Warming temperatures are chasing animals and plants to new habitats, sometimes with devastating consequences to ecosystems. But there is little evidence regarding how far and how fast the invaders might be moving.
A new study offers a glimpse of the future by looking to the past. Mosquitoes that transmit malaria in sub-Saharan Africa have moved to higher elevations by about 6.5 meters (roughly 21 feet) per year and away from the Equator by 4.7 kilometers (about three miles) per year over the past century, according to the study.
That pace is consistent with climate change and may explain why malaria’s range has expanded over the past few decades, the authors said. The results have serious implications for countries that are unprepared to cope with the disease.
“If this were random, and if it were unrelated to climate, it wouldn’t look as cleanly climate-linked,” said Colin Carlson, a biologist at Georgetown University’s Center for Global Health Science and Security and the paper’s lead author. The study was published on Tuesday in the journal Biology Letters.
Most studies on the impact of climate change on health tend to focus on the spread of disease — which can be tricky to trace to any single cause — and to be predictive. The new study is instead a retrospective look at how mosquitoes have moved.
“This really corresponds to where and how transmission is actually happening at those locations,” said Sadie Ryan, a medical geographer at the University of Florida.
The rate of displacement confirms experts’ worst fears about the impact of climate change on infectious diseases, Dr. Ryan added.
“What we expect is quite dramatic — and it does look pretty dramatic,” she said. “The fact that it’s actually doing what we are anticipating and scared of happening is very compelling.”
As the planet warms, plants and animals — particularly invertebrates — are seeking cooler temperatures, either by moving to higher altitudes or by moving closer to the poles. One meta-analysis estimated that, to date, terrestrial species have been moving uphill at a pace of 1.1 meter (3.6 feet) per year and toward the poles at a pace of 1.7 kilometers (1.1 miles) per year.
Ticks that transmit Lyme disease, for example, are dramatically expanding their range in the northern United States. Bats are also on the move, and with them diseases that they transmit, such as rabies.
In the Northeast, lobsters are dying of a fungal disease linked to warming, and fish are migrating north or into deeper waters in search of cooler temperatures. That leaves seabirds like puffins with a dwindling food supply and forces commercial fisheries to switch to new types of catch.
“Often we reduce the impacts of climate change down to the world just generally getting warmer, and we don’t often think about the vastly interconnected world in which we live,” said Morgan Tingley, an ecologist at the University of California, Los Angeles.
While species have been redistributed on the planet for millions of years in response to the climate, the changes are now “happening radically fast,” Dr. Tingley said. “That is not going to work well for a lot of species, and it’s not going to go very well in terms of the stability of ecosystems.”
In Hawaii, the invasion of new mosquito species threatens two endangered species of birds with avian malaria: the ‘akeke’e and the ‘akikiki. There are fewer than 1,000 ‘akeke’es and fewer than 50 ‘akikikis; the latter have declined precipitously in recent years and are expected to become extinct this decade, Dr. Tingley said.
He and other researchers underscored the importance of collecting data to understand exactly how and how fast mosquitoes and other disease carriers are moving across the world. Warmer climates are expected to be advantageous for mosquitoes because they, and the parasites they carry, reproduce faster at higher temperatures.
“We live in a world that is 1.2 degrees warmer, and we haven’t really checked if that is starting to happen,” Dr. Carlson said.
He and his colleagues relied on a massive database published in 2017 that chronicled the distribution of 22 species of mosquitoes in sub-Saharan Africa between 1898 and 2016. The data set mined information from entomological surveys, peer-reviewed publications, technical reports, theses and archival records.
Over that period, different species of Anopheles mosquitoes expanded their range by a cumulative average of 2,300 feet in elevation and to more than 300 miles south of the Equator, the study found.
Dr. Tingley said he had full confidence in the trend reported in the study, but “will take that rate with, like, a massive grain of salt.” That’s partly because the study may be underestimating the change by not factoring in the accelerated pace of global warming in more recent years, he said.
Some mosquito movement may also be because of changes in land use, the availability of food or a side effect of people migrating to higher elevations because of climate change, experts said. Still, disease-bearing mosquitoes are of serious concern in areas where people and institutions are unprepared.
“A heat wave is much deadlier in Detroit than it is in San Antonio at the same temperature,” Dr. Carlson said. “It’s the surprise that kills you.”
Apoorva Mandavilli
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