Pursuant to § 2.20(a), subject to reservations in § 2.20(b), and subject to policy guidance and direction by the Under Secretary, the following delegation of authority is made by the Under Secretary for Natural Resources and Environment to the Deputy Under Secretaries for Natural Resources and Environment, to be exercised only during the absence or unavailability of the Under Secretary: Perform all the duties and exercise all the powers which are now or which may hereafter be delegated to the Under Secretary for Natural Resources and Environment. Provided, that, except in the absence of both the Under Secretary and a Deputy Under Secretary, this authority shall be exercised by the respective Deputy Under Secretary only with respect to the area or responsibility assigned to him or her.