Storms of My Grandchildren (25 page)

One final mundane, but sobering, inference from the PETM: The recovery time from excess carbon in the air and ocean, and from the PETM global warming spike, was about 100,000 years. That is the recovery time predicted by carbon cycle models. The added carbon dioxide in the air increases the rate of weathering and carbon uptake, which is a negative (diminishing) feedback. Confirmation of the recovery time is a useful verification of the models. It is also a reminder that if humans are so foolish as to burn all fossil fuels, the planet will not recover on any time scale that humans can imagine.

Such was the state of PETM research, or at least my perspective on it, in mid-2007, right around the time that Bill McKibben was asking me about 450 ppm—though the most startling revelation from the PETM was yet to come. I finally promised Bill that I would give him a number at the December 2007 American Geophysical Union meeting, when I would present a talk on the rationale for the suggested carbon dioxide target.

In that talk, I emphasized carbon dioxide itself, not the carbon dioxide equivalent of all human-made gases. The perturbed carbon cycle will not recover for tens of thousands of years, and it is carbon dioxide that determines the magnitude of the perturbation. Other forcings are important and need to be minimized, and some may be easier than carbon dioxide to deal with, but policy makers must understand that they cannot avoid constraints on carbon dioxide via offsets from other constituents.

In addition to paleoclimate data, my talk covered ongoing observations of five phenomena, all of which imply that an appropriate initial target should be no higher than 350 ppm. In brief, here are the five observations.

(1) The area of Arctic sea ice has been declining faster than models predicted. The end-of-summer sea ice area was 40 percent less in 2007 than in the late 1970s when accurate satellite measurements began. Continued growth of atmospheric carbon dioxide surely will result in an ice-free end-of-summer Arctic within several decades, with detrimental effects on wildlife and indigenous people. It is difficult to imagine how the Greenland ice sheet could survive if Arctic sea ice is lost entirely in the warm season. Retention of warm-season sea ice likely requires restoration of the planet’s energy balance. At present our best estimate is that there is about 0.5 watt per square meter more energy coming into the planet than is being emitted to space as heat radiation. A reduction of carbon dioxide amount from the current 387 ppm to 350 ppm, all other things being unchanged, would increase outgoing radiation by 0.5 watt, restoring planetary energy balance.

(2) Mountain glaciers are disappearing all over the world. If business-as-usual greenhouse gas emissions continue, most of the glaciers will be gone within fifty years. Rivers originating in glacier regions provide fresh water for billions of people. If the glaciers disappear, there will be heavy snowmelt and floods in the spring, but many dry rivers in the late summer and fall. The melting of glaciers is proceeding rapidly at current atmospheric composition. Probably the best we can hope is that restoration of the planet’s energy balance may halt glacier recession.

(3) The Greenland and West Antarctic ice sheets are each losing mass at more than 100 cubic kilometers per year, and sea level is rising at more than 3 centimeters per decade. Clearly the ice sheets are unstable with the present climate forcing. Ice shelves around Antarctica are melting rapidly. It is difficult to say how far carbon dioxide must be reduced to stabilize the ice sheets, but clearly 387 ppm is too much.

(4) Data show that subtropical regions have expanded poleward by 4 degrees of latitude on average. Such expansion is an expected effect of global warming, but the change has been faster than predicted. Dry regions have expanded in the southern United States, the Mediterranean, and Australia. Fire frequency and area in the western United States have increased by 300 percent over the past several decades. Lake Powell and Lake Mead are now only half full. Climate change is a major cause of these regional shifts, although forest management practices and increased usage of freshwater aggravate the resulting problems.

(5) Coral reefs, where a quarter of all marine biological species are located, are suffering from multiple stresses, with two of the most important stresses, ocean acidification and warming surface water, caused by increasing carbon dioxide. As carbon dioxide in the air increases, the ocean dissolves some of the carbon dioxide, becoming more acidic. This makes it more difficult for animals with carbonate shells or skeletons to survive—indeed, sufficiently acidic water dissolves carbonates. Ongoing studies suggest that coral reefs would have a better chance of surviving modern stresses if carbon dioxide were reduced to less than 350 ppm.

I am often asked: If we want to maintain Holocene-like climate, why should the target carbon dioxide not be close to the preindustrial amount, say 300 ppm or 280 ppm? The reason, in part, is that there are other climate forcings besides carbon dioxide, and we do not expect those to return to preindustrial levels. There is no plan to remove all roadways, buildings, and other human-made effects on the planet’s surface. Nor will we prevent all activities that produce aerosols. Until we know all forcings and understand their net effect, it is premature to be more specific than “less than 350 ppm,” and it is unnecessary for policy purposes. It will take time to turn carbon dioxide around and for it to begin to approach 350 ppm. By then, if we have been making appropriate measurements, our knowledge should be much improved and we will have extensive empirical evidence on real-world changes. Also our best current estimate for the planet’s mean energy imbalance over the past decade, thus averaged over the solar cycle, is about + 0.5 watt per square meter. Reducing carbon dioxide to 350 ppm would increase emission to space 0.5 watt per square meter, restoring the planet’s energy balance, to first approximation.

There is a longer story and range of uncertainty for each of the five phenomena discussed above. The way science works, we must expose the caveats and keep an open mind—otherwise we will not be successful in the long run. I know you do not want a long story, so I will provide a flavor, by an example. The example also shows how people who are determined to discredit the threat of human-made climate change—I call them contrarians; others call them denialists—use uncertainties inappropriately to cast doubt on all conclusions, even those that can be made with confidence. Nobody has figured out a good way to deal with this problem, but we cannot change the way we do science, so we just have to present the data as best we can.

Through 2006, Arctic sea ice was nearly following the script predicted by climate models. Sea ice area was beginning to decrease, just a bit faster than most models predicted. Then, in 2007, the bottom fell out. There was a big melt-off that surprised everyone. The ice area at the end of the warm season was only about 4 million square kilometers; three decades earlier, when accurate satellite measurements were initiated, it was 7 to 8 million square kilometers. Climate models had not predicted such a large loss before the middle of the twenty-first century.

A few (very few) scientists then suggested that summer sea ice might be gone entirely in five or six years. Those politicians who believe that scientists are inherently reticent, understating dangers, jumped on that speculation as if it were fact. But, as you can see in figure 20, the sea ice area partly recovered in 2008 and 2009. Contrarians, as is their wont, leaped on the recovery as evidence that there is no basis for concern. They also trumpeted that Antarctic sea ice is increasing rapidly. In fact, there are reasons to expect little change in Antarctic sea ice in the near term—the discharge of cold fresh water from disintegrating ice shelves tends to increase sea ice cover, which competes with global warming’s tendency to reduce ice cover. The bottom line for Antarctic sea ice, as figure 20 shows, is that there is no meaningful trend as yet.

The Arctic is the issue. There is a strong consensus among Arctic researchers that we are faced with a clear and imminent threat to the continued existence of summer sea ice in the Arctic. I have found no Arctic researcher who believes that sea ice will survive if the world continues with business-as-usual fossil fuel use. The only questions seem to be exactly how fast the ice would be lost and how dramatic the feedbacks on tundra, methane hydrates, and Greenland would be.

The sea ice example illustrates the difficulty in communicating with the public. Contrarians spout their interpretations of data, sometimes mangling the truth, usually demonstrating a lack of insight about what is important, and often succeeding in confusing the public. Contrarians have a loud voice, out of proportion with their scientific standing, in part because of support from special interests and politicians influenced by special interests, and often aided by media, which likes to present two sides of every topic, creating the impression that the contrary opinions deserve equal respect. What can we expect the public to think when they compare a scientist who includes appropriate caveats with a contrarian who gives conclusions without hesitation? It can seem like a debate between theorists, and often the contrarians are more media savvy. It is no wonder that there is a growing gap between what is understood about global warming by the relevant scientific community and what is known about global warming by the public and policy makers.

What can be done to improve this situation? There is no simple good answer, or it would have been found by now. One suggestion that I have made repeatedly is that President Obama ask the National Academy of Sciences for a report on the status of climate science and its implications for policy makers. The academy, established by Abraham Lincoln for just such advisory purposes, is among the most respected scientific bodies in the world. Given the cacophony about global warming in the media, such authoritative guidance is needed to help define appropriate policies and to inform the public—but unless the report is specifically requested by the president, it will not have much impact.

Can scientists help improve communication so the public can better assess these matters? It is said that the public has lost interest in science, and that may be so. But we still have to try to communicate, using the same language, which requires a mutual effort. I hope that more of the public will be willing to look at straightforward scientific graphs of data. Graphs are the most compact, honest way of presenting information, allowing insights about what the data show and helping us distinguish what is significant and what is less important. They can help us assess where the climate itself is headed and how the driving factors are changing. Are human-caused climate forcings continuing on a business-as-usual course, or are they beginning to turn toward a path that can stabilize the climate? Is there evidence that amplifying feedbacks are moving toward runaway self-amplification, or are these feedbacks diminishing? Data are a work in progress because some of the most important quantities are not being measured, or are being measured with poor accuracy. Also, we are dealing with science on the fly—new quantities of importance may emerge, resulting in additional graphs. These and graphs included in this book will be updated regularly on my public Web site, with data sources provided, so the public can see how things are changing.

Global temperature must be one of the climate diagnostics, but it is a product of many driving factors and contains a good amount of variability that has nothing to do with climate forcings. By looking at the temperature data, we can avoid the common mistake of confusing local fluctuations with global climate change. For example, the summer of 2009 was unusually cool throughout much of the United States, which provided a field day for the contrarians in their efforts to confuse the public. Let’s consider the data.