Richard Wood1, Matthew Collins1, Jonathan Gregory1,2, Glen Harris1 and
Michael Vellinga1
1Hadley Centre for Climate Prediction and Research, Met Office, Exeter, UK
2NERC Centre for Global Atmospheric Modelling, Reading, UK

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The possible shutdown of the Atlantic Ocean Thermohaline Circulation (THC) has attracted considerable attention as a possible form of dangerous climate change. We review evidence for and against three common assertions, which imply that THC shutdown could pose particular problems for adaptation: first, associated climate changes would be in the opposite direction to those expected from global warming; secondly, such changes could be rapid (timescale one or two decades); and thirdly the change could be irreversible. THC shutdown is generally considered a high impact, low probability event. Assessing the likelihood of such an event is hampered by a high level of mod elling uncertainty. One way to tackle this is to develop an ensemble of model projections which cover the range of possible outcomes. We present early
results from a coupled GCM ensemble, demonstrating the feasibility of this approach, and discuss prospects for a more objective THC risk assessment in future.
1. Review of current knowledge
1.1 Impact of the THC on climate
The THC, or more precisely the meridional overturning circulation (MOC), transports around 1015W of heat northwards in the North Atlantic [1]. This heat is lost to the atmosphere northwards of about 24°N, and represents a substantial heat source for the northern hemisphere climate. The impact of this heat transport on the atmosphere has been estimated using coupled climate models. The THC can be artificially suppressed in such models by adding large amounts of fresh water to the North Atlantic to suppress deep water formation there [e.g. 2,3,4]. The resulting climate response varies in detail between models, but robust features include substantial cooling of the northern hemisphere (strongest in regions close to the
North Atlantic) and major changes in precipitation, particularly in regions bordering the tropical Atlantic.
Impacts of THC shutdown on net primary production of carbon by terrestrial vegetation are shown in Fig. 1. General cooling and drying of the Northern Hemisphere results in a reduction of 11% in hemispheric primary production. Regionally, changes are larger, and in some regions current vegetation types become unsustainable, leading to large scale ecosystem change [5].
While downscaling of the impacts of THC shutdown from global models to local scale has not been widely performed as yet, and model estimates vary in detail, there is sufficient evidence that the impacts of such a shutdown would be substantial. Fig. 2 shows the modelled effect on surface temperature of a hypothetical THC shutdown in 2049, after following a scenario of global warming up to that point [6]. We see that around the North Atlantic, the cooling effect of the THC change more than outweighs the effects of global warming, leading to a net cooling relative to the preindustrial climate in those regions. The resulting climate in the UK, for xample, would be substantially colder than that experienced during the ‘Little Ice Age’ of the 17th and 18th Centuries. It should be stressed that this is a ‘what if?’ scenario, and the model does not predict that this would actually occur.
Fig. 1: Change in net primary productivity (kg carbon per m2 per year) when the THC is artificially turned off in the HadCM3 climate model [4]. Reductions are seen over Europe (-16%), Asia (-10%), the Indian subcontinent (-36%) and Central America (-106%). The latter
figure implies that present vegetation types would become unsustainable and large scale ecosystem adjustment could be expected [5].
Fig. 2: Change in surface air temperature (°C) relative to preindustrial values, in a HadCM3 experiment in which the THC is artificially turned off in 2049, after following the IS92a greenhouse gas emission scenario up to that point [6]. Note that this is a ‘what if?’ scenario; the model does not actually predict a THC shutdown at that time.
1.2 Past rapid climate changes
A number of palaeoclimatic records point to the occurrence of rapid changes in the past. Particular events, which have been argued to show spatial coherence over a wide region, include the Dansgaard- Oeschger events during glacial periods, and more recently the so-called ‘8.2 kbp cold event’, seen in Greenland ice cores and other proxies. These events appear to have timescales of decades or even shorter, and their amplitudes are well in excess of variability seen in the later Holocene (last 8000 years). A prima facie case has been made for a link between these events and major reorganisations of the THC. See [21] for a review of the palaeoclimatic evidence.
1.3 Can the present THC exhibit multiple equilibria and rapid change?
The climatic state of the late Holocene (last few thousand years) is substantially different from the state during glacial or early post-glacial periods, when ice sheets and sea ice covered much of the northern high latitudes, allowing for strong ice-albedo feedback and potential for substantial fresh water input to the North Atlantic through ice melt. Since there is  no evidence of any O(1) changes in the THC over the past 8000 years at least, it needs to be asked whether the present (and likely future) climate states do in fact have a potential for THC shutdown.
Many simpler climate models, ranging from the box model of [7] to climate models of intermediate complexity [8,9], suggest that the present climate state may possess an alternative mode of operation with the THC weaker or absent. In many such studies increased greenhouse gas forcing can take the system beyond some threshold, after which only the ‘THC off’ state is stable. In that case, even if greenhouse gas forcing is returned to present day values, the THC remains off. Once the threshold is passed, the THC shutdown is effectively irreversible. Since the evidence for such hysteresis behaviour is largely based on more or less simplified models, it is important to ask whether such bistable behaviour exists in the most comprehensive climate models used to make climate projections (GCMs). The computational cost of coupled GCMs prohibits a complete exploration of the hysteresis curve. Experimentation has therefore concentrated on applying a temporary perturbation (usually a fresh water flux) to the models in order to turn off the THC. In most cases when the perturbation is removed, the THC recovers, implying that a stable ‘THC off’ state has not been found in that model (though it may nevertheless exist) [10-12]. However, a stable ‘THC off’ state has been demonstrated in two models [11,13]. A number of factors have been proposed as influencing the stability of the ‘off’ state, including ocean mixing [11], atmospheric feedbacks through wind stress [10] and the hydrological cycle [10,12,14]. At present it is not possible to say definitively from these model studies whether the present day THC is bistable, or whether there is a threshold beyond
which irreversible shutdown would occur.
1.4 Why is modelling the future THC so difficult?
The current state of uncertainty in modelling the future behaviour of the THC can be illustrated by comparing the THC response of a number of different climate models used in the IPCC 3rd Assessment Report, under a common greenhouse gas forcing scenario ([15], see Fig. 9.21). Under this scenario, the models suggest changes in the maximum strength of the overturning circulation, ranging from a slight strengthening to a weakening of around 50%. Even two models which show a similar response can be shown to obtain that response for different reasons, dominated in one case by thermal forcing and in the other by fresh water forcing ([15], Fig. 9.22). The response is likely to be the net result of a number of positive and negative  feedbacks. To obtain the correct net outcome it may be necessary to model each of the key feedbacks quite accurately.
Simplified models which do show the THC crossing a threshold suggest that near the threshold
predictability becomes very poor, i.e. even if we could accurately determine that the THC was near a threshold, it could be difficult to predict the timing of a shutdown (e.g. [16], [17]). At present it is not possible to identify a ‘safe’ CO2 stabilisation level that would prevent THC shutdown. It is possible that the rate of
CO2 increase, as well as the final concentration, may determine the outcome [18].
1.5 Summary: where are we now?
Comprehensive GCM climate projections suggest that the most likely response of the THC to global warming over the next century is a slowdown of around 0-50%. No models have shown a complete shutdown, or a net cooling over land areas. Hence a shutdown during the 21st Century is regarded as unlikely. Nonetheless a range of theoretical, modelling and and palaeoclimate studies shows that large, rapid changes are a possibility.
To produce a risk assessment for THC shutdown requires an understanding of both the impacts of a shutdown and the probability of occurrence. The evidence of 1.1 above points to substantial impacts (although these have not been assessed in detail). However, little can currently be said about the probability, except that it is subjectively considered low during the 21st Century, based on the results of section 1.4. To work towards a more quantitative probabilistic assessment, including information about ‘safe’ stabilisation levels, requires further development of models and methods. Some promising progress has recently been made towards this goal, and this is described in section 2 below.
2. Towards quantifying and reducing uncertainty in THC projections
2.1 Understanding what drives THC changes
The first step to reducing uncertainty is to understand the processes which contribute to the wide range of THC responses currently seen in models. A recent international initiative under the auspices of the Coupled Model Intercomparison Project (CMIP) addresses this goal by analysing a number of climate models, all subject to a number of standardised forcing experiments. Fig. 3 shows the roles of heat and water forcing in the response of the THC to 1% p.a. CO2 increase, across this range of models. The large variation in the forcing processes is apparent, although it can be seen that in all models except one the heat forcing dominates the fresh water forcing over the timescale of this experiment. More detailed analysis is required to obtain a full picture of the processes determining the THC response in each model (e.g. [18]),
but we can expect this process eventually to allow a good understanding to be developed of why the model responses are so different. This in turn will suggest targeted observational constraints than can be used to determine how much weight to give to particular model THC projections.
Fig. 3: Contributions of changes in thermal and fresh water forcing to the total THC change, following a 1% per annum CO2 increase up to four times the initial concentration, in a range of climate models. Changes are expressed as a fraction of the THC strength in the control run. The dashed line divides the regions where thermal and haline forcing dominate. Data courtesy of partners in the CMIP coordinated experiment on THC stability.

2.2 Probabilistic estimation of the future THC
Some uncertainty will inevitably remain, and in order to obtain some form of objective assessment of the likelihood of major THC changes, it will be necessary to sample the range of possible model outcomes more systematically than is possible using the few model runs shown in [15] or in Fig.3. Recent progress has been made in this area by generating ‘perturbed physics’ model ensembles (e.g. [19,20]). An ensemble of models is generated by varying a set of model parameters within a defined range. The parameter settings are chosen from a prior distribution based on expert judgement about reasonable allowable ranges.
Climate projections made using each ensemble member may then be weighted according to some chosen set of observational constraints [20], or the ensemble may be generated in such a way as to ensure a predefined goodness of fit to the observations [19]. Studies to date have used either highly simplified models [19] or atmosphere-only GCMs [20]. Here we demonstrate the feasibility of generating a coupled GCM ensemble which can exhibit a range of THC responses to a given forcing. We use an existing ensemble of atmosphere-only model runs using the HadAM3 atmospheric model [20] to generate a set of atmospheric model parameters that are likely to result in a range of different THC responses, based on detailed analysis of the coupled model HadCM3 (with standard parameter settings) [18]. An ensemble of coupled models is thus produced, and a range of THC responses can be seen. The problem of climate drift in the coupled models is overcome by one of two methods: either flux adjustment or pre-selection of parameter settings to minimise climate drift without using flux adjustment. The latter pre-selection is made on the basis of global heat budgets in the atmosphere-only ensemble. Early results show that a range of THC responses can be produced. The ensemble will now be expanded to cover as wide a region of parameter space as possible, thus allowing a plausible range of THC behaviour to be quantified. The longer term goal is to add a range of models to the ensemble (thus transcending structural constraints of any given model). This should include a spectrum of models, including compu tationally cheaper models to allow
thorough exploration of a wide parameter space (including a plausible range of stabilisation scenarios). This will allow for the first time an objective estimate of the likelihood of major THC change and identification of ‘safe’ stabilisation pathways.
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El deshielo del Ártico podría hacer que el calentamiento global sea peor


El deshielo del Ártico está liberando grandes cantidades de metano. ¿Cuán grande es la amenaza de efecto invernadero? ¿Qué puede hacerse?

Por Sarah Simpson

A young scientist with curly, reddish hair tucked beneath a knit cap stepped gingerly onto the three-day-old ice of a remote lake in northeastern Siberia. Coating the black depths like cellophane, the thin film held no promise to bear her weight, but a sudden dunk in the frigid water was a risk she had to take. Searching the lake by rickety rowboat all summer had failed, and any day winter’s first big snow would engulf the region, obscuring the lake’s surface until spring. She could not afford to wait that long.

The woman shivered in her worn, blue down jacket and glanced up at the overcast sky. After one more cautious step, she spotted her quarry: a cluster of platter-size bubbles frozen into the ice. Those pockets of gas, which had risen from thawing permafrost—formerly frozen soil—at the lake’s bottom, were the aim of her doctoral research. Long elusive, they suddenly stood out like white stars against a night sky, though less serenely. With a small pick she cracked the icy skin of one of the bubbles and remained unfazed when it hissed back like a punctured gas pipe. Leaning forward, she apprehensively struck a match just above the broken bubble and flames as high as her head burst skyward. The flammable substance was methane, a greenhouse gas that could cause more global warming than carbon dioxide (CO2).
Today, nearly seven years after igniting that first bubble, Katey Walter finds herself center stage in an environmental drama playing out across the frozen north. Now a 33-year-old assistant professor at her alma mater, the University of Alaska–Fairbanks, Walter was the first to explain the mysterious methane emissions from Arctic lakes. She isn’t shy about touting their significance as a ticking time bomb. In a complete Arctic thaw, these lakes could discharge a whopping 50 billion tons of methane: 10 times the amount already helping to heat the planet.

Whether a total or more moderate release is in store is still anyone’s guess. But pound for pound, methane in the atmosphere traps 25 times more of the sun’s heat than CO2 does. Consequently, even a modest thaw of the perennially frozen soil that lies under these ephemeral lakes and caps the dry land around them could trigger a vicious cycle: warming releases methane and creates lakes, which thaw permafrost and liberate more gas, which intensifies warming, which creates more lakes, and so on. Some Arctic lakes are growing larger, and researchers are eyeing them suspiciously as a reason why global methane concentrations shot up in 2007 and have stayed high ever since. Other signs indicate that permafrost thawing on the Arctic seafloor may be loosening the cap on large pockets of methane stored deeper down.

Walter is sounding the alarm even louder than before because global warming is taking a special toll across the far north. The region is heating up twice as quickly as the rest of the globe, rapidly melting sea ice in the Arctic Ocean as well as the permafrost, which underlies 8.8 million square miles of the Northern Hemisphere. Leading climate models already suggest greenhouse warming as a result of most of the Arctic’s permafrost thawing by 2100—and the estimates do not yet include the potentially vast additional warming imparted by methane bubbling up out of chilly waters. Walter and others are trying to determine just how much methane could be released into the atmosphere, how soon, how aggressively that release would accelerate the earth’s warming and whether anything can be done to temper the escalating threat.

Burps and Belches
Scientists know with great certainty how much methane is in the earth’s atmosphere at any given time from sampling its concentration weekly at dozens of sites worldwide. By plugging these measurements into global climate models, they know methane is responsible for a third of the current warming trend. Exactly how much gas comes from where is harder to say, which is why the Arctic lake bubbles were so long overlooked.

Deshielo del Ártico


El Ártico es muy sensible al cambio climático. Si bien el aumento de las temperaturas podría causar estragos en las comunidades del norte y los ecosistemas, la reducción del hielo marino y el permafrost amenaza con acelerar el cambio climático con efectos a escala mundial.

Del hielo marino
Durante el otoño y el invierno, una enorme porción del norte de congelación de los océanos, la formación de uno a tres metros de gruesa capa de hielo del mar. En la primavera y el verano gran parte de este hielo se derrite de nuevo. El hielo marino proporciona alojamiento y transporte para las focas, morsas, zorros árticos, los osos polares y los inuit. La parte inferior del hielo también proporciona una superficie en la que las algas pueden florecer, que forman la base de una cadena de comida rica que apoya el bacalao, char, beluga y narval.

A pesar de grandes fluctuaciones anuales y la lejanía de la región ártica hacer cambios en el hielo del mar difícil de medir, muchos estudios independientes han demostrado que el hielo marino del Ártico se está reduciendo. Los registros de satélites y los datos muestran que submarino zona de hielo marino se ha reducido de tres a cinco por ciento por década desde 1950. El año 2002 había menos hielo que cualquier otro año anterior desde las observaciones detalladas comenzó 50 años antes.

 "Es probable que la extensión del hielo del mar continuará disminuyendo durante el siglo 21 como el clima se calienta", dice Mark Serreze, de la Nacional de la Nieve y el Hielo Data Center en Boulder, Colorado (La NASA, diciembre 7,2002). "Podemos ver a un aproximado 20 por ciento de reducción en el hielo anual medio del mar en 2050, y entonces puede ser que el hielo no se acerca en lo absoluto durante los meses de verano".

Permanentemente congelada del suelo y la roca se encuentra debajo de casi el 25 por ciento de los del hemisferio norte en capas de hasta 1.400 metros de espesor. En gran parte de Canadá y el norte de Asia, cerca de una delgada capa de la superficie se derrite durante el verano, pero la tierra congelada debajo de una empresa proporciona todo fundamento año.

Cuando el permafrost se derrite, el suelo pierde su red de apoyo de cristales de hielo. Esto hace que la tierra sobre el colapso, provocando hundimientos y deslizamientos de tierra que pueden desestabilizar a los edificios, carreteras y otras infraestructuras. Aunque gran parte del Ártico está escasamente poblada, hay grandes minas, campos petroleros, oleoductos, pistas de aterrizaje y - en Rusia - una estación de energía nuclear, todos descansando en el permafrost. La mayoría de estas estructuras, diseñadas después de la década de 1940, fueron diseñados para confiar en la estabilidad del permafrost debajo. Como los cambios de suelo, pueden derrumbarse, derramando sustancias químicas peligrosas al medio ambiente y, a veces amenazando vidas humanas.

Permafrost continuarán a derretirse lentamente durante siglos como el ártico se calienta, se retira cada vez más cerca del polo. Desafortunadamente, su retirada también podría generar miles de millones de toneladas de gases de efecto invernadero.

Efectos Snowballing
El permafrost y el hielo marino dos tienen efectos de enfriamiento en el clima. Debido a que el hielo del mar es de color blanco refleja la luz lejos de la superficie de la tierra, de mantenimiento de que se enfríe. Menos hielo significa que más calor será absorbido por el océano, que además previene la formación de hielo marino, y agrava el cambio climático.

Mientras tanto, el permafrost constituye un congelador enorme ',' lleno de materia vegetal no descompuestas, uno de los mayores reservorios de carbono en la tierra. Medida que el permafrost se derrite, que la materia de la planta comienza a pudrirse, la liberación de enormes cantidades de metano - un gas de efecto invernadero 21 veces más potente que el dióxido de carbono. Muchas zonas de permafrost contienen material vegetal suficiente para mantener la generación de metano durante siglos, una vez que comienza el calentamiento.
Arctic Climate Impact Assessment
En noviembre de 2004, se publicó el estudio más extenso y autoridad en el mundo sobre el cambio climático en el Ártico. Escrito por más de 250 científicos, la Evaluación del Impacto Climático en el Ártico http://www.acia.uaf.edu/> Encontró que el cambio climático ya está afectando profundamente a las comunidades en el norte de Canadá.

Algunas de las principales conclusiones del informe incluyen:

• El clima del Ártico ya está calentando rápidamente y las temperaturas promedio se prevé que aumentará entre 3 y 9 grados centígrados en los próximos 100 años
• El retroceso de los hielos marinos reducirá el hábitat de especies emblemáticas como el oso polar, las morsas y las focas y el impacto del hábitat de las aves y otras especies marinas.
• terreno del deshielo del permafrost interrumpirá el transporte, los edificios, la extracción de recursos y otras infraestructuras,
• Los niveles elevados de radiación ultravioleta afectarán a personas, plantas y otros animales salvajes
• inuit y otros pueblos indígenas se enfrentan a grandes impactos económicos y culturales derivados de la pérdida de hábitats de caza y pesca
• Los efectos de otros factores como la contaminación química (ya presentes en niveles peligrosos en el pescado y la vida silvestre del Ártico), junto con los derrames de petróleo y la pesca pueden interactuar para provocar una serie de otros efectos aún desconocidos.

Climate change will devastate Africa, top UK scientist warns

Professor Sir Gordon Conway warns continent will face intense droughts, famine, disease and floods


John Vidal
Environment editor guardian.co.uk, Wednesday 28 October 2009 17.41 GMT 

One of the world's most influential scientists has warned that climate change could devastate Africa, predicting an increase in catastrophic food shortages.

Professor Sir Gordon Conway, the outgoing chief scientist at the UK's Department for International Development, and former head of the philanthropic Rockefeller Foundation, argued in a new scientific paper (pdf) that the continent is already warming faster than the global average and that people living there can expect more intense droughts, floods and storm surges.
There will be less drinking water, diseases such as malaria will spread and the poorest will be hit the hardest as farmland is damaged in the coming century, Conway wrote.

"There is already evidence that Africa is warming faster than the global average, with more warm spells and fewer extremely cold days. Northern and southern Africa are likely to become as much as 4C hotter over the next 100 years, and [will become ] much drier," he said.

Conway predicts hunger on the continent could increase dramatically in the short term as droughts and desertification increase, and climate change affects water supplies. "Projected reductions in crop yields could be as much as 50% by 2020 and 90% by 2100," the paper says.
Conway held out some hope that east Africa and the Horn of Africa, presently experiencing its worst drought and food shortages in 20 years, will become wetter. But he said that the widely hoped-for 8-15% increase in African crop yields as a direct result of more CO2 in the atmosphere may fail to materialise.

"The latest analyses of more realistic field trials suggest the benefits of carbon dioxide may be significantly less than initially thought," he said.
Instead, population growth combined with climate change would mean countries face extreme problems growing more food: "We are going to need an awful lot more crop production, 70-100% more food will be needed than we have at present. Part of [what is needed] is getting more organic matter into Africa's soils, which are very depleted, but we also have to improve water availability and produce crops that yield more, and use nitrogen and water more efficiently."

Sir Gordon, now professor of international development at Imperial College London, oversaw a major expansion in the UK government's support for GM research in developing countries, and said that new technologies must be part of the African response to tackling hunger and droughts. "In certain circumstances we will need GM crops because we wont be able to find the gene naturally. GM may be the speediest and most efficient way to increase yields. Drought tolerance is governed by a range of genes. It is a big problem for breeders of [both] GM and ordinary plants", he said.

He called for more research into climate change. "There is much that we do not know. The Sahel may get wetter or remain dry. The flow of the Nile may be greater or less. We do not know if the fall in agricultural production will be very large or relatively small. The best assumption is that many regions of Africa will suffer more droughts and floods with greater intensity and frequency. We have to plan for the certainty that more extreme events will occur in the future but with uncertain regularity".

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Secretario de Relaciones Exteriores, David Miliband acusa a la apatía del público sobre el cambio climático


El Secretario de Relaciones Exteriores acusó ayer al público de la falta de un sentido de urgencia frente a las consecuencias potencialmente devastadoras del cambio climático.

David Miliband, dijo que la gente se había vuelto apática acerca de la cuestión en que tenían que estar concienciadas en acción ante el cambio climático,  para la Cumbre de Copenhague en diciembre.

"Para mucha gente la moneda no ha caído y, sin embargo, este desafío del cambio climático es real y está ocurriendo ahora", dijo. "Todavía no hay esa sensación de urgencia y de unidad y de la animación sobre la conferencia de Copenhague".

Miliband y su hermano, Ed Miliband, Secretario de Cambio Climático, abríaron una exposición en el Museo de la Ciencia en South Kensington, diseñado para ilustrar el impacto potencial de la temperatura mundial, por medio de 4C. Los modelos actuales predicen que esto podría suceder de aquí a 2060 si no se toman medidas.

Miliband advirtió de agua y la escasez de alimentos, las migraciones masivas y conflictos. Un mapa presentado en la exposición mostró aumento de las temperaturas de hasta 15C en el Ártico, las mareas de tormenta golpear la costa este de Gran Bretaña y los incendios forestales y la sequía en Europa.

Ed Miliband, dijo que sólo el 18 por ciento de la población cree que el cambio climático podría afectar a sus hijos.

Defendió al Gobierno contundentemente porla difusión de información pública para promover la Ley del Gobierno sobre la iniciativa de CO2.

La Agencia de Normas de Publicidad está investigando las quejas que ha habido ya que, el anuncio de tres minutos, que muestra una secuencia de dibujos animados de las calles bajo el agua y los animales y las personas que se ahogan, es perturbador para los niños.

Sin embargo, dijo que también era vital para dar a la gente una visión positiva de un futuro de bajo carbono. "Si Martin Luther King había llegado a lo largo y dijría:" Tengo gente en una pesadilla, 'no le han seguido ", dijo.

Chris Rapley, el director del Museo de la Ciencia, dijo que una decisión de última hora se han hecho para crear la exposición en agosto después de una reunión informativa en el Departamento de Energía y Cambio Climático.

"Nos dimos cuenta de que el interés público se había aplastado y sin embargo aquí nos acercábamos a las negociaciones con más historia en la historia humana", dijo. El museo no se había previsto ejecutar una exposición el cambio climático hasta 2011.

La exposición interactiva permite a los participantes a considerar el cambio climático desde la perspectiva de los diferentes actores involucrados en las negociaciones de Copenhague, entre ellos un científico del clima, un expertos en derechos humanos y economista.


El vacío más grande en la Tierra?


Robert B. Gagosian
Un hito en el esfuerzo por racionalizar la política nacional sobre los océanos se alcanzará el 21 de mayo de 2004, cuando termina el periodo de comentarios de 50 gobernadores de los estados que están revisando el informe preliminar de la Comisión de Estados Unidos sobre Política Oceánica (http://www.oceancommission.gov).
El informe, publicado el 20 de abril, es el primer examen amplio de las cuestiones marinas en más de 30 años, motivado por el reconocimiento de que nuestros enfoques de política de los océanos no están trabajando a día de hoy. El informe fue encargado por el Congreso, con miembros de la comisión designada por el presidente. Algunos de los problemas identificados por los componentes académicos, gubernamentales y comerciales de la Comisión no son nuevas ni necesariamente exclusivo de los Estados Unidos: la sobrepesca, la erosión de las costas, la contaminación y floraciones de algas nocivas. Otros, como las zonas muertas y la introducción de especies invasoras, están creciendo en importancia. Estos problemas, agravados por la población costera, requieren nuevas estrategias y más y mejor información.
La función principal del informe es proporcionar a la falta de estrategias y el marco de la política de nuestra agenda nacional de los océanos. Desde una perspectiva científica y de investigación, el informe llega demasiado pronto. Los océanos cubren el 70% de la superficie de la Tierra, regulan el clima y la variabilidad del clima, y proporcionan la mayor parte del oxígeno del planeta. Desafortunadamente, la financiación de la investigación oceanográfica en los Estados Unidos no ha seguido el ritmo de la demanda de conocimiento del océano. Europa lidera ahora en importantes áreas de investigación del clima, el Japón está construyendo un barco de investigación capaz de perforación y muestreo en el manto superior, y Corea del Sur es una presencia emergente en el ámbito de los vehículos submarinos autónomos.

La Comisión de Política Oceánica no sólo se reconoce la importancia de mejorar la contribución de los EE.UU. a la investigación internacional y los esfuerzos de vigilancia dirigidos a este recurso mundial, sino que indica que nuestra falta de comprensión de cómo los océanos afectan específicamente a los Estados Unidos (y viceversa) puede la base de nuestra política de vacío más grande en la actualidad. El informe preliminar hace hincapié en la investigación como la base de la política de información, como requisito previo para mejorar la predicción, y como el vivero de la innovación técnica que puede dar dividendos comerciales. Las recomendaciones incluyen por tanto una duplicación de los océanos y el gasto federal de investigación costera en los próximos 5 años a US $ 1,3 mil millones por año, además de la financiación anual de $ 650 millones a US $ 750 millones para un Sistema Integrado de Observación de los Océanos, un fondo de modernización de los buques y las infraestructuras y el desarrollo de 5-año para los planes de la ciencia avanza a largo plazo en las ciencias del mar básica y aplicada y la tecnología.
La inversión propuesta en las tecnologías del océano es especialmente importante, ya que podría poner a la comunidad de investigación al alcance de los EE.UU. frente a los retos actuales, tales como la predicción del medio ambiente. Nuestra capacidad para predecir los cambios ambientales en la escala de horas a décadas no sólo ayudaría a salvar vidas, sino también miles de millones de dólares en bienes y daños a los cultivos. Sobre la predicción de los océanos ha sido limitada, en parte, por una simple falta de datos. Hemos visto que con el desarrollo de observatorios oceánicos de todo el mundo, esto está empezando a cambiar.
Nacido de los avances en electrónica, sensores, comunicaciones por satélite, y la resistencia de la batería, los observatorios son plataformas equipadas con instrumentos de medida que continuamente diversas propiedades del mar en tiempo buenas y malas. Que flotan con las corrientes, se encuentran en el lecho marino, o flotar entre dos aguas o en la superficie. Algunos son autopropulsados robots que nadan en el mar durante meses y telémetro sus datos en tierra. Una red de observatorios tales significará más ricos, los datos más continua que proporcionará una nueva comprensión de las condiciones que afectan a la pesca y ayudar a los cambios en tiempo completo y largo plazo del cambio climático. Se iluminan los patrones migratorios de los mamíferos marinos, las razones de la sequía o las inundaciones, y el destino y los efectos a largo plazo de los contaminantes. Inmediatamente a detectar la aparición de tsunamis, terremotos submarinos, erupciones volcánicas, y el clima extremo en el mar, la mejora de la predicción de sus efectos devastadores en el mar y en tierra. Por ejemplo, el valor económico de la predicción exacta de El Niño / Oscilación del Sur fenómenos EE.UU. por sí solo a la agricultura se estima en medio billón de dólares al año en mejorar las decisiones de cultivo.
Si las recomendaciones de la comisión se respeten y estas iniciativas se llevan a cabo, nuestro país estará en un nuevo curso, no sólo de la comprensión de nuestros océanos, sino en la conservación, gestión y utilizarlos sabiamente.

Robert B. Gagosian es presidente y director de la Woods Hole Oceanographic Institution en Woods Hole, Massachusetts, y un miembro de la Comisión de Política Oceánica de EE.UU. Science Advisory Panel.

El calentamiento del planeta podría sumir a Norteamérica y a Europa en una congelación profunda


Esta es una teoría que va ganando credibilidad entre muchos científicos que estudian el clima. La descongelación del hielo marino que cubre el Ártico podría alterar o incluso detener las grandes corrientes del Océano Atlántico. Sin el inmenso calor que proporcionan estas corrientes marinas — comparables a la producción de energía de un millón de centrales nucleares — la temperatura media europea podría descender de 5 a 10 grados centígrados (9 a 18 grados Fahrenheit), y algunas zonas de Norteamérica se enfriarían sólo un poco menos. Este cambio en la temperatura sería similar a las temperaturas medias del planeta hacia el final de la última era glacial, hace aproximadamente 20.000 años.

Algunos científicos creen que este cambio en las corrientes marinas puede surgir pronto, de un modo inesperado — en un período de tiempo tan corto como de 20 años — según Robert Gagosian, presidente y director de la Institución Oceanográfica Woods Hole. Otros dudan que esto llegará a ocurrir. Aun así, el Pentágono ha tomado nota. Andrew Marshall, un planificador veterano del Ministerio de Defensa, presentó recientemente un informe no confidencial que describía cómo un cambio en las corrientes marinas en el futuro próximo podría comprometer la seguridad nacional.
“Es difícil predecir qué pasará realmente”, advierte Donald Cavalieri, científico pricipal en el Centro Goddard de Vuelos Espaciales de NASA, “puesto que el Ártico y el Atlántico Norte son sistemas muy complejos, con muchas interacciones entre la tierra, el mar y la atmósfera”. Los resultados de investigaciones recientes, sin embargo, sugieren que los cambios que estamos viendo en el Ártico podrían afectar potencialmente a las corrientes que calientan Europa del Este, y este hecho mantiene a mucha gente preocupada.
El hielo es la clave
Existen varios satélites que día y noche vigilan la capa de hielo del Ártico. El satélite Aqua de NASA, por ejemplo, transporta un sensor construido por los japoneses llamado Radiómetro avanzado de barrido en microondas-EOS (Advanced Microwave Scanning Radiometer-EOS, AMSR-E). “Utilizando microondas en vez de luz visible, el AMSR-E puede penetrar las nubes y ofrecer vigilancia ininterrumpida del hielo, incluso de noche”, explica Roy Spencer, el investigador jefe del instrumento en el Centro de Hidrología y Clima Mundial en Huntsville, Alabama. Otros satélites que vigilan el hielo, dirigidos por NASA, NOAA y el Ministerio de Defensa, usan una tecnología similar.

Arriba: La circulación global oceánica entre aguas frías y profundas y aguas cálidas y superficiales influye enormemente en los climas regionales de todo el mundo.   Imagen cortesía del Laboratorio Nacional de Argonne.

La vista desde la órbita muestra claramente un descenso a largo plazo del hielo “eterno” del Océano Ártico (la parte que permanece congelada durante los meses cálidos de verano). Según un informe de 1992, de Josefino Comiso, científico del Clima en el Centro Goddard de Vuelos Espaciales de NASA, ese hielo ha estado disminuyendo desde el comienzo de las observaciones satelitales en 1978, a un promedio de un 9% por década. Los estudios con base en datos más recientes sitúan el índice en un 14% por década, sugiriendo que la desaparición del hielo del Océano Ártico se está acelerando.
Algunos científicos temen que el hielo que se funde en el Océano Ártico pueda verter una cantidad de agua dulce al Atlántico Norte suficiente como para interferir con las corrientes marinas. Parte de esta agua dulce procedería de la propia masa de hielo que se derrite, pero el principal contribuyente sería el aumento creciente de lluvia y nieve en la región. La capa de hielo que se contrae deja al descubierto una cantidad mayor de superficie oceánica, permitiendo que una mayor cantidad de humedad se evapore en la atmósfera y dé lugar a un mayor número de precipitaciones.
Debido a que el agua salada es más densa y pesada que la dulce, este “endulzamiento” del Atlántico Norte haría las capas superficiales más livianas o boyantes. Y esto es un problema, ya que el agua de la superficie necesita hundirse para impulsar un modelo primario de circulación oceánica conocido como el “Gran cinturón transportador”. El agua que está a un nivel bajo con respecto a la superficie fluye a través del suelo oceánico hacia el ecuador, mientras que las aguas superficiales cálidas de las latitudes tropicales fluyen hacia arriba para reemplazar al agua que se hunde. De esta manera el transportador se mantiene activo. Un aumento en la cantidad de agua dulce podría evitar el hundimiento de las aguas superficiales del Atlántico Norte, disminuyendo o deteniendo esta circulación.
¿Déjà Vu?
Aunque una vez fue impensable, hoy en día la noción de que el clima puede cambiar rápidamente se está convirtiendo en una teoría respetable. En un informe, de 2003, Robert Gagosian cita una “evidencia que avanza rápidamente (desde, por ejemplo, los anillos de los árboles y los núcleos del hielo) de que el clima de la Tierra cambió abrupta y enormemente en el pasado”. Por ejemplo, mientras que el mundo se calentó al final de la última era glacial hace aproximadamente 13.000 años, las capas de hielo derretido parecían haber provocado un alto repentino en el transportador, devolviendo el mundo a un período de 1.300 años de condiciones tipo era glacial llamado “Younger Dryas”.
¿Ocurrirá de nuevo? Los investigadores están intentando averiguarlo por todos los medios.
El 13 de febrero partió una expedición desde Gran Bretaña con el objetivo de colocar sensores de control en el Océano Atlántico que observarán la corriente del Golfo en busca de signos de que su velocidad ha disminuido. El viaje es el último paso de un proyecto de investigación conjunto entre Gran Bretaña y Estados Unidos llamado Cambio Climático Rápido, que comenzó en 2001. Otro proyecto internacional, llamado SEARCH (Estudio del cambio medioambiental en el Ártico), se inició en 2001 con el objetivo de evaluar con más detalle los cambios en el espesor del hielo marino del Ártico.
Según las simulaciones por computador realizadas por Thomas F. Stocker y Andreas Schmittner de la Universidad de Berna, mucho depende de la rapidez del calentamiento del Ártico. En sus modelos, un calentamiento más rápido podría cerrar por completo la principal corriente del Atlántico, mientras que un calentamiento más lento sólo provocaría una disminución de la velocidad de la corriente durante unos cuantos siglos.
Inevitablemente, la discusión apunta a los humanos. ¿La actividad industrial humana tiene mucho que ver con el calentamiento del Ártico? ¿Podríamos revertir la tendencia, si quisiéramos? No todos los científicos están de acuerdo. Algunos afirman que los cambios que están ocurriendo en el Ártico son consecuentes con los largos y lentos ciclos de comportamiento oceánico que la ciencia conoce. Otros ven un componente eminentemente humano.
“El derretimiento del hielo marino es consecuente con el calentamiento que hemos presenciado en el último siglo”, nota Spencer, pero “no sabemos qué porción de ese calentamiento se debe a las fluctuaciones naturales del clima y cuál a la emisión de gases de efecto invernadero”.
Si el Gran cinturón transportador se detiene de pronto, no importará la causa. Los europeos estarán pensando en otras cosas, por ejemplo, cómo hacer que crezcan cultivos en la nieve. Esta es la hora de averiguarlo, mientras el fenómeno es sólo una posibilidad escalofriante.

Failing ocean current raises fears of mini ice age


The ocean current that gives western Europe its relatively balmy climate is stuttering, raising fears that it might fail entirely and plunge the continent into a mini ice age.
The dramatic finding comes from a study of ocean circulation in the North Atlantic, which found a 30% reduction in the warm currents that carry water north from the Gulf Stream.
The slow-down, which has long been predicted as a possible consequence of global warming, will give renewed urgency to intergovernmental talks in Montreal, Canada, this week on a successor to the Kyoto Protocol.
Harry Bryden at the National Oceanography Centre in Southampton, UK, whose group carried out the analysis, says he is not yet sure if the change is temporary or signals a long-term trend. "We don't want to say the circulation will shut down," he told New Scientist. "But we are nervous about our findings. They have come as quite a surprise."

No one-off

The North Atlantic is dominated by the Gulf Stream - currents that bring warm water north from the tropics. At around 40° north - the latitude of Portugal and New York - the current divides. Some water heads southwards in a surface current known as the subtropical gyre, while the rest continues north, leading to warming winds that raise European temperatures by 5°C to 10°C.
But when Bryden's team measured north-south heat flow last year, using a set of instruments strung across the Atlantic from the Canary Islands to the Bahamas, they found that the division of the waters appeared to have changed since previous surveys in 1957, 1981 and 1992. From the amount of water in the subtropical gyre and the flow southwards at depth, they calculate that the quantity of warm water flowing north had fallen by around 30%.
When Bryden added previously unanalysed data - collected in the same region by the US government's National Oceanic and Atmospheric Administration - he found a similar pattern. This suggests that his 2004 measurements are not a one-off, and that most of the slow-down happened between 1992 and 1998.
The changes are too big to be explained by chance, co-author Stuart Cunningham told New Scientist from a research ship off the Canary Islands, where he is collecting more data. "We think the findings are robust."

Hot and cold

But Richard Wood, chief oceanographer at the UK Met Office's Hadley Centre for climate research in Exeter, says the Southampton team's findings leave a lot unexplained. The changes are so big they should have cut oceanic heating of Europe by about one-fifth - enough to cool the British Isles by 1°C and Scandinavia by 2°C. "We haven't seen it yet," he points out.
Though unseasonably cold weather last month briefly blanketed parts of the UK in snow, average European temperatures have been rising, Wood says. Measurements of surface temperatures in the North Atlantic indicate a strong warming trend during the 1990s, which seems now to have halted.
Bryden speculates that the warming may have been part of a global temperature increase brought about by man-made greenhouse warming, and that this is now being counteracted by a decrease in the northward flow of warm water.
After warming Europe, this flow comes to a halt in the waters off Greenland, sinks to the ocean floor and returns south. The water arriving from the south is already more saline and so more dense than Arctic seas, and is made more so as ice forms.

Predicted shutdown

But Bryden's study has revealed that while one area of sinking water, on the Canadian side of Greenland, still seems to be functioning as normal, a second area on the European side has partially shut down and is sending only half as much deep water south as before. The two southward flows can be distinguished because they travel at different depths.
Nobody is clear on what has gone wrong. Suggestions for blame include the melting of sea ice or increased flow from Siberian rivers into the Arctic. Both would load fresh water into the surface ocean, making it less dense and so preventing it from sinking, which in turn would slow the flow of tropical water from the south. And either could be triggered by man-made climate change. Some climate models predict that global warming could lead to such a shutdown later this century.
The last shutdown, which prompted a temperature drop of 5°C to 10°C in western Europe, was probably at the end of the last ice age, 12,000 years ago. There may also have been a slowing of Atlantic circulation during the Little Ice Age, which lasted sporadically from 1300 to about 1850 and created temperatures low enough to freeze the River Thames in London.
Journal reference: Nature (vol 438, p 655)

Slowing of the Atlantic meridional overturning circulation at 25° N


Harry L. Bryden1, Hannah R. Longworth1 and Stuart A. Cunningham1
The Atlantic meridional overturning circulation carries warm upper waters into far-northern latitudes and returns cold deep waters southward across the Equator1. Its heat transport makes a substantial contribution to the moderate climate of maritime and continental Europe, and any slowdown in the overturning circulation would have profound implications for climate change. A transatlantic section along latitude 25° N has been used as a baseline for estimating the overturning circulation and associated heat transport2, 3, 4. Here we analyse a new 25° N transatlantic section and compare it with four previous sections taken over the past five decades. The comparison suggests that the Atlantic meridional overturning circulation has slowed by about 30 per cent between 1957 and 2004. Whereas the northward transport in the Gulf Stream across 25° N has remained nearly constant, the slowing is evident both in a 50 per cent larger southward-moving mid-ocean recirculation of thermocline waters, and also in a 50 per cent decrease in the southward transport of lower North Atlantic Deep Water between 3,000 and 5,000 m in depth. In 2004, more of the northward Gulf Stream flow was recirculating back southward in the thermocline within the subtropical gyre, and less was returning southward at depth.


Copenhagen is fast approaching, but a deal seems further away

 Barack Obama speaks during a summit on climate change at the United Nations in New York. Photograph: Mike Segar/Reuters


Brazil, Indonesia and Norway have made positive steps in the past few weeks, illustrating the value of the bottom up approach.

It sometimes seems as if the closer we get to Copenhagen, the further we get from a deal.
There weren't many signs of progress at the last three big gatherings on climate change. Barack Obama disappointed at the United Nations by failing to press the Senate to move forward on climate change legislation, while Hu Jintao offered no specifics on how far China would go to reduce its future greenhouse gas emissions. At the G20 meeting in Pittsburgh, the industrialised economies fell short of expectations they would produce a package on climate finance. And climate talks in Bangkok this month ended in even deeper acrimony between the developing and developed economies.
But beneath the radar, there have been a few positive steps. Norway, Indonesia, and — as of this week — Brazil have all come forward with new pledges on climate action.
The country's environment minister, Carlos Minc, was quoted this week saying that Brazil would propose capping its greenhouse gas emissions at 2005 levels.
Meanwhile, the president, Lula Da Silva, said in a radio discussion that Brazil would take more aggressive measures to save the Amazon forest, aiming for an 80% reduction in deforestation in 2020. "We're in the process of preparing our proposal for Copenhagen. I foresee that by 2020 we will be able to reduce deforestation by 80%, in other words we will emit some 4.8 billion fewer tonnes of carbon dioxide," the president said. A formal announcement of Brazil's new climate position is expected early next week.
It was the third country to come forward with a new proposal since late last month. Indonesia's president, Susilo Bambang Yudhoyono, told G20 leaders on September 25 that his government was working on a plan to cut emissions by 26% in 2020 over business as usual projections.
In a copy of the speech obtained by AFP, the Indonesian leader said his country could cut emissions by as much as 41% — provided it got international support. He said the cuts would be achieved through increased investment in renewable energy, and curbing emissions from deforestation and other changes in land use.
Norway also stepped up, using the Bangkok talks to come forward with proposals to cut emissions by 30% to 40%, the most ambitious target of all developed countries.
Hilary McMahon, who works on climate policy at the World Resources Institute, says such individual pledges could become increasingly important. The international negotiations are moving towards a bottom-up approach, with individual countries setting their own targets. "It really does help us to start doing the math and adding up how far this is going to get us," she said. "Unfortunately we are still looking like we are quite short."




Types of measures of variability
With adequate measurements of climatic conditions covering many years, it is possible to define what is considered normal and what is an extreme event for any part of the world. Data gathered over the 30-year period from 1961 to 1900 define the latest Normals used for climate reference. At any given time of the year, an extreme high temperature might be defined as one that occurs only once in every 30 years. A cold winter or hot summer can be specified in a similar way, or in terms of the number of days below or above defined exceptional values. This means that when there is a succession of extremes, or more extreme events over a period such as a season, it is possible to estimate whether these extremes are part of the normal expectation for the locality, or are so unlikely that they can only be explained in terms of some more radical shift in climate.
The basic properties of any data series, for example temperature, can be defined in terms of the mean over time and the amount of variance about the mean. Other meteorological variables exhibit more complicated statistical properties. For instance, rainfall is episodic. In many parts of the world, much of the annual rainfall falls in a short rainy season. In addition, most of that rain may be concentrated in a few heavy falls and small shifts in the large-scale weather patterns from year to year may significantly alter the amount and distribution of seasonal rainfall. More complex techniques usually are needed to interpret variations in rainfall.
Some facts on global climate extremes
Since extreme meteorological events may be good markers of climatic change or variability, it is important to keep good records of such extremes. A worldwide collection of such events has been assimilated by WMO in conjunction with the University of Arizona. In 2006, the WMO Commission for Climatology has developed of a world archive for verifying, certifying and storing world weather extremes. Existing record extremes are  available to the general public on http://wmo.asu.edu. They cover temperature, pressure, rainfall, hail, aridity, wind, tornados and cyclones, and are displayed on maps of the world, the hemispheres and the continents.

From Climate extremes to disasters
On 10 February 1935 35 cm of snow fell on Laghouat on the edge of the Algerian Sahara. While this was certainly an extreme event, it was no disaster. Disasters occur frequently as a result of extreme climatic events, however, and also as a result of the accumulation of extreme events that constitute climatic variability or change. The word disaster is used to describe such events when they cause human sickness, death or migration on a large scale, or when they cause severe economic damage.

Although human misery cannot be adequately represented by statistics, it is helpful to have some measure of the global scale of the impact of weather-related disasters. Many data have been collected by the International Federation of the Red Cross and Red Crescent Societies (IFRC). The important features of these figures are:
  • droughts killed more people than all other disasters combined;
  • droughts and floods affected about an equal number of people, and far more than high winds (including hurricanes, cyclones, typhoons, storms and tornadoes);
  • floods were, however, by far the greatest cause of homelessness; and
  • for the limited time covered (1973-97), there were large variations in the numbers of people affected by different forms of disaster in successive five-year periods. This makes it difficult to draw any definite conclusions about trends, apart from noting that the number of people affected by floods appears to be rising.
Examples of Climate Events and extremes
When it comes to exploring our vulnerability to the varying climate, however, it is the extreme events that provide the most important messages. Extreme events have a disproportionate impact on human populations, and so represent a vital aspect of climate variability and change.
Tropical cyclones
Tropical cyclones are areas of very low atmospheric pressure over tropical and sub-tropical waters which build up into a huge, circulating mass of wind and thunderstorms up to hundreds of kilometres across. Surface winds can reach speeds of 200 km/h or more. On average 80 tropical cyclones form every year. They are called differently depending on where they are formed: typhoons in the western North Pacific and South China Sea; hurricanes in the Atlantic, Caribbean and Gulf of Mexico, and in the eastern North and central Pacific Ocean; and tropical cyclones in the Indian Ocean and South Pacific region.

Mid-latitude winter storms
Heavy rain and snow are dangerous for vulnerable communities. They can exacerbate rescue and rehabilitation activities after a major disaster, such as the earthquake in Pakistan in October 2005. They bring havoc to road and rail transportation, infrastructure and communication networks. An accumulation of snow can cause the roofs of buildings to collapse. Strong winds are a danger for aviation, sailors and fishermen, as well as for tall structures such as towers, masts and cranes. Blizzards are violent storms combining below-freezing temperatures with strong winds and blowing snow. They are a danger to people and livestock. They cause airports to close and bring havoc to roads and railways.

As an example, the East Asian winter is dominated by cold and relatively dry winds. The Siberian anticyclone is a persisting climatic feature that blows Arctic air over Siberia and northern China. Part of the flow sweeps out toward the North Pacific and part southward through China to the equatorial regions. The winter monsoon throughout this region is characterized by successive outbursts of cold air, called cold waves, that produce sharp falls in temperature of more than 10°C and are accompanied by snow and, in the south, rain. Snowstorms can be especially violent over northern China with sub-zero temperatures and gales lasting many days. The snowstorms can be damaging to communities and are particularly disruptive to transport, including coastal shipping. Periods of frost following the cold outbreak last several days and are a major hazard over the south of China as they have far-reaching effects on agriculture, especially on plants and crops. The frequency of cold waves varies greatly from year to year; as many as 10 per year or as few as one have been experienced. An active period is associated with higher pressure within the Siberian anticyclone and an intense low pressure system near the Aleutian Islands.
Droughts and duststorms
The primary cause of any drought is deficiency of rainfall. Drought is different from other hazards in that it develops slowly, sometimes over years, and its onset can be masked by a number of factors. Drought can be devastating: water supplies dry up, crops fail to grow, animals die and malnutrition and ill health become widespread.

Duststorms and sandstorms are ensembles of particles of dust or sand lifted to great heights by strong and turbulent wind. They occur mainly in parts of Africa, Australia, China and the USA. They threaten lives and health, especially of persons caught in the open and far from shelter. Transportation is particularly affected as visibility is reduced to only a few metres.
Floods can occur anywhere after heavy rains. All floodplains are vulnerable and heavy storms can cause flash flooding in any part of the world. Flash floods can also occur after a period of drought when heavy rain falls onto very dry, hard ground that the water cannot penetrate. Floods come in all sorts of forms, from small flash floods to sheets of water covering huge areas of land. They can be triggered by severe thunderstorms, tornadoes, tropical and extra-tropical cyclones (many of which can be exacerbated by the El Niño phenomenon), monsoons, ice jams or melting snow. In coastal areas, storm surges caused by tropical cyclones, tsunamis or rivers swollen by exceptionally high tides can cause flooding. Dikes can flood when the rivers feeding them carry large amounts of snowmelt. Dam breaks or sudden regulatory operations can also cause catastrophic flooding. Floods threaten human life and property worldwide.

The term ‘monsoon’, of Arabic origins, refers to a steady seasonal wind, and became widely associated with the Indian subcontinent and the onset of the main rainfall season. In fact, monsoon systems are a major feature of the general circulation of the atmosphere in subtropical latitudes of most regions of the world, including India, East Asia, Australia, and both North and South America ( map of the global monsoon system) .

Monsoon forecasts have improved since the early 1980s. This is the result of a growing understanding of the empirical relationships between indicators around the world and the subsequent monsoon. One reason for these advances has been the rising quality of data. Recent satellite observations have also revived interest in Himalayan snow cover as a predictor. They show that the relationship first identified by Blandford is a useful guide, but that the extent of the all-Eurasian winter snow cover was a better indicator, given the geographically uneven and variable nature of snow cover over the Himalayas, Tibet and Siberia.
Heatwaves amd cold waves/frost
Heat waves are most deadly in mid-latitude regions, where they concentrate extremes of temperature and humidity over a period of a few days in the warmer months. The oppressive air mass in an urban environment can result in many deaths, especially among the very young, the elderly and the infirm. In 2003, much of western Europe was affected by heat waves during the summer months. In France, Italy, The Netherlands, Portugal, Spain and the United Kingdom, they caused some 40 000 deaths. Extremely cold spells cause hypothermia and aggravate circulatory and respiratory diseases.