The Science of Drawing the Line

(For a much longer version of this analysis, and all the accompanying political and strategic discussion, see our recent book, Dead Heat: Global Justice and Global Warming. See Dead Heat post, or browse to the Seven Stories Press Dead Heat page, where you can actually buy a copy.)

Our goal here is hardly comprehensive. We suffer no illusion that we can summarize climate science as a whole. But we do think that can distill out the part of the science that bears most immediately on the core problem of drawing the line.

Should the climate negotiations try to cap CO2 pollution in the atmosphere at 550 parts per million (ppm), 450 ppm, or some other (hopefully lower) figure Or should we take an entirely different approach and try to cap temperature change itself, rather than CO2 pollution And what must we know about the kinds of impacts and instabilities that can be expected at any given level

We should care deeply about these questions, and about the calculations they inevitably involve, for behind them lie choices of life and death for millions of people, and survival or extinction for thousands of species. So bear with us as we discuss concentration caps, radiative forcing, climate sensitivity, and the problem of “business as usual.” In the end, we’ll try to show how “dry” science, by clarifying our real conditions of life, is the essential foundation of any just climate treaty.

Not Really the Basics

When the industrial revolution began to leverage the easy energy of fossil fuels, the atmospheric concentrations of the greenhouse gases-now greenhouse pollutants-began to rise rapidly. Since the 1700s, for example, the CO2 concentration has increased from about 275 ppm to over 370 ppm, and it continues to rise today, at about 1.5 ppm per year. The significance of this increase is measured first in terms of “radiative forcing”-which in this context is a measure of the additional energy trapped or reflected by the pollutants that humans have added to the atmosphere.

Positive radiative forcing means an increase in the solar energy absorbed from the sun, and it produces just the kinds of changes you’d expect-generally warmer temperatures, and changes in the patterns and variability of the weather. You’ve noticed those changes; now consider that they correspond to an increase in radiative forcing (for CO2 alone) of about 1.4 watts per square meter, and that in the politicians’ favorite best case future (550 ppm) it would rise to 3.7 W/m2. And remember that there are also the non-CO2 greenhouse gases-methane and all the others-which together add another watt per square meter.1

One other key introductory point: the warming effect of these gases is partially offset by a negative radiative forcing (cooling effect) caused by other pollutants called aerosols,2 particularly sulfur compounds produced by combustion. This is important because many of these cooling pollutants are extremely dangerous to human health, particularly in the local communities where they concentrate in what are called hot spots. They also lead to acid rain and must, by any reasonable measure, be quickly eliminated. Doing so, however, will mean a larger net positive radiative forcing-accelerated warming-and, in fact, this warming is probably inevitable.

How much should we care about one or two watts per square meter It depends on another key link in the chain: the overall responsiveness of the climate system to radiative forcing. Climate scientists call this the “climate sensitivity,” and by this term they mean the estimated long-run increase in global average surface temperature that would be caused by a doubling of CO2 from preindustrial levels-that is, its rise to 550 ppm, a level we could easily reach in the next fifty to a hundred years. In both the Second (1995) and Third (2000) Assessment Reports, the IPCC estimated this factor to be in a range between 1.5C and 4.5C, with a “best estimate” of 2.5C cited by the Second Assessment Report. (To compare, the Earth is now only about 5C warmer than it was 18,000 years ago at the peak of the last ice age.)

The Impacts: Present and Future

What “impacts” can we expect to see as climate change intensifies Start with the evidence that the Earth is actually warming. This could make a very long list, but if we just pick a few of the most significant bits (most of them from the IPCC’s Third Assessment Report), we have:

* Since the industrial revolution, the global average surface temperature has increased by about 0.6C. And that’s the average: The temperature is increasing much more quickly near the poles, and many scientists now expect the Arctic ice cover to be almost entirely gone by 2080.3

* Globally, the 1990s were the warmest decade and 1998 the warmest year since 1861. So far, 2001 was the second-hottest year overall, though its winter took first place. And the records just keep on coming!

* Ongoing changes in sea level, snow cover, ice extent, and precipitation are consistent with a sharp warming near the Earth’s surface. For example, there has been a widespread retreat of non-polar mountain glaciers during the twentieth century.

* The rising costs of weather damage, much of it caused by recent increases in floods and droughts, is itself a good indicator of increasing “climatic variability.” (The more farsighted insurance companies are becoming extremely worried.)

* The warming in the twentieth century is the largest of any century during the past thousand years, which can be easily seen in the famous and somewhat terrifying thousand-year temperature chart shown in figure 1 (this, by the way, is known as “the hockey stick”).


Figure 1: Global mean surface temperature record for the last thousand years, measured as annual anomaly (variation from the 1902 to-1980 mean annual temperature). Adapted from Mann, M. E., R. S. Bradley, and M. K. Hughes. 1999. Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations. Geophysical Research Letters 26:759-762.

Here you can see the Earth’s average Northern Hemispheric temperature anomalies-the annual difference from the twentieth-century mean-over the last thousand years (the dark, meandering line), a decreasing margin of error (the initially broad but thinning shaded area that surrounds that line), and a gradual long-term cooling trend that abruptly ended about 1900. There’s a lot here, but note the singularity of sudden twentieth-century warming, and note especially the steep vertical slope of the curve in the last decade.

So something is happening. But are humans really causing it Again, we defer to the IPCC, which in 1996, in their Second Assessment Report, issued the carefully crafted and oft-quoted phrase, “The balance of evidence suggests that there is discernable human influence on global climate.” By the time of their Third Assessment Report, published in 2001, the IPCC’s prose had solidified, and it told us that “there is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.”

So grant the point, and move on to the next question: what kind of climate change are we actually in for

This one is harder, for the shape of the future will depend on more than atmospheric science. It will turn as well on the character of our farms, cities, factories, and energy systems; on the nature of our economies and cultures; on the trajectory of “globalization”; on our success in avoiding a descent into hatred and endless war; and, in general, on the kind of society we have. Such things cannot be modeled, not directly. But they can be represented by “proxy” variables such as population, economic growth rates, and quantitative indicators of technological change, and these can be combined with computational models of the atmosphere, land, and ocean to yield suggestive projections of future climate change. The point to remember about such projections, though, is that they’re subject to “story-line uncertainty” as well as “scientific uncertainty.” You may have a brilliant integrated assessment model,4 but its results will nevertheless depend not only on assumptions about climate sensitivity and carbon cycling but also on assumptions about, say, the degree of regionalization in the global economy, or the faith that our children will put in technological rather than political realignments.

We can’t do much about the climate sensitivity except use the best science to try to estimate it, and in the meantime face the possibility that it will come in on the high side. The same, however, can’t be said about the story line. In fact, the story line-the tale of our common future-is quite literally up for grabs. And it’s a damn good thing, because one thing we know for certain is that we don’t want to end up where we’re currently going: Computer models looking at various plausible scenarios of the future are bringing in warming projections as high as 5.8C by 2100, and to say that such a warming would be a social and ecological disaster is to strain the limits of understatement.

How bad will it be Realize, first of all, that average temperature increases are only the beginning of the story. Actual increases will vary greatly around the average, and temperatures are projected to increase more over land, more in the higher latitudes, and much more near the poles. Further, the variability of the climate will also increase, meaning both extreme temperatures (mostly hot but also cold) and large and possibly abrupt changes in the water cycle, and thus severe droughts and floods. The expected rise in sea level (as much, in the worst case, as a full meter in the next hundred years) will have devastating and sometimes apocalyptic impacts on low-lying areas, rendering many small-island states uninhabitable and multiplying the risks that hundreds of millions of coastal residents face from increasingly severe storms and inundations. Most coral reefs will die. Large-scale tropical forest die-offs are likely, and radically increased “food insecurity” (read: starvation) is a near certainty. Rice production in Asia will be hit especially hard. Human and ecological migrations will increase, and with them political and military tension. Surprises are certain.

We could go on here, but if you really want the story of the impacts to come, go to the IPCC itself. Go, in fact, to the Summary for Policymakers of the IPCC’s Working Group 2, which focuses on “impacts, adaptation, and vulnerability.”

And there’s another thing: the risk from what the IPCC dryly calls “large-scale discontinuities.” There are a number of terrifying possibilities here: a rapid release of carbon and methane now bound within various oceanic and biological “sinks” (which remove carbon from the atmosphere), a sudden large rise in sea level caused by ice melts in Greenland or Antarctica, or the sudden collapse of the “thermohaline circulation”-the large-scale ocean current that moves warm water from the tropics toward the poles, and that warms northern Europe. None of these “discontinuities” has yet occurred, but the IPCC’s reports suggest that they could, and soon. And the fact that they’re difficult to model should not be taken as a source of solace; indeed, there’s always the possibility of true surprises: changes we didn’t even know enough to worry about.

Again, the uncertainties associated with increasing greenhouse pollution make it impossible to predict which specific impacts will follow from which concentration levels. Nevertheless, the IPCC has made an effort to produce an initial “vulnerability chart” (figure 2), and it generously repays a few moments of study. Note that the horizontal axis measures the change in degrees Centigrade from the preindustrial average.


Figure 2: Potential impacts from climate change with increasing change in global mean surface temperature. Adapted from the IPCC’s Third Assessment Report, 2001.

The chart in figure 2 shows us that:

* We’re already experiencing risks to unique and threatened ecosystems.

* The risks of extreme climate events have already risen.

* The overall impacts of climate change are already negative for some regions and the majority of people.

And it shows that if global average surface temperature rises by more than about 2C-an extremely real possibility, even a likelihood-the risks of extreme climate events will show a further large increase, and the impacts on almost all regions and economic sectors will become decisively negative. Further, with a temperature rise of around 3.5C, or even less if the more pessimistic scientists turn out to be correct, the risk of “large-scale discontinuities” (potential world-historic catastrophes) will become significant.

The Two-Degree Standard

In 1996, the European Environment Council (EEC) decided that the global average surface temperature increase should be held to a maximum of 2C above the preindustrial level, and that as a consequence the atmospheric CO2 concentration had to be held below 550 ppm. In fact, a 550-ppm concentration limit would almost certainly bring a temperature increase of far more than 2C and lead to horrific levels of destruction; it will only be “safe” if the climate sensitivity turns out to be very low, which (as we will explain) is looking unlikely. And even 2C (according to the IPCC’s Second Assessment Report) would be accompanied by significant ecosystem damage and loss of biodiversity (“whole forests may disappear”), major damage to food production in the most vulnerable parts of the world (60 to 350 million more people at risk of hunger), “significant loss of life” due to indirect health effects, particularly in developing countries, and, of course, a significant increase in sea level.

Look again at the vulnerability chart in figure 2, and understand that just because a group of politicians decides that 550 ppm would be tolerable, this hardly makes it so. Moreover, even if we’re very, very lucky and the climate sensitivity turns out to be so low that 550 ppm would map to a warming of only 2C, this would still be very grim indeed. A 2C warming would be a death sentence for tens of thousands and perhaps millions of people, a commitment to catastrophic losses of species and ecosystems, and, frankly, an invitation to a whole new level of geopolitical and military confrontation, one we hardly seem likely to manage with aplomb.

In this context, a global temperature change cap of, say, 1C would be just fine with us. But what we want to do, here, is perform a thought experiment. We want to bow to pessimism-or is it realism-and ask just what it would mean to draw the line at 2C. For if this is our goal, but seriously this time, we have to begin by asking how much greenhouse pollution we can, all of us together, emit in the next few decades. Such a calculation is necessarily uncertain, but if we did it honestly, what would it show

To answer this question, we’re going to return to the link between greenhouse gas concentrations and temperature increase. In fact, we’re going to show you the most difficult chart in this little essay. Take a look at figure 3, and notice, first of all, that each of the three diagonal lines represents (and simplifies; in reality these are not straight lines) one of the IPCC’s classic low (1.5C), best (2.5C), and high (4.5C) estimates for the Earth’s overall climate sensitivity. For each of these values, one of the diagonals shows the relationship between the atmosphere’s CO2 concentration and the expected temperature increase.


Figure 3: Relationship between CO2 concentration and equilibrium increase in global mean surface temperature for low, best, and high estimates of climate sensitivity (T).

The Low-Sensitivity Case

We’re not going to say much about this case, because it’s becoming increasingly implausible. Suffice it to say that 1.5C, the low end of the IPCC’s standard range of climate-sensitivity values, dates back to 1990’s First Assessment Report, and that few current models corroborate it.

The IPCC’s “Best” Estimate, with No Other Gases

The 2.5C climate-sensitivity diagonal, as you can see in figure 3, crosses the 2C temperature-change line when the projected CO2 concentration reaches 500 ppm (as against the preindustrial level of 275 ppm and the current level of above 370 ppm). What this tells us is that if we were willing to accept the social and ecological consequences of a 2C increase (we are not), and if the climate sensitivity is 2.5C (which could easily be quite low), and if there were no “other” greenhouse gases (there are, and they are significant), we wouldn’t want to exceed a CO2 concentration of 500 ppm.

The High-Sensitivity Case

The IPCC puts the high end of the likely climate-sensitivity range at 4.5C, but the current scientific trend is to considerably raise that upper limit. Still, let’s go, here, with the IPCC’s conservative upper estimate of 4.5C. If this turns out to be the climate sensitivity, you read this chart by following the 4.5C climate-sensitive diagonal to where it crosses the 2C temperature-change line, at 400 ppm. This is, as it happens, only 30 ppm above today’s CO2 concentration level.

The Addition of Non-CO2 Gases

Now step into the real world, and consider the additional radiative forcing from non-CO2 gases. The IPCC’s scenarios put the net non-CO2 forcing in 2050 (including the cooling from sulfate and other aerosols) at between 0.3 and 1.2 watts per square meter, equivalent to about 20 to 75 ppm of CO2. Taking a midrange value of 50 ppm and subtracting it from the numbers above, we can get a good estimate of the CO2 concentrations that, for each of our climate-sensitivity estimates, is actually likely to correspond to a warming of 2C.

The results are quite frightening. For example, if the climate sensitivity turns out to be 2.5C (which is probably low), we’d get a 2C increase at a CO2 concentration of about 450 ppm (rather than at 500 ppm if we ignore non-CO2 gases). And if it comes in at 4.5C (which is probably high, by current science), the 2C point would come at 350 ppm, rather than at 400 ppm; this would mean we’re already over the 2C line, though for a variety of reasons (the buffering effect of oceanic heat absorption, the uncertainty of aerosol cooling, and the fact that past emissions have “locked in” but not yet delivered an unknown amount of future warming) we don’t know it yet.

And It Could be Worse

This is all very rough, but believe it or not, we haven’t done violence to the facts. We’re using the IPCC’s numbers, and if we’ve had to connect the dots ourselves, it’s only because the scientists, for their own reasons-bad and good-are reticent to do so.5

When it comes to the “real” climate sensitivity, note that the IPCC’s Third Assessment Report, while still reporting the 1.5 to 4.5C range, doesn’t use its old figure of 2.5C as the “best estimate”. And note, too, that recent studies by climate modelers and recent estimates drawn from the ice-core record both suggest that the median estimate is likely to be closer to 3.5C, with significant possibilities of 5C or higher.6

And if the climate sensitivity turns out to be 3.5C, then the CO2 concentration target corresponding to a warming of 2C will, when corrected for the non-CO2 gases, be about 400 ppm, which is coming up in twenty years or less.

From Targets to Budgets

The next step in this rather grim exercise is to introduce the notion of a global emissions budget. And, in fact, this budget-how much CO2 and other greenhouse gases (GHGs) we can “safely” emit-is the real issue; ultimately, it’s only units of greenhouse gases, not units of temperature increase or carbon concentration or sea-level rise, that can be regulated.

Another word, though, about our temperature target.

We’d like nothing better than to follow Greenpeace’s carbon logic, advocate a temperature-change target of 1C, and insist that no greater warming can be tolerated. So why are we’re talking, instead and at length, about 2C Because we think that, paradoxically, doing so allows us to better underscore the severity of the situation: If we can take the real uncertainty into account (as opposed to the uncertainty as imagined by the “skeptics”), cut ourselves 2C of slack, and nevertheless conclude that we’re in serious trouble, we obviously have a pretty strong case.

In that spirit, let’s be optimistic and assume that the Earth’s climate sensitivity will turn out to be only 2.5C. This would mean that the atmospheric concentration of carbon dioxide can rise all the way to 450 ppm while still allowing us to reach a “soft-landing corridor” in which emissions don’t force the warming above 2C. Note, one last time, that this 2C would be disastrous, but noting this, move on. The next question is what it would take to make it to a 450-ppm corridor.

An Optimistic Soft-Landing Corridor

Figure 4, adapted from the IPCC’s Third Assessment Report, shows year-by-year projections of allowable emissions under a 450-ppm CO2 stabilization corridor, as calculated by one highly respected carbon cycle model.7 There are three emissions paths here, with the area between the highest and lowest shaded to show the “450 corridor.” It’s width reflects the uncertainty of carbon-cycle science, which by the way, we haven’t discussed. The headline, though, is that this uncertainty turns on questions about how and how quickly natural processes remove carbon from the atmospheric system.8


Figure 4: A 450-ppm stabilization corridor, the width of which reflects uncertainty about the global carbon cycle. Adapted from the IPCC’s Third Assessment Report, 2001.

In looking at figure 4, focus on the areas under the curves, which show, for each of the three cases, what the allowable cumulative global emissions would be over the next hundred years. Note that even on the highest and most permissive path, global carbon emissions must peak in less than twenty years, and then head steeply down. And if the low-emissions path turns out to better describe the Earth’s carbon metabolism, then there’s a whole lot less atmospheric space to go around. Indeed, we’d already be poised to overshoot the 450-ppm path, since global emissions are continuing to rise, rather than falling, as the lower path dictates.

Even in the middle path, which just about everyone uses as their soft-landing “marker scenario,” global emissions must be far, far lower in fifty years than they are today, and they must drop to these low levels even as the developing world continues to, well, develop. And this is just to meet a 450-ppm target, which we may soon recognize as substantially too high.

Again, allowable cumulative global emissions in the next hundred years are shown here as the areas under the curves, and these turn out to be roughly 750 gigatonnes of carbon (GtC) on the high path, 550 GtC in the middle, and 375 GtC on the low path. With current emissions at about 8 GtC per year, the low path would see us use the entire available carbon budget in as little as forty-five years, and that’s if annual global emissions don’t grow at all.

Despite all uncertainty, this cumulative limit tells us something we very much need to know: how much “environmental space” is actually available. The metaphor is telling: In the case of the atmosphere, we are almost literally “filling up the space,” and leaving little to our children and grandchildren. Further, and this is a politically decisive point, the North has already used up far more than its fair share.9

Look back at the middle curve. This is a plot of the numbers that the IPCC so dryly reports in its Third Assessment Report, when it tells us that stabilization at 450 ppm requires that global emissions be reduced to below current levels by 2050, and to 3 GtC annually by the end of the century. You’ll see this curve again, and not just in this book, so look hard at the trajectory it describes. Understand that this (medium) 450-ppm path requires that global emissions peak and then start dropping in less than fifteen years, and that this is quite impossible if emissions from the South continue to increase along the “business as usual” path.

“Business as Usual”

We know that things must change, but how much It’s hard to say, and for a very specific reason-we can’t discuss the size and scope of the needed changes unless we have a sense of the “baseline”-the “do nothing” case against which they can be compared. And here the issue isn’t just uncertainty but ideology as well. For example, economic models of the costs of meeting emissions targets are extremely sensitive to their baseline scenario assumptions, so sensitive that their results cannot honestly be considered apart from them. Indeed, if you want to train yourself to see through the political agendas hidden in economic and social scenarios, look first for their baseline assumptions.

Deconstruction, of course, only gets us so far, especially if the question is what we should do. And with this question in mind, models are useful tools. But consider: to model any given social or policy change, we have to compare it to a projected baseline, and how can such projections be meaningful when, in the real world, it’s difficult to predict economic or technological changes even one or two decades into the future How can we even talk about a “business-as-usual” future that lasts for most of a century

There is, in fact, no way to decisively extrapolate from current trends. The actual “present” is a dynamic one, with all sorts of factors changing simultaneously, and the interplay of all these interacting changes is quite literally impossible to predict. Even when it comes to simple, measurable quantities like energy use and carbon emissions, trends are the product of complex changes in economic structure, energy mix, and all sorts of other unpredictable and contingent factors. Historical events such as the collapse of the Soviet empire in Eastern Europe, the conversion of the British energy system from coal to gas as a result of Thatcher’s desire to break the miners’ union, and the nuclear accidents at Three Mile Island and Chernobyl all changed global energy and emissions patterns in significant and unexpected ways. Indeed, the history of the prediction of energy use should be humbling in this regard.10

Which is to say that any “baseline” scenario-in which the present is extrapolated into the future to project the consequences “if we do nothing”-must necessarily reflect beliefs and hopes and fears and politics, as well as facts. Which is, in turn, to say that “business-as-usual” scenarios generally reflect the desires, or at least the beliefs, of those who want to do business as usual.

It’s a real problem. Fortunately, the IPCC has engaged it by proposing, and promoting, a new set of scenarios-named the SRES scenarios after the “Special Report on Emissions Scenarios”-designed to make social and political assumptions more transparent. The idea here is to replace the business-as-usual approach with a new one based on a coherent set of “story lines,” each one representing a possible, narratively plausible future with distinct and explicit political and technological assumptions. And since these are defined as “non-intervention” baselines, they can, so it is hoped, make it easier for us to see what will happen in the absence of explicit climate protection policies. It’s all a very long story that we’re not going to tell, not here in any case, but we will note one thing-despite the IPCC’s claim that SRES contains no “central case” scenario, it’s pretty easy to pick out the de facto business-as-usual storyline.

For one thing, the typical business-as-usual future includes high economic growth. And because the SRES scenarios refuse to countenance worst cases like global war and global Apartheid, economic growth must also be higher in the South than in the North, so that the North can continue to get wealthier (in absolute terms) while emerging markets boom and global inequality decreases. Furthermore, following current trends, an accurate business-as-usual scenario must include substantial increases in energy efficiency, but discount the possibility of a systemic renewables revolution. And, indeed, all of these features can be found in the IPCC’s “A1 balanced” scenario (see the SRES Summary for Policymakers) in which the South develops by following the gradually more efficient path of the North. More specifically, A1B sports rapid economic growth, a “balanced” energy mix (which includes substantial increases in zero- or low-carbon energy as well as continued dependence on fossil energy), a 2050 emissions peak, and, subsequently, a population decline.

Figure 5 represents one possible A1B path. It shows the projected growth path of CO2 emissions (including emissions from forest cover and other land use changes) divided into North and South, with global emissions (the sum of North and South) tracking the top of the lighter shaded area. And, just to frighten you (though not as much as we could), we’ve overlaid (as per the discussion above) a medium-case 450-ppm soft-landing path.


Figure 5: Annual CO2 emissions North and South under IPCC’s A1 “balanced” scenario, plotted against a midrange 450-ppm stabilization pathway. Data from the IPCC’s Special Report on Emissions Scenarios, 1999.

What these curves show is that global emissions crack through the “safe” 450-ppm path by about 2005, and that Southern emissions alone (the top edge of the darker shaded area) exceed that path by about 2020. What it doesn’t show is that the more global emissions increase above the 450 path, and the higher they peak, the faster and farther they’ll have to fall to return to it. Indeed, if global emissions follow the A1B trajectory for too long, we can a expect a catastrophic temperature change upwards of 4C.

And this, dear friends, finally explains why the situation is so dire, and why half-measures will not avail us. The 450-ppm path-dangerous though it is-defines an emissions budget for the coming century that is clearly too small to accommodate “business as usual.” To make it down into the 450 corridor, we’ll have to win major changes in the trajectories of not only the developed North but the developing South as well.

Or put it another way: The North could meet its Kyoto targets, and, all else being equal, climate change would still reach extremely dangerous levels unless the curve of Southern emissions also bends down, sharply, and soon. But why-and think “real world” here-would the Southern elites strain to make such a turn unless the critically limited global emissions budget was being divided fairly between it and the North

We do not think that they would. And we conclude that any strategy that promises, at best, the begrudging support of the South will simply not do. We conclude, in fact, that as curves like this one become more and more well known, incrementalism is becoming manifestly implausible as a means of making it to a soft-landing corridor. That today, to be serious, we have to start talking about the long-term curves, and the carbon budget that they imply, and then move on explain how we plan to balance that budget.

A Per Capita Look at the Business-as-Usual World

Look, now, in Figure 6, at per capita emissions in the “A1 balanced” scenario that we presented in the last chapter. Note that average Southern emissions start at less than 1 metric ton of carbon per year in 2000 and rise to just over 1.5 tons in 2040, while Northern emissions rise to over 3 tons per person per year in 2030 before dropping gradually back to just over 2 tons. Again, keep in mind that this is a SRES “nonintervention” scenario; it assumes no climate policy whatsoever, not even the Kyoto Protocol. Any reductions in emissions are made either for strictly economic reasons (e.g., cleaner fuels are cheaper, or efficiency is increasing) or for other environmental reasons (e.g., as a side effect of reducing acid rain or “traditional” air pollutants such as particulate emissions).


Figure 6: Projected Southern and Northern per capita CO2 emissions pathways for IPCC’s A1B scenario, compared to per capita pathway for stabilization at 450 ppm. Data from the IPCC’s Special Report on Emissions Scenarios, 1999.

Note the third curve, shown here as a solid line. This is our old friend the 450-ppm path, the one that will lead to a warming of only 2C. If we’re lucky. And note that it describes a path that would be almost as demanding in the developing world as it would be here in the North.

What, then, do we know

We know that this “balanced” kind of North-South emissions convergence just isn’t going to work, not even given A1B’s rather forgiving population assumptions.11 For while such a future would arguably be fair if we had enough atmospheric space left to take it, we don’t. In fact, the 450-ppm soft-landing path, shown here in per capita terms, falls below projected Southern levels in less than two decades, before dropping later in the century to less than 0.5 metric tons per person! Even if the North magically disappeared, A1B-style Southern emissions growth alone would rapidly blow the global carbon budget.

The point in all this is hardly to blame the South, or, indeed, to make a big deal about population, which is emphatically not the problem here. The point, rather, is that a per capita analysis allows us to quickly prove a crucial point: The South can’t develop the way the North has. It’s simply not possible.12


It you’ve learned to read between the lines, you know, at this point, that this is really a story about money. Much as we’d like to see the atmosphere as, say, a celestial sphere, or as the Earth’s precious, protective membrane, the reality is that it’s also an economic resource, one to which we all have inherently easy access. And here’s another reality: only some of us have been able to use this resource to become powerful and wealthy. This was, of course, in the past, and the past is unalterable; but the point is that the same cannot be said about the future. Which presents us, finally, with a key moral and political question: Why, now that the atmosphere’s ability to absorb carbon is manifestly and critically scarce, should people in some countries continue to enjoy more of it, and thus more wealth, than others

The answer, of course, is that there are not and cannot be any such reasons, save perhaps for raw power. And this, more than anything else, is the reason why this rather dry analysis, by quantifying the pressures of scarcity, provides the rationale for a justice-based climate regime. For whatever one’s position on “the commodification of nature” or the perils of tradable pollution permits, this remains: The “right” to dump greenhouse gases into the atmosphere has significant economic value, and the economic advantages of this right cannot be ignored.

The issue here is distributive justice. But understand that in a world beset by ecological crisis, distributive justice must mean more than it has in the past. It must include not only the fair distribution of wealth, resources, and opportunities, but also the fair distribution of “impacts.” For the elemental truth is that as the storms become more violent and the droughts more fierce, some of us will be hurt far more, and far earlier, than others. The rich will be able to hide, but the poor will not, and neither will the plants and the beasts. And because the “ecosystems people”-indigenous peoples, farmers, fishermen-who rely directly on nature for their livelihoods are the most threatened of us all, the real priority is finding a politics that makes a low concentration cap possible; it’s at least as crucial as finding a fair distribution of emissions rights.

It is, as Tom Paine said, “the good fortune of some to live distant from the scene of sorry,” and this is perhaps even truer today than it was in his day. High-flown schemes will be of little use to the victims of tomorrow’s killer hurricanes, unless these schemes are tied to and enable real change and honest hope. Climate change must be minimized, but at this point severe impacts are entirely inevitable. The harm these impacts bring to the poor – always the most vulnerable – must be minimized, and then alleviated, while the “burdens” of “adapting” to climate change must be honestly addressed, fairly distributed, and adequately funded. Anything else would be unjust and lead inevitably to distrust, bitterness, and failure.

We haven’t got much time, and this isn’t going to be easy. Leave out the details for a moment and note only this: A just adaptation program will be neither simple nor cheap. Inner-city residents in Chicago are already dying from abnormal heat waves, even as South Asian peasants, along with their cousins in Central Europe, are suffering floods more terrifying than any in living memory. Disaster relief as we know it will not do, and the questions, here as everywhere, come down to democracy and money. One question, in particular, is key: Who pays13

The answer must be that the rich must pay, and not only because they’re responsible for the problem, though they are: In 1990, the industrialized countries were responsible for 75 percent of all CO2 emissions, as well as 79 percent of the CO2 still in the air and 88 percent of the human-caused warming.14 They must pay as well because their riches grew out of a past in which the world was open, and because it no longer is. And they must pay because only they can, and because if they don’t, the warming will quite certainly prove unstoppable.

It has become common to justify the need for equity in narrowly realist terms, to argue that a certain measure of equity is a matter of national or economic self-interest or even, at the extreme, of national security. Without sufficient equity, so the argument goes, there will be political and social upheaval, which will eventually “blow back” to harm us, even in our most secure enclaves. The case for equity, in other words, is often reduced to the case for its instrumental utility as a precondition of both peace (or at least stability) and environmental sustainability.

We grant the point, but only to a point. For while we fancy ourselves realists, we also believe that equity – defined specifically as equal rights to global common resources – must be affirmed as a foundational ethical and political principle, a basic element of the unfulfilled enlightenment project of human emancipation. And here’s the twist: We believe that this “moral vision” is itself part of the game, and a powerful political force. The way forward, in our view, lies in taking “the just” and “the realistic” as two sides of one single overarching imperative, the two sides of the integrated vision we so desperately need.

The large countries of the South, and China and India in particular, do not require U.S. permission to burn their coal. Nor, despite their vulnerability, must they agree to a climate treaty that they regard as patently unfair, particularly if its unfairness lies in foreclosing their “development.” Southern leaders are, moreover, entirely aware that the North owes them a huge ecological debt, that it rose to wealth and power in significant part though the high-carbon history that is now causing the climate crisis. Given this, it’s quite unreasonable – and entirely futile – to suggest that the South accept restrictions on its development that perpetuate the disproportionate pollution of the North.

And it’s just not going to happen. A climate treaty that indefinitely restricts a Chinese (or Indian) to lower emissions than an American (or European) will not be accepted as fair and, finally, will not be accepted at all. Climate equity, far from being a “preference,” is essential to ecological sustainability. At the end of the day, it’s just that simple.

— Tom Athanasiou and Paul Baer

1) For both scientific and economic reasons, the Kyoto Protocol in fact regulates a “basket” of key greenhouse gases, rather than CO2 alone. The methods for measuring and comparing these gases are complex and controversial, but they don’t affect the basic argument here.

2) Aerosols are airborne particles of various types, some of which (like sulfate aerosols) increase the reflection of sunlight and thus cause cooling (negative radiative forcing), and some of which (like soot from combustion) absorb heat as CO2 does and thus cause heating (positive radiative forcing).

3) The specific figure here is from the Norwegian Polar Institute. “Polar Bears Facing Extinction,” The Independent (London), 14 May 2002.

4) Integrated assessment models combine economic, atmospheric, and ecological models to allow quantitative projections of indicators such as emissions and average temperature, and to evaluate policies to mitigate climate change.

5) For a discussion of the issues here, see Stephen Schneider, “Can We Estimate the Likelihood of Climatic Changes at 2100″Climatic Change 52 (2002): 441-51.

6) For a discussion of the likely probability distribution across the range of possible climate sensitivity values, see N. G. Andronova and M. E.. Schlesinger, “Objective Estimation of the Probability Density Function for Climate Sensitivity,” Journal of Geophysical. Research 106 (2001):22,605-22; see also R. Knutti, T. F. Stocker, F. Joos, and G. Plattner, “Constraints on Radiative Forcing and Future Climate Change from Observations and Climate Model Ensembles,” Nature 416 (2002): 719-23.

7) The carbon cycle is the set of biological and geochemical processes by which carbon moves between living organisms, the soil, the atmosphere, and the oceans. The model used here is the Bern carbon cycle model, and the graph is from figure 25 in the Third Assessment Report Working Group 1 Technical Summary, available at

8 ) Among the critical uncertainties in the carbon cycle are the response of plants to increased atmospheric CO2, the rate of increase of decomposition of plant matter with increasing temperature and changes in precipitation, and changes in biological and chemical absorption of CO2 in the ocean. Again, consult the IPCC’s reports for details.

9) For much more on “environmental space,” a notion developed and popularized by Friends of the Earth International, see Michael Carley & Philippe Spapens, Sharing the World: Sustainable Living & Global Equity in the 21st Century (London: Earthscan Press, 1998); see also W. Sachs et al., Greening the North: A Post-Industrial Blueprint for Ecology and Equity (London: Zed, 1998). It’s important to realize that even though the concept is based on science, “environmental space” is an extremely political notion; it represents the limit on pollution or use of a resource that we collectively (and, we hope, democratically) decide to accept. In the case of “atmospheric space,” this decision will necessarily be a tough one, because there will be serious harm to humans and other species long before that “space” is “full.”

10) As should the history of prediction in general. See, for example, George Orwell on “trend chasing;” in “James Burnham and the Managerial Revolution,” Collected Essays, Journalism, and Letters, vol. 4 (New York: Harcourt Brace Jovanovich, 1968), 172-73.

11) The A1 and B1 scenario families show population peaking at 8.7 billion in 2050, and then declining back to 7 billion by 2100, which would be an unprecedented drop in the absence of war, famine, or pestilence. By contrast, the most recent “medium variant” projections from the United Nations show the population at 9.3 billion in 2050 and still growing.

12) See, for example, Living Planet Report 2002, a new report from WWF. The report, downloadable at, found that exploitation of the Earth’s renewable resources has grown by 80 percent in the past forty years and is now 20 percent higher than the natural capacity of the planet to replenish itself. But if current trends continue, it’s only until 2050 before a second Earth would be necessary to meet human demand. It’s not going to happen, of course, which is why the authors fear that global living standards will begin to plummet long before then.

13) The title was taken long ago by The Global Greenhouse Regime: Who Pays eds. Peter Hayes and Kirk Smith (New York: UN University Press, 1993).

14) Figures from Definitions of “Equal Entitlements,” Centre for Science and Environment, 1998. Based on UNFCCC, Implementation of the Berlin Mandate, Additional Proposals from Parties, 1997 (see, and, ultimately, the Brazilian proposal, which was tabled just before Kyoto.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.