Generated Summary
The document extracts and summarizes key findings related to changes in the hydroxyl radical (OH) concentration over time, as detailed in the IPCC Fourth Assessment Report: Climate Change 2007, specifically within the Working Group I: The Physical Science Basis. The report examines the factors influencing OH, a crucial component of atmospheric chemistry, and its impact on atmospheric composition. The analysis focuses on both the historical changes and future projections of OH concentrations, considering the influence of emissions, climate change, and other atmospheric variables. The methodology involves evaluating the impacts of emissions, climate change, and other atmospheric variables on OH concentrations. It uses various model studies and research findings to analyze trends and predict future scenarios. The scope of the study is global, with a focus on understanding how various factors affect OH levels.
Key Findings & Statistics
- Several model studies suggest a decline in weighted global mean OH from pre-industrial time to the present of less than 10% (Shindell et al., 2001; Lelieveld et al., 2002a; Lamarque et al., 2005a).
- Other studies have reported larger decreases in global OH of 16% (Mickley et al., 1999), 25% (Wong et al., 2004) and 33% (Hauglustaine and Brasseur, 2001).
- The model study by Lelieveld et al. (2002b) suggests that during the past century, OH concentration decreased substantially in the marine troposphere through reaction with CH4 and CO.
- Karlsdottir and Isaksen (2000) found a positive trend in OH of 0.43% yr¯¹ over the period 1980 to 1996.
- Dentener et al. (2003a,b) derive a positive trend of 0.26% yr-¹ over the 1979 to 1993 period.
- J. Wang et al. (2004) derive a positive trend in OH of 0.63% yr-¹.
- In the TAR, Prather et al. (2001) predicted that global OH could decrease by 10 to 18% by 2100 for five emission scenarios and increase by 5% for one scenario.
- Wang and Prinn (1999) also predicted an OH decrease of 16 ± 3% in 2100.
- Martinerie et al. (1995) calculated that the global mean OH concentration during the LGM was 7% lower than at present.
- Valdes et al. (2005) estimate that the cold and dry LGM climate was responsible for a 7% decrease in global OH.
- Brasseur et al. (1998) and Johnson et al. (1999) estimated that in a warmer (doubled atmospheric CO2) climate, the global and annual mean OH concentration would increase by 7% and 12.5%, respectively.
- Hauglustaine et al. (2005) use a climate-chemistry three-dimensional model to estimate a 16% reduction in global OH from the present day to 2100 accounting solely for changes in surface emissions. The effect of climate change and mainly of increased water vapour in this model is to increase global OH by 13%.
- Labrador et al. (2004) show that global OH is sensitive to the magnitude of lightning NOx emissions, and increases by 10% and 23% when global lightning is increased by a factor of 2 and 4, respectively.
- Wang et al. (1998) calculated a 10.6% increase in OH for a doubling of the source (from 3 to 6 TgN yr¯¹).
Other Important Findings
- Tropospheric OH is expected to have changed since the pre-industrial era due to its dependence on CH4 and other pollutants.
- The impact of higher CO and CH4 (decreasing OH) and increased NOx and ozone (increasing OH) on OH concentration, has led to a lack of consensus on the magnitude of this change.
- Changes in future OH depend on relative changes in hydrocarbons compared with NOx abundances.
- Future increases in greenhouse gases could induce changes in OH.
- OH will be affected by temperature, humidity and clouds or climate change effects on biogenic emissions of CH4 and other ozone precursors.
- Changes in tropospheric water could have important chemical repercussions, as the reaction between water vapour and electronically excited oxygen atoms constitutes the major source of tropospheric OH.
- Changes in lightning NOx emissions in a warmer climate may also affect OH.
Limitations Noted in the Document
- The document notes that due to the counteracting effects of higher CO and CH4 (decreasing OH) and increased NOx and ozone (increasing OH), there is little consensus on the magnitude of OH changes.
- The study acknowledges uncertainty regarding lightning emissions, and the sensitivity of OH to the total amount of N emitted.
- The document mentions that the study of J. Wang et al. (2004) does not account for interannual variability of a number of other variables that affect OH such as concentrations of NOx, tropospheric ozone and NMVOCs.
- The document indicates the importance of an improved understanding of lightning emissions to accurately simulate OH over time.
Conclusion
The assessment emphasizes the complex interplay of various factors influencing OH concentrations. Changes in OH are driven by emissions of CH4, CO, and NOx, along with climate change impacts such as temperature, humidity, and cloud cover. The report highlights the challenges in quantifying the net effect of these factors due to their opposing influences, such as the decrease in OH due to higher CO and CH4 versus the increase from NOx and ozone. Future projections show a range of possible changes in OH, depending on emission scenarios and climate models. The importance of understanding the water vapour distribution and its effect on global OH concentrations is noted. Overall, the assessment underscores the need for further research to improve the understanding of the source and dynamics of OH, especially considering the role of lightning emissions and the sensitivity of OH to the total amount of nitrogen emitted. The document concludes that an improved understanding of the uncertainties associated with OH is crucial for simulating OH accurately over time, given its role in atmospheric chemistry and climate change.