Climate change has become a "hot button" issue over the past few years, and this has only become more extreme as United States policy on climate change has diverged from much of the rest of the world and the scientific community. However, climate change is first and foremost a scientific topic. In this wikibook, the underlying science of climate change is explored in some detail. The study of climate -- sometimes called climatology or climate science -- is actually a relatively young field, but has roots in all the major branches of science. It is most easily associated with atmospheric science (and its older name, Meteorology) and oceanography. There are also strong connections with the cryosphere (glaciology), the biosphere (biology, ecology), and the lithosphere (especially through the extraction and combustion of fossil fuels). Using basic physical principles, the science of climate change and these connections to other natural sciences will emerge. We will see how computer models, current observation, and studies of ancient climates converge on a singular picture of the near future that includes continued global warming, enhancement of the hydrological cycle, decreasing sea-ice, shrinking glaciers and ice sheets, more acidic ocean water, rising sea level, and more frequent extreme climate events.
Many of the fundamental concepts of climate science are straight out of elementary physics. The equations of motion are the same fundamental equations that govern all classical fluid dynamics, much of energy transfer is based on well-known principles of radiative transfer and nuclear physics and spectrometry, and a lot of observations are based on geological, chemical and biological processes and methods. This is all to say that climate science is a multi-disciplinary field, with diverse (even disparate at times) interests and applications. It is unified only by the end goal: to understand the physical processes governing our natural world. These same physical principles are at work in understanding a changing climate. The main difference is that instead of describing the average or the natural variability of climate, climate change studies try to quantify differences or trends. Past changes and future changes are studied similarly, though sometimes using different tools, as we will see.
When the composition of the atmosphere changes, for example by changing the carbon dioxide concentration, the radiative properties of the atmosphere might also change. In the absence of an atmosphere, Earth would look a lot like a black body radiator; that is to say, the sun would shine on Earth, which would warm to an equilibrium temperature, and then a balance would be struck. That balance (radiative equilibrium) would have Earth radiating as much energy to space as the sun delivers to the surface. Mostly due to the fact that Earth is so small and intercepts so little of the total energy emitted from the sun, that radiative equilibrium temperature is much lower than the sun's temperature. Using Wien's law, we can calculate that temperature and establish that Earth is an infrared emitter.
When there is an atmosphere, like the one on Earth, some of the gases that make up the atmosphere can absorb infrared radiation. That interaction between photons and molecules increases the temperature of the atmosphere, which then emits at a slightly different wavelength. The emission from the atmosphere goes both out to space and downward, back to the ground where it is absorbed by the surface. This process, whereby energy that is emitted from the surface is absorbed by the atmosphere which then emits energy back toward the surface, is called the greenhouse effect, and it is one of the basic feedback processes in the climate system. It increases the surface temperature on Earth from the radiative equilibrium temperature to a much more life-friendly temperature. Global warming, or anthropogenic global warming, is the difference in the global mean temperature in a world with artificially elevated carbon dioxide compared to a reference state (which is usually taken as a time before the Industrial Revolution). In the rest of this part of the book, we investigate the processes involved with climate and climate change, from the sun's influence, to the natural greenhouse effect, to observed changes in the composition of the atmosphere. We will focus on feedbacks and processes that are thought important in both stabilizing and amplifying changes to the global climate.