Emissions Scenario (SRES) A1B. Use of a single scenario represents a major uncertainty for future ozone projections. Although the single baseline SRES A1B scenario is the only scenario that the Community Climate Model (CCM) modelers ran for the 2006 WMO/UNEP ozone assessment, the 2D models were run on a range of scenarios. Perhaps the authoring team could select one of the interactive 2D models from the assessment, benchmark it against the better CCMs, and then use the results from the full range of scenarios to include additional discussion of climate change scenarios. The authoring team should also explicitly state that they are not considering how drastic changes that might take place in the climate system might affect the ozone problem.

ASSESSING U.S. CONTRIBUTIONS TO OZONE-DEPLETING SUBSTANCES AND RADIATIVE FORCING

There are inconsistencies in the draft SAP in estimating U.S. contributions to production, consumption, and emissions of ozone-depleting substances (ODSs). There is also inconsistency in the draft SAP with regard to the confidence to be placed on these emission estimates:

  • Regarding production and consumption, there should not be inconsistencies in the numbers because they are obtainable from the World Meteorological Organization report, Scientific Assessment of Ozone Depletion: 2006 (WMO 2007). Lines 590-594 of the draft SAP state that “…during 1986-1994 the U.S. accounted for 24-30% of total annual production and consumption of ODSs reported by the United Nations Environment Programme (UNEP) when weighted by the ozone depletion potentials (ODPs), since 2001 this fraction has been closer to 10%”. However, the numbers are given as 25-30% and “somewhat less since” on lines 1966 and 1967; lines 938-948 state 25-30% prior to 1993 and about 10% in 2001-2005.

  • Regarding emissions, the Executive Summary of the draft SAP does not provide any numbers (E.S.3.5, L. 288-292); lines 607-614 only state that U.S. emissions have declined by 81% since the 1980s. Lines 1978-1979 state that “…this analysis suggests that the U.S. accounted for 25% of global emissions, on average, during the 1990s, and somewhat less since”. Lines 1240-1254 give a consistent number of 18-35% for the 1990s (the committee’s interpretation of line 1252), but this text does not give a number for the more recent proportion of emissions from the United States. Lines 5214-5217 state that “Between 1985 and 2005, the fraction of ozone-depleting substances weighted by ODPs emitted by the U.S. relative to the total global emissions varied from about 20% in 1985 to a maximum of about 35% in the early 1990s, to a current level of roughly 20%”.

  • Regarding the U.S. contribution to global atmospheric mixing ratios, the authoring team should correct the following inconsistencies. The Key Findings section of Chapter 2 gives the U.S. emissions to total tropospheric chlorine (on L. 658) as approximately 20 (14-32)%, and 23 (19-29)% to bromine (line 680); however, line 1984 states 16-30% for chlorine and 21-26% for bromine. Line 1634 gives a number of 21 (16-30)% for chlorine for the past decade, and on line



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