Climate malaria vector-borne diseases

The common solution

In a recent article in GRL, Emerging selection bias in large-scale climate change simulations, Kyle L. Swanson describes how experiments might go wrong as we seek a desired solution. He hypothesise a common wish to reproduce the recent warming in the Arctic, has lead to less diversity among models with convergence towards some common solution.

He writes the current generation ensembles of model simulations are statistically inconsistent with the observed shifts in in both the mean surface air temperature as well as the frequency of  extreme monthly mean temperature events due to climate warming, despite a marked reduction in the spread among ensemble members that by itself suggests convergence towards some common solution. This convergence indicates the possibility of a selection bias based upon warming rate.

I particularly liked his example of what happened after Robert A. Millikan’s original measurement of the charge of the electron: “Millikan’s original measurement was slightly erroneous due to the use of an incorrect value of the viscosity of air. In the decades following Millikan’s work and his subsequent Nobel Prize, other investigators empirically measured the electron charge. When they got a number that was too high above Millikan’s, they thought something must be wrong–and they would look for and find a reason why something might be wrong. When they got a number close to Millikan’s value they didn’t look so hard. And so they eliminated the numbers that were too far off, and did other things like that.”

In our paper in Malaria Journal, A dynamic model of some malaria-transmitting anopheline mosquitoes of the Afrotropical region. I. Model description and sensitivity analysis, we touch upon this issue by stating “A model is mental copy that describes one possible representation of a system… We present an alternative formulation of the dynamics of An. gambiae s.s. and An. arabiensis…  if malaria modelers move towards the ensemble thinking widely adopted in the climate community, this model could be one representation of historical and future changes for malaria. The aim of such an ensemble would be to deal with uncertainties in the system. Ultimately, the goal would be to produce policy-relevant information including uncertainty.”

The basic principles of climate are quite easy to understand; the sun is heating the Earth, with more radiation hitting the equator compared to the poles. Seasons are caused by the tilt of the Earth’s rotational axis away or toward the sun as it travels through its year-long path around the sun. From there you can add clouds, water, ice, land masses, aerosols, precipitation, and even mountain and building shadows if you are interested in local weather. The dynamics of malaria can be simplified in the same manner; an uninfected mosquito bites an infectious human, lives for about ten days, and from that time it can infect new humans every time it bites a human. From this simple model it has been demonstrated that killing mosquitoes is an efficient way of reducing malaria, but it is not able to tell us how efficient. To get a realistic representation of how emissions of greenhouse gasses influence climate, or how malaria is reduced by distributing bed nets we need more complex models together with observations. As we add more realism to the system, we also parameters with uncertainty, uncertainty which was there from the beginning, but which becomes visible as we describe them. If we intended to use an ensemble of models to estimate the effect of for example bed nets against malaria, we could get a very precise estimate if all models were identical, or measured their success by managing to keep malaria away from South Africa (many malaria models predict malaria is widespread there). There are still many things we do not know about malaria, and I hope modellers of malaria do not do the same mistake it seems like global climate models are moving towards; a common solution due to wrong reasons.

Climate malaria

Malaria in the UK and climate change

A couple of days ago I saw this tweet:

I agree with Tim France, but with the short tweet he also assumes UK will be a rich country with a good health system in 100 years. Below are some thoughts on how we evaluate the impacts of climate change, following up a previous post.

Every time driving a car, there is a chance you will crash. In thunderstorms you might be hit by lightning, and at any time your home could be hit by a meteor. If you drive faster the consequences of a car crash is probably more severe; if you fill your pockets with iron and stand under a tree in a thunderstorm that might be a bad idea; we can increase or reduce the risk of something bad happening. For meteors, well, there is currently not much we can do. What all of these threats have in common is the low probability they will occur, but if they do the result might be devastating.

How are these risks related to malaria in the UK and climate change? The answer is probability. Malaria was common in the in Northern Europe including UK, Sweden and Finland 250-100 years ago, but is no longer a problem. While temperatures declined or showed no trend from about 1750 to the 1920s they have risen since, and we have seen no resurgence of malaria in Europe (except from small outbreaks).

Let us go back to the car. The risk, or probability, of serious injuries in a car crash is dependent on many factors. Increasing the speed will increase the risk, but how do we compare driving a car at 40 MPH without a seat belt to driving the a car at 50 MPH with a seatbelt? From 1750 to 2013 Europe got seat belts and air bags preventing malaria; better houses (less mosquito contact), improved health systems (shorter time to recovery), the population density increased (less mosquitoes per human), and land use changed (less breeding sites for mosquitoes). These factors allowed us to increase the air temperature without increasing the risk of malaria epidemics. So what about the future?

It is virtually certain temperatures will continue to increase the next 100 years. The temperature increase alone will potentially lead to more malaria. But the real impact? Do we assume UK has the same health system in 2100 as today? Will people live in even better houses? What if there is a war in 2080? The IPCC does not make assumptions or scenarios describing alternative realities (on national scales) on how societies will develop the next 100 years. To understand the possible real, and not only theoretical, impacts of climate change, we must also dare to speculate about how society will evolve over the next 100 years. Only then we can come closer to understanding the risk of increasing the speed from 40 to 50 MPH, and know under which assumptions we are making projections about impacts of climate change.