Recently, two DKos diaries (http://www.dailykos.com/..., http://www.dailykos.com/...) have taken bitterly opposed viewpoints on how much we should try to link bad weather events to global warming. The actual differences in their viewpoints were, however, rather subtle. I believe we can reach a consensus on this issue. My motivation is partly just to avoid unnecessary friction among the good guys, but I'm also guided by a somewhat different sense of the underlying science.
Let's start with the non-controversial points. I think we all agree that:
1. Anthropogenic global warming is very real, and is going to become much more intense.
2. On general grounds, we know that weather is very sensitive to such effects, so weather patterns are going to change a lot and probably already have changed significantly, in addition to the obvious temperature increases. One systematic change can be predicted with confidence, a world-wide increase in precipitation due to warming oceans.
Now let's look at the parts where the science is a little trickier.
Some of the discussion has centered around distinguishing between individual events and statistical patterns. Causation of individual events is not really worth discussing, because weather is so extraordinarily sensitive to tiny "butterfly" effects that even the smallest change in climate or land use etc. will give an entirely different set of individual storms than would otherwise have occurred. The only meaningful subject for weather causation arguments is the statistical pattern.
We have seen a lot of claims that there will be more extreme weather, usually implying that there will be more large storms of various types. That may turn out to be true, but the scientific certainty of it is not remotely close to that of the basic warming effect. Global warming actually reduces the temperature contrast between the arctics and the tropics, so it reduces the strength of one of the principal drivers of weather patterns. Claims that AGW increases the net energy flow through the weather system are false. That flow is just set by the total amount of solar energy absorbed. AGW probably slightly increases cloud cover, so it's likely to reduce the net energy flow through the weather system. So this leads to point 3:
3. the calculations about which particular changes to expect in storm patterns are far more uncertain than the global climate calculations. That includes estimates of net number of severe hurricanes, etc.
4. So we don't want to claim that we know the changes in storm statistics are due to global warming with the same confidence that we know what's causing the warming and how much it will grow.
Nevertheless, there's another meaning to the claim that we will see "more extreme weather", captured by the phrase "global weirding". We may not know much about the details of what will happen to storm patterns, but we are very confident that they will change. Therefore:
5. People in most areas will experience weather that's weird, unfamiliar for their area, and for which their infrastructure and habits are unprepared. That's true even if the net number of severe storms worldwide doesn't change.
So when people find persistent weird weather changes, we should be open about saying that such changes could be due to global warming.
It'll be interesting to see if the various people who were slugging it out over this issue can find consensus on these points.