I was thinking about this the other day: what _is_ the best scientific statement we can make, at present, about anthropogenic global warming?
On one hand, we have theory. There's reasonably good theoretical reasons to expect that increasing carbon dioxide should increase global temperatures. However, every good scientist knows not to trust theory too much until it's been verified by experiment, especially when the system in question is so complex.
On the other hand, we have experiment. Unfortunately, we only have a sample size of one, and we don't have a control group. We tried increasing the CO2 emissions, and found that sure enough, the temperature did wind up increasing. But given there's only one planet in the sample and no planets in the control group, we certainly can't eliminate the possibility that there's no causal link. If we forget that we're looking at global temperature vs CO2 concentration and pretend we're looking at a high school kid's science project about whether playing music to plants makes them grow faster, we'd have to say that the experiment lends some support to the hypothesis but is hardly conclusive.
So what can we reasonably conclude? I think the fairest thing we can say is that it's more likely than not that increasing CO2 emissions causes an increase in global temperatures. Anyone who claims the science is "settled" though, the way it's settled for heliocentrism, evolution or relativity, is not much of a scientist.
(I am also, despite years of Year 9 science projects, still unsure whether playing music to plants makes them grow faster.)
> I was thinking about this the other day: what _is_ the best scientific statement we can make, at present, about anthropogenic global warming?
Here's what it seems like:
1. The Earth is warming at a slow rate, but at least some of this is natural as temperatures were historically considerably higher, then considerably lower than now.
2. We're still well within the bounds of normal, habitable historical temperatures.
3. Warming definitely occurs at local levels with deforesting, mining, and urbanization. This can decrease condensation and make areas less habitable to life. This can be stabilized to a large extent by planting trees, restoring soil quality, and so on.
4. Carbon dioxide currently makes up 0.3% of the greenhouse gases including water vapor, and about 5% of the non-water vapor greenhouse gases. Carbon dioxide increases are lagging temperature increases - suggesting that CO2 is not the biggest culprit of global warming.
5. That said, fossil fuels will gradually be replaced - the two current biggest hurdles are battery quality and political/public fears about nuclear power. Nuclear has gotten incredibly more stable and safe, and the advent of thorium reactors is exciting. Innovations in batteries and transitions to next generation nuclear will reduce fossil fuels a lot.
6. Much of green technology isn't so green - in many areas, turbines and solar panels take enough energy to create that they won't "pay back" their energy cost within 50 years, at which point we'll likely be able to produce energy much cheaper anyways. Some areas are very well suited for green energy - deserts with solar, very windy areas with turbines, hydroelectric on rivers, but they're brute forcing some very Earth-unfriendly alternative energy into areas it's not well suited for. Much of what's called green is marketing.
Personal judgment call based on everything I've seen? Money into R&D for hydrogen power, better batteries, fusion, and safer next-gen nuclear probably goes a lot further towards cleaning up the Earth than the current proposed solutions. Fossil fuels seem like they're going to largely be obsolete within 100-150 years or sooner, and rumors of the Earth's imminent demise appear to be greatly exaggerated.
As you say, much of your points are personal judgments. I do want to respond to some that aren't:
"2. We're still well within the bounds of normal, habitable historical temperatures."
This is a severe oversimplification, IMHO. Human society is sensitive not just to whether the Earth is overall habitable, but to impacts to the significant "sunk costs" we have in the locations of population centers. It's not much comfort to the people of California if the Earth remains habitable on average when the Sierra snowmelt disappears and the state faces a drought of unprecedented proportions.
"4. Carbon dioxide currently makes up 0.3% of the greenhouse gases including water vapor, and about 5% of the non-water vapor greenhouse gases. Carbon dioxide increases are lagging temperature increases - suggesting that CO2 is not the biggest culprit of global warming."
Where do you get that CO2 is 0.3% of greenhouse gases? It's true that water vapor is the largest contributor to the greenhouse effect, but CO2 is next. It accounts for 50% of the non-water greenhouse gases and for way more than 0.3% of the total greenhouse effect. Besides, counting water is a little spurious because the water content of the atmosphere is set by the temperature. Thus, if temperature goes up, water content goes up. So saying that CO2 doesn't matter because water vapor is much more important is wrong, it's precisely the opposite. CO2 matters more because its effect is amplified by water. (Barring controversies about cloud covers.)
CO2 is also the greenhouse gas that has the longest lifetime in the atmosphere. The others (methane, etc) break down a lot quicker.
> It's not much comfort to the people of California if the Earth remains habitable on average when the Sierra snowmelt disappears and the state faces a drought of unprecedented proportions.
It's a really long discussion, but our ability to locally adapt to climate change is actually quite good and improving regularly. Temperatures are rising really, really slowly - you can get the 2-3 degrees per 100 years back by planting trees and irrigating. Desalination has come along incredibly fast too. I can go into greater detail if you have specific questions, I know a fair bit about this stuff.
> Where do you get that CO2 is 0.3% of greenhouse gases?
"Carbon dioxide (chemical formula CO2) is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state. CO2 is a trace gas being only 0.038% of the atmosphere."
On one hand, we have theory. There's reasonably good theoretical reasons to expect that increasing carbon dioxide should increase global temperatures. However, every good scientist knows not to trust theory too much until it's been verified by experiment, especially when the system in question is so complex.
On the other hand, we have experiment. Unfortunately, we only have a sample size of one, and we don't have a control group. We tried increasing the CO2 emissions, and found that sure enough, the temperature did wind up increasing. But given there's only one planet in the sample and no planets in the control group, we certainly can't eliminate the possibility that there's no causal link. If we forget that we're looking at global temperature vs CO2 concentration and pretend we're looking at a high school kid's science project about whether playing music to plants makes them grow faster, we'd have to say that the experiment lends some support to the hypothesis but is hardly conclusive.
So what can we reasonably conclude? I think the fairest thing we can say is that it's more likely than not that increasing CO2 emissions causes an increase in global temperatures. Anyone who claims the science is "settled" though, the way it's settled for heliocentrism, evolution or relativity, is not much of a scientist.
(I am also, despite years of Year 9 science projects, still unsure whether playing music to plants makes them grow faster.)