Climate change is already having a negative impact on Americans' health, from respiratory illness to the spread of infectious illnesses like Lyme Disease, a health consortium representing over half the nation's physicians said Wednesday.

Among other health threats, rising temperatures could lead to a reemergence of malaria in the United States, the group said in a new report on the health impact of climate change.

"We're sounding the alarm," said Dr. Mona Sarfaty, director of the new consortium and professor at George Mason University in Fairfax, Virginia. "Climate change has become very real for the health of our patients. Unless we take concerted action, these harms to health are going to get much worse."

The group, the Medical Society Consortium on Climate & Health, comprising 11 medical societies representing 435,000 clinical practitioners, held its national launch Wednesday and called for policymakers to speed the nation's transition to a low-carbon economy. The group will soon submit its new report to Congress.

"Human-caused climate change is happening," Sarfaty said. "Ninety-seven percent of peer-reviewed papers by climate scientists agree with this fact."

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Higher average temperatures caused by manmade climate change lead to heat-related illnesses like heat stroke and dehydration, while air pollution increases incidents of asthma and allergy attacks, the group said.

Rising temperatures can lead to an increase in the number and geographic range of disease-carrying mosquitoes, fleas, and ticks — and may cause malaria to reemerge in the United States, according to the report.

"Ticks that carry Lyme disease have become more numerous in many areas and have expanded their range northward and westward," the report said, noting that the tick that carries Lyme Disease is now reported in 46 percent of US counties, up from 30 percent in 1998.

Higher water temperatures and more extreme weather events like storms and floods are also increasing the odds of contamination of the nation's water and food supply with bacteria and toxins, the report said.

Rising levels of carbon dioxide in the air decrease the nutritional value of certain food crops like wheat, barley, rice, and potatoes because more carbon dioxide makes those plants produce less protein and more starch and sugar as they take on less essential minerals, the report said.

Toxic algae and waterborne diseases will spread into new areas "as increasing water temperatures allow the organisms that cause these health risks to thrive," the report said.

The World Health Organization estimates that climate change will cause 250,000 additional deaths per year between 2030 and 2050 from malnutrition, malaria, diarrhea and heat stress.

One WHO document estimates that a temperature increase of 2-3 degrees Celsius (3.6-5.4 degrees Fahrenheit) will raise the global number of people at risk of infection from malaria by around 3-5 percent — meaning several hundred million people might find themselves living in a malaria zone as warmer temperatures encouraged disease-bearing mosquitoes to move into their area.

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2016 was the hottest year on record, according to US science agencies. The average global temperature for both land and ocean surfaces was 0.94C (1.69F) above the 20th century average. Sixteen out of the seventeen warmest on years on record have occurred in the 21st century, and the five warmest years have all occurred since 2010.

Wednesday's report was released amid a historic fight about national healthcare in Washington, in which the administration of US President Donald Trump and House Majority Speaker Paul Ryan are pushing a new healthcare plan that would undo the policies of former president Barack Obama.

The Trump administration has shown little concern about climate change, and Trump's Administrator of the Environmental Protection Agency, Scott Pruitt, has said he does not believe human activity is a primary contributor to global warming.

"I think that measuring with precision human activity on the climate is something very challenging to do, and there's tremendous disagreement about the degree of impact, so no, I would not agree that it's a primary contributor to the global warming that we see," Pruitt said on CNBC this month.

Pruitt's comments sparked a backlash among scientists, environmentalists, and former EPA officials.

"Anyone who denies over a century's worth of established science and basic facts is unqualified to be the administrator of the EPA," US Senator Brian Schatz, a Democrat from Hawaii and co-chair of the Senate Climate Action Task Force, said in a statement.

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