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.
Climatology, the science that studies climate, 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 "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.
For the Earth's climate, changes in pasterns (moderate shifts in the mean values of the variables) are normal and expected. Earth's climate is a dynamic system, full of complicated processes that lead to chaotic variations, changes to external forcing of the system can lead to significant changes.
Usually we think of trends in global average quantities (especially surface temperature) as indicative of climate change. But when the trend leads to a change larger than the natural variability, a statistically significant change most certainly has occurred. DEFINITIONS
This book will cover most of the normal implications of climate changes but will focus on the recent medium to long term effects on the above normal changes, normally but also confusingly labeled global warming.
The term global warming was coined by Wallace Broecker, a professor at Columbia University, in reference to the increase in the average temperature of Earth's near-surface air and oceans since the mid-20th century and its projected continuation.
Reasons for this recent shift in the general pattern have been mostly stated as being the result of actions of the human species on the planet and solar variation.
- 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 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.
- Solar variation
Solar variations are changes in the amount of radiant energy emitted by our Sun. There are periodic components to these variations, the principal one being the 11-year solar cycle (or sunspot cycle), as well as fluctuations which are aperiodic. Solar activity has been measured via satellites during recent decades and through 'proxy' variables in prior times. Climate scientists are interested in understanding what, if any, effect variations in solar activity have on the Earth. Any such mechanism is referred to as "solar forcing".
The variations in total solar irradiance (TSI) remained at or below the threshold of detectability until the satellite era, although the small fraction in ultra-violet wavelengths varies by a few percent. Total solar output is now measured to vary (over the last three 11-year sunspot cycles) by approximately 0.1% or about 1.3 W/m2 peak-to-trough during the 11 year sunspot cycle. The amount of solar radiation received at the outer surface of Earth's atmosphere varied little from an average value of 1366 watts per square meter (W/m2). There are no direct measurements of the longer-term variation and interpretations of proxy measures of variations differ; recent results suggest about 0.1% variation over the last 2000 years, although other sources suggest a 0.2% increase in solar irradiance since 1675. The combination of solar variation and volcanic effects has very likely been the cause of some climate change, for example during the Maunder Minimum. A 2006 study and review of existing literature, published in Nature, determined that there has been no net increase in solar brightness since the mid 1970s, and that changes in solar output within the past 400 years are unlikely to have played a major part in global warming. It should be stressed, the same report cautions that "Apart from solar brightness, more subtle influences on climate from cosmic rays or the Sun's ultraviolet radiation cannot be excluded, say the authors. However, these influences cannot be confirmed, they add, because physical models for such effects are still too poorly developed."
Within this wikibook, "Climate Change" is taken to primarily mean "global warming"; which is to say the warming of the Earth seen approximately since the start of the twentieth century. Of course, climate has changed before this time due to natural causes - for example the ice ages.
The United Nations Framework Convention on Climate Change defines climate change as a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods. In this sense climate change is synonymous with global warming.
However the IPCC defines climate change as a statistically significant variation in either the mean state of the climate or in its variability, persisting for an extended period (typically decades or longer). Climate change may be due to natural internal processes or external forcings, or to persistent anthropogenic changes in the composition of the atmosphere or in land use.
Whichever definition is used makes no difference to the changes in climate.
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.