July 27, 2007

Ground-Level Ozone Trends: Facts vs. Fantasy

Guest Commentary

Joel Schwartz
Visiting Fellow
American Enterprise Institute

Growing plants absorb some of the carbon dioxide emitted by human burning of fossil fuels for energy. However, according to a new study in the journal Nature, ground-level ozone (AKA “smog”) will rise during the 21st Century and stunt plant growth. This will reduce CO2 uptake by vegetation, exacerbating CO2-induced greenhouse warming.

The study, which was performed by Stephen Sitch and colleagues from England’s Hadley Centre for Climate Change Research, is based on computer modeling of current and future ozone levels. To project future emissions of greenhouse gases and air pollutants, Sitch et al. relied upon the Intergovernmental Panel on Climate Change’s (IPCC) A2 scenario. The scenario includes projections of population, economic activity, energy use, and other factors that determined future emissions.

Unfortunately, comparison of Sitch et al.’s model results with actual trends in ozone and ozone-forming pollutants show that their study has nothing to do with reality.

Sitch et al. conclude ozone will rise all over the world during the 21st Century. In reality, ozone levels have been dropping for decades in wealthy countries and will continue to do so. And while ozone will probably rise for a few decades in developing countries, these countries will reduce their ozone levels as they become wealthy enough to afford the necessary pollution controls—exactly as has already happened in the U.S. and Europe. Sitch et al. not only get the trends wrong. They also overstate actual ozone levels by a large margin.

A major problem for Sitch et al. is that the IPCC’s A2 scenario has little to do with reality. The two graphs below compare the actual trend in U.S. and European NOx (oxides of nitrogen) emissions with the A2 scenario’s assumption for OECD countries. NOx, together with volatile organic compounds (VOC), are the pollutants that form ozone. The OECD includes developed countries, such as the U.S. and European Union, plus a few mid-wealth countries such as Mexico, and is the smallest regional breakdown available in the IPCC’s scenarios report. The top graph (Figure 1) gives total emissions and the bottom graph (Figure 2) gives per-capita emissions. In both graphs, I’ve indexed the three trends to a common base value of 1.0 in 1990, which is marked by the dashed vertical line. Points for other years represent the percentage change from the 1990 baseline.

Figure 1. Actual U.S. and European trends in total NOx emissions (marked by the yellow diamonds and blue triangles, respectively) compared with IPCC A2 scenario projection for OECD countries (dashed lines and red x’s). The observed trends and the projected trends are going in nearly opposite directions.

Figure 2. As in Figure 1, except the NOx emissions are expressed as per capita values.

Note the large divergence between the IPCC A2 scenario used by Sitch et al. and actual NOx emission trends. The A2 scenario has total NOx rising almost as rapidly as it is falling back in the real world. And where per capita NOx gently rises in the A2 scenario, it is falling steeply in reality. The A2 scenario is similarly out of touch when it comes to VOC emissions, which are also falling rapidly in the real world. Sitch et al. manufactured a false appearance of rising ozone by adding lots of phantom NOx and VOC to the future atmosphere.

Here are some additional ways in which Sitch et al.’s modeling is out of touch with reality:

• According to Sitch et al.’s modeling, ground-level ozone in the U.S. in 2000 ranged from about 30-90 parts per billion (ppb), and the national-average level was roughly 55-60 ppb (based on the June-August average of diurnal—that is, 24-hour—ozone levels). But based on U.S. monitoring data, the actual U.S. average is about 37 ppb. Sitch et al. claim ozone levels for the U.S. that are nearly 70% higher than actual levels. If Sitch et al.’s ozone results were correct, they would imply that most of the U.S. exceeds the current federal 8-hour ozone standard on most summer days. But in fact, the vast majority of the country only exceeds the 8-hour standard somewhere between zero and ten days per summer and more than 80% of the nation’s ozone monitoring sites comply with the standard.

• Since they start from a way-too-high baseline ozone level, Sitch et al. greatly overestate the harm to plant life from both current and future ozone levels, and also probably overestimate the greenhouse warming contribution of ground-level ozone (ozone is a greenhouse gas). They state that ozone causes plant damage when diurnal levels exceed 40 ppb and claim that almost the entire U.S. is above this threshold. In fact, more than 80% of the U.S. is below 40 ppb.

• Sitch et al. conclude that ozone will rise by about 30% in the U.S. between 2000 and 2100, and by large amounts everywhere else in the world as well. But in fact ozone has been dropping for decades in the U.S., Western Europe, and other wealthy areas, due the NOx and VOC reductions discussed above. Ozone will continue to drop in these areas as regulators clamp down further on ozone-forming emissions. Already-adopted regulations in the U.S. and Europe require the elimination of most remaining NOx and VOC emissions during the next few decades.

• Ozone probably will rise for a few decades in less developed countries, as they burn more coal for energy, more people purchase and drive automobiles, and more goods are shipped by truck. But this process is self-limiting. Wealthier people demand and can afford to pay for lower air pollution levels and other environmental amenities. Just as in the U.S. and Europe, China, India and other developing nations will tackle outdoor air pollution as they create the wealth to do so. In fact, these countries are already beginning to address air pollution at much lower levels of real per-capita income than was the case in the U.S. or Europe.

The A2 scenario’s unrealistic NOx and VOC emissions result from more fundamental problems with this scenario. For one thing, A2 has too many people. Based on trends in fertility rates and life expectancy, the UN projects that world population will peak just above 9 billion or so, sometime in the late 21st Century. But the A2 scenario assumes a roughly linear rise, with world population passing 15 billion by the end of the 21st Century. Current world population is already nearly 300 million lower than the A2 assumption.

A2 also has too little growth in real income. Asia’s real GDP/capita grows only about 2.4% per year during the 21st Century in the A2 scenario. Yet China and India have been growing at rates several percentage points greater than this, with no sign of a slowdown.

This is a general problem with the IPCC’s scenarios for future emissions of greenhouse gases and air pollutants. By the IPCC’s own admission, its scenarios are not estimates of what is likely to happen. Instead the IPCC calls them “storylines.” They are essentially ad hoc collections of assumptions about population growth, energy use, income, and other economic and demographic factors, with little or no attempt to ensure internal consistency or realism. The IPCC does not estimate the likelihood that any of its many scenarios will be realized in practice. Yet the output of climate models driven by these scenarios is being used by climate activists to justify a wholesale reorganization of human societies.

Sitch et al. give an understated hint at the problems with the A2 scenario in the methods section of their paper:

“The [IPCC] SRES [Special Report on Emissions Scenarios] A2 scenario for 2100 was chosen as this was the standard scenario used in the OxComp study presented in Chapter 4 of the IPCC Third Assessment Report, although now some consider this to be a pessimistic scenario. Like all the SRES scenarios it assumes no emission control measures, and thus may be considered as an upper limit.”

It would be more correct to consider the A2 scenario a fantasy than an “upper limit.”

While Sitch et al. at least provide a buried hint of the lack of realism in their results, Nature’s own news article on the study suggests Sitch et al.’s have shown us what the future will look like. Titled “Carbon sinks threatened by increasing ozone,” the article begins “Rising levels of ozone pollution over the coming century will erode the ability of plants to absorb carbon dioxide from the atmosphere, a new climate-modelling study predicts.” Nature creates the false impression that the Sitch et al. study provides a prediction of likely events, rather than a wildly improbable just-so story based on demonstrably false assumptions.

The problems with the A2 scenario are probably not sufficient to explain all of the ways in which Sitch et al. departs from observed reality. For example, the overestimate of current ozone levels suggests there are additional problems with the study’s analytical methods and assumptions.

So there you have it. Another “authoritative”-but-wrong paper in a premier scientific journal that will henceforth be used to support unwarranted alarmism about climate change and air pollution.


EPA’s Air Quality System (AQS) database, http://www.epa.gov/ttn/airs/airsaqs/ (U.S. average diurnal (24-hour) ozone during June-August in 2000, estimated from nearly 900 monitoring sites around the U.S. Data downloaded from )

EPA’s ozone trend graphs at http://www.epa.gov/airtrends/ozone.html.

European Environment Agency, “Annual European Community LRTAP Convention Emission Inventory 1990-2004,” August, 2006, http://reports.eea.europa.eu/technical_report_2006_8/en.

Intergovernmental Panle on CLimate Change, 2001. Special Report on Emissions Scenarios, http://www.grida.no/climate/ipcc/emission/

Sitch, S., et al., 2007. Indirect Radiative Forcing of Climate Change through Ozone Effects on the Land-Carbon Sink, Nature, doi:10.1038/nature06059.

U.S. EPA, “Air Quality and Emissions,” http://www.epa.gov/airtrends/econ-emissions.html.

World Bank country profiles, http://web.worldbank.org/WBSITE/EXTERNAL/DATASTATISTICS/0,,contentMDK:20535285~menuPK:1390200~pagePK:64133150~piPK:64133175~theSitePK:239419,00.html.

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