A new study confirms that climate change does directly affect human health. Not addressing the problem could mean an increase in respiratory diseases, heatstroke, and mosquito-borne illnesses like chikungunya and West Nile virus.

According to a study published today in JAMA, 20 years of evidence confirms that climate change is linked to illnesses such as heatstroke and asthma. The researchers predict an increase in the number of days with extreme heat that could worsen many health conditions.

The study’s release was timed to coincide with the United Nations (UN) Climate Summit 2014 on September 23 in New York City. UN Secretary-General Ban Ki-moon has invited world leaders to help reduce emissions, strengthen climate resilience, and mobilize political will for a meaningful global climate agreement in 2015.

In the days leading up to the summit, the People’s Climate March in New York City attracted more than 300,000 demonstrators who voiced their concerns about climate change.

According to the study authors, 97 percent of climatologists maintain that climate change is caused by human activities, particularly fossil fuel combustion and tropical deforestation. This change is linked to human health. The study’s authors suggest that doctors should understand this relationship and talk about associated health risks with their patients.

Dr. Jonathan A. Patz, M.P.H., of the Global Health Institute at the University of Wisconsin and colleagues set out to provide new temperature projections for the United States. Their goal was also to review recent studies on health risks related to climate change and the benefits of efforts to reduce greenhouse gas emissions.

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The researchers predict that by 2050, many United States cities will experience more frequent extreme heat days. For example, they envision that New York City and Milwaukee may have three times their current average number of days hotter than 90°F.

This increased heat may make heat-related disorders, such as heat stress, worse. It may also reduce work capacity. Adverse health impacts of climate change may include:

  • respiratory disorders, including those made worse by pollution, such as asthma and allergic diseases
  • infectious diseases, including mosquito-borne diseases and water-borne diseases, such as childhood gastrointestinal diseases
  • food insecurity, including reduced crop yields and an increase in plant diseases
  • mental health disorders, such as post-traumatic stress disorder and depression, that are associated with natural disasters

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Dr. Braden Meason, a resident physician in Emergency Medicine at Denver Health Medical Center in Colorado, and Dr. Ryan Paterson, a staff physician in Emergency Medicine for the Kaiser Permanente Group in Colorado, reported in the Health and Human Rights Journal that global climate change leading to warmer temperatures and changes in rainfall patterns allow mosquitos to thrive in places they previously could not. This leads to the spread of mosquito-borne diseases.

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For instance, the authors say, the chikungunya virus is closely tied to weather patterns in Southeast Asia. “Extrapolation of this regional pattern, combined with known climate factors impacting the spread of malaria and dengue, [paint] a dark picture of climate change and the spread of this disease from south Asia and Africa … As drought and heavy rainfall events increase with climate change and disease vectors spread, chikungunya prevalence is likely to increase, with the possibility of becoming endemic worldwide.”

Outbreaks of chikungunya have spread from Africa, Asia, Europe, and the Indian and Pacific Oceans. In late 2013, chikungunya virus was found for the first time in the Americas on islands in the Caribbean. Last July, the first U.S. case was identified in Florida.

Chikungunya virus likely will continue to spread to new areas in North America, Central America, and South America through infected people and mosquitoes, according to the Centers for Disease Control (CDC).

Learn More About West Nile Virus »

West Nile virus transmission has been documented in Europe and the Middle East, Africa, India, parts of Asia, and Australia. It was first detected in North America in 1999, and has since spread across the continental United States and Canada. As of September 16, a total of 45 states and the District of Columbia had reported West Nile virus infections in people, birds, or mosquitoes. Overall, 725 cases of West Nile virus disease in people have been reported to CDC.

Researchers from the Center for Tropical Research, at UCLA’s Institute of the Environment and Sustainability, note the most important climate variables predicting West Nile virus rates in a given year are temperature and rainfall.

In an article published in the journal Global Change Biology, they state that in 2012, there were more than 5,500 human cases of the disease reported in 48 states, the highest number in more than a decade. They predict that in California, approximately 68 percent of the state’s area will have an increase in the probability of West Nile virus by 2050.

On September 23, the Washington Post reported that the unprecedented drought in California might be responsible for at least 311 human cases of the West Nile virus currently in that state. “Droughts, along with warm weather, can produce the conditions necessary for an abundance of the insects,” stated the report.