Robin Bell, Ken Caldera, Bill Easterling, Stephen Schneider
In the list of world challenges, global warming might be at once the most alarming and the most controversial. According to some predictions, climate change caused by human activity could cause mass extinction in the oceans, redraw the planet’s coastlines, and ravage world food supplies. At the same time, a significant portion of the American public questions whether global warming will really cause any major harm; many still doubt that human-driven warming is happening at all. How can we settle the debate? And can we intervene in the process or find ways to adapt to the new conditions? In conjunction with the National Science Foundation and the San Francisco Exploratorium, DISCOVER brought together four experts to discuss the reality and meaning of climate change. In a highly nuanced exchange of ideas, these researchers weighed the various scenarios and laid out a road map for navigating the warmer world to come. The conversation was moderated by DISCOVER’s editor in chief, Corey S. Powell.
POWELL: One question I hear all the time is whether the current change in climate is truly extraordinary. Even if humans are contributing to global warming, isn’t this just like the natural variations that have happened many times in the past?
Robin Bell: A little background first. I spend a lot of time studying the ice sheets at the bottom of the planet—how they form and how they collapse. The poles are like the planet’s air conditioner. When things are working well, the poles keep the planet nice and cool and we don’t think about it. When things stop working, the poles can start to melt and there’s a puddle on the floor. Today both poles are getting warmer; in Greenland and Antarctica you can see the surface of the ice dropping, and you can see there’s less mass when you measure the ice from space. The process has been ongoing, but it looks like it’s happening faster than it was. We know the ice sheets have come and gone in the past. Why is this any different? One of the most compelling reasons is that in the past the ice sheets from the two poles didn’t move together—one would lead and the other would follow. This time, both the north and south are spewing ice into the global ocean, accelerating at the same time.
Ken Caldeira: Another indication of how unusual all this is can be seen by looking at ocean chemistry. When we drive our car and carbon dioxide comes out of the tailpipe, within a year it has spread throughout the atmosphere and is integrated with the surface ocean. When carbon dioxide dissolves in seawater, it forms carbonic acid, and in high enough concentrations carbonic acid is corrosive to the shells and skeletons of many marine organisms. To measure the impact, people go out in ships and drill holes in the ocean floor, where shells of marine organisms have settled throughout geologic history. What we see is that if we continue in our current trends in burning fossil fuels, the ocean will become more acidic than it has been at any time in the past 65 million years. The last time the ocean was as acidic as it has the potential to become in the coming decades, we saw a mass extinction event.
POWELL: Yet as you note, the earth got warm in the past, too.
Caldeira: That’s true, but it got warm over millions of years, and ecosystems had a chance to adapt. What we’re seeing are rates of increase in greenhouse gases and warming that exceed natural rates by a factor of 100. So what we’re doing is really unusual when seen from a geologic perspective.
POWELL: Humans are doing in centuries what natural processes do over millions of years?
Caldeira: Yes, and the other timescale mismatch is that what we do over the next decades will affect life on this planet for hundreds of thousands of years, if not millions of years. We are at a critical juncture in earth history. If we don’t do the right thing and there are geologists around 50 million years from now, they’ll be able to look at cores and see the remnants of a civilization that developed advanced technology but didn’t develop the wisdom to use it wisely.
POWELL: What about the impact of global warming on agriculture? As the climate changes, will people have enough to eat?
Bill Easterling: One of the most remarkable achievements of the 20th century was the way we were able to increase the global food supply in pace with unprecedented population growth. We will have to raise the food supply another two times to feed all of the people that we think will be alive by the latter third of the 21st century. We have reason to be somewhat sanguine about doing it if climate stays more or less the same, but how will we do it with the climate change? Based on our simulations and on 25 years of research, what bothers us most is that in the tropics, where the majority of poor people live today, crops are currently raised at temperatures pretty close to their photosynthetic optimums.
POWELL: Meaning that higher temperatures will make it difficult for us to produce the amount of food we need?
Easterling: If you go any higher, yields begin to fall. On the other hand, in the midlatitudes—in the temperate zones where we live and where many of the grain belts of Europe and North America are located—a little bit of warming in some cases is not a bad thing, at least not at first. These are regions where crops are currently cold-limited. In other words, if you warm the temperatures, you actually might get a little bit of additional yield. So you’ve already begun to set up a kind of haves and have-nots, an imbalance where the poorest people in the world who vitally depend on agriculture as a development tool, in addition to providing food security, are now being even further disadvantaged. But there really aren’t many winners in the long run because even if the higher latitudes are given an advantage, they still are faced with moving food across large distances and making sure that it’s done in such a way that the farming systems in the receiving countries are not put out of business because of the inundation of free or subsidized food.
POWELL: Climate change is such a huge issue that people tend to feel paralyzed by it. Stephen, you’ve framed it in a helpful way as a problem of risk management. What does that mean?
Stephen Schneider: I often testify before Congress and talk to the media, and they always ask the same question: Is the science settled enough for us to have policy? Do we know enough to spend money fixing this? But science, and especially system science, is very complicated. Now, in any system that’s complicated there are some components that are well established. In other words, they’re relatively settled. We know that the world is now 0.75 degrees Celsius warmer than it was a century and a half ago. We know that the ice sheets are decreasing. But then there are other components with competing potential outcomes—for instance, will a change of three degrees make crop yields go up or down?
POWELL: So how should we separate out the well-established parts, and how do we evaluate the ones that are not so certain?
“With any one line of evidence, there might be a 25% chance it’s random. But the probability of all these events’ lining up is pretty darn low—unless it is global warming.”
Schneider: When you’re covering climate change, you don’t get somebody from a deep ecology group to tell you we’re near the end of the world and then somebody from the Competitive Enterprise Institute who’s going to tell you carbon dioxide is a fertilizer while forgetting about ocean acidification. If you do that, the two lowest-probability outcomes get most of the time in the media and you get this dumbed-down debate. It’s bipolar, and that’s not how system science works. There are multiple potential outcomes. What we do is whittle out the relative likelihood of each of these outcomes so we can make a value judgment about whether or not the risks are adequate to move forward. Risk is what can happen, multiplied by the probability of its happening. That’s what we call an objective or scientific assessment. We try to make the risk aspects clear and then leave the risk management where it properly belongs, which is out among the public and in the political world.
POWELL: In the news, climate change is often described in terms of legislation and treaties. So I was surprised, Ken, when I heard you call it a “hardware problem.” Can you explain that?
