El impacto que mató a los dinosaurios solo gratinó la Tierra/Dinosaur-killing impact only set Earth to broil

Un nuevo estudio parece sugerir que el asteroide que impactó hace 65 millones de años acabando con la vida de los dinosaurios, no incineró la vida sobre la superficie del planeta, sino que simplemente la "gratinó". El trabajo responde algunas preguntas preocupantes acerca de la teoría que sostiene que el impacto produjo incendios mortales en todo el mundo, pero origina nuevas cuestiones acerca de lo que realmente condujo a la extinción masiva de fines del periodo Cretácico.
La extinción de los dinosaurios y de la mayoría de las especies del planeta, se achaca al impacto de un asteroide de 10 km de diámetro. Los primeros modelos por computador demostraron que más de la mitad de los residuos llevados al espacio por el impacto, volverían a la atmósfera a las ocho horas.
Los modelos predijeron que esa lluvia de residuos calientes irradiaría, entre 20 minutos y dos horas, el mismo calor de un horno cuando "gratina" (260ºC). Este calentamiento intenso y durante tanto tiempo, llevaría la madera al punto de ignición originando incendios a nivel global.
Sin embargo, algunas especies sobrevivieron y el estrato con los restos del impacto no contiene tanta ceniza como cabría esperar de la quema de todos los bosques del mundo, lo que hace cuestionarse acerca de la magnitud de los incendios post-impacto.
Para explicar esta discrepancia, Tamara Goldin de la Universidad de Viena y Jay Melosh del Purdue Institue de Indiana, han estudiado como la caida de las eyecciones a través de la atmósfera podría afectar a la transferencia de calor desde lo alto de la atmósfera a la superficie terrestre. Los primeros modelos solo tenían en cuenta a los gases de efecto invernadero como absorbentes del calor.
El estudio revela que los primeros restos que volvieron a entrar en la atmósfera a los pocos minutos del impacto, ayudaron a proteger la superficie terrestre de residuos posteriores. Según dijo Goldin a New Scientist, "las mismas eyecciones se pusieron en medio de la radiación térmica (en la atmósfera) y protegieron la Tierra".
Como resultado, la superficie sufrió un intenso calor procedente del cielo durante unos pocos minutos. Según fueron cayendo más partículas, bloquearon más y más el calor que venía de arriba, impidiendo que los bosques ardieran. De acuerdo a Goldin "es realmente dificil comenzar una ignición con un pulso corto de calor intenso" en un área lejos de la zona del impacto.
La vida sobre la superficie podría haberse gratinado, pero no quedó como una "patata frita". Los animales que consiguieron refugiarse bajo tierra o en el agua, podrían haber sobrevivido a ese corto periodo de intenso calor, lo que explicaría el que no toda la vida muriera.
Wendy Wolbach de la DePaul University en Chicago, que propuso en 1985 que la ceniza hallada a fines del cretáceco provenía de incendios globales, está de acuerdo. Según dijo a the The New Scientist, el efecto apantallador "tiene sentido".
No obstante, sin incendios globales se necesitan otros mecanismos para explicar la extinción masiva. Entre ellos está la idea de que el polvo atmosférico interceptó la radiación solar originando un "invierno del impacto" que duró años hasta que las emisiones posteriores al impacto produjeran un calentamiento global.
La lluvia ácida que siguió al impacto, también podría haber jugado su papel en la extinción, al igual que el estress climático originado por las masivas erupciones volcánicas que tuvieron lugar hace 65 millones de años en la región india del Deccan .

The asteroid impact that ended the age of dinosaurs 65 million years ago didn't incinerate life on our planet's surface – it just broiled it, a new study suggests. The work resolves nagging questions about a theory that the impact triggered deadly wildfires around the world, but it also raises new questions about just what led to the mass extinction at the end of the Cretaceous period.
The impact of a 10-kilometre asteroid is blamed for the extinction of the dinosaurs and most other species on the planet. Early computer models showed that more than half of the debris blasted into space by the impact would fall into the atmosphere within eight hours.
The models predicted the rain of shock-heated debris would radiate heat as intensely as an oven set to "broil" (260 °C) for at least 20 minutes, and perhaps a couple of hours. Intense heating for that long would heat wood to its ignition temperature, causing global wildfires.
Yet some species survived, and the global layer of impact debris doesn't contain as much soot as would be expected from burning the world's forests, raising questions about the extent of post-impact wildfires.
To explain the discrepancy, Tamara Goldin of the University of Vienna and Jay Melosh of Purdue University in Indiana studied how ejecta falling through the atmosphere might affect heat transfer from the top of the atmosphere to the ground. Earlier models considered only how atmospheric greenhouse gases would absorb heat.
The study reveals that the first debris to re-enter the atmosphere just a few minutes after the impact helped protect the surface from the debris that followed. "The actual ejecta themselves were getting in the way of the thermal radiation [in the atmosphere] and shielding the Earth," Goldin told New Scientist.
As a result, the surface felt the full heat from the sky for only a few minutes. As more particles drifted down, they blocked more and more of the heat from above, preventing the world's forests from igniting. "With the short pulse [of intense heat], it's really hard to get ignition" far from the impact site, Goldin says.
Surface life would have been broiled, but not burnt to a crisp. Animals that were able to take refuge underground or in the water were likely able to survive the short period of intense heat, explaining why not all life was killed.
Wendy Wolbach of DePaul University in Chicago, who in 1985 proposed that soot found at the end of the Cretaceous came from global wildfires, agrees. The heat shielding effect "makes sense", she told New Scientist.
Without global wildfires, other mechanisms are needed to explain the mass extinction. These include the idea that dust in the atmosphere cut off sunlight in an "impact winter" that lasted for years before emissions released after the impact caused long-term global warming.
Acid rain following the impact may also have played a role in the extinction, as could the additional stress on global climate from the massive volcanic eruptions that occurred 65 million years ago in India's Deccan Traps.

Tomado de/Taken from New Scientist

Resumen de la publicación/Abstract of the paper
Self-shielding of thermal radiation by Chicxulub impact ejecta: Firestorm or fizzle?
Tamara J. Goldin and H. Jay Melosh1
Geology 37 (2009) 1135-1138
Abstract As hypervelocity ejecta from the Chicxulub (Yucatán, Mexico) impact fell back to Earth, the surface may have received a deadly dose of thermal radiation sufficient to ignite global wildfires. Using a two-phase fluid flow code, which includes ejecta and air opacities in a radiative transfer calculation, we modeled the atmospheric reentry of spherules arriving at distal sites. The models predict a pulse of thermal radiation at the surface peaking at 5–15 kW/m2, analogous to an oven set on “broil” (~260° C). Previous calculations, which ignored spherule opacity, yielded >10 kW/m2 sustained over >20 min and such an extended pulse is thought to be required for wood ignition. However, the new modeling suggests that fluxes only exceed the solar norm for ~30 min and are only >5 kW/m2 for a few minutes. Previous models failed to consider the self-shielding effect of settling spherules, which block an increasing proportion of downward thermal radiation emitted by the later-arriving spherules. Such self-shielding may have prevented widespread wildfire ignition, although the thermal pulse may have been sufficient to ignite localized fires and kill fauna lacking temporary shelter. An opaque cap of submicron dust in the upper atmosphere could, however, override the self-shielding effect.

El cambio climático modificará la aparición de incendios/Climate Change will modify the wildfire occurrence

El cambio climático traerá un cambio en la pauta de los incendios a nivel mundial, y estos cambios llegan rápido, según el primer análisis de este tipo, realizado por investigadores de la Universidad de California en Berkeley en colaboración con científicos de la Universidad Tecnológica de Texas.
Los investigadores usaron datos de sensores infrarrojos y térmicos tomados entre 1996 y 2006 por satélites de la Agencia Espacial Europea para su estudio de pirogeografía: la distribución y comportamiento de los incendios, para determinar las características ambientales comunes asociadas con el riesgo de los mismos. Luego han incorporado estas variables en proyecciones acerca de cómo futuros escenarios climáticos, podrán influir en la aparición de los incendios a nivel mundial.
El estudio ha encontrado que gran parte del planeta sufrirá cambios en la actividad incendiaria, y esto implica tanto el aumento como la disminución de la probabilidad de aparición de incendios. Hay áreas en las que raramente se producían incendios en el pasado, que experimentarán un gran aumento de los mismos en el período 2010-2039. Por el contrario, otras regiones donde los fuegos eran comunes en el pasado, serán consideradas como áreas de riesgo en disminución.
Estos estudios preliminares muestran punbtos calientes de aumento en el oeste de los Estados Unidos y en la meseta tibetana, mientras que regiones tales como el noroeste de China o el África central serán menos proclives a los incendios en las próximas décadas.

Climate change will bring about major shifts in worldwide fire patterns, and those changes are coming fast, according to a first-of-its-kind analysis led by researchers at the University of California, Berkeley, in collaboration with scientists at Texas Tech University.
Researchers used thermal-infrared sensor data obtained between 1996 and 2006 from European Space Agency satellites in their study of pyrogeography – the distribution and behavior of wildfire – on a global scale. They not only got a global view of where wildfires occur, but they determined the common environmental characteristics associated with the risk of those fires. They then incorporated those variables into projections for how future climate scenarios will impact wildfire occurrence worldwide.
The study has found that much of the planet will incur changes in fire activity, and this includes increases as well as decreases in the likelihood of fire. Specific areas where wildfire occurrence was rare in the past are projected to experience large increases in fire activity in the period 2010-2039. Regions where fire was common in the past and projected to experience a large decrease were considered areas at risk of fire retreat.
These preliminary results show hotspots of fire invasion forming in parts of the western United States and the Tibetan plateau, while regions including northeast China and central Africa may become less fire-prone in the coming decades.

Tomado de/Taken from Science Daily; University of Berkeley

Resumen de la publicación científica/Abstract of the research paper
Krawchuk MA, Moritz MA, Parisien M-A, Van Dorn J, Hayhoe K (2009) Global Pyrogeography: the Current and Future Distribution of Wildfire. PLoS ONE 4(4): e5102. doi:10.1371/journal.pone.0005102
Climate change is expected to alter the geographic distribution of wildfire, a complex abiotic process that responds to a variety of spatial and environmental gradients. How future climate change may alter global wildfire activity, however, is still largely unknown. As a first step to quantifying potential change in global wildfire, we present a multivariate quantification of environmental drivers for the observed, current distribution of vegetation fires using statistical models of the relationship between fire activity and resources to burn, climate conditions, human influence, and lightning flash rates at a coarse spatiotemporal resolution (100 km, over one decade). We then demonstrate how these statistical models can be used to project future changes in global fire patterns, highlighting regional hotspots of change in fire probabilities under future climate conditions as simulated by a global climate model. Based on current conditions, our results illustrate how the availability of resources to burn and climate conditions conducive to combustion jointly determine why some parts of the world are fire-prone and others are fire-free. In contrast to any expectation that global warming should necessarily result in more fire, we find that regional increases in fire probabilities may be counter-balanced by decreases at other locations, due to the interplay of temperature and precipitation variables. Despite this net balance, our models predict substantial invasion and retreat of fire across large portions of the globe. These changes could have important effects on terrestrial ecosystems since alteration in fire activity may occur quite rapidly, generating ever more complex environmental challenges for species dispersing and adjusting to new climate conditions. Our findings highlight the potential for widespread impacts of climate change on wildfire, suggesting severely altered fire regimes and the need for more explicit inclusion of fire in research on global vegetation-climate change dynamics and conservation planning.