The global public health emergency involving deformed babies emerged in 2015, the hottest year in the historical record, with an outbreak in Brazil of a disease transmitted by heat-loving mosquitoes. Can that be a coincidence?
Scientists say it will take them years to figure that out, and pointed to other factors that may have played a larger role in starting the crisis. But these same experts added that the Zika epidemic, as well as the related spread of a disease called dengue that is sickening as many as 100 million people a year and killing thousands, should be interpreted as warnings.
Over the coming decades, global warming is likely to increase the range and speed of the life cycle of the particular mosquitoes carrying these viruses, encouraging their spread deeper into temperate countries like the United States.
Recent research suggests that under a worst-case scenario, involving continued high global emissions coupled with fast population growth, the number of people exposed to the principal mosquito could more than double, to as many as 8 billion or 9 billion by late this century from roughly 4 billion today.
“As we get continued warming, it’s going to become more difficult to control mosquitoes,” said Andrew Monaghan, who is studying the interaction of climate and health at the National Center for Atmospheric Research in Boulder, Colo. “The warmer it is, the faster they can develop from egg to adult, and the faster they can incubate viruses.”
Already, climate change is suspected — though not proved — to have been a factor in a string of disease outbreaks afflicting both people and animals. These include the spread of malaria into the highlands of eastern Africa, the rising incidence of Lyme disease in North America, and the spread of a serious livestock ailment called bluetongue into parts of Europe that were once too cold for it to thrive.
In interviews, experts noted that no epidemic was ever the result of a single variable.
Instead, epidemics always involve interactions among genes, ecology, climate and human behavior, presenting profound difficulties for scientists trying to tease apart the contributing factors. “The complexity is enormous,” said Walter J. Tabachnick, a professor with the Florida Medical Entomology Laboratory, a unit of the University of Florida in Vero Beach.
The epidemics of Zika and dengue are cases in point. The viruses are being transmitted largely by the yellow fever mosquito, Aedes aegypti. That creature adapted long ago to live in human settlements, and developed a concomitant taste for human blood.
Cities in the tropics, the climate zone most favorable to the mosquito, have undergone explosive growth: Humanity passed a milestone a few years ago when more than half the population had moved to urban areas. But spending on health care and on basic public health infrastructure, like water pipes and sewers, has not kept pace. Mosquito control has also faltered in recent decades.
The mosquito lays its eggs in containers of water, of a sort that are especially common in the huge slums of Latin American cities. With unreliable access to piped water, people there store water in rooftop cisterns, buckets and the like. Old tires and other debris can also become mosquito habitat.
Water storage near homes is commonplace in areas where Zika has spread rapidly, like the cities of Recife and Salvador in northeastern Brazil, and where dengue experienced a surge in 2015, like São Paulo, Brazil’s largest state.
Altogether, dengue killed at least 839 people in Brazil in 2015, a 40 percent increase from the previous year. Worldwide, dengue is killing more than 20,000 people a year.
Several experts said in interviews that a main reason for the disease outbreaks was most likely the expansion of the number of people at risk, through urbanization, population growth and international travel. They see the changing climate as just another stress on top of a situation that was already rife with peril.
While they do not understand to what degree rising temperatures and other weather shifts may have contributed to the outbreaks, they do understand some of the potential mechanisms.
The mosquitoes mostly live on flower nectar, but the female of the species needs a meal of human blood to have enough protein to lay her eggs. If she bites a person infected with dengue, Zika or any of several other diseases, she picks up the virus.
The virus has to reproduce in the mosquito for a certain period before it can be transmitted to another person in a subsequent bite. The higher the air temperature, the shorter that incubation period. Moreover, up to a point, higher temperatures cause the mosquitoes to mature faster.
With rising temperatures, “You’re actually speeding up the whole reproductive cycle of the mosquitoes,” said Charles B. Beard, who heads a unit in Fort Collins, Colo., studying insect-borne diseases for the Centers for Disease Control and Prevention in Atlanta. “You get larger populations, with more generations of mosquitoes, in a warmer, wetter climate. You have this kind of amplification of the risk.”
Aedes aegypti is present across the southern tier of the United States. Brief outbreaks of dengue have occurred recently at the warmest margins of the country, and one is underway in Hawaii. But with pervasive window screens and air-conditioning, the risk of disease transmission is far less for most Americans than for people in poorer countries.
The mosquito does not thrive in areas with cold winters. Some research suggests that continued climatic warming could allow the mosquito to colonize more of North America in coming decades, though how much of a disease risk that would represent is anybody’s guess.
The yellow fever mosquito competes with a cousin, the Asian tiger mosquito, that has also colonized the United States, and is more tolerant of cold weather. Whether one would beat out the other in a hotter climate is unclear. Likewise, it is unclear how effective the Asian tiger mosquito might become at transmitting Zika or dengue viruses.
In principle, the risk from continued global warming applies not just to temperate countries, but to cities at high altitude in tropical countries. Researchers are keeping a close eye on Mexico City, for instance.
With 21 million people in the city and its suburbs, Mexico City is the largest metropolis of the Western Hemisphere. While the lowlands of Mexico are plagued by yellow fever mosquitoes and the viruses they transmit, the country’s capital sits on a mountain plain that has — up to now — been too cold for the mosquitoes.
But temperatures are rising, and the mosquitoes have recently been detected in low numbers near Mexico City. “The mosquito is just down the hill, literally,” Dr. Monaghan said. “I think all the potential is there to have virus transmission if climatic conditions become a bit more suitable.”