Historical perspective: climate change can be life-changing.
Societies come and go. Jared Diamond's 2005 bestseller Collapse: How Societies Choose to Fail or Succeed provides a fascinating set of historical case studies of how seemingly highly successful social orders fell as a result of poor choices and bad planning.
External factors also played a role, one being climate change. For example, pivotal to the demise of the Anasazi in the U.S. Southwest was the onset of a drought. (In one of TheGreenGrok's travelogue posts, I discussed a similar event leading to the disappearance of the Sinagua.)
A paper in last week's Proceedings of the National Academy of Sciences strikes a similar theme. In his inaugural article, which marks his election to the U.S. National Academy of Sciences, author Tony McMichael of the Australian National University, Canberra, writes about "insights from past millennia into climatic impacts on human health and survival." He provides a broad, sobering account of how major human and social disruptions over the past 10,000 years correlate with, and thus may very likely have been caused at least in part by, climatic shifts.
History Indicates Three Types of Climate-Related Health Risks
According to McMichael, climate change has hit human health in three general ways:
- directly, through extreme weather events like heat waves and floods;
- indirectly, through impacts to crop yields, water supplies, and changes in the range or seasonality of infectious diseases; and
- even more indirectly, through the social and economic costs resulting from the impacts in #2, such as death, under-nutrition, infections, migrations, and despair and depression and hostilities coping with limited resources.
The cascading impacts that have had the most damaging effects on human societies, McMichael writes, are under-nutrition and starvation, infectious disease outbreak, and conflict and warfare.
A Focus on Four Time Scales of Climatic Change
McMichael argues that climate change can affect human health and well-being on six general time scales: 1) over millennia via evolution, 2) at times of major climate shifts that mark transitions in human culture and ecology (like the onset of continental glaciation 80,000 years ago and the subsequent end about 11,000 years ago), 3) long-term climatic changes that span centuries, 4) medium-term or multidecadal climatic changes, 5) short-term or multiyear climatic changes, and 6) acute climatic/weather events.
McMichael's paper focuses on the last four.
Long-Term, Multi-century Climate Change -- Demise of the Mayan Civilization
Among McMichael's historical examples of long-term climate change is the ninth century collapse of the Mayans who inhabited a large part of Mesoamerica, specifically the northern Yucatan Peninsula through to Guatemala and El Salvador -- an example also covered by Diamond.
Sediments and pollen indicate that the region was hit by three decadal droughts (in 760-800, 840- 870, and 890-920 CE) that resulted in part from "a weakening and shift in the summer monsoon," and quite possibly prompted the collapse of one of the great civilizations of humankind.
Medium-Term -- Black Plague
One of the most interesting examples to me was the 14th century spread of the bubonic plague, which helped wipe out about one third of Europe's population. It was an event, McMichael posits, caught between a set of climatic events and the devastating plague that ensued. Most of us learned about this devastating plague back in grade school; refresher courses came with Ingmar Bergman's The Seventh Seal (1957) and Albert Camus's The Plague (1947). Even so, that there might be a climate change connection was new to me.
It's believed that the bubonic plague -- which, I also hadn't known, actually began in Central Asia and China and spread eastward to Europe -- moved from infected rats to fleas to humans. McMichael argues that a "trophic cascade," triggered by a sequence of climatic shifts, proliferated and spread the disease.
More specifically, he writes that decades of ideal climate, which started around 1300 CE and supported abundant plant growth (and thus food for humans and rats), were followed by first a cooling period and then catastrophic flooding in central China. Those subsequent floods and likely crop failures, he argues, dispersed both wild rat populations and people, making contact between the two more likely and aiding the spread of the disease. (It's interesting to note that health officials predict that a similar but far less dangerous trophic cascade will lead to a rise in Lyme disease outbreaks this year.)
Short-Term -- Great Famine
When Europe's Great Famine peaked between 1315 and 1322, "dire weather prevailed," McMichael writes, "with incessant and often torrential rain, floods, mud, and cold." He notes that a diverse set of factors, from economic disruptions (from land-policy changes) to multi-year changes in climate that would have "imposed extra stress on a vulnerable population," caused the famine, whose death toll reached an estimated 10 percent of northern Europeans and a full third of the populations in hard-hit areas.
Acute -- Yellow Fever
Another connection McMichael highlights in his paper -- and one that perhaps hits closest to home for us modern folks who are keenly familiar with acute weather events -- is that between the 1793 outbreak of yellow fever in Philadelphia and weather. An unusually hot summer, arriving on the heels of a strong El Nino, enabled the epidemic to flourish in what would normally be well beyond the northern limit of the of the disease-carrying mosquitoes. A month before the yellow fever broke out, Philadelphia had seen an influx of people -- slaves who'd escaped an epidemic and rebellion in what is now Haiti and brought with them the disease.
Implications for Today
An interesting paper, but how relevant to our contemplations of and plans for a world that is continuing to warm globally?
1. Anecdotal Evidence: We should keep in mind that each instance cited by McMichael (and Diamond for that matter) is anecdotal in nature -- a clear-cut cause-and-effect linkage between climate and societal outcomes is not really possible for complex systems like climate. I find the arguments compelling and convincing; others may not.
2. Cold Often More Damaging than Warm: In McMichael's examples, more often than not the culprit tended to be cooling trends, not a warming trend, such as what we're witnessing now. Good news? Perhaps. But ...
3. Beware Drought: McMichael concludes that "the greatest recurring health risk has been from impaired food yields, mostly due to drying and drought" and that "drought has been the dominant historical cause of hunger, starvation, and consequent death." If global warming brings more severe and/or more frequent drought -- and there is evidence that such a change is already afoot (see here and here) -- such a scenario, in McMichael's words, would cast "an ominous shadow over this coming century."
4. Who's Better Prepared to Survive Climatic Disruption? This interesting question is hard to answer. Some data suggest that folks like most of us living in the developed world -- with infrastructure to protect and shelter us and supply systems for sustenance -- are less vulnerable to environmental events than people living in less developed areas under impoverished conditions (and, by extension, than the people who were confronted by climate change in the past). McMichael argues the opposite:
"Although modern societies might expect to be less vulnerable to climatic stress in view of their stocks of knowledge, physical resources, technological interventions, and good governance, there are limits to that coping capacity. Further, in several respects modern societies may be at heightened vulnerability."
Should we be worried? Well, there's gotta be lots of speculation and inference going on in an analysis like this, and what's happened in the past is not necessarily what has to happen in the future. Indeed, as Diamond points out in Collapse, societies that planned and acted in a proactive way can survive the various slings and arrows tossed at them. But, even so, yeah, I'd be worried, especially given recent events that suggest we don't seem much inclined to confront -- through mitigation or adaptation -- the likely climate changes in the coming decades.
There is a bright side: Despite all the calamities we have faced as a species, we've managed to survive. I suspect we will survive a few centuries adapting to global warming. However, I doubt if it's going to be an especially pleasant experience for those doing the surviving -- just try channeling the folks who lived through the Black Death. If it's OK with you, I'll pass on that one.
Crossposted with TheGreenGrok.