Joseph Romm interviews Michael MacCracken

Joseph Romm: The 2007 IPCC report said that warming of the climate in recent decades is unequivocal. That is a very strong statement. Can you sort of explain how we are so certain that the planet is warming.

Michael MacCracken: Well over the past time of civilizations or the past few centuries, we've seen variations occurring. But what is happening now are changes where we're getting to unprecedented conditions. We've actually had a relatively--although there've been variations--relatively stable time over the past few thousand years and society has formed based on sea level being where it is and based on temperatures and storm tracks providing precipitation in particular places. And we're adapted to that condition, and those are starting to change. It used to be it was just variation in one place up and another place down, and now every place is basically moving up. They went through a lot of statistical tests trying to figure out what are the potential causes, and we believe there are causes for temperature change and for climate change. So you look at solar radiation, you look at volcanic eruptions, you look at greenhouse gases, you look at other things. And each has a particular fingerprint that you can see happening, and that tends to exclude the other ones to some degree. So they saw that we're having statistically significant changes and that most of these changes are due to human activities.

Romm: And I'm wondering if you can comment a little more specifically on this notion of attribution. How do we know that it is human emissions that are causing these changes?

MacCracken: Well the first thing to do is sort of detect the change. So it's a two part process. Detect that there's change, and then you have to figure out--you know, and that's like saying there was a crime committed--and then you have to figure out who did it. OK. And so you go look at what the possibilities are. One of the popular ones that people talk about is changes in the amount of solar radiation coming out. If the sun were getting hotter, that would presumably warm up the atmosphere. It would warm up not just the lower atmosphere, the surface, but it would warm the upper atmosphere. The interesting thing is that we have observations showing the lower atmosphere, the surface are warming, but the upper atmosphere is cooling, sort of in direct contradiction to what solar radiation increase would be expected to do. We also have measurements now from satellites that indicate how big that range is, that give a sense of how big a change could occur. We look for volcanic eruptions. Volcanic eruptions when they happen put aerosol in the stratosphere, reflects some solar radiation away, so they lead to global cooling. And so we're really warming, you could say, well the volcanic eruptions going away would lead to warming. Well that already happened. That happened in some times in the 20th century when we didn't have volcanic eruptions for a long time. And we're yet we're still getting warming greater than that kind of condition. So it's sort of hard to see how it could be that cause. If you look at this change in atmospheric composition, what you see is that should create warming in the lower atmosphere, but cooling in the stratosphere because it's basically able to radiate the heat away more rapidly. And that's indeed what's happening. And the magnitude of the change is about ten times as large as the changes in solar radiation. So it just looks like it has to be the very dominant cause.

Romm: And what about this argument that you're always hearing, "well the climate's always changing, it's been changing for hundreds of thousands of years." Why should we worry now?

MacCracken: Well over the earth's history the climate has been very different. So back 65 billion years ago, CO2 concentrations were really high, the world was very warm, dinosaurs roamed all the way to the poles. And we've had ice ages. So life on Earth will survive a range of conditions. Most of those changes occurred pretty slowly so species could adapt. The one big one when an asteroid came in and hit the earth there was such rapid change that they couldn't adapt and you had a lot of species wiped out. Well what we're doing now is starting to change the climate, change the zones where species and plants grow, change the conditions under which society was set up. And we're doing it relatively rapidly in sort of geological terms. So if we really start melting, for example the Greenland ice sheet or the Antarctic ice sheet and everything, and we've had sea levels higher before, go back 125,000 years and it was maybe 15-20 feet higher. So the world has existed with higher sea levels, but if you live in New York or you live in Washington, DC, you're not going to have a city there. You might say, well, I'll be like the Dutch and I'll put up sort of dikes and levies around and that will protect it, but you can only sort of do so much of it. And then there's some places like Florida where you can't do that because the underlying geology is limestone, so that's like building a dam on top of a sponge or something, water's just going to keep seeping in underneath. And so there's a whole bunch of places you won't be able to protect. And we're going to have to relocate. And that will be a very expensive, disruptive process.

Romm: There are people out there who just don't believe scientists, who believe this is some sort of, I don't know, hoax, fabrication, conspiracy. What do you say when you run into those people and they say, "I just think that there's no truth to this."

MacCracken: Well scientists try to be, to not to be believers. We try not to use the word, believe, something. We try and look at what the evidence is telling us. So I think what we need to do is basically explain the sort of fundamental few points that indicate that we're going to have dramatic changes. The first thing to understand is that the earth's climate has been different, I mean it hasn't been stable, you see changes in the past that have occurred. We think we're understanding the reasons for them occurring. And so if those reasons or something like them or equivalent to them occur, you're going to expect the climate to change. It is hard to accept that driving cars around is going to cause Greenland to melt, or cause changes in the circulation of the ocean, and that running power plants on coal is going to change where plants grow and the ranges of plants. But there's so many of us now on the planet who are doing so much that that's where the evidence is pointing and we can't find a reason about why the climate would stay the same when all these changes are occurring, because in the past, when changes like this have occurred, we've had big changes in climate.