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Study finds rising temperatures increase risk of unhealthy ozone levels absent sharp cuts in precursors

Ozone pollution across the continental United States will become far more difficult to keep in check as temperatures rise, according to new work led by the National Center for Atmospheric Research (NCAR). The study shows that Americans face the risk of a 70% increase in unhealthy summertime ozone levels by 2050, assuming continued greenhouse gas emissions with resultant significant warming (IPCC Scenario A2 and RCP (Representative Concentration Pathway) 8.5.)

However, the study also showed that a sharp reduction in the emissions of ozone precursors would lead to significantly decreased levels of ozone even as temperatures warm. Without those cuts, almost all of the continental United States will experience at least a few days with unhealthy air during warmer summers, the research shows. Heavily polluted locations in parts of the East, Midwest, and West Coast in which ozone already frequently exceeds recommended levels could face unhealthy air during most of the summer.

Scenario A2 and RCP 8.5
The A2 emissions scenario is one of the marker scenarios developed through the IPCC; the A2 scenario is at the higher end of the SRES (Special Report on Emissions Scenarios) emissions scenarios (but not the highest).
The A2 story line is characterized by heterogeneity. Self reliance and local identities are emphasized, and population increases continuously to more than 10 billion by 2050. Economic development is regionally oriented and economic and technological development is relatively slow, compared to the other story lines.
Cumulative CO2 emissions by the middle and end of the 21st century are projected to be about 600 and 1850 GtC respectively, and expected CO2 concentrations (in parts per million, ppm) for the middle and end of the 21st century in this scenario are about 575 and 870 ppm, respectively. (Preliminary global mean CO2 concentrations in February 2014 were 398.06 ppm, according to figures from NOAA’s Earth System Research Laboratory.)
Methane and nitrous oxide increases grow rapidly in the 21st century. Sulfur dioxide increases to a maximum value just before 2050 (105 MtS/yr) and then decreases in the second half of the century (60 MtS/yr by 2100).
The IPCC selected and defined RCPs by their total radiative forcing (cumulative measure of human emissions of GHGs from all sources expressed in Watts per square meter) pathway and level by 2100. The RCPs were chosen to represent a broad range of climate outcomes, based on a literature review.
The RCP 8.5 is characterized by increasing greenhouse gas emissions over time representative for scenarios in the literature leading to high greenhouse gas concentration levels. The underlying scenario drivers and resulting development path are based on the A2r scenario.

The detailed research is one of the first of its type to be conducted with new, advanced geoscience supercomputing capabilities. It will be published online this week in the Journal of Geophysical Research-Atmospheres, a journal of the American Geophysical Union.

Ozone formation. Ozone pollution is not emitted directly, but instead forms as a result of chemical reactions that take place between nitrogen oxides and volatile organic compounds in the presence of sunlight. These gases come from human activities such as combustion of coal and oil as well as natural sources such as emissions from plants.

Unlike ozone in the stratosphere, which benefits life on Earth by blocking ultraviolet radiation from the Sun, ground-level ozone can trigger a number of health problems. These range from coughing and throat irritation to more serious problems, including aggravation of asthma, bronchitis, and emphysema. Even short periods of unhealthy ozone levels can cause local death rates to rise. Ozone pollution also damages crops and other plants.

Warmer temperatures and other changes in the atmosphere related to a changing climate, including higher atmospheric levels of methane, spur the chemical reactions that lead to ozone.

The study. To examine the impacts of climate change on ozone pollution, NCAR scientist Gabriele Pfister and her colleagues looked at two scenarios. In one, emissions of nitrogen oxides and volatile organic compounds from human activities would continue at current levels through 2050. In the other, emissions would be cut by 60-70%. Both scenarios assumed continued greenhouse gas emissions with significant warming.

The researchers found that, if emissions continue at present-day rates, the number of eight-hour periods in which ozone would exceed 75 parts per billion (ppb) would jump by 70% on average across the United States by 2050. The 75 ppb level over eight hours is the threshold that is considered unhealthy by the Environmental Protection Agency. (The agency is considering tightening the standard to a value between 65 and 70 ppb over eight hours.)

Overall, the study found that, 90% of the time, ozone levels would range from 30 to 87 ppb in 2050 compared with an estimated 31 to 79 ppb in the present. Although the range itself shifts only slightly, the result is a much larger number of days above the threshold now considered unhealthy.

There are three primary reasons for the projected increase in ozone:

  • Chemical reactions in the atmosphere that produce ozone occur more rapidly at higher temperatures.

  • Plants emit more volatile organic compounds at higher temperatures, which can increase ozone formation if mixed with pollutants from human sources.

  • Methane, which is increasing in the atmosphere, contributes to increased ozone globally and will enhance baseline levels of surface ozone across the United States.

In the second scenario, Pfister and her colleagues found that sharp reductions in nitrogen oxides and volatile organic compounds could reduce ozone pollution even as the climate warms. In fact, 90% of the time, ozone levels would range from 27 to 55 ppb. The number of instances when ozone pollution would exceed the 75 ppb level dropped to less than 1% of current cases.

Our work confirms that reducing emissions of ozone precursors would have an enormous effect on the air we all breathe.

—Gabriele Pfister

Pfister and a nationwide scientific team expect to learn more about the sources, chemistry, and movement of air pollutants this summer when they launch a major field experiment known as FRAPPÉ (Front Range Air Pollution and Photochemistry Experiment) along Colorado’s Front Range. The FRAPPÉ campaign will involve a series of coordinated NSF/NCAR C-130 flights and ground-based measurements that investigate the factors controlling Front Range surface ozone and assess whether or not current emission controls are sufficient to reduce ozone levels below the NAAQS ( National Ambient Air Quality Standard).

FRAPPÉ is a collaborative effort between the Colorado Department of Public Health and the Environment, the University of Colorado and Colorado State University, UC Berkeley, and other university collaborators, local projects and agencies including local school districts, NASA, NOAA, and NCAR.

Supercomputing. The new study was among the first conducted on the new 1.5-petaflop Yellowstone supercomputer. The IBM system, operated by NCAR and supported by funding from the NSF and the University of Wyoming, is one of the world’s most powerful computers specifically dedicated to research in the atmospheric and related sciences.

Scientists were able to simulate pollution levels hour by hour for 39 hypothetical summers. This allowed the team to account for year-to-year variations in meteorological conditions, such as hot and dry vs. cool and wet, thereby getting a more detailed and statistically significant picture of future pollution levels.

To simulate the interplay of global climate with regional pollution conditions, the scientists turned to two leading atmospheric models, both based at NCAR and developed through broad collaborations with the atmospheric science community. They used the Community Earth System Model, funded primarily by the Department of Energy and NSF, to simulate global climate as well as atmospheric chemistry conditions. They also used an air chemistry version of the multiagency Weather Research and Forecasting model to obtain a more detailed picture of regional ozone levels.

We use a regional coupled chemistry-transport model to assess changes in surface ozone over the summertime U.S. between present and a 2050 future time period at high spatial resolution under the A2 climate and Representative Concentration Pathway (RCP) 8.5 anthropogenic precursor emission scenarios. Predicted changes in regional climate and globally enhanced ozone are estimated to increase surface ozone over most of the U.S.; the 95th percentile for daily 8 h maximum surface ozone increases from 79 ppb to 87 ppb. The analysis suggests that changes in meteorological drivers likely will add to increasing ozone, but the simulations do not allow separating meteorological feedbacks from that due to enhanced global ozone.

Stringent emission controls can counteract these feedbacks; if implemented as in RCP8.5, the 95th percentile for surface ozone is reduced to 55 ppb. A comparison of regional to global model projections shows that the global model is biased high in surface ozone compared to the regional model and compared to observations. On average, both the global and the regional model predict similar future changes but reveal pronounced differences in urban and rural regimes that cannot be resolved at the coarse resolution of the considered global model. This study confirms the key role of emission control strategies in future air quality projections and demonstrates the need for considering degradation of air quality with future climate change in policy making.

—Pfister et al.

Even with Yellowstone’s advanced computing speed, it took months to complete the complex simulations.

This research would not have been possible even just a couple of years ago. Without the new computing power made possible by Yellowstone, you cannot depict the necessary detail of future changes in air chemistry over small areas, including the urban centers where most Americans live.

—Gabi Pfister

The work was funded by the National Science Foundation (NSF), which is NCAR’s sponsor, and the US Department of Energy. In addition to NCAR, the study co-authors are from the Pacific Northwest National Laboratory; University of Colorado, Boulder; and North-West University in South Africa.


  • Pfister, G. G., S. Walters, J.-F. Lamarque, J. Fast, M. C. Barth, J. Wong, J. Done, G. Holland, and C. L. Bruyère (2014) “Projections of future summertime ozone over the US,” J. Geophys. Res. Atmos., 119, doi: 10.1002/2013JD020932

  • Nakicenvoic et al. (2000) “Special Report on Emissions Scenarios. A Special Report of Working Group III of the Intergovernmental Panel on Climate Change.” Cambridge University Press: Cambridge. 599 pp.



Not so sure that the majority or the 97% really cares.

They seem to worry a lot more about how to reach the 3% wolf pack?

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