Huge Amazonian fires may spawn fierce tempests in South America
Source: San Francisco Chronicle
Keay Davidson, Chronicle Science Writer
Smoke from giant Amazonian fires could disrupt the climate across much of South America, perhaps spawning more severe thunderstorms and hailstorms, scientists warn today.
Impoverished rain forest inhabitants typically set the fires to create revenue-generating pastures and cropland. As the blazes incinerate the jungles, oceans of bluish-gray smoke particles ascend, interact with ordinary rain clouds and absorb their moisture, thus wiping them from the sky as surely as an eraser wipes chalk from a chalkboard, researchers say in today's issue of the journal Science.
Such cloud erasures could worsen dry spells and -- since clouds normally cool Earth by shielding it from the sun -- might contribute to global warming. Paradoxically, the same smoke could trigger taller, more violent thunderstorms by altering the cooling rate of "bubbles" of warm, moist rising air that fuel the storms, the scientists speculate.
A possible result: severe updrafts in thunderstorms, whose gales could wreak widespread devastation. They might also breed big chunks of hail -- not the BB-size, fast-melting kind of hail that surprised San Franciscans on Thursday, but, rather, the hefty, hardball kind that flattens crops, cattle and villages for miles around.
"This is a smoking gun connecting climate change and forest loss ...," said Jennifer Krill, campaign director at the San Francisco-based Rainforest Action Network. Amazonian infernos impress even Californians inured to countywide grass fires.
"Several hundred thousand deforestation and agricultural fires burn in Amazonia during the dry season each year, covering vast areas with dense smoke," scientists report in one of three Science articles today on the South American blazes.
An Amazon blaze "is horrible, but it's spectacular in its horribleness," said Yoram Kaufman, the co-author of another one of the three Science articles. He works at NASA's Goddard Space Flight Center in Greenbelt, Md.
During research expeditions, the scientists witnessed "heavy smoke from forest fires" effacing clouds or preventing them from dropping rain, according to a team led by M.O. Andreae of the biogeochemistry department at the Max Planck Institute in Mainz, Germany.
This happens because the clouds' tiny molecules of water vapor tend to condense onto the abundant smoke particles. Hence, the water vapor molecules are isolated from each other and thus are less likely to accumulate into water droplets plump enough to plunge as rain.
With special instruments aboard aircraft, the scientists observed the rising smoke "reduce cloud (water) droplet size and so delay the onset of precipitation" as high as 4 miles, the Andreae team reports.
Normally, clouds form from the condensation of water vapor in a bubble of warm, moist air as it lofts into the sky, where it expands, cools and condenses into glistening water droplets. In giant fires, though, smoke prevents warm clouds of water vapor from condensing at low altitudes; hence, in some cases, condensation is delayed until the bubble has reached a greater height, the Andreae team theorized.
As a result, they say, smoke-influenced thunderclouds might grow taller than normal. That's bad news, because tall thunderstorms tend to generate more violent updrafts and ground-scouring winds.
More ominously, updrafts loft water droplets to great heights. There, in the subzero temperatures of the upper air, the droplets condense into chunks of hail that repeatedly rise and fall, as if on a roller coaster, steadily acquiring thicker coats of ice until they're so fat that they drop to Earth. (When one slices into a chunk of hail, its onion-like layers hint at how many times it has risen and fallen inside the storm.)
Outside experts reacted to the Science articles with interest, but caution. It's too early to say if and how Amazonian fires might generate more dangerous storms, although the possibility deserves close study, they say. Even ordinary precipitation continues to be a subject plagued by puzzles, they point out.
Consider the warm maritime clouds that hover over oceans. To this day, scientists haven't figured out how these clouds condense into raindrops within 15 minutes, as they often do, instead of 40 minutes as computer models claim they should, says one outside observer, Daniel Breed, a cloud physicist at the National Center for Atmospheric Research in Boulder, Colo.