A natural process
The Greenhouse Effect is a natural process that warms the Earth, and, in fact, is quite necessary for our survival. Gases in the atmosphere, like water vapor (clouds), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) act as a natural blanket by preventing the sun’s heat energy from radiating back into space, much like a greenhouse traps the sun’s energy to warm someone’s plants even in the middle of winter. The natural greenhouse effect helps warm the Earth’s surface by as much as 33oC, and without it, our planet would be too cold for humans to survive.
The diagram below illustrates the basic processes behind the greenhouse effect. As the sun’s energy hits the Earth, some of that energy is absorbed by the earth’s crust and by the oceans, warming the planet. The rest of the energy is radiated back toward space as infrared energy. While some of this infrared energy does radiate back into space, some portion is absorbed and re-emitted by water vapor and other greenhouse gases in the atmosphere. This absorbed energy helps to warm the planet’s surface and atmosphere just like a greenhouse.
Factors affecting the Greenhouse Effect
There are three main factors that directly influence the greenhouse effect: (1) the total energy influx from the sun, which depends on the earth's distance from the sun and on solar activity, (2) the chemical composition of the atmosphere (what gases are present and in what concentrations), and (3) albedo, the ability of the earth's surface to reflect light back into space. The only factor that has changed significantly in the last 100 years is the chemical composition of the atmosphere—and that is because of human activity.
Human activity has changed the concentration of certain greenhouse gases in the atmosphere since the beginning of the Industrial Revolution, around 1750. The three graphs below show that the amounts of carbon dioxide (in parts per million), methane (in parts per billion) and nitrous oxide (in parts per billion) in the earth's atmosphere have increased dramatically over the past 1,000 years, with the most rapid increase occurring over the past fifty years.
Global warming: Anthropogenic (human-cased) forcing
Although the greenhouse effect is a naturally-occurring process, humans have recently amplified the natural effect by increasing the concentration of CO2 and other greenhouse gases in the atmosphere (primarily through fossil fuel use in the energy and transportation sectors). According to the most recent report by the Intergovernmental Panel on Climate Change (IPCC’s 4th Assessment Report), there is greater than a 90% chance that human activity is causing the increase in Earth’s global average temperatures observed since the mid-21st century.
The major greenhouse gases are listed in the figure below, along with their chemical formula. Although some of the gases listed have a larger global warming potential, carbon dioxide (CO2) is the most important greenhouse gas because of its abundance in the atmosphere. Today, atmospheric CO2 concentrations measure over 380 parts per million (ppm), mostly due to fossil fuel use in the energy and transportation sectors.