; and (B) for retail natural gas suppliers shall increase gradually from 0.25 percent of sales in 2020 to 0.5 percent of sales in 2025 and each year thereafter through 2030. (4) The targets are cumulative. Each year’s savings shall be achieved in addition to the previous years’ savings. (d) Required percentages after 2030 The Secretary may, upon petition or upon the Secretary’s own initiative, increase the required percentage of end-user savings for years after 2030. (e) Market-Based Trading System The Secretary shall allow suppliers to achieve the targets under subsection (a) through a market-based trading system. (f) Rule of construction Nothing in this section shall be construed to preempt or limit State actions to enhance renewable energy generation or energy efficiency. . III Science-Based Reductions 301. Emission reduction targets Not later than 2 years after the date of enactment of this Act, the Administrator of the Environmental Protection Agency (in this title referred to as the Administrator ) shall promulgate annual emission reduction targets for each of calendar years 2030 through 2050, so as to ensure that the quantity of United States greenhouse gas emissions— (1) in 2035, is at least 40 percent below the quantity of such emissions in 1990