South America’s biggest and wealthiest city is waiting for the rain. If it doesn’t come soon, São Paulo could run out of water before 2015. Residents have been warned to brace for a “collapse like we’ve never seen before” by the president of Brazil’s Water Regulatory Agency.

How this could happen? The answer may surprise you.

It starts with deforestation of the Amazon, more than 2,000 kilometers away. As trees in the rainforest are cut down, the forest’s ability to release humidity into the air is reduced. Air currents carrying water vapor, known as “flying rivers,” fail to flow south, changing the climate of the region. (Watch President Obama speak on the importance of fighting climate change.)

People carry plastic bottles to gather water from a community kiosk in a city outside São Paulo. (© AP Images)

Billions of liters of water have been lost. What’s more, deforestation in the Amazon has risen 29 percent over the past year. (That’s why some people are using mobile phones to monitor deforestation.)

Already 14 million residents of São Paulo have been affected by the rain shortages, which have dried up key reservoirs. An executive of the state-owned water utility that serves the city said São Paulo has about two weeks of drinking water supplies left. Other parts of Brazil face different threats, including reduced coffee, sugarcane and other top export crops.

Deforestation has serious and far-reaching effects. Reduce the destruction of forests by supporting sustainable production. See the devastation caused by deforestation with NASA’s satellite images and consider volunteering to plant trees in your community.