During the third week of January, a series of heavy rains in Brazil killed over 700 people and left over 14,000 without homes. Much of the devastation was caused by massive mudslides and surging floods that collapsed roofs and carried away automobiles, buildings, and even people. These problems were most pronounced in towns near the mountainous Serrana region in the state of Rio de Janeiro. The Rio area’s particular susceptibility to the devastation caused by mudslides can be easily explained by its geological makeup; its deep layer of solid bedrock sits almost 3,000 feet above sea level and is covered by a thin layer of topsoil. In the case of heavy downpours, waterlogging results in sodden, loosened soil, causing extreme instability. Loose mud begins to hurtle down the steep slopes, resulting in widespread destruction.
Though the amount of heavy rain seems extremely strange, William Boos, a Yale Assistant Professor of Geology and Geophysics specializing in Atmosphere, Oceans, and Climate Dynamics, has a possible explanation. According to Boos, the heavy Brazil rainstorms might have been a byproduct of robust La Niña conditions in the Southern Hemisphere. La Niña, an oceanic and atmospheric phenomenon where sea surface temperatures can be lowered by four to five degrees Celsius, has reportedly been developing since September 2010 and may possibly have played a major role in these disasters. On January 19, a major extra-tropical storm rolled across Southern Brazil approximately 22 degrees south of the Equator, and this corresponded to the heaviest full day of rains in both Rio and Sao Paulo. However, it is likely too early to make any definitive analyses, since the Earth’s storm activity does possess a large random component.
So, what could altered climate patterns in the Southern Hemisphere mean for areas of the United States traditionally susceptible to mudslides – in particular, regions of California? Boos believes that many implications are conceivable. Perturbation of jet streams by La Niña and its counterpart climate pattern, El Niño, can significantly alter global weather as in the past. La Niña has resulted in increasingly wet patterns across the Northeast in the United States, while El Niño has been more of a southwestern phenomenon.
The possible risks to California are, however, somewhat counterintuitive. “It’s generally predicted that as the climate slowly warms, it’s going to become more rainy in rainy regions, and more dry in dry regions,” Boos explains. Although it may seem that the increased risk of devastating mudslides would be caused by general and overall increases in Californian precipitation levels, the exact opposite may be the case. In California, dangerous mudslides often occur in drier, hilly areas where raging wildfires have cleared vegetation off steep slopes. In these areas, soil is no longer held firmly in place, and such perennially drier areas are extremely sensitive to the occasional downpour. It is estimated that areas affected by heavy wildfire remain extremely susceptible to mudslides for over three years.
Given that California, unlike Brazil, does not typically demonstrate monsoon-like circulation patterns, El Nino-induced dryness may actually be a possible cause for increased risk for and sus¬ceptibility to mudslides. According to a California weather risk assessment report issued by the California Climate Change Center in 2009, “if tem¬peratures rise into the medium warming range, the risk of large wildfires in California could increase by as much as 55 percent.” Boos further elaborates that “heavy rains can happen for other reasons, not just monsoons.” It is clear that if and when they do in these areas, devastation could result.
There is some upside though – California is not nearly as susceptible to widespread death and property damage as the rural Brazilian areas afflicted in January. The infrastructure of these Californian areas greatly surpasses that of many towns in the Rio de Janeiro state, where a significant portion of homes are shoddily and often illegally built, increasing the risk for destruction. Still, the need for caution and constant critical analysis of global climate patterns in weather in general is essential “Studying the weather is not only scientifically interesting but also very societally relevant,” Boos says. “It’s rewarding because you often get to help many vulnerable populations.”