CHAPTER TWELVE
Ice Ages and the Future
It is worth reiterating here something that was pointed out in the first chapter of this book but may have drifted into the background since: the Earth is still in an ice age. We are in a warm period, one of the many interglacial intervals that have occurred throughout the Pleistocene Ice Age, but even so, there are significant amounts of permanent ice in polar regions. It is easy to forget that this may be just a short respite before another glacial interval begins. Or perhaps the respite may be longer than we anticipate. Man’s activities may intervene and confound our ability to use the natural climate variations of the past as a tool to predict the future.
In spite of the uncertainty about the future, we do know that the past few million years of the Pleistocene Ice Age have been fairly unrepresentative of the long history of our planet. Over that four and a half billion years, only a few major ice ages have been identified, separated by hundreds of millions of years of warmer climates when no major ice sheets blanketed the continents. We have touched on some of the possible causes for these unusual episodes in previous chapters: variations in the shape of the Earth’s orbit around the sun, and the tilt and wobble of its axis of rotation; the positions of the continents as they move about on the Earth’s surface due to plate tectonics; and the concentration of greenhouse gases, especially carbon dioxide, in the atmosphere. It is widely agreed that within the Pleistocene Ice Age, it is the astronomical parameters that have regulated the repeated cycles of glacier growth and decline. And by analogy, the course of ancient ice ages may have been similarly controlled by these parameters. But it is not at all clear that any conjunction of these astronomical conditions would on its own be sufficient to tip the Earth into a cold period and initiate an ice age. Much more likely, most scientists now believe, is that a combination of factors—including at the least the astronomical parameters, greenhouse gases, and plate tectonics—must play a role. Under the right conditions, a relatively small change in one of these—perhaps a fluctuation in the Earth’s orbit—may act as a trigger to initiate a cold interval. An additional important ingredient seems to be positive feedback, some process that amplifies the effect of the other factors.
Recent concern about global warming, and especially the impact of mankind’s addition of greenhouse gases to the natural atmospheric inventory, has led to increased awareness of the role these gases may play in starting or ending ice ages. Data from ice cores have further stimulated interest, because they show that on almost every timescale, the concentration of greenhouse gases has tracked changes in temperature. Rapid addition of these substances to the atmosphere may have triggered some of the abrupt temperature rises recorded in the ice cores, and it has also become clear that for the future greenhouse gases will be a major factor in determining when the Earth will slip into the next glacial period of the Pleistocene Ice Age. It is just possible that the greenhouse gas content of the atmosphere is the critical necessary condition that controls when an ice age will begin or end.
What, exactly, are the important greenhouse gases, and how do they affect the Earth’s temperature? There are quite a few, but the most important in terms of their ability to trap solar energy in the atmosphere are water vapor (H2O), carbon dioxide (CO2), and methane (CH4). All three are molecules that absorb the heat energy radiated up into the atmosphere from the Earth’s warm surface. The surface is heated by incoming solar energy—the energy you feel as you lie on a beach in the sun—which is short-wavelength radiation that is not absorbed by the greenhouse gases. Because of their structures, these gases only absorb energy of specific longer wavelengths, which coincidentally match those of the energy radiated away from the heated surface. The greenhouse molecules not only absorb this energy, they also re-radiate much of it back into the atmosphere. Although the analogy is not quite perfect, they act something like window glass that lets the sun’s energy in but blocks the heat in a warmed-up room from making the return journey into the outside air.
All of the greenhouse gases are quite minor constituents of our atmosphere, but without them much of the solar energy incident on the Earth would be lost almost as quickly as it is gained, and we would have a much colder home—with an average temperature of about – 18°C, far below the freezing point of water. Of the three top greenhouse gases, water vapor is by far the most important. However, the atmosphere’s water vapor content tends to follow temperature changes rather than cause them, because higher temperatures promote evaporation and thus higher water vapor concentrations, while lower temperatures have just the opposite effect. Water vapor is thus more likely to be an amplifying factor than a trigger, with low concentrations helping to maintain the cold temperatures of an ice age and much higher values during warm times tending to keep average temperatures high. Methane, on the other hand, has the potential to cause large temperature changes because it is a very efficient greenhouse gas—on a weight-for-weight basis, it is much more effective than carbon dioxide. There is currently a lively debate about whether or not changes in atmospheric methane were responsible for some of the rapid temperature fluctuations recorded in the ice cores, especially abrupt warming events such as the one at the end of the Younger Dryas cold period. As we shall see, there is good evidence that large quantities of methane have been released quickly in the past through natural processes; however, methane is a relatively unstable molecule in the atmosphere and is fairly quickly (in roughly a decade for the average methane molecule) destroyed through chemical reactions with other compounds. The effect of even a large increase is therefore of limited duration, although it has been argued that even a short-lived temperature spike could push the climate over a threshold and into a new state.
That leaves carbon dioxide as the greenhouse gas most likely to be involved in the initiation—and perhaps also the end—of ice ages. There are a number of natural processes that produce or consume this gas, and thus have the potential to change its atmospheric concentration. And unlike methane, it is not rapidly destroyed by chemical reactions with other compounds and so has a long lifetime in the atmosphere. Svante Arrhenius, as we saw earlier, recognized long ago that CO2 could be important for regulating the Earth’s climate. All one needs to do to understand its effectiveness is to consider our sister planet Venus. Much like the Earth in some ways, Venus has a very different atmosphere—one composed almost entirely of CO2. There is so much CO2 that the atmospheric pressure is almost one hundred times that on Earth, causing a “super greenhouse” that maintains a surface temperature about twice as hot as your kitchen oven turned to its highest setting. On October 22, 1975, the Russian Venera 9 spacecraft landed on the scorching surface of Venus. It reported a temperature of 485°C and an atmospheric pressure ninety times that on Earth. Although the lander had been designed to withstand high temperatures—it has been described as having been “built like a tank”—Russian scientists were nevertheless elated when the camera on board survived the roasting temperature long enough to send back the first images of Venus’s rocky surface.
The sources and sinks of atmospheric CO2 on Earth, the processes that could change its concentration and thereby put our planet on a path toward the beginning or end of an ice age, were touched on in chapter 8 in connection with Snowball Earth. In the short term, carbon dioxide is regulated by biologic processes. During photosynthesis plants take CO2 from the atmosphere, use the carbon to make organic material, and release oxygen into the atmosphere. When the plants die, bacteria go to work to oxidize the carbon and convert it back into CO2, which is returned to the atmosphere. The photosynthetic cycle is rapid enough that there are actually small but easily measurable annual cycles in the atmosphere’s carbon dioxide content, with more CO2 consumed in the warm, bright summer months, less in the winter. There can also be significant lags in the return of CO2 to the atmosphere if the organic matter produced during photosynthesis is sequestered, for example by burial, and not quickly oxidized. There is some evidence that this happened startin
g around 400 million years ago when the vascular plants, with thick stems capable of carrying water and nutrients to branches and leaves, first evolved. Their rapid proliferation and growth of the Earth’s first forests meant that large amounts of carbon-rich organic material were produced, all ultimately from atmospheric CO2. Much of the carbon was incorporated into soil—then also a new feature of the evolving Earth—or peat that was buried and converted into coal, instead of being oxidized and returned to the atmosphere. As a consequence the CO2 content decreased significantly. Because this happened in the run up to the Permo-Carboniferous Ice Age, it may well have been the factor that set the stage for that cold period. As we saw in chapter 8, the time when this ice age occurred is characterized in the geologic record by repeated cycles of coal deposits in some parts of the world. When we burn that coal today we are returning some of that long-buried CO2 to the atmosphere.
Although biological sources control the short-term cycling of CO2, in the long term the main source of CO2 is volcanism. When volcanic lavas erupt on the surface, they bring with them a variety of gases from the Earth’s interior, carbon dioxide among them. All those holes in a piece of pumice are nothing more than gas bubbles trying to escape from the frothy, molten rock as it cools, and a part of the gas in the bubbles is CO2. Over geologic time, it is the balance between volcanic supply and consumption by the chemical weathering of rocks—which we shall come to shortly—that has provided our planet with a relatively stable climate. Venus shows us what can happen if supply far outstrips consumption. Fortunately for us, our watery planet has a mechanism for achieving a rough balance over long time periods.
The process that keeps our atmospheric carbon dioxide in check is the chemical weathering of rocks. When rain falls through the atmosphere, it dissolves a small amount of CO2, making carbonic acid. The higher the carbon dioxide content of the atmosphere, the more acidic the rainwater—at today’s levels (the atmosphere contains slightly less than 0.04 percent CO2) rain is quite acidic. As a result, it can actually dissolve solid rock. The process is slow, and each rainstorm dissolves only a tiny amount, but over time, the erosion becomes quite substantial. If you have any doubt about this, just visit an old graveyard. Some types of rock dissolve so quickly that tombstone inscriptions become illegible in a century or less, especially in places with heavy rainfall.
It would be quite interesting to follow a molecule of CO2 from the atmosphere through its journey to the ocean and beyond. Our molecule would form a new compound, carbonic acid, when it dissolved in a drop of rainwater. No harm done when the raindrop splattered on a rock at the Earth’s surface, the molecule of carbonic acid would get to work dissolving the rock. In that process, it would release various chemical elements—calcium for example—and itself be transformed again, into a compound referred to as a bicarbonate ion, but still incorporating the original CO2 molecule. Both the calcium dissolved from the rock and the bicarbonate ion would be present, in a dissolved state, in the moisture that seeped away through the soil and eventually made its way into streams and the ocean. Planktonic organisms living in the surface waters of the sea use the dissolved substances in seawater to make their calcium carbonate shells, so our carbon dioxide molecule might find itself combined with a calcium atom to form a molecule of calcium carbonate in one of these shells. When the organism died, the shell would fall to the seafloor, and, together with millions of others, be buried in ocean sediment and turned into limestone. The net result of this long journey is that our molecule of CO2, originally vented into the atmosphere during a volcanic eruption, would be stored in limestone on the seafloor. This is the process that keeps the CO2 supplied by volcanism in balance over the long term, maintaining the atmospheric content at a low level—and gradually building up huge amounts of limestone at the Earth’s surface. If all the CO2 bound up in that limestone were to be released, our atmosphere would be more like that of Venus, and the Earth too would be scorchingly hot because of the greenhouse effect.
With an understanding of how atmospheric CO2 is controlled, it is reasonable to ask whether any of the processes we’ve discussed can be implicated in starting or ending specific ice ages. The Permo-Carboniferous, as already indicated, may be tied to the lowering of the carbon dioxide content by vascular plants. We know, too, that the amount of volcanic activity, the source of atmospheric CO2, has varied significantly through geologic time, but to the extent that it’s possible to track these changes, there is no evidence that connects any of the known ice ages with reduced volcanism. However, there is an interesting and plausible scenario that has been proposed for the initiation of the Pleistocene Ice Age, one that links it with increased chemical weathering and a consequent decrease in atmospheric CO2. This particular suggestion—by no means proven, and at this point very much a working hypothesis—also illustrates once again the multiple interconnections among geologic processes, because it invokes the creation of the Himalayan Mountains and uplift of the Tibetan Plateau as its starting point. Formation of the Himalayas is well understood: it is the result of plate tectonics. Three hundred million years ago, India was part of the Gondwanaland supercontinent near the South Pole, and was partly covered with the ice sheets of the Permo-Carboniferous Ice Age. As the large continent broke up, India slowly drifted north and eventually crashed into Asia, with the crumpled and pushed up rocks caught up in the collision forming the Himalayas. The collision began about 50 million years ago and continues today—India is still pushing north against Asia. The timing of mountain building in the Himalayas coincides closely with the beginning of glaciation in the Antarctic 35million years ago, and uplift continued throughout the gradual cooling of the planet toward the Pleistocene Ice Age.
It’s possible that the close agreement in timing between Himalayan mountain building and global cooling toward the Pleistocene Ice Age is pure coincidence. But there are several reasons why the idea is worth serious consideration. One is that high-altitude regions are cold and better able to sustain glaciers than lowlands in any state of the global climate, and in the specific case of the Himalayas and the Tibetan Plateau, the area at high altitude is enormous, almost half the size of the United States. As ice cover increased, so did the positive feedback effect of reflected solar energy. The sheer size and height of the Tibetan Plateau also means that its presence completely altered the global wind pattern, resulting in significant regional changes in climate, although these would have been only indirectly involved in the initiation of glaciation. However, the cornerstone of the proposal is that mountainous regions are sites of intense chemical weathering. Because this process removes CO2 from the atmosphere, a significant increase in chemical weathering would cause global cooling by reducing the greenhouse effect.
There is no question that chemical weathering is more rapid in the mountains than on flat plains. Anyone who has spent time in mountain ranges, especially geologically young ones such as the Himalayas, or the Rockies or Alps, knows that there are shattered and broken up rocks everywhere. Piled up against every steep cliff is a mound of talus, loose fragments of broken rock that are much more easily attacked by acidic rainwater than is the solid bedrock of flatter regions. Furthermore, mountains are natural rainmakers. When moistureladen winds are forced upward along mountain fronts, the air cools, water vapor condenses to liquid drops, and it rains. This general principle is accentuated in the Himalayas, because as the vast Tibetan Plateau heats up in the summer sun, the air above it rises, pulling in moist air from the tropical Indian Ocean. The result is the famous Indian monsoons, which drench the mountain front and further accelerate weathering.
All of the available evidence suggests that uplift of the Himalayas has caused chemical weathering in the region to increase greatly. Today, the rivers draining the mountain range all carry much higher amounts of dissolved material than most other world rivers; in spite of the fact that their watersheds cover a relatively small fraction of the Earth’s total surface area, they deliver almost a quarter of all the dissolved materi
al flowing into the oceans. The great pile of sediments in the Bay of Bengal, at the mouth of the Ganges, further documents the erosive power of the monsoon rains.
Thus it is likely that uplift and weathering of the Himalayas has affected atmospheric CO2 and played a part in the gradual cooling of our planet over the past 35 million years. In the most comprehensive computer simulations of climate the influence of greenhouse gases is quite clear: the coldest temperatures and greatest ice cover always appear in trials with the lowest amounts of CO2. If mountain building is implicated in the initiation of the Pleistocene Ice Age through its effect on chemical weathering, then it may have played a role in ice ages of the more distant past, too.
And if greenhouse gases are so important for ice age climate, what does that say about the future? Today, of course, the concern is about increasing CO2 and global warming, not cooling. Over the past few thousand years, and accelerating over the past few hundred years, clearing land for agriculture, burning forests, and especially burning fossil fuels, has added CO2 to the atmosphere more quickly than it can be taken up by weathering or photosynthesis or dissolution in the ocean. Its concentration has risen by about one third, and is still increasing rapidly, now almost entirely due to burning of fossil fuel. During the last interglacial period, some 120,000 years ago, CO2 levels (and temperatures) were similar to those of today. But the greenhouse gas content of the atmosphere did not rise any higher—in fact, it decreased as the Earth began cooling into the glacial interval that culminated only 20,000 years ago. At present, already at a peak concentration, CO2 is being added to the atmosphere a hundred times faster than it was during any of the natural increases that can be observed in ice cores.
Even critics of global warming don’t dispute the greenhouse property of CO2—it is a well-known fact of physics. It is difficult to comprehend how further additions to the atmosphere at current rates could fail to raise global temperatures and possibly influence the course of the Pleistocene Ice Age. And there is an additional possible source of climate surprise that may arise because of the increasing temperatures: methane. We saw in chapter 8 that release of large amounts of methane has been suggested as a cause for the end of Snowball Earth ice ages and the very warm periods that immediately followed. Today, there are great stores of methane locked up as “hydrates”—icy crystals that contain large amounts of methane in a relatively small volume. The methane in this unusual form of ice comes primarily from bacterial action, and it forms solid layers in the permafrost of cold regions and also in ocean sediments along the margins of continents. The hydrate crystals can only exist over a restricted range of low temperatures and moderate pressures, and decompose easily when conditions change.
Frozen Earth: The Once and Future Story of Ice Ages Page 27
