The main climate state change is between periodical glacial and interglacial cycles in Earth history, studied from climate proxies. The climate system is responding to the current climate forcing and adjusts following climate sensitivity to reach a climate equilibrium, Earth's energy balance. Model simulations suggest that the current interglacial climate state will continue for at least another 100,000 years, due to CO2 emissions - including complete deglaciation of the Northern Hemisphere.
The orbital forcing from Milankovitch cycles is a periodical factor to determine Earth's energy budget and responsible for the glacial cycles on Earth, depending on the radiative equilibrium. Other factors include processes and change in geospheric systems. These include oceanic processes (such as oceanic circulation), biotic processes, variations in solar radiation received by Earth, plate tectonics, volcanism, albedo vegetation changes and human-induced alterations of the natural world.
The greenhouse has been the dominant state in Earth's past. Recovered ocean sediments of the past 120 million years contain evidence of the long-term transition from a greenhouse to icehouse climate state. A time when there are no glaciers on Earth is considered a greenhouse climate state. An ice age implies the presence of extensive ice sheets at Earth polar regions. The time during an Ice Age glacial period, when glaciers reach their maximum extent is referred to as icehouse climate state. There have been five known ice ages in the Earth's past, with the Earth experiencing currently an interglacial period (warming) during the present Quaternary Ice Age, identified as the "marine isotope stage 1" (MIS1) in the Holocene epoch (or recently the Anthropocene epoch).
The current climate state and evidence from the past of the climate system are important in determining the future evolution of climatic anomalies. Dansgaard–Oeschger events are considered switches between states of the climate system. Tipping points in the climate system describe thresholds, such as ice-albedo feedback which can cause abrupt climate change, and possibly leading to a new state. The climate state affects the formation processes of large volcanic provinces.
In the past the planet's climate has been fluctuating between two dominant states: the Greenhouse and the Icehouse state.
- The Huronian glaciation, is the first known glaciation in Earth's history, and lasted from 2400-2100 million years ago.
- The Cryogenian glaciation lasted from 850-635 million years ago.
- The Andean-Saharan glaciation lasted from 450–420 million years ago.
- The Karoo glaciation lasted from 360–260 million years ago.
- The Quaternary glaciation is the current glaciation period and begun 2.58 million years ago.
In the past, weathering of silicate rocks sequestered CO2, a negative feedback loop which maintained Earth’s climate within a habitable range over millions of years. When atmospheric CO2 concentration rise, temperature and precipitation increase and thereby enhance chemical weathering. The last time global temperature reached a long-term maximum was during the Early Eocene Climatic Optimum, 51–53 million years ago (See also Paleocene–Eocene Thermal Maximum). Only over the past 34 million years have CO2 concentrations been low, temperatures relatively cool, and the poles glaciated. This long- term shift in Earth’s climatic state resulted, in part from differences in volcanic emissions, which were particularly high during parts of the Palaeocene and Eocene epochs (about 40–60 million years ago) but have diminished since then.
Conceivable levels of human-made climate forcing could yield the low-end runaway greenhouse. A forcing of 12–16 W m−2, which would require CO2 to increase by a factor of 8–16 times, if the forcing were due only to CO2 change, would raise the global mean temperature by 16–24 °C with much larger polar warming. That would melt all the ice on the planet, and probably thaw methane hydrates and scorch carbon from global peat deposits and tropical forests. This forcing would not produce the extreme Venus-like baked-crust greenhouse state, which cannot be reached until the ocean is lost to space. A warming of 16–24 °C produces a moderately moist greenhouse, with water vapour increasing to about 1% of the atmosphere's mass, thus increasing the rate of hydrogen escape to space. However, if the forcing is by fossil fuel CO2, the weathering process would remove the excess atmospheric CO2 on a time scale of 104–105 years, well before the ocean is significantly depleted. Baked-crust hothouse conditions on the Earth require a large long-term forcing that is unlikely to occur until the sun brightens by a few tens of per cent, which will take a few billion years.
Hansen et al. 2013 suggests that the Earth could become in large parts uninhabitable and noted that this may not even require burning all of fossil fuels, because of higher climate sensitivity (3–4 °C or 5.4–7.2 °F) based on a 550ppm scenario.
Based on climate proxies paleoclimatologists study the different climate states originating from glaciation. In climate science distinction is made between the background state (Today), the initial state, the equilibrium state and the paleo state, when considering climate sensitivity and climate forcing. In order to understand future climate projections, interactions of feedbacks and thus the calculated climate sensitivity with the background climate state has become a top priority in climate science. Feedbacks in the climate system, and thus climate sensitivity, may depend in an unknown non-linear manner on the climate state before perturbation (‘background climate state’) and on type of forcing. However, decadal to inter-decadal climate variability can be interpreted from past variability, and the identification of the involved dynamical processes can help to understand the chain of events which are characteristic for shifts in the climate state, today.
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