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Global DimmingGlobal dimming is a gradual reduction in the amount of sunlight reaching the Earth's surface since the 1950s. The effect varies by location but globally is of the order of a 10% reduction over the last four decades. This cooling effect may have 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 and other particulates in the atmosphere. It is thought that the water droplets in clouds coalesce around the particles, resulting 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. During the day the interception of sunlight predominates giving a cooling effect but at night the reflection and absorption and reradiation of heat from the Earth is the only effect so they keep us warm. Hence while clouds reduce the sunlight reaching the earth, it is possible, but not inevitable, for them to have a net warming effect over a 24 hour cycle. The height and reflectivity of the clouds affects the balance between cooling and warming. However it has never been shown that a cloud has remained in a fixed shape or location so the possibility for error in these conclusions renders them somewhat less than absurd. Research Global dimming may have been reported by Atsumu Ohmura in 1989; it was certainly reported by Stanhill and Moreshet in 1992. 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) http://www.realclimate.org/index.php?p=110
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 http://www.google.co.uk/search?q=cache:uaKeKseXPHcJ:www.unep.org/GC/GC22/Document/Executive%2520Summary.pdf+VEERABHADRAN+RAMANATHAN+maldives&hl=en . Prior to the research being undertaken, predictions were of a 0.5% to 1% effect from particulate matter; the variation from prediction is 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, i.e. aircraft contrails have been raising nighttime temperatures and/or lowering daytime temperatures by much more than previously thought. 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, caused by Northern hemisphere pollution cooling the Atlantic. This is not universally accepted and is very difficult to prove. The solution to global dimming would appear to be to cut atmospheric pollution. This has been taking place in Europe for some years, and there is evidence that global dimming had indeed decreased as a result. European data from 1990 onwards which has shown that chemical scrubbing of exhaust gases from power plants and catalytic converters on car exhausts has improved air quality but may have accelerated the rate of warming. However, the rise in average temperatures in Europe in the early years of the 2000s - which is presumed to be a consequence of the reduction in pollution - resulted in forest fires in Portugal, nearly 15,000 heat-related deaths in France, and increased the melting of alpine glaciers during the summer of 2003. Relationship to global warming Some scientists now consider that the effects of global dimming have masked the effect of global warming, and that resolving global dimming may therefore have a major and previously unpredicted impact on temperatures and sea levels. Initial work to incorporate the effects of global dimming suggest that world temperatures may rise by 2 C by 2030, and as much as 10 C by 2100; this is a doubling of the widely accepted figure of a 5 C rise in global temperature this Century. If this were to be so, such large increases would lead to the melting of the Greenland icecap, major reductions in the extent of tropical rainforests, and significant rises in sea levels. A further speculation is that such a rise in temperature would trigger a rapid and irreversible release of the huge deposits of methane hydrates currently locked beneath the ocean floor, releasing methane gas, one of the most powerful of the greenhouse gases. A similar mechanism is one of the theories proposed to explain the Permian-Triassic extinction event approximately 252 million years ago, and the extinctions associated with the Paleocene-Eocene Thermal Maximum around 55 million years ago. It is estimated that it took the planet as long as 100,000 years to recover to a "normal state" following the Thermal Maximum. See also References - Stanhill, G. & Cohen, S. 2001. Global Dimming: a review of the evidence for a widespread and significant reduction in global radiation with discussion of its probable causes and possible agricultural consequences. Agricultural and Forest Meteorology 107, 255-278.
- Roderick, Michael L. and Graham D. Farquhar, 2002. The cause of decreased pan evaporation over the past 50 years. Science Vol. 298, No 5597, pp. 1410-1411, November 15, 2002.
- BBC Horizon TV broadcast of January 13, 2005 BBC Horizon
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