Cite as: 502 U. S. 437 (1992)
Opinion of the Court
through the first four months of 1989. See Report of Special Master 7-8. Those charts reveal that during the years 1981 through 1984, the four Oklahoma utilities purchased virtually 100% of their coal requirements from Wyoming sources. These purchases decreased slightly, if at all, in 1985 and 1986 following the adoption of the original concurrent resolution. After January 1, 1987, the effective date of the Act, these utilities reduced their purchases of Wyoming coal in favor of coal mined in Oklahoma.
Unrebutted evidence demonstrates that, since the effective date of the Act, Wyoming has lost severance taxes in the amounts of $535,886 in 1987, $542,352 in 1988, and $87,130 in the first four months of 1989.6 These estimates are based on an equivalence of British Thermal Unit (BTU) ratings, thus accounting for the hotter burning propensities of Oklahoma coal.7 Other unrebutted submissions confirm that Wyoming has a significant excess mining capacity, such that
6 See Affidavit of Richard J. Marble, Director, Minerals Tax Division, Wyoming Department of Revenue and Taxation 3 (Exh. B to Appendix to Motion of Wyoming for Summary Judgment). Oklahoma does not contradict these estimates. Instead, its expert, an economist familiar with energy and coal-related issues, emphasizes only that Wyoming experienced a more severe loss in severance tax revenues due to its reduction of the severance tax rate and a decline in coal market prices. Affidavit of David M. Weinstein 2-3 (Exh. G to Appendix to Motion of Oklahoma for Summary Judgment). At best, Oklahoma's counteraffidavit suggests that the estimate of lost severance tax revenues is a bit too high, pointing to the slight percentages of Oklahoma coal purchased prior to the Act as indicative that Wyoming did not provide 100% of the coal purchased. Id., at 3.
7 A coal's BTU rating reflects the heat-generating efficiency of the coal when burned. Coal extracted from Wyoming's Powder River Basin—the source of coal shipped to Oklahoma since 1980—has a lower average BTU rating than the Oklahoma coal delivered to the utilities. Accordingly it takes less Oklahoma coal by weight to generate the same amount of energy as the Wyoming coal. Because sulfur content factors into Oklahoma's later argument, we note here as well that Wyoming coal has a lower average sulfur content than Oklahoma coal, thus less sulfur escapes and pollutes the air when Wyoming coal is burned.
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