Human beings are causing mountains in California to rise, according to a new study published yesterday in scientific journal Nature. Geologists monitoring the mountains of the California Coast and Sierra Nevada ranges found that the vertical positions of more than 500 GPS sensors have been steadily increasing by between one and three millimeters every year. The authors of the study blame the rise on the huge amount of water extracted from the area by human hands. More than 160 cubic kilometers of water has been taken from the region for irrigation and other purposes in the last 150 years, a process that has contributed to a net raise in the height of the mountains, and may lead to more frequent earthquakes in the state.

Mountains naturally rise and fall in height with the seasons, as snow and rain compress them in winter, before drier spells allow them to rise again by melting snow and evaporating water. But the geologists behind the Californian study say that the removal of groundwater for human use has reduced the weight on the Earth's crust, and caused the mountains to flex upward at an inflated rate. Colin Amos, lead author of the study, said he and his team had considered other tectonic causes for the year-on-year rise in height, but came to the conclusion that people were involved after modelling the crust in the area. "It looks like the net vertical motion of the mountains upward can be explained just by humans sucking the water out of the ground."

The rise of the mountains may lead to an increase in seismic activity

The study also says the rise of the ranges may cause more earthquakes in the area. Central Californian seismic activity is more common during dry seasons, as mountains flex upward and impact the nearby San Andreas fault, a process described as "unclamping." But commenting on the study in Nature, NASA's Paul Lundgren says that while quakes could be more frequent, they could also be smaller. Lundgren questions the link between the rise in mountain height and San Andreas seismic activity, suggesting that it's unclear whether unclamping will have a long-term impact on the fault, and clarifying that the study "does not imply that a large earthquake is imminent." But Lundgren points to its findings to show the activities of humans can make unexpected changes to our planet. "Whether it's water withdrawal or things like oil extraction," he says, our actions "can potentially have some other unintentional effects."