next whole dollar amount. (D) National emissions target attainment year For purposes of subparagraph (A), a calendar year is a national emissions target attainment year if the level of greenhouse gas emissions in the United States for the calendar year does not exceed 20 percent of the level of greenhouse gas emissions in the United States for calendar year 2005 as determined by the Secretary in consultation with the Administrator of the Environmental Protection Agency. (c) Taxable carbon substance For purposes of this subchapter, the term taxable carbon substance means— (1) coal (including lignite and peat), (2) petroleum and any petroleum product (as defined in section 4612(a)(3)), and (3) natural gas, which is extracted, manufactured, or produced in the United States or entered into the United States for consumption, use, or warehousing. (d) Substance taxed only once No tax shall be imposed by subsection (a) with respect to a taxable carbon substance if the person who would be liable for such tax establishes that a prior tax imposed by such section has been imposed with respect to such substance. 4693. Refunds or credits (a) Sequestered carbon Under regulations prescribed by the Secretary, if a person uses a taxable carbon substance so that the carbon associated with such substance will not be emitted, then an amount equal to the amount of tax in effect under section 4692(b) with respect to such substance for the calendar year in which such use begins shall be allowed as a credit or refund (without interest) to such person in the same manner as if it were an overpayment of tax imposed by section 4692. (b) Previously taxed carbon substances used To make another taxable carbon substance Under regulations prescribed by the Secretary, if— (1) a tax under section 4692 was paid with respect to any taxable carbon substance, and (2) such substance was used by any person in the manufacture or production of any other substance which is a taxable carbon substance, then an amount equal to the tax so paid shall be allowed