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Published: Tuesday 13 August, 2013

canada goose coats on sale canada goose coats on sale Do you think we are seeing the signs of a global dimming

Since measurements began in the 1950s, scientists have discovered that there has been a decline of sunlight reaching the Earth; they called it global dimming. But according to a paper published in the journal Science this month, the dimming did not continue into the 1990s and indeed since the 1980s scientists have observed a widespread brightening. What caused the dimming to go down and what effect will it have, if any, on climate change?Robyn Williams: Did you see the Four Corners program about global dimming: the idea that pollution forms a blanket over the Earth and has masked the effects of climate change until recentl canada goose coats on sale y? Well a paper in the journal Science this month shows that dimming went down after 1990 and it was interesting to watch sceptics leap on this as if it negates the whole proposition of warming. Not so. Dr Martin Wild heads the team based at the Institute for Atmospheric and Climate Science in Zurich in Switzerland. So why has dimming gone down?

Martin Wild: One of the reasons for this is that the air has become cleaner again. We see that sunlight reaches again the Earths surface easier than it did before and we think one of the reasons for this is that air pollution has somewhat reduced over the 1990s and that a canada goose coats on sale ir pollution measures have become more effective. Also, the breakdown of the former communistic countries led to a reduction of emissions over a large part of the northern hemisphere, which again helped. But also there are some indications that the cloud cover has decreased over the 1990s, which also favours increase in solar radiation at the Earths surface.

Robyn Williams: How might this affect global warming, because there have been predictions, notably of course as we saw in a Four Corners film here on ABC television, that when this dimming was reduced we would get an increase, a rapid increase in global warming. What do you think of that idea?

Martin Wild: The change in sunlight at the Earths surface does not by itself accelerate warming. However, I would rather say that they do no longer mask the greenhouse effect, so the greenhouse induced warming becomes visible in its full size since there is no global dimming anymore, which would counteract and mask the global warming.

Robyn Williams: And so you would yourself expect global warming to be more severe as a result of that and the signs already to be present?

Martin Wild: That is true. I think we have seen that already. We see that between 96 and 98 where we had significant dimming there was no increase in surface temperature measured at land surfaces. There was hardly any change in temperature to the Earths surface, however, starting in the 80s and 90s when the dimming was no longer present the temperature of the Earths surface increased by quite significantly. So I think this effect we have already seen in the past decades.

Robyn Williams: Well some people are quoting that, you know, two years ago in Europe there was a huge heatwave and lots of people in France and other places died, would you expect that sort of thing to be linked to that effect?

Martin Wild: I think the European heatwave was rather local, it has affected the whole of Europe but on average there has been no increase in temperature over the whole globe. So, I think this is an effect linked to the dynamics of the atmosphere which may made high pressure system stay over Europe for a very long time.

Robyn Williams: So its a local spike, yes.

Martin Wild: Its made a very hot summer in Europe but for example a very bad summer in Japan,.

Global dimming is the gradual reduction in the amount of global direct irradiance at the Earths surface that was observed for several decades after the start of systematic measurements in the 1950s. The effect varies by location, but worldwide it has been estimated to be of the order of a 4% reduction over the three decades from 19601990. However, after discounting an anomaly caused by the eruption of Mount Pinatubo in 1991, a very slight reversal in the overall trend has been observed.1

It is thought to have been caused by an increase in particulates such as sulfate aerosols in the atmosphere due to human action. The switch from a global dimming trend to a brightening trend in 1990 happened just as global aerosol levels started to decline.

Global dimming has interfered with the hydrological cycle by reducing evaporation and may have reduced rainfall in some areas. Global dimming also creates a cooling effect that may have partially masked the effect of greenhouse gases on global warming.

Deliberate manipulation of this dimming effect is now being considered as a geoengineering technique to reduce the impact of global warming.

The incomplete combustion of fossil fuels such as diesel and wood releases black carbon into the air. Though black carbon, most of which is soot, is an extremely small component of air pollution at land surface levels, the phenomenon has a significant heating effect on the atmosphere at altitudes above two kilometers 6,562 ft. Also, it dims the surface of the ocean by absorbing solar radiation.27

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.28 Prior to the research being undertaken, predictions were of a 0.51% 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.

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.29

However some research shows that black carbon will actually increase global warming, being second only to CO2. They believe that soot will absorb solar energy and transport it to other areas such as the Himalayas where glacial melting occurs. It can also darken Arctic ice reducing reflectivity and increasing absorption of solar radiation.30

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 neartotal shutdown of civil air traffic during the three days following the September 11, 2001 attacks afforded a unique opportunity in which to observe the climate of the United States absent from the effect of contrails. aircraft contrails may have been raising nighttime temperatures and/or lowering daytime temperatures by much more than previously thought.26

Airborne volcanic ash can reflect the Suns rays back into space and thereby contribute to cooling the planet. Dips in earth temperatures have been observed after large volca canada goose coats on sale no eruptions such as Mount Agung in Bali that erupted in 1963, El Chichon Mexico 1983, Ruiz Colombia 1985, and Pinatubo Philippines 1991. But even for major eruptions, the ash clouds remain only for relatively short periods. canada goose coats on sale

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