The greenhouse effect refers to a natural process that occurs in the Earth's atmosphere. If this process is disrupted, then it could contribute to global warming. As the sun's rays hit the Earth, heat is bounced back to the atmosphere where these gases contain the heat and keep it there to warm the planet. This is an important natural process and allows life forms to flourish and survive. Problems occur only when these gases multiply and build-up, containing heat too efficiently and thus warming the Earth's atmosphere. As the Earth's average temperature rises, effects in its landmasses and sea water level become apparent. Polar ice caps melt along with glaciers, contributing to higher and warmer sea levels. By the end of the century, it is estimated that sea levels can increase from 4 inches to a high of about 40 inches if global warming continues unabated.