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Edited on Mon Feb-20-06 05:37 PM by Lisa
While we know that there were cooling and warming intervals in earlier times (the most recent example of a sustained warming trend was probably in the Middle Ages, which lasted several centuries) -- the researchers investigating this noted that even that situation was intermittent. There were a lot of local disparities, so some regions were warming up while others weren't.
(That's what the seminar was mainly about ... they figured that the "climate curve" of the past few centuries actually had more jiggles than had been suspected, and that both the Medieval Warm Period and the Little Ice Age were, respectively, warmer and colder. They only LOOK relatively flat because we have been taking global rather than regional average temperatures.)
The climate change models are predicting that large areas of the planet would be in synch, if warming continues this century -- so more regions would be affected, than during portions of the Medieval Warm Period, or the climate amelioration (after the Subatlantic Deterioration of 500-400 BC) which warmed up the Alps in Hannibal's time.
This in itself is interesting, because there seem to be a lot of actual impacts being reported, in many places around the world -- not just temperature or precipitation measurements, but real-life changes in the way things are. Melting permafrost and the disappearance of sea ice, some species getting scarce and other ones showing up where they didn't live before -- the Inuit, for example, have a lot of experience looking at these things, and they know how variable conditions can be in the Arctic. But now they're saying that it's not the way it was before. Scientists say that the environment "integrates" conditions over a number of years ... so one or two abnormal seasons probably won't have much of an effect, but if things go on changing, then we start to see things "where the rubber meets the road".
The more rapid the changes are, and the larger the areas being affected, the more it suggests a change in the fundamental factors which shape our climate. If we could blame this on shifts in the amount of energy being given off by the sun, or the amount reaching the earth's surface (due to orbital cycles, or sub-atmospheric stuff like clouds, etc.) -- this should have been detectable by now. Scientists call the sorts of things which can affect climate "forcings", and the one that seems to make the most sense is the increase of energy-retaining gases in the atmosphere, which we've been observing for half a century. I asked some of the researchers I work with if they have seen any evidence for an orbital or even sunspot "forcing" that might account for the recent warming, and they drew a blank.
If it were just temperature, that would actually help some species and some areas (as the original poster pointed out) -- growing seasons would start earlier, conditions would be less harsh, etc. However, a bunch of other variables would be changing at the same time.
For example, higher evapotranspiration and the loss of mountain glaciers would mean drier summers in a lot of places. So the net result of warmer climates could be less soil moisture -- which would cancel out any advantages to a jump-started growing season, in many places. (Then there is the concern about more pests surviving milder winters, which is bad for crops and forests.) Russia and Canada used to believe that global warming would be good for their economies ... but as we've started totalling up the damage to various sectors (replacing coastal infrastructure, fixing railways as the permafrost starts melting, etc.), it doesn't seem it'll be a boon for most of us, even if the winters up here do get less severe.
What makes matters even more complicated is that some of the changes would have a kind of feedback effect. There could be negative feedbacks (where the changes have a natural dampening effect ... such as, more evaporation means more clouds, and maybe that'd cool things off a bit; or maybe the increase in CO2 would stimulate plant growth, and more CO2 would be taken out of the atmosphere). But there seems to be more evidence for positive feedbacks, where the changes would accelerate warming. For example -- warmer conditions would increase decay rates, and more CO2 would be emitted from the soil -- plus the increased risk of forest firs. Up north, the decrease of snow cover would result in more sunlight being absorbed by the surface. Researchers are still trying to figure out what these feedbacks are, and how much they might add.
No, you're not a fool. That's why they have refereed journals and peer review for examining these kinds of questions -- scientists know that an inital theory, or a study done using just one temperature indicator, may be mistaken or oversimplified. It's also why it took several decades for the majority of climate scientists to decide that global warming could pose some serious problems.
Something else to consider is that in Roman times, the world population was less than half a billion people. In fact, it was probably similar to the number of people living in North America today. That was for the whole planet.
We're now looking at 6 billion, going on 7 billion ... and by the end of the century, it will be 9 billion if we're lucky and we don't have major famines, wars, and disasters. We've already put most of the accessible land under cultivation. That gives us less margin for error ... we can't just abandon large portions of continents, and migrate on to wetter more fertile areas (as the Central Asian nomadic people did at the end of the Roman era, arguably contributing to the end of that particular empire). As it is, many areas are having water shortages (e.g. the overpumping of the Great Plains aquifers), without having this extra issue.
p.s. As a climate researcher, I'd be happy if global warming weren't happening, because I could focus on things like El Nino prediction, and looking at obscure historical records to unravel questions just like the one about what climate was like in Hannibal's time. I really don't want to worry about disputes over water, or whether our area's economy will be threatened by the mountain pine beetle expansion, or whether the new national park we just spent millions of dollars acquiring will be submerged by the end of the next century! Let alone having to consider things like how much to spend on shoreline protection, or whether emissions trading systems are workable. But the way things are shaping up, it looks like I won't have much of a choice.
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