By now, everyone has heard about global warming, although the extent of the effect is still under debate, as is the impact of human activities on the level of warming. The latest issue in the debate is the hockey-stick curve (OGJ, July 11, 2005, p. 68).
It seems that the only thing that everyone agrees with is that the Earth’s atmosphere is an incredibly complex system.
Recently, another lesser-known global effect making headlines is a phenomenon called “global dimming.” The theory basically states that the amount of sunlight reaching the Earth’s surface decreased 2-3%/decade during 1960-90. Global dimming has a cooling effect. Many scientists believe that it may have lessened the degree to which global warming has affected the Earth.
According to recent research, a reversal in the global dimming trend began in 1990, which could partially explain the record-high global temperatures in the late 1990s.
Global dimming
First reported in the late 1980s, and measured with data from the 1950s to the present, global dimming varies by location. The largest increases in global dimming occurred in the northern hemisphere’s middle latitudes.
The effect is most likely due to higher levels of particulates and aerosols in the atmosphere. Small water droplets develop around these particulates and form clouds that are more reflective than other clouds, sending more sunlight back into space.
Because the clouds reflect sunlight into space and also intercept the heat radiating from the Earth, the net effect is difficult to predict. The effects vary with the time of day and cloud location and height.
Some researchers even theorize that aircraft contrails contribute to the global dimming effect, although this theory is difficult to test.
Atsumu Ohmura, a geography researcher at the Swiss Federal Institute of Technology in Zurich, was the first to document evidence of global dimming in 1989. Other researchers confirmed his research in the early 1990s.
The early reports of global dimming drew much skepticism from some climatologists who blamed the variations on inaccurate data recording devices.
In 2001, however, Gerald Stanhill (who coined the term global dimming) and Shabtai Cohen of the Volcani Centre, Bet Dagan, Israel, published research based on all the available data. They proved that the amount of solar radiation reaching the Earth’s surface had decreased by 0.23-0.32%/year during 1958-90.
Graham Farquhar with the Australian National University in Canberra, who used a different method to estimate solar radiation, confirmed the conclusions of Stanhill and Cohen.
Good pollution?
In the May 6, 2005, issue of Science magazine, Martin Wild, a climatologist at the Swiss Federal Institute of Technology, and his colleagues reported that global dimming reversed its long-term trend in 1990. Since then, the Earth’s surface has actually brightened.
Wild suggested that decreasing aerosol emissions, especially in the US and Europe, could be the reason behind the global brightening trend. A few other areas of the world, like India, are still experiencing global dimming.
Wild and his group used surface observations to calculate the degree of global dimming. The data, separated into two time periods, 1950-90 and 1985-2000, showed increases in solar radiation.
The surface observations were separated into 32 regions. In the first time period, 24 areas experienced decreasing solar radiation, and 8 areas increased. In the second time period, only 6 regions had decreasing solar radiation (none of which were statistically significant), and 26 showed increases.
The drawback in Wild’s research was limited data because 70% of the Earth’s surface is covered with water and many other areas are unreachable.
Rachel Pinker, a professor of meteorology at the University of Maryland, who also published an article in the same issue of Science magazine, used satellite data since 1983 to measure global dimming. Although data were available for a shorter time period, they do include data for oceans.
Her research showed a linear increase of 0.1%/year in solar radiation. Based on a second-order polynomial, the data showed a slight dimming effect until 1992, at which time solar radiation started increasing.
As with global warming, discovering all the causes and effects of global dimming is an extremely complex effort. For example, higher temperatures should increase evaporation, cloud formation, and therefore more global dimming.
If the larger trends hold true, however, the world could be in for a brighter future.✦
