from 20°N to 90°N (NAM) or 20°S to 90°S (SAM). Dansgaard–Oeschger cycles – occurring on roughly 1,500-year cycles during the Last Glacial Maximum Ocean current changes The oceanic aspects of climate variability can generate variability on centennial timescales due to the ocean having hundreds of times more mass than in the atmosphere, and thus very high thermal inertia. For example, alterations to ocean processes such as thermohaline circulation play a key role in redistributing heat in the world's oceans. Ocean currents transport a lot of energy from the warm tropical regions to the colder polar regions. Changes occurring around the last ice age (in technical terms, the last glacial) show that the circulation is the North Atlantic can change suddenly and substantially, leading to global climate changes, even though the total amount of energy coming into the climate system didn't change much. These large changes may have come from so called Heinrich events where internal instability of ice sheets caused huge ice bergs to be released into the ocean. When the ice sheet melts, the resulting water is very low in salt and cold, driving changes in circulation. Life Life affects climate through its role in the carbon and water cycles and through such mechanisms as albedo, evapotranspiration, cloud formation, and weathering. Examples of how life may have affected past climate include: glaciation 2.3 billion years ago triggered by the evolution of oxygenic photosynthesis, which depleted the atmosphere of the greenhouse gas carbon dioxide and introduced free oxygen another glaciation 300 million years ago ushered in by long-term burial of decomposition-resistant detritus of vascular land-plants (creating a carbon sink and forming coal) termination of the Paleocene–Eocene Thermal Maximum 55 million years ago by flourishing marine phytoplankton reversal of global warming 49 million years ago by 800,000 years of arctic azolla blooms global cooling over the past 40 million years driven by the expansion of grass-grazer ecosystems External climate forcing Greenhouse gases Whereas greenhouse 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 , 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