Our Global Sunscreen
By Mike Carlowicz
t’s not often that scientists get to conduct experiments that seem like they come out of a science fiction novel or a video game. Yet, that is what some researchers at NASA did a few years ago. Atmospheric physicists Paul Newman and Luke Oman built a simulation of the Earth’s atmosphere and then proceeded to strip away our protective ozone layer. Their computer model reproduced the chemistry and circulation of the air; natural variations in temperatures and winds; and minor changes in the energy received from the sun. Newman and Oman then added ozone-destroying chemicals to the atmosphere at a rate of 3% more per year—on top of what was already in our 1970s atmosphere. For several months, they ran their model on a supercomputer and reproduced about 80 years of simulated Earth time. They called their experiment “The World Avoided.” By the year 2020 in the simulation, 17% of the Earth’s protective ozone layer vanished. Holes in the ozone layer formed not just over Antarctica—as they currently do each spring—but over the Arctic, too. By 2040, the ultraviolet (UV) index, the measure of the sunburn-causing radiation reaching the Earth’s surface, rose as high as 15 on summer days in mid-latitude cities such as Washington, D.C. (A UV index of 10 is considered very high today and quickly leads to sunburn if you don’t wear sunscreen.) In the simulated future, two-thirds of the planet’s ozone layer disappeared by 2065. Ozone holes swirled over both poles all year long, and most ozone disappeared from the tropics, too. The intensity of UV radiation reaching the Earth’s surface doubled—levels that would increase DNA mutations in human and animal cells, suppress our immune systems, and increase the incidence of cataracts and skin cancer. In another demonstration of the effects of a world without an ozone layer, Newman’s group exposed a basil plant to intense UV radiation. In a lab at NASA’s Goddard Space Flight Center, they set up a special lamp that simulates the intensity of sunlight when testing satellite hardware. They put a basil plant in front of that lamp and a camera behind it and watched for 27 hours. Without the protective shielding of an ozone layer, the plant started browning and developing spots within an hour. It was dead in a day. “We simulated a world avoided, and it’s a world we should be glad we avoided,” Newman said.
A good idea with a bad outcome
The experiments run by Newman and colleagues were actually extensions of an experiment that humans have been unwittingly running with Earth for nearly a century. Humans have been depleting the ozone layer with chemical products. The unintentional experiment started in the late 1920s, when Thomas Midgley Jr. and other industrial chemists began to produce chlorofluorocarbons (CFCs), nontoxic compounds that improved refrigeration. Other manufacturers later put these chemicals to work as propellants in spray cans and as solvents for cleaning greasy residues. CFCs and similar compounds are mostly inert (nonreactive) at sea level and so were found to be extremely useful for all of these tasks. But the progress came with a cost: People who used CFC products were releasing more chlorine into the environment than could be removed by natural processes. Unknown to the chemists who developed them, CFCs were accumulating and dispersing through the atmosphere. At high altitudes, where conditions are different from the Earth’s surface, those chlorine compounds were destroying ozone, the gas that absorbs and scatters UV light from the sun.
“We simulated a world avoided, and it’s a world we should be glad we avoided”
12 ChemMatters, APRIL 2013
NASA AND SHUTTERSTOCK
Surface-level Ozone (SMOG)
While ozone can be found through the entire atmosphere, it is mostly concentrated at an altitude of about 15 miles. This band of ozone-rich air, called the ozone layer, protects all life forms...