Over the past century global average temperature has increased by about 0.8 degrees C. Any substantial change in the Earth's temperature must be the result of a perturbation, or so-called radiative forcing, of the planet's energy balance. The equilibrium global temperature response to a particular amount of radiative forcing is termed the Earth's climate sensitivity. Once the Earth's energy balance is perturbed, feedbacks arise that act either to enhance or suppress the perturbation. The positive feedback arising from changes in the water vapor level in the atmosphere resulting from a change in temperature is key. In this article, we evaluate the possible causes of the recent warming of the Earth. We review measured variations in the solar output, as well as the historical paleoclimate record, especially the glacial-interglacial cycles. Atmospheric greenhouse gases, carbon dioxide, methane, and nitrous oxide, have increased substantially over the last century; the radiative forcing resulting from these increases can be accurately calculated. The forcing attributable to the increase in levels of airborne particles (aerosols) is considerably more uncertain. All these factors must be taken into account to arrive at an explanation of the recent warming.