|Credit: Nathan Walker, www.nathanwalker.net|
climate change—and with it chains of storms, floods, heat waves,
droughts, and even cold snaps—is now virtually inevitable, and will
bring widespread ecological destruction, extinction, and human
suffering. Continued dithering will lead to climatic instability on a
truly terrifying scale. We’re entirely dependent on science to
understand the climate problem and the demands it makes upon our
responses. Some strategies will work and others won’t, and we need a
good grasp of both the science and the politics to tell the difference.
So bear with us, if you will, as we discuss concentration caps, radiative forcing, climate sensitivity, and increased climatic variability. We’ll try to make this as painless as we can.
The IPCC’s assessments
The global-warming crisis has given rise to a unique scientific body, the Intergovernmental Panel on Climate Change (IPCC). The IPCC brings together thousands of scientists from around the world to provide assessments of the threat, the science that allows us to know the threat, and the uncertainty of that science.
So-called skeptics have attacked the integrity of the IPCC, but a recent report by the US National Academy of Sciences—one the Bush administration itself requested—strongly endorsed both the IPCC’s process and its assessments, and denied that they have become politicized. Indeed, the NAS report Climate Change Science: An Analysis of Some Key Questions forced the administration into a position in which it either had to admit the seriousness of the climate change problem or be seen as turning its back on science. Only days after its release in June 2001, George W. Bush conceded that “the National Academy of Sciences indicates that the [temperature] increase is due in large part to human activity,” but nevertheless went on to repudiate the Kyoto Protocol as “fatally flawed.”
This move—from “the science is uncertain” to “the treaty is flawed”—is being forced upon the fossil-fuel cartel. “Climate skepticism” has come to remind even mainstream observers of a tobacco company PR campaign.
The Bush people hate the IPCC, and there are reasons why. The IPCC’s tasks include the preparation of “state of the science” assessments every five years, and these reports have played a crucial role in making the situation clear.
Greenhouse gas basics
Human-generated (anthropogenic) greenhouse gases such as CO2 warm the Earth by trapping additional solar radiation within the atmosphere. Many gases besides CO2 act as greenhouse gases, most notably water vapor, but also methane (CH4), nitrous oxide (N2O), and fluorocarbons (some of which, the chlorofluorocarbons, or CFCs, are being phased out due to their ozone-destroying properties). The greenhouse effect is natural. Before humans started increasing the concentrations of these gases in the atmosphere, these gases worked together to create a “blanket” that kept the Earth far warmer than it would otherwise have been.
Unfortunately, once the Industrial Revolution began to leverage the cheap energy of fossil fuels, atmospheric concentrations of greenhouse gases began to rise rapidly. Since the 1700s, CO2 alone has increased from about 275 parts per million (ppm) to over 370 ppm, and it continues to rise at about 1.5 ppm per year. The significance of this increase is measured first in terms of radiative forcing—the amount of additional energy trapped or reflected by the greenhouse gases that humans have added to the atmosphere.
Positive radiative forcing means an increase in the energy absorbed from the sun, and it produces the kinds of changes you’d expect—generally warmer temperatures, and changes in the patterns and variability of the weather. The current increase in radiative forcing attributable to humans for just CO2 is about 1.4 watts of solar energy per square meter, almost half of the increase expected from a doubling of CO2 concentrations from their preindustrial level. And since the non-CO2 greenhouse gases add about one watt per square meter, they are also extremely significant.
The warming effect is partially offset by cooling caused by other pollutants called aerosols, especially sulfur compounds produced by combustion. Many of these cooling pollutants are extremely dangerous to human health, particularly where they concentrate. They also lead to acid rain and they must be quickly eliminated. Doing so, however, will mean a larger net positive radiative forcing, and, in fact, this positive forcing is inevitable—unless we dump larger amounts of aerosols into the air—because they have much shorter atmospheric lifetimes than the major greenhouse gases.
How much should we care about one or two watts per square meter? That depends on another key link in the chain: the overall responsiveness of the climate to radiative forcing. Scientists call this climate sensitivity, and express it in terms of the estimated long-run increase in global average surface temperature that would be caused if doubling occurs of atmospheric CO2 from preindustrial levels—that is, to 550 ppm, a level we could easily reach in the next 50-100 years. The IPCC estimates this factor to be in a range between 1.5°C and 4.5°C, with a best estimate of 2.5°C cited by its Second Assessment Report.
Impacts: present and future
Let’s glance at the evidence that we’ve already changed the climate, and consider the impacts we can expect to see as the changes intensify. Start with the evidence that the Earth is actually warming.
Since the Industrial Revolution, the global average surface temperature has increased by about 0.6°C. The temperature is increasing much more quickly near the poles, and many scientists now expect Arctic ice cover to be almost entirely gone by 2080.
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.
Ongoing changes in sea level, snow cover, ice extent, and precipitation are consistent with a warming climate near the Earth’s surface. For example, there was a widespread retreat of non-polar mountain glaciers during the 20th 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 far-sighted insurance companies are becoming extremely worried.)
The warming in the 20th century is the largest of any century during the past thousand years.
So something is happening. But are humans really causing it? The IPCC, in their 1996 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: “There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.”
The skeptics, of course, still argue that the observed temperature increase and other signs of climate change haven’t been proven to be a result of humanity’s greenhouse gas emissions. Is this a reasonable objection? Could the warming be a result of natural variability?
No. For while the climate varies over a wide variety of time scales—from El Niño cycles of a few years, to glacial cycles of tens of thousands of years, to the even longer cycles of the Earth’s deep history—none of these explains the trend so clearly visible in the last century.
But basic laboratory physics does. We know that greenhouse gases trap solar energy, and we can measure the rapidly increasing concentrations of these gases with grim precision. Given this, we actually expect significant warming, and in much the same pattern that it actually appears (for example, the warming peaks are at the poles and in the winters).
It’s not “proof,” but it will do. Uncertainties remain, but they no longer justify doubt that we’re changing the climate. In fact, the skeptics’ traditional comment in reply to the evidence for human-induced climate change is that computer models project temperature increases higher than those actually observed. Alas, with the recent inclusion of cooling aerosols into these models, the projections have converged with the observed record.
So let’s just accept it, and go on to the next question: What kind of climate change are we in for? This one is harder, for the shape of the future will depend on a good deal more than atmospheric science. It will turn as well on the character of our farms, 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 militarism; and, in general, on the kind of society we have. 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 5.8°C by 2100. To say that such a warming would be a social and ecological disaster is to strain the limits of understatement.
How bad would it be?
Remember that average temperature increases are only the beginning of the story. Actual increases will vary greatly. Temperatures are projected to increase more over land and in the higher latitudes—much more near the poles. Climate variability will also increase, meaning extreme temperatures, and 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 effects on low-lying areas, rendering many small islands 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 increased 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.
And we have to mention the risk from what the IPCC drily calls “large-scale discontinuities.” There are a number of terrifying possibilities here: a rapid release of huge amounts of carbon and methane now bound in oceanic and biological sinks, 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 they could, perhaps even within our children’s lifetimes.
Again, the uncertainties associated with increasing greenhouse pollution make it impossible to predict which specific impacts will follow from which concentration levels. Nevertheless, 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 if global average surface temperature rises by more than about 2°C—an extremely real possibility, even a likelihood, given the way things are going—the likelihood of extreme climate events will show a further large increase, and the impacts on almost all regions and economic sectors will become negative. Further, with a temperature rise of around 3.5°C, or even less if the more pessimistic scientists turn out to be correct, the risk of “large-scale discontinuities” will become significant.
The two-degree standard
In 1996, the European Environment Council (EEC) decided the global average surface temperature increase should be held to a maximum of 2°C above the preindustrial level, and that as a consequence, the CO2 concentration had to be held below 550 ppm. Unfortunately, such a 550-ppm concentration limit would be “safe” only if the climate sensitivity turns out to be very low, which is quite unlikely. Even more unfortunately, the number quickly became a popular one. Soon thereafter, Bill Clinton also announced that his policy was to stabilize at 550 ppm, and, no doubt coincidentally, rumors began to fly that the United States was pressuring the IPCC, then busy drafting its Third Assessment Report, to feature 550 ppm as its principal mitigation scenario.
Fortunately, the pressure failed. A CO2 level of 550 ppm would almost certainly bring a temperature increase of far more than 2°C. In all likelihood—according to the IPCC’s Second Assessment Report—it would be accompanied by significant ecosystem damage and loss of biodiversity (“whole forests may disappear”), significant 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 a significant increase in sea level.
Moreover, even if we’re very lucky and the climate sensitivity turns out to be so low that 550 ppm would map to a warming of only 2°C, this would still be very grim indeed. A 2°C warming would be a death sentence for tens of thousands—perhaps millions—of people, a commitment to catastrophic losses of species and ecosystems, and, frankly, an invitation to a dangerous new exacerbation of geopolitical and military instability, one that we hardly seem likely to manage with aplomb.
The real point is that we simply must not burn all of the fossil fuels at our disposal, or even all the gas and oil, and that even burning most of them would produce an ecological holocaust. A warming of 0.6°C has already occurred, and the rise will reach 1.0°C very soon, and Greenpeace counsels us that “temperature changes above 1.0°C above preindustrial levels could bring about rapid and unpredictable changes to ecosystems.”
Just what would it mean to draw the line at 2°C? If that were our goal, how much greenhouse pollution could we, 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 look again at the IPCC’s classic low, middle, and high estimates for the Earth’s overall climate sensitivity (1.5°C, 2.5°C, and 4.5°C, respectively). For each of these values, there is a different relationship between the atmosphere’s CO2 concentration and the expected temperature increase.
We’re not going to say much about the low-sensitivity case, because the case for low climate sensitivity is becoming increasingly implausible. Suffice it to say that 1.5°C, 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 2.5°C climate-sensitivity diagonal crosses the 2°C 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 2°C increase (which we aren’t), and if the climate sensitivity is 2.5°C (which could easily be low), and if there were no other greenhouse gases (and there are), we wouldn’t want to exceed a CO2 concentration of 500 ppm.
The IPCC’s upper estimate of climate sensitivity is 4.5°C, but the current scientific trend is to considerably raise that upper limit. If 4.5°C turns out to be the climate sensitivity, then the world will see 2°C temperature change at 400 ppm. This is, as it happens, only 30 ppm above today’s CO2 concentration level.
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 2°C.
The results are quite frightening. For example, if the climate sensitivity turns out to be 2.5°C (which is probably low), we’d get a 2°C 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 4.5°C (which is probably high), the 2°C point would come at 350 ppm, rather than at 400 ppm. This would mean we’re already over the 2°C line, though for a variety of reasons (absorption of heat by the oceans, 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.
When it comes to “real” climate sensitivity, note that the IPCC’s Third Assessment Report, while still reporting the 1.5 to 4.5°C range, doesn’t use its old figure of 2.5°C as the “best estimate” of climate sensitivity. 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 3.5°C, with significant possibilities of 5°C or higher.
The bottom line is not pretty. Both the climate system and our civilization have a great deal of inertia and will be difficult to turn. Atmospheric CO2 is increasing year by year, and the increase isn’t going to be easy to stop. And none of these numbers account for those dreaded “non-linearities” that, set off by the heat-related die-back of tropical forests or the release of methane currently bound up in oceanic sinks, would take all these fine projections off the table and leave us, instead, in climate hell. In short, we haven’t got much time. Z
Tom Athanasiou is author of dozens of essays on environmental and techno-scientific politics. Paul Baer is a Ph.D. candidate in the Energy and Resources Group at the University of California, Berkeley. Tom and Paul are co-authors of Dead Heat, Globalization and Global Warming (Seven Stories, 2002), from which this article is adapted.
Take action: Athanasiou and Baer’s NGO, EcoEquity, works to catalyze an honest public debate about the real demands of global environmental justice. EcoEquity publishes Climate Equity Observer, a free online newsletter available at www.ecoequity.org or by e-mail.
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