Storms of My Grandchildren (6 page)

Can these disparate perceptions of Connaughton be squared up? I will try to do so, but I should first describe my presentation to CEQ and its reception.

It was a good, friendly discussion at the White House, with Connaughton and about twenty-five others, including representatives from OMB and the Office of Science and Technology Policy (OSTP). Marburger was not there—he was on a plane, it was explained—which was just as well, as his deputy, Kathie Olsen, was present. Olsen, more engaged and engaging than Marburger, had been chief scientist at NASA headquarters from 1999 to 2002.

In the beginning of my talk I said that there was bad news and good news about global climate. The bad news: It has become clear that Earth’s climate is very sensitive to climate forcings, and we are close to driving the system into a region with dangerous consequences for humanity.

The good news: It is not too late to solve the problem, and there would be multiple side benefits to doing so. I noted that the United States, under George Bush the elder, helped bring about the United Nations Framework Convention on Climate Change, which has the specific objective of stabilizing atmospheric greenhouse gases at a level that avoids dangerous human-made interference with climate. I argued that the actions required to stabilize climate, which require addressing both non–carbon dioxide and carbon dioxide emissions, would likely have economic benefits and would certainly be beneficial for energy security and national security.

During my talk, I noted a quote from the president’s June 2001 Rose Garden speech: “We will be guided by several basic principles. Our approach must be consistent with the long-term goal of stabilizing greenhouse gas concentrations in the atmosphere. Our actions should be measured as we learn more from science and build on it. Our approach must be flexible to adjust to new information…We will act, learn, and act again, adjusting our approaches as science advances and technology evolves.” I suspected these words had flowed from the pen of Ari Patrinos, but they fit my talk perfectly, because, I argued, I now had improved information about the “global warming time bomb” threat and the actions needed to avert it.

The most important scientific insight that I hoped to convey at the White House meeting was based on Earth’s climate history, that is, paleoclimate. Climate history is our best source of information about how sensitive the climate system is, and, it turns out, the climate is remarkably sensitive—large climate changes can occur in response to even small forcings.

I mentioned in the preface to this book that understanding climate forcings, imposed perturbations of the planet’s energy balance, would be the most difficult science you would need to deal with. Sorry. I was leading you on a bit, hoping to get your toes wet.

Paleoclimate and climate sensitivity might give you more trouble—perhaps not; it is pretty simple. But you are facing a case of double or nothing—you need to decide whether you are willing to learn about them, or whether you prefer not to bother.

But if you are willing to expend a modicum of effort, you can take a big step toward appreciating the degree to which we are living on a planet in peril. Additional steps will be needed, but this first one—learning about climate sensitivity and paleoclimate—is essential to developing a realistic understanding of the potential implications of climate change for your children and grandchildren.

All right, it’s true that the initiation fee—the “modicum of effort”—is not necessarily small, depending on your training and the time you have to concentrate. If you prefer not to pay these dues, at least not at the moment, perhaps rather than casting the book aside, you might skip to page 51, near the end of this chapter.

If you will stick with it, I will be your docent on a short excursion through the remarkable world of climate change. You will be able to understand, for example, how in natural climate oscillations, the temperature change must precede the carbon dioxide change. You will also gain a quantitative appreciation of implications for human-made climate change. In return, I hope you will help spread the knowledge. Remember that the fate of our grandchildren depends on a better public appreciation of the situation.

Paleoclimate, especially the waxing and waning of ice ages, is something that you should know about anyhow. Just twenty thousand years ago, most of Canada was under a huge ice sheet, as much as three kilometers (two miles) thick. That ice sheet pushed south, over the U.S. border, covering the areas of Seattle, Minneapolis, and New York.

Ice sheets have continually expanded and retreated for millions of years. While advancing, they shove before them massive amounts of soil—most of the topsoil in Iowa was robbed from Minnesota and Canada by the glaciers. The farmhouse I was born in sat on topsoil so deep that I assumed it went all the way to China. The town I grew up in, Denison, Iowa, is on a hill that is an end moraine, a dirt pile left at the snout of a glacier before it melted.

The size of continental-scale ice sheets is mind-boggling. Although thinner toward the edges, ice over New York towered several times higher than the Empire State building—thick enough to crush everything in today’s New York City to smithereens. But not to worry—even though we sometimes hear geoscientists talk as if ice ages will occur again, it won’t happen—unless humans go extinct. Forces instigating ice ages, as we shall see, are so small and slow that a single chlorofluorocarbon factory would be more than sufficient to overcome any natural tendency toward an ice age. Ice sheets will not descend over North America and Europe again as long as we are around to stop them.

Temperatures at many places around the world are obtained in analogous ways. Ocean sediments that pile up over the years contain the shells of microscopic animals, which reveal the temperature of the water in which the shells were formed. Mineral properties in stalagmites, formed by dripping water in a cave, also preserve a record of temperature changes over hundreds of thousands of years.

The important point revealed by the data from many places around the world is that the large climate variations are global in extent. But the amplitude of temperature change depends on location. Temperature changes at the equator are typically one third as large as polar changes. The global average change is about one half as large as the change at the poles.

The same ice cores that yield the Antarctic temperature allow us to measure atmospheric composition from bubbles of air trapped when the snow compressed into ice. The amount of carbon dioxide, shown in the middle curve in figure 3, is larger during the warm periods. This is as expected, because a warmer ocean releases carbon dioxide into the air. Part of the carbon dioxide release is due to decreased solubility as temperature rises (just as warm soda releases its fizz), and part is due to other mechanisms including reduced storage of biological carbon in the deep ocean as ocean circulation speeds up in interglacial periods.

Close examination shows that temperature changes precede the carbon dioxide changes by several hundred years. Carbon dioxide change in response to climate change is an important feedback process that affects climate sensitivity, as I will discuss momentarily. But note here that the sequence (carbon dioxide change following temperature change) and the delay (several hundred years) are as expected for these natural climate changes. The length of the delay of the carbon dioxide response to temperature change is due to the ocean turnover time, which is several centuries.

When ice sheets melt, the water ends up in the ocean, and sea level rises. The bottom curve in figure 3 shows that sea level changes are large. Twenty thousand years ago, sea level was 110 meters (about 350 feet) lower than it is today, exposing much of the present continental shelves. The rate of sea level rise can be rapid once ice sheets begin to disintegrate. About 14,000 years ago, sea level increased 4 to 5 meters per century for several consecutive centuries—an average rate of 1 meter every 20 or 25 years.

The descent out of Eemian warmth into ice age conditions must have been stressful on humans, even though it took thousands of years. Indeed, the final descent into full ice age conditions 70,000 years ago was rapid and coincided with the one near extinction of humans; as few as one thousand breeding pairs are estimated to have survived during the population bottleneck. A popular theory for the cause of both this rapid cooling and population decline is the colossal eruption of the Toba supervolcano at about that time. Geologic records indicate that Toba ejected at least eight hundred cubic kilometers of material, compared with four cubic kilometers from the 1991 Pinatubo eruption, the largest volcanic eruption of the past century. Regardless of the validity of the Toba theory, it is likely that the rapid global cooling at that time played a role in the population bottleneck.

The huge sea level changes illustrated by the lower curve in figure 3 have played an important role in the development of human societies. Low sea level during the last glacial period produced the Bering land bridge connecting eastern Siberia and Alaska. This grassland steppe region, sometimes called Berengia, was up to a thousand miles wide from north to south. Asians that migrated into Berengia became isolated from ancestor Asian populations. Glaciers that had blocked the path southward began to melt 16,000 or 17,000 years ago, enabling human migration into the Americas.

It was actually the absence of sea level change that helped lead to the development of complex human societies. The social hierarchies of complex societies require food yields sufficient to support the non-food-producing component. Curiously, almost all of the first known population centers, on several continents, date to about 6,000 to 7,000 years ago, when the rate of sea level rise slowed markedly. Until then, as shown in figure 3, sea level had increased continually (not continuously) at an average rate of more than one meter per century for several thousand years. Most human settlements were either coastal or riverine, often in delta regions. Coastal biologic productivity and fish populations are low while sea level is changing, but they can increase an order of magnitude with stable sea level. Thus it has been hypothesized that the high-protein fish diets that become possible with stable sea level account for the near-simultaneous development of complex societies worldwide. This near-simultaneity is surely exaggerated by the fact that earlier settlements were simply flooded or washed away by rising seas. But there is little doubt that our civilizations would have had much greater difficulty getting started, and probably would be less developed today, if sea level had not stabilized. As we shall see, however, the period of near-stable sea level is about to end.