Last year, an anxious, depressed 17-year-old boy was admitted to the psychiatric unit at the Royal Children's Hospital in Melbourne. He was refusing to drink water. Worried about drought related to climate change, the young man was convinced that if he drank, millions of people would die. The Australian doctors wrote the case up as the first known instance of "climate change delusion."

Robert Salo, the psychiatrist who runs the inpatient unit where the boy was treated, has now seen several more patients with psychosis or anxiety disorders focused on climate change, as well as children who are having nightmares about global-warming-related natural disasters.

Such anxiety over current events is not a new phenomenon. Worries about contemporary threats, such as nuclear war or AIDS, have historically been woven into the mental illnesses of each generation. But global warming could have a broader and deeper effect on mental health, even if indirectly.

"Climate change could have a real impact on our psyches," says Paul Epstein, the associate director for the Center for Health and the Global Environment at Harvard Medical School.

Over this century, the average global temperature is expected to rise between 1 degrees and 6 degrees Celsius. Glaciers will melt, seas will rise, extremes in precipitation will occur, according to scientists' predictions.

There is evidence that extreme weather events, such as droughts, floods, cyclones, and hurricanes, can lead to emotional distress, which can trigger such things as depression or post-traumatic stress disorder, in which the body's fear and arousal system kicks into overdrive.

After Hurricane Katrina, rates of severe mental illness - including depression, PTSD, anxiety disorder, panic disorder, and a variety of phobias - doubled, from 6.1 percent to 11.3 percent, among those who lived in affected regions, a 2006 study by the Hurricane Katrina Community Advisory Group said.

Rates of mild-to-moderate mental illness also doubled, from 9.7 percent to 19.9 percent.

"After a disaster, people can feel inadequate, like outside forces are taking control of their lives," said Joshua Miller, a professor at the Smith College School for Social Work who responds to disasters worldwide. "They can't see a positive future. They tend to lose hope or become depressed."

Severe disasters also destroy the infrastructure needed to provide mental health care, and forcibly displace people, severing social connections when people need them most, Miller said.

Climate change is expected to create about 200 million environmental refugees by 2050, according to the Intergovernmental Panel on Climate Change, the international body established within the United Nations to evaluate causes and consequences of global warming.

Of course, no one can predict what effect warming will have on our psyches. The links between mental illness and the weather can be tenuous or even downright contradictory. Depending on which studies you read, suicide is more common, less common, or equally common in hot weather. Ditto dry weather.

But even in the face of uncertainty, specialists say the indirect effects of global warming could be substantial.

Though much of the anxiety centers on the possibility of extreme weather events, global warming will also transform the natural environment in a more gradual way, they say. These changes could have their own effect on mental health.

"It's not all trauma," said Carol North, a psychiatrist who runs the trauma and disaster program at the Dallas VA Medical Center. "Some of it's a quiet decline of quality of life."

Indeed, climate change may eventually deplete natural resources, make it more difficult for people to live off the land, and disrupt the global food supply.

"That will mean declining socioeconomic status and quality of life across the world," North said, and "depression, demoralization, disillusionment."

In India and Australia, where severe droughts have already taken a toll on agriculture, researchers have noted an uptick in suicides among farmers.

On the other side of the globe, the changing Arctic climate is expected to make hunting and fishing far more difficult for the people who live there. The Arctic Climate Impact Assessment says that such changes threaten Inuit culture, and that increases in domestic violence, drug and alcohol abuse, and suicide may result.

Glenn Albrecht, director of the Institute for Sustainability and Technology Policy at Australia's Murdoch University, has examined the psychological distress people experience in the face of this kind of slow, but chronic, change in their environments. His work with Australian communities living in areas changed by strip mining or drought revealed that people felt disconnected from nature, were no longer able to find solace in it, and they felt helpless.

"Climate change is a massive driver of change in people's home environment," Albrecht said. "These changes become sources of chronic stress."

Albrecht and his colleagues developed and verified an Environmental Distress Scale, designed to identify stresses related to the degradation of external environments.

"We tend to consider ourselves highly mobile global citizens, but we have a very profound connection to our environment," Albrecht says. "We tend to take that for granted."

So what's to be done? We need to train people to administer "psychological first aid," Smith's Miller said. That means making sure people feel safe after a natural disaster, and educating them about the kinds of psychological responses they might experience.

In the long term, we may also derive some psychological benefit from banding together with other citizens to mitigate the effects of global warming. Taking action might not only give us back a sense of our own sense of efficacy against a powerful outside force, but also help us build community and social ties that offset stress, said Epstein and other specialists.

"Getting involved can be an antidote to the depression that can come from the overwhelming realizations that we have to face . . . ," Epstein said. "It can be empowering to realize that what you do is effective."

Emily Anthes can be reached at emily@emilyanthes.com.