Climate change:Climate change is a change in the statistical distribution of weather over periods of time that range from decades to millions of years. It can be a change in the average weather or a change in the distribution of weather events around an average (for example, greater or fewer extreme weather events). Climate change may be limited to a specific region, or may occur across the whole Earth.
In recent usage, especially in the context of environmental policy, climate change usually refers to changes in modern climate (see global warming). For information on temperature measurements over various periods, and the data sources available, see temperature record. For attribution of climate change over the past century, see attribution of recent climate change.
Causes:Factors that can shape climate are often called climate forcings. These include such processes as variations in solar radiation, deviations in the Earth's orbit, mountain-building and continental drift, and changes in greenhouse gas concentrations. There are a variety of climate change feedbacks that can either amplify or diminish the initial forcing. Some parts of the climate system, such as the oceans and ice caps, respond slowly in reaction to climate forcing because of their large mass. Therefore, the climate system can take centuries or longer to fully respond to new external forcings.
Plate tectonics:Over the course of millions of years, the motion of tectonic plates reconfigures global land and ocean areas and generates topography. This can affect both global and local patterns of climate and atmosphere-ocean circulation.
The position of the continents determines the geometry of the oceans and therefore influences patterns of ocean circulation. The locations of the seas are important in controlling the transfer of heat and moisture across the globe, and therefore, in determining global climate. A recent example of tectonic control on ocean circulation is the formation of the Isthmus of Panama about 5 million years ago, which shut off direct mixing between the Atlantic and Pacific Oceans. This strongly affected the ocean dynamics of what is now the Gulf Stream and may have led to Northern Hemisphere ice cover. Earlier, during the Carboniferous period, plate tectonics may have triggered the large-scale storage of carbon and increased glaciation. Geologic evidence points to a "megamonsoonal" circulation pattern during the time of the supercontinent Pangaea, and climate modeling suggests that the existence of the supercontinent was conducive to the establishment of monsoons.
More locally, topography can influence climate. The existence of mountains (as a product of plate tectonics through mountain-building) can cause orographic precipitation. Humidity generally decreases and diurnal temperature swings generally increase with increasing elevation. Mean temperature and the length of the growing season also decrease with increasing elevation. This, along with orographic precipitation, is important for the existence of low-latitude alpine glaciers and the varied flora and fauna along at different elevations in montane ecosystems.The size of continents is also important. Because of the stabilizing effect of the oceans on temperature, yearly temperature variations are generally lower in coastal areas than they are inland. A larger supercontinent will therefore have more area in which climate is strongly seasonal than will several smaller continents and/or island arcs.
Solar output:Variations in solar activity during the last several centuries based on observations of sunspots and beryllium isotopes.
The sun is the predominant source for energy input to the Earth. Both long- and short-term variations in solar intensity are known to affect global climate.
Early in Earth's history the sun emitted only 70% as much power as it does today. With the same atmospheric composition as exists today, liquid water should not have existed on Earth. However, there is evidence for the presence of water on the early Earth, in the Hadean and Archean eons, leading to what is known as the faint young sun paradox. Hypothesized solutions to this paradox include a vastly different atmosphere, with much higher concentrations of greenhouse gases than currently exist Over the following approximately 4 billion years, the energy output of the sun increased and atmospheric composition changed, with the oxygenation of the atmosphere being the most notable alteration. The luminosity of the sun will continue to increase as it follows the main sequence. These changes in luminosity, and the sun's ultimate death as it becomes a red giant and then a white dwarf, will have large effects on climate, with the red giant phase possibly ending life on Earth.
Solar output also varies on shorter time scales, including the 11-year solar cycle and longer-term modulations. The 11-year sunspot cycle produces low-latitude warming and high-latitude cooling over limited areas of statistical significance in the stratosphere with an amplitude of approximately 1.5°C. But although "variability associated with the 11-yr solar cycle has a significant influence on stratospheric temperatures. ...there is still no consensus on the exact magnitude and spatial structure". These stratospheric variations are consistent with the idea that excess equatorial heating can drive thermal winds. In the near-surface troposphere, there is only a small change in temperature (on the order of a tenth of a degree, and only statistically significant in limited areas underneath the peaks in stratospheric zonal wind speed) due to the 11-year solar cycle. Solar intensity variations are considered to have been influential in triggering the Little Ice Age, and for some of the warming observed from 1900 to 1950. The cyclical nature of the sun's energy output is not yet fully understood; it differs from the very slow change that is happening within the sun as it ages and evolves, with some studies pointing toward solar radiation increases from cyclical sunspot activity affecting global warming.
Orbital variations:Slight variations in Earth's orbit lead to changes in the amount of sunlight reaching the Earth's surface and how it is distributed across the globe. The former is similar to solar variations in that there is a change to the power input from the sun to the Earth system. The latter is due to how the orbital variations affect when and where sunlight is received by the Earth. The three types of orbital variations are variations in Earth's eccentricity, changes in the tilt angle of Earth's axis of rotation, and precession of Earth's axis. Combined together, these produce Milankovitch cycles which have a large impact on climate and are notable for their correlation to glacial and interglacial periods, their correlation with the advance and retreat of the Sahara, and for their appearance in the stratigraphic record.
