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Global Warming & Human Survival
Discussions of global warming center on two basic issues: is the climate warming up, and what will that mean for the several billion humans on this planet? For those who agree that the climate is warming, the next questions are Why? And What can we do about it?
By all the good, solid, scientific evidence, the global mean temperature is in fact rising. Even staunch opponents of the global warming hypothesis are finally facing the facts and admitting that yes, the planet's climate is warming up. Those who do not want to attempt any intervention to slow or stop the warming are now saying "Well, the planet used to be warmer...the global mean temperature throughout the planet's history was higher...so higher is normal."
Normal does not equal desirable. Or even survivable. "Normal warmness" did not sustain a human population over 2 billion.
The point is not whether the climate has changed, whether it has been warmer or cooler in the past, but whether preventable/slowable warming now will have a good or bad effect on the human population, via its effect on the biological basis of our survival.
A surprising number of people who are not biologists (or for that matter farmers) seem to have an inordinate fear of cold, which makes them fear an Ice Age more than anything else. You hear such ridiculous notions as "We need to get hotter so another Ice Age won't come" (while serious climatologists have pointed out that under some conditions getting hotter could bring on another Ice Age by changing the planet's albedo.) Serious sudden prolonged cold would also cause us problems, of course. Humans have thrived and developed civilization in a fairly narrow range of temperatures, compared to the actual global means this planet has produced and could produce again. But humans have demonstrably survived an Ice Age; we have not yet demonstrated the capacity to survive in an extremely warm climate.
What would be ideal to sustain the largest possible human population is a global mean temperature which a) limits extremes of year to year variation and b) allows maximum primary productivity of vegetation including all crops needed for human nutrition, and those primary oxygen sources which provide us with the necessary for breathing.
A static mean climatic temperature would help with a). If your rains (whatever they are) and your frost dates are easy to predict, you can adjust your agriculture. If the sea level is constant (also a factor here) you can then begin management of coastal lands with some hope of maximizing human success. Changes in global mean temperature (up or down) add more variability and thus lessen agricultural success, as some areas get too much rain one year and too little the next, while others become consistently dryer (existing deserts will expand); such changes also affects sea level, which affects coastal management strategies.
Critically for the global fresh water supply, rising sea level coupled with increased withdrawal from near-coast subsurface water leads to contamination of those aquifers with salt water, a condition that does not go away nearly as quickly when sea level drops. The corrosive effect of salt water on the rocks that hold the aquifer also leaches out undesirable (often toxic) minerals. So global warming means rising sea-levels, which means less land for agriculture and less fresh water in coastal aquifer. If you go back to the really warm periods of Earth's history, the whole center of the United States was under water. Rising sealevels already threaten heavily populated (and food-producing) lowlands around the world, in all continents.
Leaving aside the problems with variability and sea level, consider the regions of the planet that grow the most food for human consumption, and look at the effect of warming on those basic food plants (grains, legumes, potatoes, etc.) Each has its required set of environmental characteristics, and unless you're Lysenko you don't think the zygote is smart and able to reset itself in a generation to new conditions. Each has a set of water requirements. Transpiration in plants (water loss through the leaves) is closely related to temperature: plants need more water in hot weather, but they also have limits (inherent in each plant's leaf structure) on how much water they can lose by transpiration without damage. For all plants, higher temperatures mean increased need for water. Yet, in a warmer world, less fresh water will be available for irrigation of crops.
Woody plants (such as fruit trees) native to temperate climates require winter chill (so many hours below a given temperature) and many also require insect pollination. These pollinating insects have their own climatic requirements. Temperate-derived grain crops require a certain rate of change in day-length, and are set up to use soil minerals within a particular range (which relates to summer temperature and the rate of transpiration.) Tropic derived crops don't bother with daylength, and can't handle frosts or freezes, but do have seasonal water requirements that trigger flowering and reproduction.
The soils these plants grow and produce best on have developed over the past thousands of years...you cannot stick a plant evolved to grow in an acidic, wet, cool soil into an alkaline, dry, hot soil and expect anything but a dead plant. And vice versa. Temperature alone--not even counting differences in amount and timing of rainfall--changes the availability of minerals in the soil, in part because the soil flora and fauna--the invertebrates, bacteria and fungi in the soil, which are a major part of the biogeochemical cycle--have their own preferred temperature ranges, and operate most efficiently in this and not that temperature.
Thus an entire, invisible (except to those who study it) web of interconnected living organisms support the plants and animals we depend on for food, and this web has developed over tens of thousands of years in a climate cooler than the one we're headed for. When ecologists talk about an ecological collapse, they are not talking just about the extinction of a few "minor" species, but about the loss of hundreds of species, thousands of species, in a short time, a cascade of losses equivalent to that following catastrophic event like a large meteor strike. If the soil micro-organisms die, the soil is less fertile, and will produce less.
And that doesn't even begin to deal with the biomedical effects of climate change, which have been discussed with some rigor in medical journals the past several years. The movement of tropical diseases into temperate climates is already happening...partly as a result of climate change and partly as a result of rapid human transport.
So whether you consider a past maximal temperature to be "normal" or not, the fact is that the earth's maximal temperature did not sustain what we now wish to sustain. Whether the temperatures we're headed toward can sustain a large human population is...going to be an interesting discovery. Not something to be shrugged off with "It's been there before."
The earth has.
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This essay ©2003 Elizabeth Moon