What is the greenhouse effect? According to McElroy, the greenhouse effect occurs when gases such as CO2 are released into the earth's atmosphere, adding to the existing gases that normally absorb infrared heat radiating off the earth. With more gas in the atmosphere, heat that is usually able to escape back into space is trapped, thus creating a greenhouse effect and increasing the temperature of the earth. Increased temperatures on the earth cause water to evaporate from the ocean, thus compounding the greenhouse effect.

By analyzing air trapped in arctic snow 450,000 years ago, scientists are able to create a historical record of the increase of CO2in our atmosphere. Rising from a count of 200 parts per million (ppm) in the Ice Age, to 280 ppm in warm periods and increasing since the Industrial Revolution, the CO2 count now stands at 360 ppm. McElroy says that we can confidently predict that if we don't change our environmental policies, we're going to see levels that exceed 500 ppm within a few decades, and levels exceeding 750 or even 1,000 ppm during the next half or three-quarters of our century. The level of CO2 in the atmosphere today is higher than it has been at any time in 450,000 years, probably higher than it's been in 1,000,000 years. This change in CO2 levels, says McElroy, is driven by the burning of fossil fuels as well as by tropical deforestation in Brazil, Indonesia, and central Africa. We now add six billion tons of CO2 to the environment every year: three billion tons goes into the atmosphere and another three billion tons goes into the oceans.