are dissolved in cold water in higher latitudes. This water sinks down and brings the carbon into the deeper ocean levels, where it can stay for anywhere between decades and several centuries. Ocean circulation events cause this process to be variable. For example, during El Nino events there is less deep ocean upwelling, leading to lower outgassing of carbon dioxide into the atmosphere. Biological processes also lead to ocean-atmosphere carbon exchange. Carbon dioxide equilibrates between the atmosphere and the ocean's surface layers. As autotrophs add or subtract carbon dioxide from the water through photosynthesis or respiration, they modify this balance, allowing the water to absorb more carbon dioxide or causing it to emit carbon dioxide into the atmosphere. Geosphere Carbon is generally exchanged very slowly between the atmosphere and geosphere. Two exceptions are volcanic eruptions and the combustion of fossil fuels, both of which release high amounts carbon into the atmosphere very quickly. Fresh silicate rock that is exposed through geological processes absorbs carbon from the atmosphere when it is exposed to air by the processes of weathering and erosion. Anthropogenic sources Human activities change the amount of carbon in the atmosphere directly through the burning of fossil fuels and other organic material, thus oxidizing the organic carbon and producing carbon dioxide. Another human-caused source of carbon dioxide is cement production. The burning of fossil fuels and cement production are the main reasons for the increase in atmospheric CO2 since the beginning of the industrial era. Other human-caused changes in the atmospheric carbon cycle are due to anthropogenic changes to carbon reservoirs. Deforestation, for example, decreases the biosphere's ability to absorb carbon, thus increasing the amount of carbon in the atmosphere. As the industrial use of carbon by humans is a very new dynamic on a geologic scale, it is important to be able to track sources and sinks of carbon in the atmosphere. One way of doing so is by observing the proportion of stable carbon isotopes present in the atmosphere. The two main carbon isotopes are 12C and 13C. Plants absorb the lighter isotope, 12C, more readily than 13C. Because fossil fuels originate mainly from plant matter, the 13C/12C ratio in the atmosphere falls when large amounts of fossil fuels are burned, releasing 12C. Conversely, an increase in the 13C/12C in the atmosphere suggests a higher biospheric carbon uptake. The ratio of the annual increase in atmospheric CO2