Global Dimming: A New Perspective on Climate Change
Increasing evidence indicates human activity is the major culprit in global climate change. But the world’s not just getting hotter. Recent discoveries reveal that human pollutants may have sparked a global dimming phenomenon that resulted in lower temperature averages in the mid-20th century. Ironically, the identification of this global cooling reveals more than ever the need for international reduction of the greenhouse gas emissions associated with global warming.
Thirty years ago, it was not melting ice caps but rather speculations of an emerging ice age that fostered public anxiety. In 1975, Newsweek even ran an article on the topic, “The Cooling World.” The hype drew energy from observations that the world’s average surface temperature had been gradually decreasing for three decades. In the 1980’s, long range studies of world climate showed as much as a 10% reduction in sunlight in some areas of the U.S. and as much as 30% in parts of Russia from the 1950’s to 1980’s. Consequently, the world’s surface had received about .23 - .32% less solar energy each year from 1958-1992, cooling the overall temperature of the earth.
Particulate matter (PM’s), and sulfate aerosols especially, emerged as the likely culprits in this “global dimming.” PM’s increase the number of water particles in clouds, increasing the sunlight reflected back into space. Also, with diameters less than 10 micrometers, PM’s fall into the EPA’s list of top 6 most undesirable air pollutants in the Clean Air Act. They readily catch in the lungs and can even enter the bloodstream, causing and complicating various heart and respiratory diseases.
The main problems with global cooling, besides the generations of asthmatics spawned by inhaling PM’s, were the possible negative effects on global food production. NASA reports that lower mean rainfall from 1931-1968 and 1968-1997 over Africa’s prolific Sahel Belt correlated closely with the cooling effects of sulfate aerosols over the North Atlantic. Millions of people starved to death in the subsequent droughts throughout the 1970’s and 1980’s.
While global temperatures show an upward trend since 1860, dimming and cooling started to outweigh the effects of global warming in the late 1940’s. Then starting in 1970 with the Clean Air Act in the United States and similar policies in Europe, atmospheric sulfate aerosols declined significantly. The EPA reports that in the U.S. alone from 1970 to 2005, total emissions of the six principal air pollutants, including PM’s, dropped by 53 percent. In 1975, the masked effects of trapped greenhouse gases finally started to emerge and have dominated ever since.
While a few dissenters hold on to alternate theories of natural variance or solar influence on climate change, the official report by the Intergovernmental Panel for Climate Control (IPCC) in 2001 attributed global warming to human greenhouse gas emission, and predicted rises up to 5.8° C in temperature by 2100. Similarly, according to data collected by the National Oceanic and Atmospheric Administration (NOAA) Paleoclimatology Program from Antarctic ice cores in 2004, global temperature over the last 740,000 years correlate significantly with greenhouse gas levels in the atmosphere.
The continued success of global aerosol clean-up since the 90’s may have contributed to a rise in sunlight reaching the earth over the last decade, and hint that global warming may now increase faster and at even greater magnitudes than we previously thought. Some scientists recently revised their predictions of climate change to an increase in 10° C by the year 2100. This could transform large tracts of continents to inhospitable deserts.
Granted, the science of climate change suffers from a host of uncertainties, and lacks specific predictive power. At best, scientists can draw large trend correlations. But some studies show that humans can have unbelievable effects on local climate. The most interesting study detailed the effects of airplane contrails in the wake of 9/11, when all but a few military planes dotted the sky for 3 days. NASA’s Langley Research Center found that these few contrails dissipated into cirrus clouds covering thousands of square miles. Researchers at the University of Wisconsin then analyzed climate variation in the air traffic-heavy Midwest within the same period. They observed average daily temperatures variations of roughly 1° C hotter in the day and cooler at night in the absence of contrails, the largest deviation in three decades for the area.
These data pose a tempting solution: get some planes to spray clouds of particulate matter into the atmosphere and voilà; we cleverly average the effects of our pollutants. This could delay or prevent the financial loss associated with changing the very foundation of the world’s economic motor. Especially in countries like India and China, converting to green technologies could cost untold billions of dollars and hinder economic development. Additionally, slowing climate change in either direction circumvents major problems in world resource infrastructure, like Africa’s famine. Or in the case of global warming, halting the rapid rise in temperature could augment ecological problems such as species loss and flooding.
But given the relationship between PM’s and death from heart and respiratory disease, future politicians are unlikely to include “more pollution” in their campaign slogans. Current global policy emphasizes the precautionary principle, which aims to reduce greenhouse gas emissions despite comprehensive knowledge of climate change mechanisms. This guides the Kyoto protocol, which assumes any level of action fares better than the off-chance climate change will become disastrous and irreversible.
The global dimming episode should remind us that adopting this “ready, shoot, aim” attitude toward climate change is not necessarily the best approach. While doing the world a favor by getting rid of one pollutant, we may have exacerbated the arguably greater problem of global warming. Conversely, the accelerated onset of global warming affords even less time to get the facts straight before we take action.
So what do we do now? Where policy makers lack the heart to increase PM output to shield us from the sun, they must muster strength to take proactive measures in reducing greenhouse emissions. Globally, this should include a revision of Kyoto’s zero-accountability role for India and China, who will emerge as the main polluters in the coming century. Domestically, the U.S. could follow Europe’s lead in taxing gasoline to help fund public transportation instead.
As the protective mask of sulfate aerosols declines and the sun breaks through once more, the globe may find itself even more vulnerable to the greenhouse effect. So whether it’s cutting tax breaks for SUV’s or instituting industrial emissions caps and trade systems, global dimming more than ever calls on the world to pay attention to global warming.