TCS Daily

Polluted Climate

By Joel Schwartz - April 8, 2005 12:00 AM

Climate change is one environmental issue that hasn't had much traction at the federal level. Congress has refused to ratify the Kyoto Protocol, while the Bush administration has opposed explicit carbon dioxide reduction requirements. Thus, it should come as no surprise that activists have tried to stir up political support for legislated carbon dioxide (CO2) reductions by trying to tie climate change to matters of more immediate public concern, such as air pollution.

For example, activists, scientists, and government officials have claimed that global warming will cause ozone to rise in the future. This claim is false. Upcoming large reductions in emissions of ozone-forming pollutants will reduce future ozone levels, regardless of climate change. Even more important, higher temperatures will decrease levels of airborne particulate matter. Thus, to the extent temperatures do rise in the future, the net result will be a decrease in air pollution health risks.[1]

Last year the Natural Resources Defense Council (NRDC) published Heat Advisory: How Global Warming Causes More Bad Air Days. The report claimed that by 2050 increasing temperatures would cause a 50 percent rise in days exceeding the federal 8-hour ozone standard each year.[2] The federal government's 2002 Climate Action Report also cited potential increases in air pollution due to higher temperatures.[3] And the Massachusetts Attorney General, whose bid to force EPA to regulate CO2 as an air pollutant will be argued today in federal court, puts ozone increases near the top of his list of harms from global warming.[4] Even academic and government scientists have entered the debate as activists -- NRDC's Heat Advisory was written by public health professors from Johns Hopkins and Columbia, and atmospheric and environmental scientists from Yale, the State University of New York at Albany, the University of Wisconsin, NASA, and the U.S. Department of Agriculture. Some of these scientists also published their Heat Advisory results in the prestigious refereed journal Environmental Health Perspectives.[5]

All else equal, higher temperatures do indeed mean higher ozone levels. But all else won't remain equal. EPA and state regulators have already taken actions that will eliminate most remaining ozone-forming emissions -- volatile organic compounds (VOC) and nitrogen oxides (NOx) -- during the next 20 years or so.[6] At worst, warming will cause ozone to decrease slightly less than it otherwise might.

The activists and scientists who claim that higher temperatures will necessarily increase ozone fail to note that a roughly one-degree-Fahrenheit temperature rise during the last 30 years was accompanied by nationwide declines in ozone levels. Where more than 80 percent of ozone monitors violated the federal 8-hour ozone standard during the late 1970s, only about 35 percent do so today. The average number of 8-hour-ozone exceedance days per year has declined more than 70 percent.

Despite a record of decreasing ozone, Heat Advisory's authors managed to manufacture future ozone increases by assuming ozone-forming emissions in 2050 would be the same as they were in 1996. Yet by the time Heat Advisory was published in 2004, VOC and NOx had already declined 50 and 25 percent, respectively, below 1996 levels.[7] Heat Advisory claims to predict how rising temperatures will affect future ozone levels. In fact, the report merely estimates what ozone levels would have been back in 1996 had average temperatures been a few degrees warmer.

Temperature isn't even the climate variable with the greatest effect on ozone. Wind speed and mixing height -- the height to which air and pollutants can mix upward from the ground -- are more important. Gentler winds and lower mixing heights allow ozone to build up. Higher temperatures are associated with lower winds, but also with higher mixing heights. It's not clear which effect would dominate.

Rainfall also affects ozone. More-frequent rains would tend to reduce the number of high-ozone days. Precipitation has increased in the United States during the last century and climate models predict precipitation will increase if the planet warms. Heat Advisory deals only with temperature, and doesn't assess the potential effects of other meteorological changes on ozone. Neither does Massachusetts's Attorney General. He claims warming will increase summer rainfall 10 percent by 2100 in his state, but doesn't make the connection that more rain could offset the ozone-increasing effect of higher temperatures.[8]

Climate activists have focused on global warming's potential effects on ozone, but in terms of health a far more important result of warmer temperatures would be a reduction in airborne particulate matter (PM). About one-third to one-half of all fine particulate matter (PM2.5, the type of greatest regulatory and health concern) is made up of "semi-volatile" nitrates and organic compounds. These compounds evaporate as temperature rises, reducing PM2.5 levels. A recent study of the Los Angeles metropolitan region concluded that, all else equal, a nine degree (F) increase in temperature would reduce peak PM2.5 by 25 percent.[9]

The epidemiological studies cited by EPA and environmental activists to support more stringent air pollution regulations have linked PM2.5 to far more widespread and severe health effects than ozone, including much higher rates of premature mortality. EPA attributes more than 90 percent of the health benefits of the Clean Air Act to PM reductions and less than one percent to ozone reductions.[10] Of course, just as for ozone, already-adopted requirements will eliminate most remaining human-caused PM2.5 during the next two decades. Whatever the effects of warming on air pollution, they will be irrelevant by the time they occur. But this doesn't change the fact that within the activists' own paradigm -- a paradigm that assumes great harm from current levels of air pollution, no change in future pollution emissions, and consideration only of the effect of temperature among all meteorological variables -- the net effect of global warming would be a reduction in harm from air pollution; just the opposite of what NRDC and its allies in government and academia have been claiming.

Heat Advisory claims to be about the future, but is really a what-if tale about a past that never existed. When it comes to discussions of climate change and air pollution, it seems safe to predict that this game of let's pretend will continue.

The author is Visiting Fellow, American Enterprise Institute.

[1] For a more detailed discussion of the issues addressed in this column, see Air Quality False Alarm, a report I co-authored with George Taylor.

[5] See "Assessing Ozone-Related Health Impacts under a Changing Climate,"

[6] For a list of several of the measures that will eliminate most remaining air pollution, see Table 1 in For an analysis of air pollution emission trends, see

[7] For the evidence, see the previous note, as well as a letter to the editor of Environmental Health Perspectives I co-authored with Pat Michaels and Bob Davis:

[9] J. Aw and M. J. Kleeman. (2003). Evaluating the First-Order Effect of Inter-Annual Temperature Variability on Urban Air Pollution. Journal of Geophysical Research - Atmospheres 108: 7-1 - 7-18.

Heat Advisory cites this study, but both misstates and sidesteps its PM2.5 results. Heat Advisory claims "Ozone and nonvolatile secondary PM will generally increase at higher temperatures because of increased gas-phase reaction rates. Interannual temperature variability in California, for example, can increase peak O3 [ozone] and 24-hour average PM2.5 by 16 percent and 25 percent, respectively, when other meteorological variables and emissions patterns are held constant."

In fact, the Los Angeles study concluded that, all else equal, higher temperatures increase ozone, but decrease PM2.5. The 25 percent figure mentioned in Heat Advisory actually represents a decrease in PM2.5, rather than an increase. The study also predicted that substantial ozone increases would occur over a small area, while substantial PM2.5 decreases would occur over a large area. The 16 percent ozone increase mentioned in Heat Advisory was predicted to occur only in a very localized portion (the northwest corner) of the Los Angeles region. The vast majority of the region would experience ozone increases of at most a few percent and many areas would experience no change or even a decrease in ozone. On the other hand, those same higher temperatures were predicted to result in substantial decreases in PM2.5-typically 10 to 25 percent-throughout most of the region, while only a small area along the coast might experience an increase of less than one percent. In other words, far more people would benefit from the PM2.5 reductions, than would be harmed by the ozone increases.

[10] EPA's two studies on the benefits and costs of the Clean Air Act were published in 1997 and 1999, and can be downloaded from


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