natural

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Across
  1. 2. trapping the sun's warmth in a planet's lower atmosphere, due to the greater transparency of the atmosphere to visible radiation from the sun than to infrared radiation emitted from the planet's surface.
  2. 6. A gradual increase in the overall temperature of the earth's atmosphere is generally attributed to the greenhouse effect caused by increased levels of carbon dioxide, chlorofluorocarbons, and other pollutants.
Down
  1. 1. consisting largely of methane and other hydrocarbons occur naturally underground
  2. 3. is derived from petroleum, especially for use as a fuel or lubricant
  3. 4. is a combustible black or dark brown rock consisting mainly of carbonized plant matter viscous liquid
  4. 5. is relates to or denotes energy derived from the sun's rays