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The Habitable Planet: A Systems Approach to Environmental Science 

Energy Challenges Interview with John Holdren

Interviewer: Would you begin by explaining the IPAT relation. How does it work? And what strategies does that therefore imply?

JOHN: About thirty-five years ago, Paul Ehrlich, the Stanford population biologist that I’ve worked with for a long time, and I published a very simple relation for thinking about the impact of human activities on the environment. It’s called the IPAT relation, in which I stands for impact. Then I equals population times affluence times technology — the number of people times the level of economic activity per person, usually measured as GDP (gross domestic product) per person, times the amount of impact per unit of economic activity, which we call the technology factor because you can change that impact by changing technology, for example, changing from coal-burning to wind power as an example. If you think about the particular problem, which is very important in the energy domain, of the influence of the energy system on the global climate, what we’re most interested in under the impact heading is emissions of carbon dioxide. So the relationship you have is emissions of carbon dioxide, impact on the climate, equals the number of people, times the amount of GDP per person, times the amount of carbon dioxide emitted per unit of GDP, per dollar of GDP. And that’s the technology factor, which we are at liberty to adjust by changing the way we get our energy or by changing the amount of energy we use to make a dollar of GDP.

The relation is very useful because it allows you to think separately about the different leverage points that we have on the factors that are causing environmental problems. One leverage point is population; you can do various things to reduce population growth. And if the population of the world in 2100 were to be eight billion people rather than say twelve billion people, a great many of these problems would be easier to solve. The energy problem would be easier to solve. The water problem would be easier to solve. The agriculture problem would be easier to solve for eight billion than for twelve. So population is an important lever.

The middle term, GDP per person, is not a lever that most people want to use to reduce impact, because it’s generally agreed that more is better. Nonetheless, if bringing the environmental costs of what we do into the balance sheets slows the rate of economic growth a bit, all that’s really doing is telling us that under the previous rate of economic growth we really weren’t getting rich as fast as we thought because the growth was coming at the expense of consuming the environmental underpinnings of well being. And that’s a bad idea. It’s like eating your capital and saying you’re getting richer even though your bank account is getting smaller. But the technology term, the third term, in the case of energy and climate change, has probably the largest leverage in it, because we can change very substantially the amount of energy it takes to make a dollar of GDP by making our whole economy more efficient. More efficient cars. More efficient trucks. More efficient planes. More efficient refrigerators. More efficient air conditioners. More efficient manufacturing processes and so on. That’s probably the cheapest, cleanest, fastest, safest leverage we’ve got on the energy problem and the climate aspect of the energy problem. Beyond that though, you do need to do something about the technologies of energy supply, because even a Prius still uses liquid fuel. And even a compact fluorescent bulb still uses electricity. And you have to figure out how you’re going to get it. What we need to do is change those technologies of energy supply to reduce the amount of carbon dioxide that’s released to the atmosphere for each liter of fuel or each kilowatt-hour of electricity that’s delivered to the economy. We can do that by changing coal-burning technologies to the advanced technologies that can capture the carbon dioxide and sequester it away from the atmosphere. We can switch to nuclear or renewable forms of electricity generation and so on.

Interviewer: I want to ask you to speak about energy use, climate change, and the environment because that’s your area of expertise.

JOHN: Let me start by saying that I think that environment is the most challenging dimension of the energy problem. And energy is the most challenging dimension of the environment problem. That is, if you ask what are the biggest environmental problems that humans are causing around the planet, an astonishing fraction of those environmental problems — air pollution, acid rain, water pollution, radioactivity, oil in the oceans, climate change — are largely associated with the ways we get energy in our society. If you look at what the big energy challenges are, how are we going to supply the energy to meet human needs and aspirations all over the world? You find that the toughest part of that is doing it without wrecking the environment, without aggravating that huge array of environmental problems already associated with energy supply.

Within the array of environmental problems connected with energy, the most challenging by far is energy’s impact on global climate. The reason that that is the most challenging environmental problem of energy supply and indeed, the most challenging environmental problem of any kind that we face is that climate is the envelope within which all other environmental conditions and processes have to function. The hydrologic cycle, the nitrogen cycle, the pollination of crops, the purification of air and water, the maintenance of biodiversity, every one of these environmental issues has to function within the envelope of climate. If you distort the global climate enough, if you distort that envelope enough, all of the environmental conditions and processes within it will be disrupted. Everything we depend on the environment for is at risk.

So why does climate change matter? It matters because climate governs the productivity of farms and forests and fisheries. Climate governs the prevalence of oppressive heat and humidity, and thus, the livability of our great cities in the summer. Climate governs the geography of disease. What pathogens and disease vectors can live in what abundance, in what places? Climate governs the damages that we have to expect from floods and droughts and wildfires and intense tropical storms, hurricanes, and typhoons. Climate governs the damage we have to expect from increasing sea level. It governs the distribution and abundance of species, the organisms we value. The organisms we hate. All of that is at risk when you disrupt the climate. And we are disrupting the climate in a degree and at a pace that is capable of disrupting all of those environmental conditions and processes that matter so much to us.

Interviewer: What do you see as the biggest problems that will be facing us in the future?

JOHN: I think some of the biggest potential train wrecks in the climate domain include, first of all, rising sea level at more rapid rates than most people have thought likely. We know from the study of climate over Earth’s history that there have been periods when sea level on the Earth went up by several meters per century. And we know that that could happen again, although no one knows exactly what the probability is that it will happen again under the sorts of warming influences that humans are now exerting. But you cannot rule out that sea level could rise by two, three, four meters in the twenty-first century, if catastrophic melting and disintegration processes were to occur in Greenland and the west Antarctic ice sheet. You think about what that means — two, three, four meters of sea level in a century. Basically Bombay is gone. London is gone. New York is gone. Washington, D.C. is gone. Shanghai is gone. A tremendous impact on the world.

The second huge issue is fresh water supplies. If you look at what’s happening now on the Tibetan Plateau, where the glaciers that feed the great rivers of China and India originate, these glaciers are now estimated to be disappearing at fifty percent per decade. When those glaciers are gone, the fluctuations of flood and drought that have always plagued the great rivers of India and China are going to be far, far worse. And India and China, by the way, are already figuring this out. Their own climate models, their own measurements of those glaciers, have underlined how much at risk those countries are. The old attitude that climate change is a problem the rich countries caused and the rich countries have to fix and the developing countries might come along later is changing very rapidly as China and India figure out how directly at risk they are because of their water supplies.

People tend to think that in a warmer world you get more evaporation, so you get more precipitation, so there’s going to be more water availability. But there are two flaws in that understanding. The first flaw is that the global climate is very uneven. And even while the global average in precipitation is going up, there are parts of the world where precipitation is going to go down. And particularly what you see if you look at what the climate models are telling us is that drought is going to increase across Southern Europe, across much of the Southern United States, across much of Central Africa, places that already are plagued by drought or are going to be even more plagued by drought in a climate-changed world.

The other flaw in the picture is that when precipitation increases under global warming, the amount of precipitation that falls in extreme events actually goes up. So you have more heavy downpours. And when you have heavy downpours, what you have is larger losses to storm runoff. The water escapes to the ocean before it can do any good either for people or ecosystems. So even though total precipitation has gone up, useable water has gone down. And so we’re in for big water problems in the case of continuing climate change at the rates that we are now pushing it.

Another big issue, which I think is kind of a sleeper, is wildfires. Wildfires are extremely sensitive to temperature, to humidity, to soil moisture, and to windiness, all of which are affected by climate change. We have had over the past few years a number of episodes of extreme wildfires in California and Colorado that burned huge areas and that would have burned even larger areas if the weather hadn’t changed just in time and just enough to make them controllable. We can’t count on that continuing to be true in the future. As those areas get drier and hotter and the soil moisture goes down, we may have wildfires becoming essentially uncontrollable in parts of the world that have always been plagued by wildfires.

Another big potential issue is agricultural productivity. When the Intergovernmental Panel on Climate Change issued its 2001 report, they said basically that under continuing climate change, agricultural productivity’s going to go down in some places. It’s going to go up in some places, as well. It probably will be a wash overall out to 2050 or so when the temperature gets to two and a half, three degrees Centigrade, on a global average basis, above the pre-industrial level. That already is pretty alarming. The fact that it would go down in some places, particularly vulnerable places, between now and 2050 is bad enough. But since the IPCC report came out in 2001, we have learned more about the vulnerability of agriculture to climate change. It looks like the CO2 fertilization effect, which was expected to help, is weaker than people thought in 2001. It looks like the adverse effects of higher temperatures are worse than people thought in 2001. And so the current models of climate change impacts on agriculture are showing earlier and more rapid declines in agricultural productivity, both in the tropics and in the temperate zone. And it now looks like we could be in trouble with agriculture under global average temperature increases of one and a half to two degrees Celsius rather than two and a half to three, which is a much more pessimistic picture. Even so, the models of climate change influence on agriculture do not yet adequately incorporate the effects of a warmer, wetter world on crop pests and pathogens at all. We just don’t understand that well enough to model it quantitatively. But every ecologist will tell you that crop pests and pathogens do better in a warmer, wetter world and that’s going to further aggravate the prospects for global agriculture under climate change.

The last one I’ll mention as a potential tipping point of great import is one that influences all the others. It’s the possibility that the release of carbon dioxide and methane from the northern soils, from the permafrost and from the tundra, could drastically accelerate climate change all over the world. There is as much carbon under the tundra and permafrost as there is in the atmosphere and that carbon that’s stored under the tundra and permafrost is vulnerable to being released as carbon dioxide and methane in a warming world. Again, nobody knows for sure how much temperature rise it would take to release a large part of that currently stored carbon into the atmosphere. It could happen when the global average surface temperature gets two degrees C. above the pre-industrial level. It could happen at three degrees. It might not happen until four degrees. But, if we are unlucky and it happens sooner rather than later, then we are in for a sharp acceleration of the onset of all of the kinds of problems I’ve been talking about because of this burst of additional carbon dioxide and methane into the atmosphere. And you have to realize that methane is about twenty times as potent per molecule as carbon dioxide is as a greenhouse gas.

Interviewer: You mentioned tipping points. Have we already reached the tipping point? Is there a way to avoid the worst? What should our strategy be? And is there any hope for us?

JOHN: The UN Framework Convention on Climate Change to which a hundred and eighty-eight countries, including the United States, are parties, has as its goal avoiding dangerous human interference in the climate system. My view is that that goal is already out of reach. We are already experiencing dangerous human interference in the climate system. And we’re experiencing it in increased incidents of heatwaves, of floods, of droughts, of intense tropical storms, of accelerated sea-level rise. It’s already dangerous. The issue now is whether we can avoid catastrophic human influence on the climate system. I believe we still have a good shot at avoiding catastrophic interference. Nobody knows exactly where the cliffs are. I sometimes say that the situation we are in is like driving a car with bad brakes toward a cliff in the fog. The fog represents the uncertainties that still surround many important aspects of the science and make it impossible to say exactly how far away the cliff is. But the prudent strategy is to start putting on the brakes in that car. The fog is not a consolation if you’re driving a car with bad brakes toward a cliff. Some of the skeptics say this, that, or the other thing is uncertain, so we shouldn’t do anything. No, we should put on the brakes. I believe that if we start putting on the brakes now, if we take the measures that are available to us to reduce the human emissions of greenhouse gases into the atmosphere, the emissions both from fossil fuel burning and from tropical deforestation, we have a good chance of not driving the car over the cliff. We can’t be absolutely sure, because we don’t know where the cliff is. But prudence requires that we take the possibility of not driving over the cliff by starting to put on those brakes.

The interesting thing is that it’s not that expensive. It’s not that difficult to put on the brakes. You know we keep hearing we’re not going to change this or we’re not going to change that because it will hurt the economy. What everybody needs to understand is that not addressing the risks of climate change is likely to have a far higher cost to the economy than addressing it. It’s much cheaper to take preventive action than to try to cope with disaster after it has occurred. And indeed, there are many options for addressing climate change that we often call win/win options, because we can reduce greenhouse gas emissions and reduce human influences on climate while getting other benefits, while preserving biodiversity, while expanding opportunities for sustainable employment, by promoting innovation that develops new products that can deliver the goods and services that people want in climate-friendly ways.

We’ve done this in the past. The United States actually succeeded between 1970 and 2005 in cutting in half the amount of energy it takes to make a dollar of Gross National Product in the U.S. economy. And we did that basically at a profit. We saved a huge amount of money on the energy we didn’t have to buy. We generated a huge amount of innovation in improved industrial processes, improved refrigerators, improved air conditioners. Refrigerators today in the United States take a tiny fraction of the energy that refrigerators used twenty years ago. They have more inside space and they’re cheaper than they were twenty years ago. I mean the notion that we have to suffer in order to address this problem is just wrong. We can benefit while addressing this problem in sensible ways.

Interviewer: Is there anything you’d like to add that I didn’t get to ask you?

JOHN: The one thing that I’ve gotten a lot of mileage out of lately, and so I’ll say it again, is about the choices. Basically, in the situation we face with respect to climate change, we have three choices. Mitigation, that is, taking steps to reduce the pace and magnitude of climate change that we’re causing; adaptation, which means adjusting to the climate changes that do occur by changing agricultural practices, by building dams and dikes and so on; and the third choice is suffering. What needs to be understood is we’re going to have a mix of the three. We’re going to do some mitigation. We’re going to do some adaptation. We’re going to do some suffering. But what the mix is, is still up to us. You need to understand that adaptation, while it’s a good thing and an important thing, and we need to do more of it, becomes more difficult and more expensive and less effective the bigger the climate changes are to which you’re trying to adapt. That means that the more you can do on mitigation, the less pressure you put on adaptation, and the less suffering there’s going to be. That is the choice we have.

Series Directory

The Habitable Planet: A Systems Approach to Environmental Science 


Harvard Smithsonian Center for Astrophysics in association with the Harvard University Center for the Environment. 2007.
  • ISBN: 1-57680-883-1