October 3, 2011

Malaria Declines Despite Local Warming

Filed under: Adaptation

“Spreading tropical disease” is high on the list of bad things that are going to happen as the world warms—if you believe the doomsayers. And topping their list of spreading tropical diseases is malaria.

But, as we have on many past occasions pointed out, malaria is neither “tropical” nor is it “spreading.”

In fact, back in the late 19th century, malaria was thought to be endemic in most regions east of the Rocky Mountains—brought to the U.S. in the 16th and 17th century by European colonists and African slaves and spreading across the country with the migration of those populations (Zucker, 1996).


Figure 1. Region of the U.S. where malaria was thought to be endemic in 1882 (source: Zucher, 1996).

Malaria transmission was still a problem in some parts of the U.S. into the mid-20th century, and, in 1946, when the CDC was established (back then it was known as the Communicable Disease Center) its primary goal was to eradicate malaria from the U.S.—which it successful achieved within a few years (with the help of DDT).

All the while, the average temperature in the U.S. was on the rise, increasing by some 1 to 2 degrees between 1882 and the early 1950s.

Clearly, a warming climate did not leading to an expansion of malaria in the U.S.

Nor does it appear to be doing so elsewhere.

About 10 years ago, a paper by Simon Hay and colleagues was published in Nature magazine that made this abundantly clear. Hay et al. analyzed the occurrence of malaria in the east Africa highlands and found the number of cases to be expanding rapidly, however, and here is the kicker, try as they might, they could find no evidence of a driver in the form of climate change. They concluded:

The absence of long- and short-term change in the climate variables and the duration of [malaria transmission] suitability at these highland sites are not consistent with the simplistic notion that recent malaria resurgences in these areas are caused by rising temperatures.

That would seem to throw some cold water on the heated rhetoric of the global warming/tropical disease crowd.

But wait. Within a few months time, Nature published a comment by noted global warming/tropical disease advocate Jonathan Patz and colleagues who challenged Hay’s conclusions—not about the increase of malaria occurrence (of course), but that the climate in the African highlands had not changed to become more suitable for malaria transmission. Patz et al.’s basic contention was that:

• The Hay et al. weather data were interpolated over mountainous terrain.

• Because of mosquitoes’ response to climate thresholds, you don’t need a significant trend in climate—climate variability is important, too;

•Nevertheless, based on a different analysis, regional warming trends do exist that match the increase in malaria.

And of course, the press loved this take, for now everything was right again in the world of global warming doom-and-gloom. For example, Reuters reporter Patricia Reaney wrote:

Climate change could be causing more than higher temperatures—it may also be helping to fuel a rise in Malaria in East Africa…Earlier research had suggested the upsurge was due to drug resistance and population growth, and not global warming. But scientists in the United States and Britain say it may not be just a coincidence that the rise in malaria parallels East African warming trends.

So take that, you naysayers.

At the time, Hay et al. answered their critics, and continued to stand by their conclusions writing in response to Patz et al.:

“Evidence against the epidemiological significance of climate change in the recent malaria resurgences in Africa is mounting and remains unmatched by any contrary evidence.”

Now forward the clock to the present day.

Several members of the original team gathered by Simon Hay have gotten back together to see how malaria and climate have evolved in East Africa over the intervening 10 years since their Nature publication. Their results have just been published in the scientific (open access) journal PLoS ONE. The lead author of the new study is David Stern from the Crawford School of Economics and Government, Australian National University.

What Stern et al. found was quite interesting. With 10 more years of climate data, they have identified significant increases in the temperature throughout their study region. Stern explains that “We conclude that there is now clear evidence of increased temperatures in highland East Africa especially in the last 15 years.”

So perhaps their critics were correct all along, the climate has indeed been warming in East Africa concomitant with the spread of malaria?

Not so fast.

It turns out, that over the past decade or so, the occurrence of malaria in the region has plummeted!

That’s right, despite rising temperature, malaria cases have bottomed out to historically low levels.

Stern postulates that the rise in malaria cases in the region during the 1990s “was probably due to resistance to chloroquine, an older antimalarial drug, and not climate change.”

And the decline in malaria cases in the face of rising temperature is further proof that climate change is not a major player in the rates of malaria transmission in the region. No greenhouse-gases-fuelled apocalypse here. Time to turn your attention elsewhere.

But before leaving, Stern gets in a parting shot.

This screenshot below is taken from Stern’s weblog posting on September 17, 2011 in which he seems to give some advice to the IPCC.

Nice idea, but call us cynical as we seriously doubt that we’ll be seeing this “iconic” figure in the IPCC’s next Assessment Report (AR5)!

References:

Hay, S.I., et al., 2002. Climate change and the resurgence of malaria in the East African highlands, Nature, 415, 905–909.

Hay, S.I., et al., 2002. Hay et al. reply, Nature, 420, 628.

Patz, J.S., et al., 2002. Regional warming and malaria resurgence, Nature, 420, 627–628.

Stern, D., et al., 2011. Temperature and malaria trends in Highlands East Africa. PLoS ONE, 6, e24524.

Zucker, J.R., 1996. Changing patterns of autochthonous malaria transmission in the United States: A review of recent outbreaks. Emerging Infectious Diseases, 2, 37-43.




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