(CNN) — Climate change and antimicrobial resistance are two of the greatest threats to global health, according to a new report from the United Nations Environment Programme.

The report, titled "Bracing for Superbugs," highlights the role of climate change and other environmental factors contributing to the rise of antimicrobial resistance. It was announced Tuesday at the Sixth Meeting of the Global Leaders Group on Antimicrobial Resistance in Barbados.

Antimicrobial resistance or AMR happens when germs such as bacteria, viruses and fungi develop the ability to defeat the medications designed to kill them.

"The development and spread of AMR means that antimicrobials used to prevent and treat infections in humans, animals and plants might turn ineffective, with modern medicine no longer able to treat even mild infections," the UN Environment Programme said in a news release.

Roughly 5 million deaths worldwide were associated with antimicrobial resistance in 2019, and the annual toll is expected to increase to 10 million by 2050 if steps aren't taken to stop the spread of antimicrobial resistance, according to the report.

In the US, there are nearly 3 million antimicrobial-resistant infections each year, and more than 35,000 people die as a result, the US Centers for Disease Control and Prevention says.

Antimicrobials are commonly used in cleaning products, plant pesticides and medications to kill and prevent the spread of germs among people, animals and crops.

Drug resistance can develop naturally, but experts say the overuse of antimicrobials in people, animals and food production has accelerated the process. The microorganisms that survive these chemicals are stronger and more powerful, and they can spread their drug-resistant genes to germs that have never been exposed to antimicrobials.

The focus so far has largely been on excessive antimicrobial use, but experts say there is growing evidence that environmental factors play a significant role in the development, transmission and spread of antimicrobial resistance.

"Climate change, pollution, changes in our weather patterns, more rainfall, more closely packed, dense cities and urban areas -- all of this facilitates the spread of antibiotic resistance. And I am certain that this is only going to go up with time unless we take relatively drastic measures to curb this," said Dr. Scott Roberts, an infectious diseases specialist at Yale School of Medicine, who was not involved with the new UN report.

Climate change and antimicrobial resistance

The climate crisis worsens antimicrobial resistance in several ways. Research has shown that increased temperatures increase both the rate of bacterial growth and the rate of the spread of antibiotic-resistant genes between microorganisms.

"As we get a more extreme climate, especially as it warms, the gradients that drive the evolution of resistance will actually accelerate. So, by curbing temperature rises and reducing the extremity of events, we can actually then fundamentally curb the probability of evolving new resistance," Dr. David Graham, a professor of ecosystems engineering at Newcastle University and one of the UN report's authors, said at a news conference ahead of the report's release.

Experts also say severe flooding as a result of climate change can lead to conditions of overcrowding, poor sanitation and increased pollution, which are known to increase infection rates and antimicrobial resistance as human waste, heavy metals and other pollutants in water create favorable conditions for bugs to develop resistance.

"The same drivers that cause environmental degradation are worsening the antimicrobial resistance problem. The impacts of antimicrobial resistance could destroy our health and food systems," Inger Andersen, the UN Environment Programme's executive director, said at the news conference.

Making diseases harder to treat

Environmental pressures are creating bugs that thrive in the human body, which experts say is unusual for some species.

"There's one hypothesis from a prominent mycologist who suggests that the reason the body's temperature is 98.6 is because that is the temperature where fungi can't grow that well. And so, now we're seeing Candida auris and some of the other new microbes that have come up that really grow quite well -- even at temperatures of 98.6 in the human body. And so I think climate change, really selecting for these organisms to adapt to a warmer climate, is going to increase the odds that there's infection in humans," Roberts said.

Such opportunistic infections jeopardize medical advancements like joint replacements, organ transplants and chemotherapy — procedures in which patients have a significant risk of infection and require effective antibiotics.

Drug-resistant infections can make treatment difficult or even impossible. Roberts says that resorting to "last-ditch treatments" is "never a good scenario from the patient level because there are reasons we don't use them up front," such as organ toxicity and failure.

"When somebody does present with a drug-resistant bacteria or fungus and we really need to rely on one of these last-line antibiotics, it's usually a challenge to treat from the outset. And so the patients really don't do as well as a result," he said. "In rare circumstances, we run out of options entirely, and in that case, there's really nothing we can do. Fortunately, those cases remain quite rare, but I am certain that with this growing antibiotic resistance problem, we'll see these increasing frequency over time."

Actions to fight antimicrobial resistance

Experts say that both climate change and antimicrobial resistance have been worsened by and can be improved by human actions. One critical step is to limit antibiotic overuse and misuse.

"Antibiotics and antifungals do not work on viruses, such as colds and the flu. These drugs save lives. But, anytime they are used, they can lead to side effects and antimicrobial resistance," the UN report's authors wrote.

The authors also emphasize that the health of people, animals, plants and the environment are closely linked and interdependent, and they call on governments to identify policies to limit antibiotic use in agriculture and reduce environmental pollution.

Finally, experts say, steps to reduce climate change are steps to limit antimicrobial resistance.

"Whatever we can do on an individual level to kind of reduce the impact of climate change, really, that's kind of only worsening this problem, as well as pollution and urbanization and in dense, crowded areas. Although I know from the individual level that's a hard thing to change," Roberts said.

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