LIKE California, much of Brazil is gripped by one of the worst droughts in its history. Huge reservoirs are bone dry and water has been rationed in S漀 Paulo, a megacity of 20 million people; in Rio; and in many other places.

Drought is usually thought of as a natural disaster beyond human control. But as researchers peer deeper into the Earth’s changing bioclimate — the vastly complex global interplay between living organisms and climatic forces — they are better appreciating the crucial role that deforestation plays.

Cutting down forests releases stored carbon dioxide, which traps heat and contributes to atmospheric warming. But forests also affect climate in other ways, by absorbing more solar energy than grasslands, for example, or releasing vast amounts of water vapor. Many experts believe that deforestation is taking place on such a large scale, especially in South America, that it has already significantly altered the world’s climate — even though its dynamics are not well understood.

“A lot of people are scrambling to make observations in the Amazon this year, with the expected big El Ni漀 coming,” said Abigail L. S. Swann, an eco-climatologist at the University of Washington. “It’s expected to drive significant drought over the Amazon, which will change how much water trees have available.”

Humans have long settled in places where there is adequate and predictable precipitation, and large forests play a crucial role in generating dependable amounts of rainfall. Trees take up moisture from the soil and transpire it, lifting it into the atmosphere. A fully grown tree releases 1,000 liters of water vapor a day into the atmosphere: The entire Amazon rain forest sends up 20 billion tons a day.

The water vapor creates clouds, which are seeded with volatile gases like terpenes and isoprene, emitted by the trees naturally, to form rain. These water-rich banks of clouds travel long, wind-driven distances, a conveyor belt for the delivery of precipitation that scientists call flying rivers.

The sky-borne river over the Amazon carries more water than the Amazon River itself. It begins as moisture that builds over the Atlantic Ocean, and then flows westward over the emerald crown of the Amazon, where it picks up far more moisture. The laden clouds eventually bump up against the Andes and are steered south and then east, which means rain for Bolivia and Brazil.

One way forests may move water is known as “biotic pumping.” As water transpires into the atmosphere above the forest, the theory holds, it creates a low-pressure system that sucks in air surrounding it, eventually and continually pumping moisture inland from the ocean. Cutting down forests degrades these low-pressure systems, essentially turning off the pump. Large-scale deforestation is thus believed to be a major contributor to the extreme drought in Brazil.

Scientists have long known that vegetation has a profound effect on weather. In 1907, officials built a 2,000-mile-long fence across Australia to keep invasive rabbits from crossing from the wild outback into farms. On the side with native vegetation, rain clouds formed in the sky above, but the farm-field skies were clear. The “bunny-fence experiments” charted a decline in rainfall of 20 percent on the cultivated side. Researchers are still trying to explain why, but the leading theory is that the darker native plants absorb more heat and release it into the atmosphere, along with energy and water vapor to form clouds.

Today’s researchers mainly rely on computer modeling to understand the effects of deforestation, a difficult task because there are so many complex pathways through which trees control climate: precipitation, carbon storage, large clouds of complex chemical emissions and absorption of the sun’s energy.

“This area is a frontier,” said David Schimel, an eco-climatologist at NASA’s Jet Propulsion Laboratory and the author of “Climate and Ecosystems,” “but a frontier because it’s difficult, not because it’s neglected.”

In the last year alone some 2,000 square miles of the Amazon — roughly the size of Delaware — were lost to clearing, largely for planting soybeans and raising cattle. A growing number of scientists are warning that wide-scale deforestation — about 20 percent of the Amazon forest is gone already and nearly that much is degraded — may already be directing precipitation away from places long accustomed to it.

One Princeton study suggested that deforesting the Amazon could potentially contribute to drought in places as far away as California, while other research indicated that recent droughts in Texas and New Mexico might be linked to cutting in the Amazon. Despite the uncertainty embedded in these and other studies, “There’s lots of evidence that changing the water cycle in the Amazon would have global consequences,” Dr. Schimel said. “It’s a fairly robust notion.”

AND its impact could potentially accelerate. In a recent report, Antonio Donato Nobre, a veteran climatologist with Brazil’s National Institute for Space Research, warned that if just 40 percent of the Amazon region is deforested there could be an abrupt large-scale shift to grasslands, which could substantially alter global weather patterns “and cause a breakdown of the current climate system.” If deforestation continues, he has said, S漀 Paulo will most likely “dry up.”

In the broadest sense, scientists say, forests represent a kind of ecological infrastructure that helps maintain comfortable living conditions on the planet, whether by taking up and holding carbon dioxide, cleaning water through their roots, preventing floods by stabilizing soil — or, in this case, by regulating climate.

Dr. Nobre and other climate experts are urging an immediate halt to deforestation, as well as large-scale planting of new forests, as a way to essentially nurse the Amazon back to full health and stabilize its pivotal role in climate.

Gordon Bonan, a scientist at the University Corporation for Atmospheric Research in Boulder, Colo., and the author of “Ecological Climatology,” said reducing deforestation and replanting forests should be priorities not just in Brazil but in North America and beyond for many reasons, including the health of climate systems. “The pace of change is far outpacing our understanding of what the change is doing,” he said, “and by the time we do understand it’s probably going to be too late.”

While it is true that vast tree planting, which reroutes groundwater on a huge scale and absorbs far more energy than an unforested landscape, can have complex and potentially negative effects, “On balance,” if done properly, “it’s a positive strategy for climate change,” he added.

Some people aren’t waiting for further research and are hoping to geoengineer local climates with new forests. Bishop Fredrick Shoo, the bishop elect of the Evangelical Lutheran Church in Tanzania, has been planting trees with 100,000 of his parishioners upwind of Mount Kilimanjaro for 12 years, in hopes of cooling the hot, dry winds that are melting the mountain’s glaciers. During that time, he estimates, they have planted 3.7 million trees.

“My hope is we’ll be able to restore the forests of Kilimanjaro and save the water sources of Kilimanjaro,” said Bishop Shoo, known as the tree bishop. “We have a moral obligation to take care of creation and to be sure coming generations have a good place to live.”