PrintMore than Just Temperature
The past confirms much more about our understanding. The major events in Earth’s history were identified first by their influence on living things, including extinctions. A huge amount of additional research was required to learn that changing climate was responsible for many of those events, and perhaps for almost all of them. This long history of climatically caused extinctions supports our scientific expectation that continuing climate change risks extinctions in the future. We also expect that the CO2 we put up will continue to affect the climate for a long time, based on models and understanding that are well-confirmed by the geologic history.
The biggest of the climate changes of the past were much larger than the changes humans have caused so far. But, if we continue to burn the available fossil-fuel resource, we can cause a change that is as more-or-less as large as, and much faster than, the biggest natural events (except for the meteorite that killed the dinosaurs, which caused large changes very very rapidly).
The geologic record highlights another major issue. Science always involves uncertainty. All measurements have some “plus and minus”—Dr. Alley is within an inch of 5’7” and weighs within a few pounds of 145, for example, but he surely is not known to be exactly those measurements. And, when measurements are used to drive models that project climate changes that are used to estimate economic impacts, many sources of uncertainty are involved, and we cannot in any way be exactly certain what the future holds.
In assessing those uncertainties, though, we find evidence of an asymmetry that you probably could have figured out from common sense. In ordinary life, breaking things is almost always easier than building them. If you want to build a new house, you will need a lot of different materials and tools and know-how. But, if you want to tear down a house, you can do it with just a wrecking ball or an exploding stick of dynamite.
When we survey the history of climate, we see something similar. We don’t find evidence of Eden, a time when changing CO2 and climate had turned the whole Earth into paradise. Deserts and ice have grown and shrunk, so some times may have been “nicer” than others, with no guarantee that we now live in the best of all possible worlds. But, hazards existed at all times.
We do find evidence of occasions that were much closer to Hell, with up to 95% of the known species becoming extinct. A species might survive from just a single pregnant female or a few eggs or seeds even if all other individuals are killed, so the extinction times were very bad indeed.
If we continue to rapidly change the atmospheric concentration of CO2, we have a best estimate of likely impacts, which will be discussed further just below and in additional material later in the course. Uncertainties are real, and the future may be somewhat better than expected, or somewhat worse. But, we don’t see any reasonable chance that the changes will be much better than expected—cranking up CO2 is very, very unlikely to make Eden. And, the history of climate suggest the possibility that things will be much worse than expected—cranking up CO2 might break things we really care about.
If you drive somewhere, you face a similar situation. What you expect is very far on the "good" side of what is possible, as shown in this short piece...
Video: Possibilities (5:10)
PRESENTER: So I'm this really lucky person. I get to ride my bicycle when I go places. And that's a great thing. But suppose you have to drive a car.
You may run into problems. And you might have very few problems at really low. And you might have bad problems way over here on the right. So this is problems getting worse.
And this is how likely-- this is highly likely you're really going to get this. And this is rare down here. So what we're going to look at is, what does a commuter encounter if you go out in your car in the real world.
Well, the most likely thing that happens-- and so we show way up here because it's likely. It's that you get caught in traffic. And you kill some time.
And you turn on the radio, and it's just sort of boring. It's not something that you really wanted to listen to. That's really what most of us experience when we have to drive somewhere.
Be perfectly honest. It is possible that you will get to a situation that nobody's in your way. And you turn on the radio and they're playing the Beach Boys festival. And you're just grooving as you run down the road. It's a wonderful thing, and you're having a ball.
It is also possible that you get stuck in lots of traffic. You're sitting there for an hour. You turn on the radio, and they're testing the Emergency Broadcast System. And they're screaming out of the radio. And this is no fun at all.
But recognize that there's a slight possibility that you're sitting there stuck in traffic listening to the Emergency Broadcast System. And a drunk driver comes running over the top of you. And you know, you get-- I'm sorry. You could be seriously damaged, or you could end up dead. And that is indeed possible. It's not very likely, but it's possible.
Well, what do we do about that? We buy cars that have airbags in them, that have crumple zones. We put on our seat belts. If we have kids, we put them in a kid's seat.
We take out catastrophic insurance. We pay Mothers Against Drunk Driving to try to reduce drunks. We pay engineers to make the roads safer. We put a fair chunk of our transportation budget into something that we do not expect to happen, because it's so devastating if it happens.
Now, when we start talking to Congress, or to what have to you, about the cost of global warming, we have a best estimate. What is the most likely thing? And when we take those problems that go with that best estimate and you put them in an economic model, we are better off if we deal with it than if we pretend it doesn't exist.
Now, be very clear. This is science. It is not revealed truth.
It is indeed possible that we will see smaller or slower changes. Absolutely correct, that could happen. It's also possibly we could see larger or faster changes.
We simply do not see any way that simply adding CO2 to the air will turn the earth into the greatest place to live that could possibly be imagined. You can't make Eden with just one thing, because building paradise would take getting a lot of things right.
So there's no really not much chance that we get wonderful, no problems, great benefits, just from cranking up CO2. But there's a slight chance that we actually make the tropics too hot to live in for unprotected people, that we could have dead zones belching out poison gas, that we could shut down the North Atlantic and dry out the monsoon belts, that we could dump and ice sheet in the ocean and flood the coasts in a hurry.
These are all considered to be very unlikely at this point. But we can't rule them out. And CO2 might, by itself, do that. And so if you look at the picture, yes, it could be a little better. It could be a little worse. It could be a lot worse. But we don't see any way to make it a lot better.
Now, this is an opinion. But the last times that I have sort of talked to high policymakers about this, that I've testified to Congress or what have you, my impression is that we've spent a lot of time having this argument.
I present what we know best from the science. And someone says, it could be better. This is our best estimate. It could be better. This is our best estimate, this could be better.
Yes, that is not both sides. Be very clear, the best scientific evidence versus don't worry is not showing you both sides. And if we scientists are wrong, it's more likely to be on the bad side than it is on the good side.