Human-generated emissions of carbon dioxide and other greenhouse gases affect important global climate processes. Understanding the greenhouse effect, carbon cycle, climate forcing, and climate feedbacks helps to explain how and why climate is changing.
The greenhouse effect is a natural warming process. Carbon dioxide and certain other gases are always present in the atmosphere. These gases create a warming effect that has some similarity to the warming inside a greenhouse, hence the name "greenhouse effect."
Increasing the amount of greenhouse gases intensifies the greenhouse effect. Higher concentrations of these gases trap more infrared energy in the atmosphere than occurs naturally. The additional heat further warms the atmosphere and Earth's surface.
Image based on a figure from US EPA. For more information, see America's Climate Choices.