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Across 3. process that warms the earth's surface by trapping heat 10. increase in nutrients in rivers or lakes causing overgrowth of plants and algae 11. a greenhouse gas from burning fossil fuels 12. mainly human faeces and urine, can pollute water if left untreated 13. the contamination of the environment with a substance which harms habitats and living organisms. Caused by humans.

Down 1. the breaking down of dead or waste material by microorganisms 2. the loss of water-soluble plant nutrients from the soil, due to rain and irrigation. 4. this substance is used up in respiration by microorganisms 5. pollutant in the air which causes acid rain 6. a poisonous gas that prevents oxygen from binding to haemoglobin 7. nitrogen containing mineral needed for plant growth 8. what farmers add to their fields to help plants grow 9. the cutting down of trees in a large area