The National Academy of Sciences recently released a 150 page report on global temperature change over the last 2000 years. It's available free online here. This kind of reconstruction is, of course, tricky, in that we only have decent human temperature measurement records for the last 400 years or so, and near the beginning of that period the measurements weren't as accurate and weren't taken at widely dispersed enough locations. To fill the gap, scientists have tried to use data from tree rings, ice cores, coral, and so forth. Here's one of the resulting charts:
As you can see, the last hundred years are highlighted in red and show an upward trend, though not an unbroken one. However, there are a couple apparently problems. One, as noted by the blue "?LIA?" bar, is that this modeling fails to detect the 'little ice age' of the might nineteenth century, when for a while the Thames regularly froze over in London, and record cold winters were recorded world-wide. Also, they weren't able to project these method back far enough to compare the current warming trend with the one during the early middle ages (approx 800-1100) . Although that warming tend was clearly the result of natural processes while this one is at least in part the result of human greenhouse gas emissions, seeing the relative scale would be of immense use in getting a sense of perspective on current trends. Some hint is provided by this graph of Greenland's average temperature over the last 2500 years:
The vertical axis if a little confusing, since it shows the number of years ago. Thus, we are currently at 0. The height of the medieval warm period was around 900 AD, which ties well with Norse sagas, which describe Greenland as much more hospitable when it was first settled around that time. Current temperatures in Greenland aren't even half-way to the medieval high point yet, though the rate of ascent is steeper.
I must admit I'm a little fuzzy on the claim that one hears quite a bit that at some specific point the "process will become irreversible". All climate trends in the Earth's past have reversed at one point or another. CO2 can be precipitated out as limestone in the ocean, and can also be consumed by plants. So it's not like once we get the stuff in the air we can never get it out again. I'll agree that having the sea level rise 20ft would pose problems, but if it happens over the course of 500 years, I think those problems would be pretty small. We'd have plenty of time to adjust, and our technology will be so different just 100 years from now, forget 500, that making straight-line judgments seems a little silly. Picture someone in 1500 trying to plan a strategy to deal with a crisis occurring by 2000, but assuming that technology and political trends would remain roughly the same.
Prof Bill McGuire, of the Benfield Hazard Research Centre, University College London, said: "Last year a six metre [20ft] rise was thought to be at least 1,000 years away. This year it is 500 as the Greenland ice sheet continues to fall apart at an accelerating rate."....
The studies show that greenhouse gas increases over the next century could warm the Arctic by between three and five degrees Celsius in summer. This would make the Arctic free of sea ice by 2100. The process will become irreversible some time in the second half of this century.
"We need to start serious measures to reduce greenhouse gases within the next decade," said Prof Overpeck. "If we don't do something soon, we're committed to four to six metres of sea level rise in the future."
What sort of things might we do about it? Firstly, using fossil fuels will become increasingly costly during the next few hundred years, so there will be additional (and unavoidable) pressure to move to alternate energy sources. But assuming that we get some critical level of CO2 into the atmosphere over the next 50 years, there are some pretty interesting things we could try to do to deal with that if it was clearly causing major problems. One would be oxygen farming on a massive scale. One of the obvious ways to get rid of CO2 is to have plants consume it and turn it into oxygen. Forestation is pretty, but about 50% of the CO2 consumption on Earth (and 45% of the oxygen production) is performed by algae. Massive algae farming could help reduce CO2, and some genetic modification might even help to produce specialized algae which would be even more efficient at it.
That said, there are a couple basic things that it's surprising environmental advocates aren't a lot more interested in. The number one thing is nuclear power. Currently only 20% of the US's electricity production comes from our 100 nuclear plants. And yet, nuclear power is by far the best solution to reducing the 40% of our annual CO2 productions which comes from fossil-fuel-burning power plants. Significantly reducing the regulatory and PR hurdles to building more nuclear plants could make a major difference in US greenhouse gas production. An additional 200-300 US nuclear plants would make a world of difference. We would want US and European nuclear power production to be as high as possible. The greatest per capita increases in CO2 production over the coming century will be in the developing world, where people are just getting electricity and transportation -- and many of those areas are unstable enough we don't necessarily want them running nuclear reactors yet.
The other place we could make a difference is in the 40% of US CO2 production which comes from transportation. Usually this leads people to start shouting about gas taxes and penalizing SUV buyers. However, there's something that would help at least as much (perhaps more) that no one seems to talk about: diesel. We think of diesel engines and nasty, smokey, low powered things from the 70s and 80s, but modern efficient diesel engines produced in Europe and Asia frequently offer significantly better mpg than US gasoline engines. Horse power is often slightly lower, but the torque is often higher. The VW Jetta TDI gets 44 miles per gallon, on a par with most hybrid cars, and built around a simpler, more powerful engine which can be expected to last waaaay longer than the complex systems that make a hybrid work. (The basic design of the diesel engine is simpler and less subject to mechanical failure than the gasoline engine design.) Yet the government puts major disincentives and restrictions on diesel cares in the US, keeping them to a tiny market share compared to the 30-40 percent of the road they own in Europe and Asia.
It's all very well to go around your house turning off extra lights as Roger Ebert said he did after watching Al Gore's movie (though if you make less than Roger Ebert you're probably already doing that to try to save money) but nuclear power and diesel vehicles (which would probably be a better sell to the tough-car-buying American public than hybrids) are far more likely to actually make a difference in the long run. And even if the pessimistic scenarios are correct, 500 years is a very, very long time to watch the tide come in. Pick up your copy of Hitchhikers Guide to the Galaxy and remember: Don't Panic!