SEA LEVELS ARE RISING AND TREES ARE FLOWERING EARLIER, signs that climate change is already here. And that change arrives with troubling questions about what dangers lie in store for humans. Disease patterns will shift, and cases of heat-related illnesses are likely to rise—problems that will affect how medicine is practiced in the 21st century. “We’re already seeing the adverse effects of climate change on health,” says Barry Levy.

Levy, an adjunct professor of public health at Tufts University School of Medicine in Boston, served as an epidemiologist at the Centers for Disease Control and Prevention and president of the American Public Health Association. Most recently, with Jonathan Patz, he co-edited Climate Change and Public Health, which was named the 2015 Environmental Health Book of the Year by the American Journal of Nursing.

Q: Who are the victims of climate change?
A: Climate change is causing higher temperatures, so there are more heat-related disorders, which can involve serious complications and death. During a heat wave in France in 2003, about 15,000 people died; in a heat wave in India in 2015, more than 2,500 died. People with chronic diseases are at especially high risk, as are older people who are socially isolated, those who work outdoors, and people who live in hot environments.

Higher temperatures can also spark respiratory and allergic disorders, because they increase ground-level ozone, a strong respiratory irritant, and the production of pollen,

Q: What about infectious diseases?
A: Along with higher temperature, there is increased rainfall in many areas and more frequent downpours. That can increase the breeding of mosquitoes, ticks, and other vectors that transmit diseases such as malaria, Lyme disease, and West Nile virus. These vector-borne diseases are being spread more widely in temperate zones and to higher elevations in the tropics, because climates are becoming more favorable to their hosts.

More frequent heavy downpours can also result in flooding. That in itself can cause injury and deaths, as we saw over the summer in Baton Rouge and Houston. And drinking water can become contaminated by sewage, which can cause gastrointestinal illnesses.

Q: What effects can we expect to see on global health?
A: People throughout the world are being affected by climate change, although there are great inequalities. In general, the countries that have produced the most greenhouse gases that drive climate change—the United States, China, and Western European countries, for instance—are suffering the least from its consequences. And those that have produced the least are suffering the most.

Climate change is threatening food production and access to safe water in many low-income countries, because of droughts, floods, and the rising sea level. This is forcing many people to migrate, a displacement that will likely uproot tens of millions of people. You can imagine that such a movement will create socioeconomic and political instability, even collective violence. And these will result in additional, adverse consequences to health and security.

Q: So what needs to be done?
A: We need to better educate health professionals, policymakers, and the general public about climate change. Because it occurs gradually the consequences may not be apparent to many people.  But climate change is likely to affect them, their families and their communities. And there are things they can do to reduce their carbon footprint and address the problem.

As for clinicians and their institutions, they will need to prepare for increased occurrence of illnesses and injuries induced by climate change. And public health workers and their organizations need to work with the communities they serve. They will need to implement preventive measures, develop preparedness plans for heat waves and other extreme weather events, and build individual and community resilience.

We as a society need to substantially reduce the emission of greenhouse gasses. The consequences of doing nothing are too great.