1980s onwards, scientists independently began 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 coined the term “dimming”. Dimming exists in sites all over the Former Soviet Union].

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

It should be noted 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.

Some climate scientists have theorized that aircraft contrails (also called vapor trails) are implicated in global dimming, but the constant flow of air traffic previously 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 of the trend

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

The Baseline Surface Radiation Network (BSRN) has been collecting surface measurements. BSRN was started in the early 1990s and updated the archives in this time. Analysis of recent data reveals that the surface of the planet has brightened by about 4% in the past decade. The brightening trend is corroborated by other data, including satellite analyses.

Effects on rainfall pattern

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. Because of this, the Tropical rain belt may not have risen to its northern latitudes, thus causing an absence of seasonal rains. This claim is not universally accepted and is very difficult to test.

This figure shows the level of agreement between a climate model driven by five factors and the historical temperature record. The negative component identified as “sulfate” is associated with the aerosol emissions blamed for global dimming.It is also concluded that the imbalance between global dimming and global warming at the surface leads to weaker turbulent heat fluxes to the atmosphere. This means globally reduced evaporation and hence precipitation occur in a dimmer and warmer world, which could ultimately lead to a more humid atmosphere in which it rains less.

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