Effective Control of Global Warming by
Simultaneous Reduction of NOx and CO Emissions
- Development of a Method for Assessing the Impact of
Air Pollutant Gases on Global Warming -

May 2, 2001

Dr. Hajime Akimoto (Program Director) and Dr. James Oliver Wild from the Atmospheric Composition Research Program - a division of the FRSGC, which is a joint project of NASDA (President: Mr. Shuichiro Yamanouchi) and JAMSTEC (President: Mr. Takuya Hirano), have teamed together with Dr. Michael J. Prather (University of California, Irvine) to develop for the first time the method for quantitatively assessing the impact of air pollutant gases such as nitrogen oxides (NOx) and carbon monoxide (CO) on global warming using a three-dimensional global chemical transport model (CTM).

The analysis obtained by this method revealed that a global-scale implementation of simultaneous reduction of NOx and CO emissions as air pollution control measures would also be effective in curbing global warming (Fig. 1).
The results were published in the American Geophysical Union's journal, Geophysical Research Letters (Vol. 28, No. 9, p. 1719), on May 1, 2001.


The 2001 Report of the Intergovernmental Panel on Climate Change (IPCC) states that in addition to the gases targeted in the Kyoto Protocol, the contribution of tropospheric ozone (O3) to the greenhouse effect is also important. The report further states that in order to curb global warming it is necessary to reduce the emissions of both greenhouse gases and other gases that control their concentration. Air pollutants such as nitrogen oxides (NOx), carbon monoxide (CO), and non-methane hydrocarbon (NMHC) produce OH radicals, which affect tropospheric ozone and methane levels, and hence they are called indirect greenhouse gases. A method for quantitatively assessing the impact of these indirect greenhouse gases on global warming has not been developed until now.


Dr. Hajime Akimoto (Program Director) and his colleagues have developed a method for analyzing the response of concentrations of ozone (O3) and methane (CH4) to changes in the quantity of air pollutant emissions, using a three-dimensional global chemical transport model (CTM) that combines a global climate model with a photochemical reaction model (Fig. 2). The results obtained from preliminary calculations using this method demonstrated that although the production of ozone is suppressed when only NOx emissions are reduced, the concentration of methane rises, thereby possibly accelerating global warming in the long term. When NOx and CO emissions are reduced simultaneously, however, it has been shown that the build up of methane is abated and that this approach is thus effective in mitigating global warming (Figs. 3 and 4).

Our aim now is to conduct more detailed research into atmospheric pollutant emissions incorporating factors characteristic to each region (Asia, etc.) and to investigate the role of other air pollutant gases. We also intend to promote research that will contribute to the creation of an index (standard) that would enable integrated measures against air pollution and global warming.


Joint Promotion Office, Frontier Research System for Global Change
Mr. Hishida (TEL: +81-45-778-5615)
Mr. Kawasaki (TEL: +81+45-778-5700)

JAMSTEC (Japan Marine Science and Technology Center)
Public Relations, Training and Education Division of Administration Department
TEL: +81-468-67-5547
URL: http://www.jamstec.go.jp/ (JAMSTEC)

NASDA (National Space Development Agency of Japan)
Public Relations Office
TEL: +81+3-3438-6111
URL: http://www.nasda.go.jp/ (NASDAQ)


1. Research Context
    Until now, a quantitative discussion of the impact of short-lived air pollutants such as NOx, CO, and NMHC on global warming has been extremely difficult. These gases do not have greenhouse effect of their own, but since they control the concentration levels of the main greenhouse gases - methane, ozone, CFC substitutes (HCFC) - they are called indirect greenhouse gases. The recently released IPCC Report stated that, along with greenhouse gases, the reduction of gas emissions that affect their concentrations is necessary for stabilizing radiative forcing.
    In order to analyze the influence of these indirect greenhouse gases on global warming, we first distinguished between the short-term changes in ozone and the long-term changes in methane, as caused by the impact of altering emission levels of indirect greenhouse gases. The method for assessing the influence on global warming was then established by combining the data. The three-dimensional global chemical transport model used in the study was developed at the University of California, Irvine.
2. Mechanism Causing Air Pollutant Gases to Contribute to Global Warming (Fig. 1)
    (1) Air pollutants known as indirect greenhouse gases (NOx, CO, NMHC, etc.) trigger a photochemical reactions when they are irradiated by ultraviolet light in the troposphere, thereby leading to the production of ozone and OH radicals.

    (2) Since ozone is a potent greenhouse gas, reducing tropospheric ozone by controlling air pollutant emissions is effective in curbing global warming. It is already known that the reduction of tropospheric ozone is most effectively achieved by controlling NOx concentrations. Since ozone has a short life cycle (1 to 2 weeks in summer, and approximately 2 months in winter), the abatement of global warming due to a decrease in NOx levels takes effect almost concurrently with the reduction of NOx.

    (3) On the other hand, OH radicals react with another greenhouse gas, methane that has the effect of eliminating methane from the atmosphere. When NOx levels are lowered OH radicals also decrease, thereby raising methane concentrations in the atmosphere. Rather than mitigating the problem, global warming is advanced due to methane's greenhouse effect. Moreover, methane has a long life cycle in the atmosphere (approximately 10 years), so the escalation in global warming caused by the reduction of NOx will continue for some decades after NOx levels have been lowered.

    (4) It follows that an assessment of the warming effect induced by a reduction of NOx requires an evaluation of the short-term depletion of ozone together with the long-term increase in methane. By conducting a quantitative assessment combining these two factors, lowering NOx levels only has a positive greenhouse effect for a period of several decades and could result in accelerated global warming.

    (5) There are other substances apart from CH4 that produce a photochemical effect with OH radicals in the troposphere. If these substances are reduced, the relative level of OH radicals that react with CH4 rises and the increase in CH4 concentration is abated. We know that, of these substances, variations in carbon monoxide (CO) levels exert the greatest impact on OH radical concentrations.
    The findings show that if NOx is reduced together with CO, ozone concentration is further lowered, but at the same time, the increase in methane production is curbed. Therefore, an assessment covering several decades indicates a negative greenhouse effect and the possibility of mitigating global warming.
3. Extract from Summary for Policymakers, the Third Assessment Report of Working Group I of the Intergovernmental Panel on Climate Change (IPCC)
    * Model calculations of the concentrations of the non-CO2 greenhouse gases by 2100 vary considerably across the SRES [Special Report on Emission Scenarios] illustrative scenarios, with CH4 changing by -190 to +1,970 ppb (present concentration 1,760 ppb). N2O changing by +38 to +144 ppb (present concentration 316 ppb), total tropospheric O3 changing by -12 to +62%, and a wide range of changes in concentrations of HFCs [hydrofluorocarbons], PFCs [perfluorocarbons] and SF6 [sulfur hexafluoride], all relative to the year 2000. In some scenarios, total tropospheric O3 would become as important a radiative forcing agent as CH4 and, over much of the Northern Hemisphere, would threaten the attainment of current air quality targets.

    * Reductions in greenhouse gas emissions and the gases that control their concentration would be necessary to stabilize radiative forcing. For example, for the most important anthropogenic greenhouse gas, carbon cycle models indicate that stabilization of atmospheric CO2 concentrations at 450, 650 or 1,000 ppm would require global anthropogenic CO2 emissions to drop below 1990 levels, within a few decades, about a century, or about two centuries, respectively, and continue to decrease steadily thereafter. Eventually CO2 emissions would need to decline to a very small fraction of current emissions.

    * The total amount of O3 in the troposphere is estimated to have increased by 36% since 1750, due primarily to anthropogenic emissions of several O3-forming gases. This corresponds to a positive radiative forcing of 0.35 Wm-2. O3 forcing varies considerably by region and responds much more quickly to changes in emissions than the long-lived greenhouse gases, such as CO2.
4. Explanation of Terminology
    * Radiative forcing
    Perturbation to the balance of the energy budget expressed as the amount of radiation per unit area at the tropopause. This is caused by factors such as incoming solar radiation, atmospheric greenhouse gas concentrations, and cloud quantity. The concept of radiative forcing can be used to quantitatively compare the contribution of these factors to climate change. Radiative forcing is also used as a gauge for expressing the degree of the greenhouse effect.