because trees normally soak up CO2 to make food.Scientists say higher CO2 levels strengthen the “greenhouse effect” and could increase the Earth's temperature.②In fact，CO2 levels have risen by 30 percent since the Industrial Revolution of the 18th and 19th centuries. Over the past three years，satellite measurements have shown a sea level rise of about a quarter inch worldwide.③“If nothing is done to reduce fossil­fuel pollution and global warming，sea levels will rise even more.” says geologist Richard Alley.