Given the massive movements of the atmosphere and the oceans, and the varied locations of large continents and mountain ranges, it can certainly be hard to imagine that the limited activities of humans can modify the global climate. Yet, we also can observe that the change in location of the warm waters of the Pacific Ocean (called an El Nino) and the gases and dust injected by volcanic eruptions can indeed change global weather and the average global temperature. On even finer scales, we experience the effects of additional water vapor in the atmosphere when we stay warmer through humid nights than night when the air is dry and clear. Earth history also indicates that the CO2 concentration can be different than at present, and that, along with changes in other factors, these changes can contribute to climate change (e.g., the Cretaceous—the time of the dinosaurs—had a CO2 concentration several times as high as today—and palm trees in arctic regions).We also know that the surface of Venus is very hot, and not because it is closer to the Sun—the reason instead is that the atmosphere of Venus has a lot of carbon dioxide, and this large amount traps the solar radiation that is not reflected to space and makes the surface scorching hot (because of its bright clouds—which is why we can see it--Venus absorbs less solar radiation per square meter than the Earth). Studies of all of this have been going on since the 19th century, and the various proposals have since then stood up to withering criticism because they are confirmed by observations, by past climatic behavior, by theoretical analyses, and, yes, by computer models of the Earth’s climate system that are based on the rigorous physical laws of conservation of mass, momentum, energy, etc. (plus some empirically determined relationships). The problem being so complex, the nations of the world created the Intergovernmental Panel on Climate Change to draw together scientific understanding from the scientific community around the world. They have now issued four major assessments that have been accepted UNANIMOUSLY by the nations of the world and also by all of the major national academies of science. So, while the notion of humans causing global warming may seem counter-intuitive and not be completely understood, its basis is very soundly based, as explained in the assessments, and, for policymakers, the risks created are sufficiently clear and serious that it is time (or far past time) that they be addressed in a significant and serious manner.