Climate Change/Introduction
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[edit] What is Climate?
Climate is a broad term, but it always describes a long-term average of a system. Often 'climate' is used to mean the long-term mean state of the atmosphere, including temperature, humidity, and wind. In other contexts, 'climate' can include the oceanic state, the cryosphere (snow and sea-ice), the biosphere, and sometimes even the lithosphere (Earth's crust). Meteorologists and atmospheric scientists often say that climate is what you expect; weather is what you get.
[edit] What is Climate Change?
Although dynamical systems, like Earth's climate system, are full of complicated processes that lead to chaotic variations, changes to external forcing of the system can lead to significant changes. For the Earth's climate, we usually think of trends in global average quantities (especially surface temperature) as indicative of climate change. When the trend leads to a change larger than the natural variability, a statistically significant change most certainly has occurred. DEFINITIONS
[edit] Anthropogenic Climate Change
Anthropogenic means "human caused," form "anthro-", meaning "human," and "genic," meaning "produced by, origin, cause". The term anthropogenic climate change is used to attribute changes in Earth's climate to activities of humans. In recent times, this has been taken as implying mainly the emission of "greenhouse gases" into the atmosphere, usually by burning fossil fuels.
How can humans change Earth's climate? Even as far back as Arrhenius[1] people have been aware that the composition of the atmosphere affects the climate. Some gases, like carbon dioxide, have molecular structure that allows the absorption of certain wavelengths of light. In the case of "greenhouse gases," that means absorbing infrared radiation. The distinguishing characteristic of a greenhouse gas is that it absorbs infrared radiation better than it does visible radiation; this allows sunlight to penetrate through the gas (the atmosphere) and warm Earth's surface. The Earth then radiates as a blackbody, emitting infrared radiation that is then trapped in the atmosphere. This is the greenhouse effect.
If humans change the composition of the atmosphere, say by burning fossil fuels which release carbon dioxide, then more energy goes into the atmosphere than would have otherwise. More energy leads directly to higher temperature, hence climate change.
[edit] Historical Footnote
Climatology is a young science, with modern climate science only emerging from meteorology, oceanography, and geology in the late 20th Century. Of course, people have been interested in the natural world, including movements of air and water, for a very long time. One early example of a theory for anthropogenic climate change is George P. Marsh's[2] "The Earth as Modified by Human Action," published in 1874. The science in this early effort is far from the level of climate science today, but Marsh does link land use change, including deforestation and irrigation, to changes in the local climate.
[edit] A Wikibook digression
One contributing author to this wiki book stated that decreased nocturnal cooling may never have been "considered in any debate about global warming." The argument was stated as
All planets with rotational days unequal to their orbital years absorb their sun's heat during their day and release it at night. In the case of planet Earth, however, not only are we adding to the sun's heat in the daytime; The ever-increasing tendency away from regular day-night cycles of work, play and sleep means that at night, the time when our Earth should be shedding its excess heat, we are still adding to it.
This is a fair argument at first blush, but it does not hold up under scrutiny. While Earth cools much more efficiently at night at the surface, the better cooling does not continue into the upper troposphere very well. That means that most of the energy from the cooling will still end up where it would during the day: either absorbed in the troposphere or emitted to space. Also, the argument seems to imply that increased nocturnal activity by humans makes the cooling less efficient, but it is an extremely small effect. The more efficient cooling at night is due almost entirely to the absence of sunlight. Think of the evolution of the surface temperature as dT = S - F to zeroth order, where S is the solar energy absorbed at the surface and F is the cooling by infrared emission. At nighttime, S = 0, so dT is all due to cooling by emission. During the day, the warming offsets the cooling. We are not adding to the sun's heat, as the contributor states, but just trapping it in the troposphere. That trapping has no diurnal cycle, since there is a negligible diurnal cycle in the concentration of atmospheric constituent gases. Let this be a lesson for the reader: critical thinking should always accompany learning about new topics.
