Obviously, global warming is going to change climates in ways that increase the frequency and/or severity of heat waves in some places. Some people die in heat waves, some health conditions are worsened in heat waves. I think, however, that there will be other much more important epidemiological impacts of global warming.
We know that the sea level will rise. Indeed, scientists tell us that it was hundreds of feet higher than today in during periods in the geological record. There is also increasing evidence that as sea surface temperature rises, at least in some places there will be more and more frequent hurricanes and typhoons. The New Orleans catastrophe clearly demonstrated the dangers, and indeed people drowned in New Orleans during Katrina. The recent deaths in coastal Pakistan as a result of heavy monsoon rains, much less severe than hurricane conditions, illustrate that in poor countries the impact of storm conditions can be much worse that those we experienced in the United States.
Think about the New Orleans experience, however, which involved hundreds of thousands of refugees moving to other parts of the country, and hundreds of thousands of people staying in the devastated city. Think of the rises of crime rates that occurred in the cities that accepted the refugees. Then extrapolate that thinking to the extreme poverty of coastal Asia. How many will die in the aftermath of floods, hurricanes, typhoons and other events that would not have occurred were it not for global warming?
As climates change, so too will the prevalence of disease vectors. New hyper-endemic and endemic areas are likely to be created for malaria, dengue, yellow fever, and other insect borne diseases. I would expect there to be a significant spike in incidence of these diseases as populations with little or no exposure are newly exposed, and as public health systems gear up to meet with historically unprecedented epidemics.
It has been estimated that 80 percent or more of human communicable disease is the result of diseases that have jumped from another species to man. Often human communicable diseases have reservoirs in non-human species. Thus every once and a while a new flu strain arises in birds, is modified in the first people it infects, and becomes pandemic. The flu pandemics kill a million or two people, or perhaps tens of millions as occurred with the Spanish flu during World War I. New diseases seem especially likely to emerge in human populations when people come into unprecedented contact with other species. Thus viral hemorrhagic fevers that are extremely lethal have been observed when people came into unusual close contact with rodent populations, sometimes due to extreme whether conditions, or to people moving into previously unoccupied lands. It seems to me that it is likely that climate change is going to create new circumstances with increasing frequency where such events can happen. How important will the health impact be? Think of HIV/AIDS which seems to have jumped from another primate species into human populations a few decades ago. Think of the plague as rat populations changed and plague in the rodent population became an epidemic in humans, killing one-third the population of Europe.
Speaking of flu, everyone knows that there is a flu season, and indeed there are seasons where epidemics of viral respiratory diseases are much more likely to occur. Change the climate, you are likely to change the occurrence of these seasons. Create weather conditions where viral diseases can become epidemic where they were very infrequent or non-epidemic in the past, and the epidemics will find populations with no herd immunity. The likely impact -- far greater incidence of more severe disease.
It is also the case that diarrheal diseases in developing nations are weather related. Rains come, disease agents get washed into places (like the water supplies) where people are exposed. Global warming is going to put more moisture in the air, and one assumes lots of places will get more rain. It is not a great leap of logic to suppose that many of them will get more diarrheal disease incidence as a result. Indeed, if rainfall greatly increases, the public works infrastructure may not be able to cope in many places, and again disease frequency may increase.
Of course, those areas which become dryer than they are as a result of climate change may also face new health problems. A lot of people are still dependent on surface water, and when there is less of that there will be consequences for those people.
Agriculture is climate sensitive. If the climate changes enough, the crops that were grown in a place will no longer grow there. Indeed, trying to grow crops in an area that will no longer support that agriculture can lead to long term environmental degradation; desertification is not easily reversible! For Americans, with a complex food chain bringing food not only from all over the continent but all over the world, the dislocations of agriculture due to climate change are an economic problem. For subsistence farmers in Africa, they may become a survival problem. Hunger is already a huge problem among the half the world's population that live on an income of less than US$2 per day, and even more for the billion or so people who live on less than one dollar per day. How will subsistence farmers in countries with dysfunctional governments react to climate change? How will already hungry children and families react to changes in food availability? Not with improving health!
All of the above focuses on direct consequences of climate change for health. The indirect consequences must also be considered. People will migrate due to climate change. Farmers who can no longer farm in the new climate and residents of coastal zones that become inhabitable due to sea level rises and increasing storm action are examples. (It is estimated that ten percent of the world's population live in coastal zones that will be affected.) Many will move to urban areas, moving to slums and facing huge social and economic challenges -- not circumstances always conducive to good health. Some will move to other countries, legally or not.
I have always felt that the war between Honduras and El Salvador a couple of decades ago was due to the large number of Salvadorans who had migrated to Honduras. Large scale population migrations can cause social instability. Indeed, I suspect weak governments will not be made stronger by the impacts of climate change on their citizens.
One of the sad facts of public health is that poor health conditions have cummulative impacts. Little kids who get frequent infections tend to be undernourished, even if food is available, and food is often not that available in subsistence households. Undernourished kids have weakened immune systems, and are more susceptible to diseases. Families facing a high likelihood of their kids dying have more kids. Having more kids in the household makes it still harder to feed them all, and these households have more frequent bouts of illness as there are more immunologically naive members to bring home communicable diseases and closer contacts among the crowded household members. Frequently hungry and ill kids don't learn as much in school and frequently hungry and ill parents don't produce as much at work. And so it goes....
Jeff Sachs, in the current Scientific American writes:
Environmental sustainability is already very difficult to achieve with today’s 6.6 billion people and average economic output of $8,000 per person. By 2050 the earth could be home to more than nine billion people with an average output of $20,000 or more.More people means more health problems. But unfortunately, even if the world reaches an average output of $20,000 person, not everyone will have that much money. Indeed, over the last half century the inequality of distribution of the world's wealth and income have gotten much worse. Indeed,large areas of the world have gotten poorer for decades at a time not only by comparison but also in real terms. The global demographic and economic changes, unfortunately can not be depended upon to prevent climate change from having severe health impacts.
People in rich countries may think about global warming in terms of their air conditioning bills and hope for less snow to shovel. Poor people in poor countries may have to think about it in terms of disruption of their lives and increased suffering from diseases and deaths.
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