The sentence of imprisonment imposed on a person convicted of an offense under the Controlled Substances Act (
21 U.S.C.
801 et seq.) or the Controlled Substances Import and Export Act (
21 U.S.C.
951 et seq.), involving methamphetamine or any listed chemical that is defined in section 102(33) of the Controlled Substances Act (
21 U.S.C.
802
(33), shall, if the offense is committed under the circumstance described in subsection (b), be increased by a consecutive term of imprisonment of not more than 15 years.