gases released by the biosphere is often seen as a feedback or internal climate process, greenhouse gases emitted from volcanoes are typically classified as external by climatologists. greenhouse gases, such as co2, methane and nitrous oxide, heat the climate system by trapping infrared light. volcanoes are also part of the extended carbon cycle. over very long (geological) time periods, they release carbon dioxide from the earth's crust and mantle, counteracting the uptake by sedimentary rocks and other geological carbon dioxide sinks. since the industrial revolution, humanity has been adding to greenhouse gases by emitting co2 from fossil fuel combustion, changing land use through deforestation, and has further altered the climate with aerosols (particulate matter in the atmosphere), release of trace gases (e.g. nitrogen oxides, carbon monoxide, or methane). other factors, including land use, ozone depletion, animal husbandry (ruminant animals such as cattle produce methane), and deforestation, also play a role. the us geological survey estimates are that volcanic emissions are at a much lower level than the effects of current human activities, which generate 100–300 times the amount of carbon dioxide emitted by volcanoes. the annual amount put out by human activities may be greater than the amount released by supereruptions, the most recent of which was the toba eruption in indonesia 74,000 years ago. orbital variations milankovitch cycles from 800,000 years ago in the past to 800,000 years in the future. slight variations in earth's motion lead to changes in the seasonal distribution of sunlight reaching the earth's surface and how it is distributed across the globe. there is very little change to the area-averaged annually averaged sunshine; but there can be strong changes in the geographical and seasonal distribution. the three types of kinematic change are variations in earth's eccentricity, changes in the tilt angle of earth's axis of rotation, and precession of earth's axis. combined, these produce milankovitch cycles which affect 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. during the glacial cycles, there was a high correlation between co2 concentrations and temperatures. early studies indicated that co2 concentrations lagged temperatures, but it has become clear that this isn't always the case. when ocean temperatures increase, the solubility of co2 decreases so that it is released from the ocean. the exchange of co2 between the air and the ocean can also be impacted by further aspects of climatic change. these and other self-reinforcing processes allow small changes in earth's motion to have a large effect on climate. solar output variations in solar activity during the last several centuries based on observations of sunspots and beryllium isotopes. the period of extraordinarily few sunspots in the late 17th century was the maunder minimum.the sun is the predominant source of energy input to the earth's climate system. other sources include geothermal energy from the earth's core, tidal energy from the moon and heat from the decay of radioactive compounds. both long term variations in solar intensity are known to affect global climate. solar output varies on shorter time scales, including the 11-year solar cycle and longer-term modulations. correlation between sunspots and climate and tenuous at best. three to four billion years ago, the sun emitted only 75% as much power as it does today. if the atmospheric composition had been the same as today, liquid water should not have existed on the earth's surface. 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. over the next five billion years, 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 any life on earth that survives until that time. volcanism in atmospheric temperature from 1979 to 2010, determined by msu nasa satellites, effects appear from aerosols released by major volcanic eruptions (el chichón and pinatubo). el niño is a separate event, from ocean variability. the eruptions considered to be large enough to affect the earth's climate on a scale of more than 1 year are the ones that inject over 100,000 tons of so2 into the stratosphere. this is due to the optical properties of so2 and sulfate aerosols, which strongly absorb or scatter solar radiation, creating a global layer of sulfuric acid haze. on average, such eruptions occur several times per century, and cause cooling (by partially blocking the transmission of solar radiation to the earth's surface) for a period of several years. although volcanoes are technically part of the lithosphere, which itself is part of the climate system, the ipcc explicitly defines volcanism as an external forcing agent. notable eruptions in the historical records are the 1991 eruption of mount pinatubo which lowered global temperatures by about 0.5 °c (0.9 °f) for up to three years, and the 1815 eruption of mount tambora causing the year without a summer. at a larger scale—a few times every 50 million to 100 million years—the eruption of large igneous provinces brings large quantities of igneous rock from the mantle and lithosphere to the earth's surface. carbon dioxide in the rock is then released into the atmosphere. small eruptions, with injections of less than 0.1 mt of sulfur dioxide into the stratosphere, affect the atmosphere only subtly, as temperature changes are comparable with natural variability. however, because smaller eruptions occur at a much higher frequency, they too significantly affect earth's atmosphere. 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. during the carboniferous period, about 300 to 360 million years ago, plate tectonics may have triggered 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. 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 or islands. other mechanisms it has been postulated that ionized particles known as cosmic rays could impact cloud cover and thereby the climate. as the sun shields the earth from these particles, changes in solar activity were hypothesized to influence climate indirectly as well. to test the hypothesis, cern designed the cloud experiment, which showed the effect of cosmic rays is too weak to influence climate noticeably. evidence exists that the chicxulub asteroid impact some 66 million years ago had severely affected the earth's climate. large quantities of sulfate aerosols were kicked up into the atmosphere, decreasing global temperatures by up to 26 °c and producing sub-freezing temperatures for a period of 3–16 years. the recovery time for this event took more than 30 years. the large-scale use of nuclear weapons has also been investigated for its impact on the climate. the hypothesis is that soot released by large-scale fires blocks a significant fraction of sunlight for as much as a year, leading to a sharp drop in temperatures for a few years. this possible event is described as nuclear winter. humans' use of land impact how much sunlight the surface reflects and the concentration of dust. cloud formation is not only influenced by how much water is in the air and the temperature, but also by the amount of aerosols in the air such as dust. globally, more dust is available if there are many regions with dry soils, little vegetation and strong winds. evidence and measurement of climate changes paleoclimatology is the study of changes in climate through the entire history of earth. it uses a variety of proxy methods from the earth and life sciences to obtain data preserved within things such as rocks, sediments, ice sheets, tree rings, corals, shells, and microfossils. it then uses the records to determine the past states of the earth's various climate regions and its atmospheric system. direct measurements give a more complete overview of climate variability. direct measurements climate changes that occurred after the widespread deployment of measuring devices can be observed directly. reasonably complete global records of surface temperature are available beginning from the mid-late 19th century. further observations are derived indirectly from historical documents. satellite cloud and precipitation data has been available since the 1970s. historical climatology is the study of historical changes in climate and their effect on human history and development. the primary sources include written records such as sagas, chronicles, maps and local history literature as well as pictorial representations such as paintings, drawings and even rock art. climate variability in the recent past may be derived from changes in settlement and agricultural patterns. archaeological evidence, oral history and historical documents can offer insights into past changes in the climate. changes in climate have been linked to the rise and the collapse of various civilizations. proxy measurements variations in co2, temperature and dust from the vostok ice core over the last 450,000 years. various archives of past climate are present in rocks, trees and fossils. from these archives, indirect measures of climate, so-called proxies, can be derived. quantification of climatological variation of precipitation in prior centuries and epochs is less complete but approximated using proxies such as marine sediments, ice cores, cave stalagmites, and tree rings. stress, too little precipitation or unsuitable temperatures, can alter the growth rate of trees, which allows scientists to infer climate trends by analyzing the growth rate of tree rings. this branch of science studying this called dendroclimatology. glaciers leave behind moraines that contain a wealth of material—including organic matter, quartz, and potassium that may be dated—recording the periods in which a glacier advanced and retreated. analysis of ice in cores drilled from an ice sheet such as the antarctic ice sheet, can be used to show a link between temperature and global sea level variations. the air trapped in bubbles in the ice can also reveal the co2 variations of the atmosphere from the distant past, well before modern environmental influences. the study of these ice cores has been a significant indicator of the changes in co2 over many millennia, and continues to provide valuable information about the differences between ancient and modern atmospheric conditions. the 18o/16o ratio in calcite and ice core samples used to deduce ocean temperature in the distant past is an example of a temperature proxy method. the remnants of plants, and specifically pollen, are also used to study climatic change. plant distributions vary under different climate conditions. different groups of plants have pollen with distinctive shapes and surface textures, and since the outer surface of pollen is composed of a very resilient material, they resist decay. changes in the type of pollen found in different layers of sediment indicate changes in plant communities. these changes are often a sign of a changing climate. as an example, pollen studies have been used to track changing vegetation patterns throughout the quaternary glaciations and especially since the last glacial maximum. remains of beetles are common in freshwater and land sediments. different species of beetles tend to be found under different climatic conditions. given the extensive