Feedback del cambio climático

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El feedback del cambio climático es importante en la comprensión del calentamiento global debido a que los procesos de retroalimentación (feedback) pueden amplificar o disminuir el efecto de cada forzante radiativo, y por lo tanto juegan un papel importante en la determinación de la sensibilidad climática. En general el feedback es el proceso en el cual los cambios en una cantidad producen un segundo cambio, y este cambio en ese segundo realimenta al primero. Una realimentación positiva amplifica el cambio en la primera cantidad mientras una realimentación negativa lo reduce.[1] [2]

Por definición, los forzantes son externos al sistema climático, mientras que los feedbacks son reacciones internas, en esencia, las retroalimentaciones representan los procesos internos del sistema. Algunas retroalimentaciones pueden actuar de forma aislada con respecto al resto del sistema climático, mientras que otras pueden estar estrechamente unido, por lo que puede ser difícil saber hasta qué punto un determinado proceso contribuye.[3] Forzantes, feedbacks y la dinámica del sistema climático determinarán la cantidad y la rapidez con que cambia el clima. La retroalimentación principal positiva en el calentamiento global es la tendencia del calentamiento para aumentar la cantidad de vapor de agua en la atmósfera, que a su vez conduce a un mayor calentamiento.[4] La retroalimentación negativa principal proviene de la ley de Stefan–Boltzmann: la cantidad de calor irradiado por la Tierra hacia el espacio cambia con la cuarta potencia de la temperatura de la superficie de la Tierra y de la atmósfera.

Algunos de los efectos observados y potenciales del calentamiento global son feedbacks positivos, que contribuyen directamente a un mayor calentamiento global. El Cuarto Informe de Evaluación del IPCC, del Grupo Intergubernamental de Expertos sobre el Cambio Climático (IPCC) afirma que "el calentamiento antropogénico podría llevar a algunos efectos que serían abruptos o irreversibles, dependiendo de la velocidad y la magnitud del cambio climático."[5]

Positivas[editar]

Feedbacks del ciclo del carbono[editar]

Ha habido predicciones, y alguna evidencia, que el calentamiento global podría causar pérdida de carbono de los ecosistemas terrestres, dando lugar a un aumento de la contaminación atmosférica y de los niveles de CO2. Varios modelos climáticos indican que el calentamiento global durante el siglo 21 podría ser acelerado por la respuesta del ciclo del carbono terrestre al calentamiento.[6] Los once modelos del C4MIP study found that a larger fraction of anthropogenic CO2 will stay airborne if climate change is accounted for. By the end of the twenty-first century, this additional CO2 varied between 20 and 200 ppm for the two extreme models, the majority of the models lying between 50 and 100 ppm. The higher CO2 levels led to an additional climate warming ranging between 0,1° a 1,5 °C. However, there was still a large uncertainty on the magnitude of these sensitivities. Eight models attributed most of the changes to the land, while three attributed it to the ocean.[7] The strongest feedbacks in these cases are due to increased respiration of carbon from soils throughout the high latitude boreal forests of the Northern Hemisphere. One model in particular (HadCM3) indicates a secondary carbon cycle feedback due to the loss of much of the Amazon Rainforest in response to significantly reduced precipitation over tropical South America.[8] While models disagree on the strength of any terrestrial carbon cycle feedback, they each suggest any such feedback would accelerate global warming.

Las observaciones muestran que los suelos en Inglaterra han estado perdiendo carbono a un ritmo de cuatro millones de toneladas al año durante los últimos 25 años[9] according to a paper in Nature by Bellamy et al. in September 2005, who note that these results are unlikely to be explained by land use changes. Results such as this rely on a dense sampling network and thus are not available on a global scale. Extrapolating to all of the United Kingdom, they estimate annual losses of 13 million tons per year. This is as much as the annual reductions in carbon dioxide emissions achieved by the UK under the Kyoto Treaty (12.7 million tons of carbon per year).[10]

It has also been suggested (by Chris Freeman) that the release of dissolved organic carbon (DOC) from peat bogs into water courses (from which it would in turn enter the atmosphere) constitutes a positive feedback for global warming. The carbon currently stored in peatlands (390–455 gigatonnes, one-third of the total land-based carbon store) is over half the amount of carbon already in the atmosphere.[11] DOC levels in water courses are observably rising; Freeman's hypothesis is that, not elevated temperatures, but elevated levels of atmospheric CO2 are responsible, through stimulation of primary productivity.[12] [13]

Tree deaths are believed to be increasing as a result of climate change, which is a positive feedback effect.[14] This contradicts the previously widely held view that increased natural vegetation would lead to a negative-feedback effect.

Liberación de metano del Ártico[editar]

Warming is also the triggering variable for the release of carbon (potentially as methane) in the arctic.[15] Methane released from thawing permafrost such as the frozen peat bogs in Siberia, and from methane clathrate on the sea floor, creates a positive feedback.[16] [17]

Liberación de metano a partir de la fusión del permafrost de turberas[editar]

Western Siberia is the world's largest peat bog, a one million square kilometer region of permafrost peat bog that was formed 11,000 years ago at the end of the last ice age. The melting of its permafrost is likely to lead to the release, over decades, of large quantities of methane. As much as 70,000 million t de metano, an extremely effective greenhouse gas, might be released over the next few decades, creating an additional source of greenhouse gas emissions.[18] Similar melting has been observed in eastern Siberia.[19] Lawrence et al. (2008) suggest that a rapid melting of Arctic sea ice may start a feedback loop that rapidly melts Arctic permafrost, triggering further warming.[20] [21]

Liberación de metano de los hidratos[editar]

Methane clathrate, also called methane hydrate, is a form of water ice that contains a large amount of methane within its crystal structure. Extremely large deposits of methane clathrate have been found under sediments on the sea and ocean floors of Earth. The sudden release of large amounts of natural gas from methane clathrate deposits, in a runaway global warming event, has been hypothesized as a cause of past and possibly future climate changes. The release of this trapped methane is a potential major outcome of a rise in temperature; it is thought that this might increase the global temperature by an additional 5° in itself, as methane is much more powerful as a greenhouse gas than carbon dioxide. The theory also predicts this will greatly affect available oxygen content of the atmosphere. This theory has been proposed to explain the most severe mass extinction event on earth known as the Permian–Triassic extinction event, and also the Paleocene-Eocene Thermal Maximum climate change event. In 2008, a research expedition for the American Geophysical Union detected levels of methane up to 100 times above normal in the Siberian Arctic, likely being released by methane clathrates being released by holes in a frozen 'lid' of seabed permafrost, around the outfall of the Lena River and the area between the Laptev Sea and East Siberian Sea.[22] [23] [24]

Aumento abrupto de metano en la atmósfera[editar]

Literature assessments by the Intergovernmental Panel on Climate Change (IPCC) and the US Climate Change Science Program (CCSP) have considered the possibility of future projected climate change leading to a rapid increase in atmospheric methane. The IPCC Third Assessment Report, published in 2001, looked at possible rapid increases in methane due either to reductions in the atmospheric chemical sink or from the release of buried methane reservoirs. In both cases, it was judged that such a release would be "exceptionally unlikely"[25] (menos de un 1% de probabilidad, en base a la opinión de expertos).[26] The CCSP assessment, published in 2008, concluded that an abrupt release of methane into the atmosphere appeared "very unlikely"[27] (menos del 10% de probabilidad, en base a opiniones expertas).[28] The CCSP assessment, however, noted that climate change would "very likely" (greater than 90% probability, based on expert judgement) accelerate the pace of persistent emissions from both hydrate sources and wetlands.[27]

Descomposiciones[editar]

Organic matter stored in permafrost generates heat as it decomposes in response to the permafrost melting.[29] This is significant mainly due to its effect on Arctic methane release.

Descomposicion de turba[editar]

La turba, occurring naturally in peat bogs, is a store of carbon significant on a global scale. When peat dries it decomposes, and may additionally burn. Water table adjustment due to global warming may cause significant excursions of carbon from peat bogs.[30] This may be released as methane, which can exacerbate the feedback effect, due to its high global warming potential.

Sequías en selvas[editar]

Las selvas, most notably tropical rainforests, are particularly vulnerable to global warming. There are a number of effects which may occur, but two are particularly concerning. Firstly, the drier vegetation may cause total collapse of the rainforest ecosystem.[31] For example, the Amazon rainforest would tend to be replaced by caatinga ecosystems. Further, even tropical rainforests ecosystems which do not collapse entirely may lose significant proportions of their stored carbon as a result of drying, due to changes in vegetation.[32]

Fuegos forestales[editar]

The IPCC Fourth Assessment Report predicts that many mid-latitude regions, such as Mediterranean Europe, will experience decreased rainfall and an increased risk of drought, which in turn would allow forest fires to occur on larger scale, and more regularly. This releases more stored carbon into the atmosphere than the carbon cycle can naturally re-absorb, as well as reducing the overall forest area on the planet, creating a positive feedback loop. Part of that feedback loop is more rapid growth of replacement forests and a northward migration of forests as northern latitudes become more suitable climates for sustaining forests. There is a question of whether the burning of renewable fuels such as forests should be counted as contributing to global warming.[33] [34] [35] Cook & Vizy also found that forest fires were likely in the Amazon Rainforest, eventually resulting in a transition to Caatinga vegetation in the Eastern Amazon region.

Desertificación[editar]

La desertificación es una consecuencia del calentamiento global, en algunos ambientes.[36] Desert soils contain little humus, and support little vegetation. As a result, transition to desert ecosystems is typically associated with excursions of carbon.

CO2 en los océanos[editar]

Cooler water can absorb more CO2 than warmer water. As ocean temperatures rise the oceans will absorb less CO2 resulting in more warming. Conversely when cooler the oceans have absorbed more CO2, resulting in further cooling. There is about 50 times more carbon in the oceans than there is in the atmosphere.[37]

In addition to the water itself, the ecosystems of the oceans also sequester carbon. Their ability to do so is also expected to decline as the oceans warm: Warming reduces the nutrient levels of the mesopelagic zone (about 200 to 1000 m deep), which limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.[38]

Realimentaciones nubosas[editar]

Warming is expected to change the distribution and type of clouds. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud. These details were poorly observed before the advent of satellite data and are difficult to represent in climate models.[39]

Salidas de gases[editar]

Release of gases of biological origin may be affected by global warming, but research into such effects is at an early stage. Some of these gases, such as nitrous oxide released from peat, directly affect climate.[40] Others, such as dimethyl sulfide released from oceans, have indirect effects.[41]

Feedback del albedo del hielo[editar]

La imagen muestra, en Groenlandia, la diferencia entre la cantidad de luz solar reflejada, en el verano de 2011, comparada con el porcentaje promedio reflejado entre 2000 a 2006. Prácticamente toda la capa de hielo muestran algunos cambios, con algunas zonas que reflejan la luz cercana al 20 % menos que hace una década. Data: observaciones de espectrorradiómetros de imágenes de resolución moderada (MODIS), instrumentos de Terra de NASA y satélites Aqua.
Foto aérea mostrando una sección de banquisa. The lighter blue areas are melt ponds and the darkest areas are open water, both have a lower albedo than the white sea ice. The melting ice contributes to ice-albedo feedback.

When ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues. During times of global cooling, additional ice increases the reflectivity which reduces the absorption of solar radiation which results in more cooling in a continuing cycle.[42] Considered a faster feedback mechanism.[43]

Medición de la banquisa 1870-2009 en el hemisferio norte en millones de kilómetros cuadrados. El sombreado azul indica la era presatélite, con datos menos fiables. En particular, el nivel casi constante en otoño hasta 1940 refleja falta de datos, en lugar de una verdadera falta de variación

El cambio de albedo es también la razón principal de porqué el IPCC predice Tº polares en el hemisferio norte subiendo el doble a lo que sucederá en el resto del mundo, en un proceso conocido como amplificación polar. En septiembre de 2007, the Arctic sea ice area reached about half the size of the average summer minimum area between 1979 to 2000.[44] [45] Also in September 2007, Arctic sea ice retreated far enough for the Northwest Passage to become navigable to shipping for the first time in recorded history.[46] The record losses of 2007 and 2008 may, however, be temporary.[47] Mark Serreze of the US National Snow and Ice Data Center views 2030 as a "reasonable estimate" for when the summertime Arctic ice cap might be ice-free.[48] The polar amplification of global warming is not predicted to occur in the southern hemisphere.[49] The Antarctic sea ice reached its greatest extent on record since the beginning of observation in 1979,[50] but the gain in ice in the south is exceeded by the loss in the north. The trend for global sea ice, northern hemisphere and southern hemisphere combined is clearly a decline.[51]

Ice loss may have internal feedback processes, as melting of ice over land can cause eustatic sea level rise, potentially causing instability of ice shelves and inundating coastal ice masses, such as glacier tongues. Further, a potential feedback cycle exists due to earthquakes caused by isostatic rebound further destabilising ice shelves, glaciers and ice caps.

The ice-albedo in some sub-arctic forests is also changing, as stands of larch (which shed their needles in winter, allowing sunlight to reflect off the snow in spring and fall) are being replaced by spruce trees (which retain their dark needles all year).[52]

Feedback del vapor de agua[editar]

Si la atmósfera se calienta, la saturación de la presión de vapor se incrementa, y la cantidad de vapor de agua en la atmósfera tiende a aumentar. Since water vapor is a greenhouse gas, the increase in water vapor content makes the atmosphere warm further; this warming causes the atmosphere to hold still more water vapor (a positive feedback), and so on until other processes stop the feedback loop. The result is a much larger greenhouse effect than that due to CO2 alone. Although this feedback process causes an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.[39] Climate models incorporate this feedback. Water vapor feedback is strongly positive, with most evidence supporting a magnitude of 1,5 a 2,0 W/m2/K, sufficient to roughly double the warming that would otherwise occur.[53] Considered a faster feedback mechanism.[43]

Negativos[editar]

Ciclo del carbono[editar]

Principio de Le Chatelier[editar]

Siguiendo el principio de Le Châtelier, el equilibrio químico del ciclo del carbono de la Tierra cambiarán en respuesta a antropogénicas emisiones de CO2. The primary driver of this is the ocean, which absorbs anthropogenic CO2 via the so-called solubility pump. At present this accounts for only about one third of the current emissions, but ultimately most (~75%) of the CO2 emitted by human activities will dissolve in the ocean over a period of centuries: "A better approximation of the lifetime of fossil fuel CO2 for public discussion might be 300 years, plus 25% that lasts forever".[54] However, the rate at which the ocean will take it up in the future is less certain, and will be affected by stratification induced by warming and, potentially, changes in the ocean's thermohaline circulation.

Temperización química[editar]

La meteorización química en periodos geológicos extensos actúa en remover CO2 de la atmósfera. La biosecuestración también captura y almacena CO2 por procesos biológicos. La formación de conchas por biomineralización de parte de organismos en el océano, over a very long time, removes CO2 from the oceans.[55] The complete conversion of CO2 to limestone takes thousands to hundreds of thousands of years.[56]

Productividad primaria neta[editar]

Net primary productivity changes in response to increased CO2, as plants photosynthesis increased in response to increasing concentrations. However, this effect is swamped by other changes in the biosphere due to global warming.[57]

Lapse rate[editar]

The atmosphere's temperature decreases with height in the tropósfera. Since emission of infrared radiation varies with temperature, longwave radiation escaping to space from the relatively cold upper atmosphere is less than that emitted toward the ground from the lower atmosphere. Thus, the strength of the greenhouse effect depends on the atmosphere's rate of temperature decrease with height. Both theory and climate models indicate that global warming will reduce the rate of temperature decrease with height, producing a negative lapse rate feedback that weakens the greenhouse effect. Measurements of the rate of temperature change with height are very sensitive to small errors in observations, making it difficult to establish whether the models agree with observations.[58] [59]

Blackbody radiation[editar]

As the temperature of a black body increases, the emission of infrared radiation back into space increases with the fourth power of its absolute temperature according to Stefan–Boltzmann law.[60] This increases the amount of outgoing radiation as the Earth warms. The impact of this negative feedback effect is included in global climate models summarized by the IPCC.

Véase también[editar]

Referencias[editar]

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  56. Prologue: The Long Thaw: How Humans Are Changing the Next 100,000 Years of Earth's Climate by David Archer
  57. Por favor, pon la referencia que aparece aquí.
  58. National Research Council Panel on Climate Change Feedbacks (2003) (Limited preview). Understanding climate change feedbacks. Washington D.C.: National Academies Press. ISBN 9780309090728. http://books.google.com/?id=X13aXHpni2MC&printsec=frontcover. 
  59. http://geotest.tamu.edu/userfiles/216/dessler09.pdf
  60. Yang, Zong-Liang. «Chapter 2: The global energy balance». University of Texas. Consultado el 15-02-2010.

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