ACROSS

Greenhouse Gases (10 + 5 letters)

Deforestation (13 letters)

Landfill (8 letters)

Climate Change (7 + 6 letters)

Microplastics (13 letters)

Environment (11 letters)

DOWN

Atmosphere (10 letters)

Fossil Fuels (6 + 5 letters)

(9 letters) Solar Power (5 + 5 letters)

Global Warming (6 + 7 letters)

Recycling (9 letters)

The unscrambled word is "pollution".

Clue: 

The release of harmful substances into the environment, causing damage and degradation.