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COMMENTARY

Don Paul: NASA study illuminates link between warming, disease

Don Paul

According to most models, we are in a weak El Niño right now that will persist for months to come. But the last El Niño was a biggie – the biggest in at least 50 years, occurring from late 2015 into 2016.

Tropical disease specialists, entomologists, epidemiologists and public health scientists have long known a mean warming climate will impact the epidemiology of tropical diseases, allowing them and their carriers, such as insects, to spread into regions vulnerable to warming and changes in precipitation patterns. The recent massive El Niño has served as a global laboratory over a short span of time as to what can happen with marked and rapid warming. Just to be clear, El Niños have occurred prior to the era of the current warming climate. There may be an increase in the frequency of El Niños in this warming era, but such a tie-in between the warming climate and the increased frequency has not yet been firmly established. But since additional mean warming is inevitable due to the continued increases in the greenhouse gases carbon dioxide, methane and water vapor (the latter increase due to more evaporation from the heating due to human activity), a new study on what happened with disease spread during the “super” El Niño can serve as a basis for study of disease pattern spread over a shorter period of time with more intense short-term warming.

A just-released NASA study is the first of its type to monitor disease patterns related to short-term warming and regional changes in precipitation. While it is not an exact fit for what will occur during the more gradual mean warming, it does serve up quite a number of verified case examples.

As NASA stated, “During the 2015-2016 event, changes in precipitation, land surface temperatures and vegetation created and facilitated conditions for transmission of diseases, resulting in an uptick in reported cases for plague and hantavirus in Colorado and New Mexico, cholera in Tanzania, and dengue fever in Brazil and Southeast Asia, among others.” Lead author Assaf Anyamba noted the analysis of satellite data, model output and public health records made it possible to get a handle on numerous disease events on a global basis. The study was able to utilize NASA datasets on land temperature, soil moisture and vegetation changes in imagery gathered by NASA’s Terra Satellite, along with NASA and NOAA precipitation datasets. Here, for example, is high-resolution imagery of the spread of moisture and flooding over a region in Australia between January and early February:

Based on monthly outbreak data from 2002 through 2016 in Colorado and New Mexico, reports of plague were at their highest by late 2015 and hantavirus cases peaked in 2016. The weather link was an El Niño-driven increase in both temperatures and rainfall that spurred more vegetative growth. This environment provided more food for the rodents that carry these two potentially fatal diseases. There was more human contact, mainly through more fecal and urine contamination. More rodents meant more of the fleas they carry and the diseases the fleas carry.

In East Africa’s Tanzania, cholera cases were the second and third highest in 2015 and 2016 over an 18-year period. Increased rainfall driven by El Niño allowed more fecal contamination in sewage and the fresh water supply, as well as on foodstuffs. Cholera, a potentially fatal bacterial disease, doesn’t leave a region quickly. The peak has lingered into 2017 and 2018.

Dengue fever, a potentially fatal disease carried by mosquitoes, increased in Brazil and Southeast Asia during the El Niño. In these two regions, the drier weather associated with a strong El Niño drove mosquito populations into urban areas where they could find more standing water supplied by people. The water is needed for laying eggs. The warmth led to more rapid sexual maturity among the now-urban mosquito population, and led to more aggressive biting of humans.

The less frequent “super” El Niños are generally better forecasted months in advance. This improved forecasting of the 2015-16 El Niño led to USDA entomologists being better able to advise East African nations to vaccinate cattle to avoid a Rift Valley fever outbreak tied to El Niño-driven warming and precipitation. These vaccinations likely prevented thousands of human Rift cases and thousands of livestock deaths.

The rapid accrual of heating in tropical central and eastern Pacific waters during a super El Niño has weather impacts around the globe. The release of heat and water vapor from these warmer waters changes the upper level flow and positions of dry and wet weather outbreaks, along with regions of warming, on a global basis.

El Niños are not microcosms of the more gradual mean global warming climate. But this study does confirm the modeled and predicted spread and increased rates of occurrence of numerous tropical diseases associated with a warming climate, on a shorter timescale. Similar changes in the epidemiology of tropical disease, geographical coverage and rates of occurrence tied directly to a mean warming climate independent of El Niños will be more gradual. While the frequency of El Niños may be on the increase in some way related to the warming climate, that is still not confirmed in long-term studies.

What does seem more apparent is a shorter and lesser relative reduction in global temperature rates of increase in the period following peak El Niño flare-ups. When such a flare-up occurs, the following year has to statistically show a slowing in mean warming by comparison. If, say, ibuprofen drops a fever from 104 degrees to 100, that’s a significant drop-off. Nonetheless, you still have a fever.

The NASA study has provided vital datasets in tropical medicine, entomology, virology, bacteriology and epidemiology for what much of humanity can expect on a more gradual basis for at least the next century to come.

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