Calidad del Aire, Sequía y Erosión del Suelo

Quien contamina paga. Ojalá fuera cierto pero es mera propaganda política. Sin embargo va a resultar que la atmósfera se toma la justicia por su cuenta. Recientes estudios muestran que la contaminación del aire generada por las actividades industriales incrementa el número de partículas en suspensión atmosférica, pero a costa de disminuir su tamaño. Hablamos de aerosoles. Estos últimos actúan como nucleadores del vapor de agua que, al convertirse en gotas, terminan por caer al suelo en forma de precipitaciones. No obstante si el tamaño de los aerosoles es demasiado pequeño, también lo es su potencial para condensar el vapor de agua. Dicho de otro modo, las gotas son demasiado pequeñas como para formar las nubes que desprenden precipitaciones. Y al acaecer tal proceso, no se reduce la cantidad de agua precipitada, sino que cuando llueve, lo hace de un modo más torrencial en detrimento de la de naturaleza más suave. En otras palabras, se induce un aumento de la torrencialidad. Como corolario, los suelos en lugar de empaparse lentamente hasta saturarse, sufren el impacto de tormentas intensas que incrementan la escorrentía superficial, el sellado del suelo y la erosión. Resultado: para una misma cantidad de precipitación anual se incrementa el déficit hídrico (sequía), disminuyendo la producción de cosechas y fomentando la erosión de los recursos edáficos. Tal narración pudiera parecer a algunos “el cuento de caperucita”. Pues no es el caso. Investigaciones recientes llevadas a cabo en China “parecen” demostrar que la sequía que padecen extensas zonas del NE de aquél  país es debida a la contaminación del aire que aqueja a sus ciudades, y no a cambios en la circulación atmosférica, ni al vapor de agua que contienen.   

Contaminación del aire en China. Fuente: Treehugger

Es palmario que las investigaciones actuales sobre el cambio climático no van a lograr frenar el aumento en el efecto de invernadero. De eso se encargan con fervor inusitado la economía bobalizante y sus defensores. Sin embargo, vamos a aprender algo más a cerca de cómo se ha generado todo este proceso catastrófico, así como mejorar ciertos procesos biogeosféricos subyacentes. En este contexto, diversos estudios constatan como las partículas de materia en suspensión aérea desempeñan papeles muy relevantes sobre el clima y algunos eventos atmosféricos que hace años ni vislumbrábamos. Hemos intentado varias veces hablar de este tema. Sin embargo, resulta harto complejo, por cuanto no depende del número de partículas o aerosoles que contiene el aire si no que también influyen su composición,tamaño, etc.

Ya sabemos que la mala calidad del aire genera lluvia ácida, a demás de afectar gravemente la salud de la población que la padece. Ahora bien, lo que acabamos de comenzar a narrar nos parece bastante más insólito y novedoso, por cuanto quedan vinculadas contaminación del aire, lluvias útiles para la producción agropecuaria, tormentas que puedan dar lugar a erosión e inundaciones, así como el agravamiento de los problemas de desertización. ¡Vaya lío!. Digamos que en China su falta de cuidado por la contaminación atmosférica induce pobreza y falta de agua en el agro, poniendo en riesgo su seguridad alimentaria. Y todo ello sin que quede afectada la cantidad total de agua precipitada. Ósea que por los designios biogeosféricos, el país que contamina paga, aunque el peso directo de tal desastre recae, como casi siempre, sobre las espaldas de los más desheredados, en este caso los campesinos. A demás de obtener menores cosechas, el gobierno de este país les recortan los suministros de agua, con vistas a que la China capitalista siga contaminando. Esto es a lo que yo llamo justicia bobalzante. Abajo os dejamos dos noticias con abundantes detalles. La primera, más corta, fue publicada por Terradaily, mientras que la más extensa por Sciencedaily. Resumiendo, que poquito sabemos de las conexiones biogeosféricas.

Juanjo Ibáñez      

Air pollution reducing good rains in China: study

by Staff Writers; Stockholm (AFP) Aug 31, 2009

Air pollution has over the past 50 years led to a reduction of the light rainfall in China essential for the country’s agriculture and water resources, a Chinese researcher said Monday.

A recent study conducted in Sweden, the United States and China showed that the number of days with light rainfall (less than 0.1 millimetre per day) dropped by 23 percent in China from 1956 to 2005. That was because of air pollution particles that reduced the formation of rain clouds, researcher Deliang Chen of Gothenburg University told AFP.

«The particles of air pollution, named aerosols, make the drops of water considerably smaller and that makes them more difficult to aggregate into rain clouds that can release rain,» he said. While the amount of rain has remained stable over the years, the reduction of light rainfalls, or «good rainfalls», poses a threat to China as the country drags its feets on reducing greenhouse gas emissions, he said. «Light rains in a country like China are counted upon because they’re easy for the soil to absorb. But heavy rains make the soil run off and the water is not absorbed and can cause problems such as erosion or floodings,» he said.

«Now we have evidence that air pollution is not only bad for the climate but also for the agriculture and the economy,» he added. The study, entitled «Heavy pollution suppresses light rain in China,» was published recently in the US Journal of Geophysical Research. Northern China is the region most exposed to drought. It is currently experiencing its worst drought in 60 years, leaving nearly five million people short of water, state media have reported.

Our Polluted World and Cleaning It Up

The Sky Is Not Falling: Pollution In Eastern China Cuts Light, Useful Rainfall

ScienceDaily (Aug. 17, 2009) — New research shows that air pollution in eastern China has reduced the amount of light rainfall over the past 50 years and decreased by 23 percent the number of days of light rain in the eastern half of the country. The results suggest that bad air quality might be affecting the country’s ability to raise crops as well as contributing to health and environmental problems.

The study links for the first time high levels of pollutants in the air with conditions that prevent the light kind of rainfall critical for agriculture. Led by atmospheric scientist Yun Qian at the Department of Energy’s Pacific Northwest National Laboratory, the study appears August 15 in the Journal of Geophysical Research-Atmospheres. «People have long wondered if there was a connection, but this is the first time we’ve observed it from long-term data,» said Qian. «Besides the health effects, acid rain and other problems that pollution creates, this work suggests that reducing air pollution might help ease the drought in north China.»

Drier Times

China‘s dramatic economic growth and pollution problems provide researchers an opportunity to study the connection between air quality and climate. Rain in eastern China — where most of the country’s people and pollution exist — is not like it used to be. Over the last 50 years, the southern part of eastern China has seen increased amounts of total rainfall per year. The northern half has seen less rain and more droughts. But light rainfall that sustains crops has decreased everywhere. A group of climate researchers from the U.S., China and Sweden wanted to know why light rain patterns haven’t followed the same precipitation patterns as total rainfall.

Previous work has shown that pollution can interfere with light rain above oceans, so the team suspected pollution might have something to do with the changes over land. Light rain ranges from drizzles to 10 millimeters of accumulation per day and sustains agriculture. (Compared to heavy rain that causes floods, loss of light rain has serious consequences for crops.). While the light rains have diminished, pollution has increased dramatically in China in the last half of the 20th century. For example, while China’s population rose two and a half times in size, the emissions of sulfur from fossil fuel burning outpaced that considerably — rising nine times.

Sky Gremlins

Air pollution contains tiny, unseen particles of gas, water and bits of matter called aerosols. Aerosols — both natural and human-caused (anthropogenic) — do contribute to rainfall patterns, but the researchers needed to determine if pollution was to blame for China’s loss of rain and how. To find out, the team charted trends in rainfall from 1956 to 2005 in eastern China, which has 162 weather stations with complete data collected over the entire 50 years.

From this data, the team determined that both the north and south regions of eastern China had fewer days of light rain — those getting 10 millimeters per day or less — at the end of the 50 year timespan. The south lost more days — 8.1 days per decade — than the north did, at 6.9 days per decade. However, the drought-rattled north lost a greater percentage of its rainy days, about 25 percent compared to the south’s 21 percent. «No matter how we define light rain, we can see a very significant decrease of light rain over almost every station,» said Qian.

Up Up & In the Way

To probe what caused the loss of rainfall, the team looked at how much water the atmosphere contained and where the water vapor traveled. Most parts of eastern China saw no significant change in the amount of water held by the atmosphere, even though light rains decreased. In addition, where the atmosphere transported water vapor didn’t coincide with light rain frequency. These results suggested that changes in large-scale movement of water could not account for the loss of the precipitation. Some of pollution’s aerosols can seed clouds or form raindrops, depending on their size, composition and the conditions in which they find themselves. Because these skills likely contribute to rainfall patterns, the researchers explored the aerosols in more depth.

Cloud droplets form around aerosols, so the team determined the concentration of cloud droplets over China. They found higher concentrations of droplets when more aerosols were present. But more droplets mean that each cloud droplet is smaller, in the same way that filling 10 ice cream cones from a quart of ice cream results in smaller scoops than if the same amount were put in only five cones. This result suggested that aerosols create smaller water droplets, which in turn have a harder time forming rainclouds. The team verified this with computer models of pristine, moderately polluted or heavily polluted skies. In the most heavily polluted simulation, rain fell at significantly lower frequencies than in the pristine conditions. An examination of the cloud and rain drops showed that these water drops in polluted cases are up to 50 percent smaller than in clean skies. The smaller size impedes the formation of rain clouds and the falling of rain.

Qian said the next step in their research is to examine new data from the DOE’s Atmospheric Radiation Measurement Climate Research Facility in the central eastern Chinese city of Shouxian. The data was collected from April to December of 2008. «This work is important because modeling studies of individual cases of pollution’s effect on convective clouds have shown varying results, depending on the environmental conditions,» said coauthor Ruby Leung. «The ARM data collected at Shouxian should provide more detailed measurements of both aerosols and clouds to enable us to quantify the impacts of aerosols on precipitation under different atmospheric and pollution conditions.»

The work was supported by the Office of Biological and Environmental Research within the DOE Office of Science under a bilateral agreement on regional climate research with the China Ministry of Science and Technology.

Journal reference:Qian et al. Heavy pollution suppresses light rain in China: Observations and modeling. Journal of Geophysical Research, 2009; 114D00K02 DOI: 10.1029/2008JD011575

Adapted from materials provided by DOE/Pacific Northwest National Laboratory. | More