Climate Change: When Dengue Reaches Helsinki

Old Church Park in Helsinki
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Dengue-infected mosqutoes could reach Helsinki by the end of the century if global emissions continue climbing at the current rate. (Image: I99pema / Wikimedia)

NEW HAVEN, CONNECTICUT – Although nearly half the world’s population now faces the risk of dengue, a mosquito bite in Beijing, Berlin or Baltimore remains a nuisance, not a threat.

But for how long?

To answer this sort of question—that is, to project the health effects of rising global temperatures—researchers are increasingly using complex mathematical models.

Among them is Swedish mathematician and epidemiologist Joacim Rocklöv, PhD. Working with colleagues in Sweden, Singapore, the US and Brazil, the Umeå University scientist has used modeling to project the spread of dengue. At a talk earlier this month at Yale University School of Public Health, Rocklöv showed dengue maps for 2 possible futures: one in which nations come close to achieving the goals of the Paris climate accord, and one in which global emissions climb at current rates.

Even in the more optimistic scenario (technically, a pathway called RCP 2.6), by the last decade of the 21st century, someone drinking an iced coffee at an outdoor café in Rome or Madrid will risk contracting dengue. The picture is far worse in a future in which emissions have followed current pathways (RCP 8.5). In that scenario, even a sidewalk café as far north as Helsinki or Amsterdam might be plagued in summer by dengue-infected mosquitoes, primarily Aedes aegypti, but also Aedes albopictus. Both mosquito species also transmit chikungunya, yellow fever and Zika infections. “It’s quite alarming,” Rocklöv said, adding that he expects that maps of the United States would show a similar trend: dengue moving north.

Maps like these might serve to convey the urgency of climate change, says Rocklöv, a professor of epidemiology and public health: “If people knew the difference between a Paris-agreement future and a business-as-usual future, I think they would be very eager to do a lot of mitigation and transform the society,” he said.

Assessing the future risk of dengue geographic spread of dengue is part of an international collaboration of 100 modelling groups called the Inter-Sectoral Impact Model Intercomparison Project. Initiated by research institutes in Austria and Germany, the consortium aims to show how the world will look under different climate-change scenarios, modeling the effects of different emissions pathways on, for example, fisheries, permafrost, water resources, tropical cyclones and health. Rocklöv noted that average global temperature increases will under-predict how much hotter the world will become for human beings: If temperature increases are calculated based on where human populations are concentrated, the increases for most people will likely be 2X the global average.

Modelers also use data to describe what has already happened. Rocklöv was part of a large international group that studied how ambient temperatures in 13 countries have affected mortality. The researchers looked at 74 million deaths during selected periods between 1985 and 2012. The concluded that cold has caused more “excess” mortality than heat has. Further, they found that most deaths due to ambient temperature didn’t occur during spells of extreme heat or cold, but rather during periods of milder non-optimal weather.

In his talk, Rocklöv also discussed how investigators might use “big data” for prevention. During a dengue outbreak in Yogyakarta, on Java, Indonesia, Rocklöv and colleagues used Twitter messages to detect linkages among residents of different neighborhoods in the city. Using Twitter traffic to illuminate these social networks, they watched for parallels in the spread of infection. “Mobility seems to explain a lot of the intra-city dynamics of dengue,” he said. Maps of interactions within a community can be integrated with climate information, such as sea surface temperatures, to project where outbreaks might occur and to curb disease spread.

Rocklöv showed an image of an iceberg to represent current knowledge of the implications of climate change on health: the ice above the surface—what we know—represents only a small part of the picture. “There are a lot of indirect effects and complex interactions that are hard to disentangle,” he said.

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