Climate change, mosquitoes, human health and uncertainty - a case study

Alistair Woodward

Wellington School of Medicine and Health Sciences

University of Otago

International Summit on Science and the Precautionary Principle, Lowell 2001

Biting back New Scientist 23 Sep 00

Malaria is marching north, and global warming will make it worse,with mosquito armies colonising Europe, the US and highland regions of the South. That's the picture being painted by a panel of UN scientists and several national governments. But for one of the world's senior entomologists, this is not honest science. Paul Reiter is chiefentomologist at the US government's dengue research lab in Puerto Rico. And he's afraid that attributing the spread of malaria to global warming could detract attention from much-needed efforts to combat the disease itself and save lives now.

Scientific views on climate change and mosquito-borne diseases

• Differences are over-stated in the popular media

• Nevertheless, there are disagreements

• If we understand the reasons, we may get a clearer picture of major sources of uncertainty in assessments of this kind

Why climate change and mosquito-borne disease is a concern

• The evidence for accelerated global warming is strong

• Mosquitoes and the pathogens they transmit are temperature-sensitive

• Mosquito-borne diseases affect hundreds of millions of people each year

20

15

1900 21002000

14

16

17

18

13

19

Temperature (OC)

Year

2.5oC

Bestestimate

Low

High

IPCC estimates(2001): c. 1-5oC

1.2oC

2050

Relationship between temperature and malaria parasite development time inside mosquito (“extrinsic incubation period” or EIP). EIP shortens at higher temps, so mosquitoes are infectious sooner.

Climate change and mosquito-borne disease: IPCC

McCarthy JJ, Canziani OF et al(eds). Climate Change 2001: Impacts, Adaptations and Vulnerability. Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, Cambridge University Press 2001.

TRANSMISSION POTENTIAL

0

0.2

0.4

0.6

0.8

1

14 17 20 23 26 29 32 35 38 41

Temperature (°C)

Incubation period

0

10

20

30

40

50

15 20 25 30 35 40

(day

s)Biting frequency

0

0.1

0.2

0.3

10 15 20 25 30 35 40

Temp (°C)

(per

day

)

Survival probability

0

0.2

0.4

0.6

0.8

1

10 15 20 25 30 35 40

(per

day

)

P.vivaxP.falciparum

Temp (°C) Temp (°C)

Climate change and mosquito-borne disease: IPCC

Health impact Confidence

Move to higher altitudes Medium - high

Move to higher latitudes Medium - low

Extended transmissionseason

Medium - high

Increased population inareas of potentialtransmission

Medium - high

Decreased transmissionwhere temperatures high

Low - medium

“tend to increase in range and incidence … actual occurrence strongly influenced by local conditions”

Climate change and mosquito-borne disease: Reiter

Climate change and mosquito-borne disease. Env Health Perspect 2001; 109: 141-161

Climate change and malaria: temperatures without fevers? Science 2000; 289: 1697-8

From ague to West Nile. Scientific American 2000; 283(6): 10

The changing distribution of malariaThe changing distribution of malariaa) The current distribution according to WHO

b) The distribution of malaria in 1850-1870

Historical Distribution of Malaria Cases

“climate has rarely been the principal determinant [in the past]; human activities and their impact on local ecology have generally been much more significant. It is therefore inappropriate to use climate-based models to predict future prevalence”

Climate change and mosquito-borne disease: Reiter

Sources of uncertainty for Sources of uncertainty for assessing CC-attributable assessing CC-attributable

burden-of-diseaseburden-of-disease

Change in climate

Emissions

GHG concentrations

Climate/weather-healthrelationships

Impact modelImpact model

Socio-economic scenarios: modulating effects

Climate change and mosquito-borne disease: where there is not disagreement

• Climate change is happening

• Vectors are temperature-sensitive

• The etiology of these diseases is complex; social and economic factors are critical

• Climate has not been the major determinant over the last thousand years

Climate change and mosquito-borne disease: some major differences

• Disease potential v. disease prediction

It is inappropriate to use climate based models to predict future prevalence. (Reiter)

The objective is not to propose testable long-range hypotheses (in a once-only global experiment). It is to provide indicative forecasts of a critically important consequence, to guide pre-emptive policy-making. (McMichael)

Climate change and mosquito-borne disease: some major differences

• Assumptions about a future world

“if the present warming trend continues, human strategies to avoid these temperatures are likely to become more prevalent” (Reiter)

GlobalisationGlobalisation

RegionalisationRegionalisation

Markets,Markets,ConsumerismConsumerism

Community,Community,conservationconservation

A1. WorldMarkets

B1. GlobalSustainability

A2. ProvincialEnterprise

B2. LocalStewardship

Very high economic growthVery high economic growth2100 population: 7 billion2100 population: 7 billionMedium mitigation, high adaptationMedium mitigation, high adaptationTemp (2050s) +1.6Temp (2050s) +1.6o o CCRainfall: + 11% winter, -7% summerRainfall: + 11% winter, -7% summer

High economic growthHigh economic growth2100 population: 7 billion2100 population: 7 billionHigh mitigation, low adaptationHigh mitigation, low adaptationTemp (2050s) +0.8Temp (2050s) +0.8o o CCRainfall: + 7% winter, -1% summerRainfall: + 7% winter, -1% summer

Moderate economic growthModerate economic growth2100 population: 15 billion2100 population: 15 billionNo mitigation, low adaptationNo mitigation, low adaptationTemp (2050s) +2.2Temp (2050s) +2.2o o CCRainfall: + 14% winter, -10% summerRainfall: + 14% winter, -10% summer

Low economic growthLow economic growth2100 population: 10 billion2100 population: 10 billionVariable mitigation and adaptationVariable mitigation and adaptationTemp (2050s) +1.6Temp (2050s) +1.6o o CCRainfall: + 11% winter, -7% summerRainfall: + 11% winter, -7% summer

Climate change and mosquito-borne disease: some major differences

• Assumptions about a future world

“re-establishment of the disease [in Italy] is unlikely unless living standards deteriorate drastically”.(Reiter)

“malaria could become established again [in Europe] under the prolonged pressures of climatic and other changes if a strong public health infrastructure is not maintained” (IPCC)

An attempt to characterize two world views

IPCC

A cautious approach that does not take historic social advances for granted, and sees humans are relatively minor players on a very large ecological stage

Reiter

Places greater weight on the human capacity to shape and control environments, with confidence that past achievements will be sustained and extended

Climate change and mosquito-borne disease: some major differences

• A matter of scale?

The causes of localized variations may not be the same as those that affect long-term disease trends

Changes in risk of malaria due to various environmental changes caused by increased demand for food and energy

ENERGY USE

REGIONALPOPULATION GROWTH

FOOD PRODUCTION

CLIMATE CHANGE

Changes in Mosquito Habitat

Migration of Populations

Forest Clearing

Risk of acquiring malaria

Urban Crowding

Malnutrition

Increased Geographic Range of Mosquito and Malarial Parasite

severity

Demandfor Food

CO2 emissions

CO2 emissions

Contributions to uncertainty

• Language

“the level of CO2 in the Earth’s atmosphere has risen by 30% in the last 100 years” (IPCC)

“a measurable increase in atmospheric CO2 from around 0.029% in 1890 to 0.037% today (Reiter)

Contributions to uncertainty

• Disciplinary effects– Differences between disciplines

“economists know little about the intricate web of natural ecosystems, whereas scientists know equally little about the incredible adaptability of human economies”’ (Nordhaus 1994)

Contributions to uncertainty

• Disciplinary effects– the dynamics of multi-disciplinary groups

“doubts and uncertainties of core specialists are diminished by the overlaps and interpenetrations with adjacent disciplines .. The net result is a more secure collective belief” (Wynne 1996)

Climate change and mosquito-borne diseases: where is the uncertainty?

• The most important disagreements are not to do with data or methods of analysis

• Conclusions differ because the problem is framed in different ways

• The causes of uncertainty lie not only in the quality of the science but also with basic assumptions and values

Some of the factors that influence the way a scientific problem is framed

Myth of naturefragile robust

Orientationecocentric anthropocentric

Attitude to riskrisk averse risk seeking

Van Asselt and Rotmans, GEC 1996; 6: 121-157