11
SHOULD WE FEAR THE OXYGEN FUTURE?

We have come to the end of history, if history can be counted as something that has already happened. In this last chapter, let’s gaze into the future. In keeping with the rest of this book, we will simply ask: can oxygen levels be expected to stay the same, or will they undergo wild swings as they have for the past 540 million years?

The future stretches before us not as one long dark tunnel but as a series of vignettes of variable clarity, like a long avenue punctuated by streetlights of differing luminosity. This century and at least the next will continue to be a time of warming from greenhouse gases produced by humanity. It would be nice if the volcanoes of our world would politely stop outgassing carbon dioxide as well, at least until we humans get our act together and curtail our carbon dioxide production. But the volcanoes just keep spewing this gas into the air, as they have since Earth began. It is our addition to that natural input that is the problem.

We are warming our world, rapidly. A hundred years from now the planet will have returned to its atmospheric condition during the Late Cretaceous through Eocene. Happily, those were times when oxygen was at about its present level, or was even slightly higher. But can we foresee a time farther in the future, when oxygen levels might change? If they do, will they be higher or lower? The fate of oxygen is fixed by



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 229
Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere 11 SHOULD WE FEAR THE OXYGEN FUTURE? We have come to the end of history, if history can be counted as something that has already happened. In this last chapter, let’s gaze into the future. In keeping with the rest of this book, we will simply ask: can oxygen levels be expected to stay the same, or will they undergo wild swings as they have for the past 540 million years? The future stretches before us not as one long dark tunnel but as a series of vignettes of variable clarity, like a long avenue punctuated by streetlights of differing luminosity. This century and at least the next will continue to be a time of warming from greenhouse gases produced by humanity. It would be nice if the volcanoes of our world would politely stop outgassing carbon dioxide as well, at least until we humans get our act together and curtail our carbon dioxide production. But the volcanoes just keep spewing this gas into the air, as they have since Earth began. It is our addition to that natural input that is the problem. We are warming our world, rapidly. A hundred years from now the planet will have returned to its atmospheric condition during the Late Cretaceous through Eocene. Happily, those were times when oxygen was at about its present level, or was even slightly higher. But can we foresee a time farther in the future, when oxygen levels might change? If they do, will they be higher or lower? The fate of oxygen is fixed by

OCR for page 229
Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere the rates at which organic compounds and sulfur-containing compounds and minerals (such as pyrite) are buried, or not, and the rate at which they are weathered or not. Many things control burial and weathering rates. One of these is continental position, and it is this that can best be predicted for the future. PLATE TECTONICS AND THE FATE OF GEOGRAPHY We have been in an ice age now for more than 2.5 million years, and there is a real possibility, based on past experience, that our planet awaits at least that much time again in the seemingly endless cycles of ice and warming for another 2.5 million, or 5 million, or even 10 million years. But eventually the ice ages will end, and indeed with humans producing greenhouse gases at such a prodigious rate it may well be that the time of ice is over, perhaps on timescales of geological periods (tens of millions of years). While global temperature is a function of many factors (including the presence of eroding high mountains, such as the Himalayas, which serve to increase the rate at which the global thermostat removes carbon dioxide from the atmosphere), geography also plays a huge role. Our planet is heading for greater warmth caused both by the increased energy of the sun and by a new global geography. Continental drift is moving enough of the northern landmasses out of high latitudes and moving Antarctica out of the extreme southern latitudes, to end the tyranny of ice. We have a good idea of how and even when that will come about, based on powerful new computer simulations of plate movements. Plate movements for the past 600 million years are fairly well established (although ambiguity increases for older time). Certainly the positions of continents for the past 200 million years are very well known. Five hundred million years ago, at the time of the major animal diversification known as the Cambrian Explosion, the continents were widely dispersed along the equator. For the next 200 million years, large-scale drift and continental collision resulted in the formation of ever-larger land bodies and major mountain chains, including the Appalachians of the eastern United States. By about 300 million years ago the major continents had coalesced into a single united block, a super-continent named Pangea. By about 200 million years ago, the huge

OCR for page 229
Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere expanse of land began to break apart and drift in separate ways, creating the Atlantic Ocean as North America split away from Europe and South America from Africa. By 120 million years ago, the southern continents broke apart as well, with Africa, Antarctica, India, and Australia moving in divergent directions and culminating in the continental positions of the modern world. Given the large number of years that Earth has existed, it is no surprise that the continents have wandered extensively through time. We believe this wandering will not end any time soon, and thus we can confidently predict that continental drift will continue. Furthermore, there is enough information from present-day drifting to allow prediction of future motions. Future motion will have enormous effects on future climate and on the fate of future life. The best scenarios for understanding future continental change come from labs and analyses that have studied and modeled past motions. We fully expect future directions and rates of plate movements as measured in the present-day to continue into the future. While geographic reconstructions become more problematical and error prone the farther into the future we look, there is good agreement for at least the next 250 million years among the several independent groups of investigators who have examined this problem. The most detailed examination of future continental positions comes from C. Scotese and his Paleomap Project. For nearly two decades Scotese and his coworkers have been compiling maps of continental position in the past and have recently turned their attention to the future. They have arrived at 10 reconstructions of plate positions for the next 250 million years. They are convinced that at the end of this vast period of time there will again be a supercontinent, a return to the state last experienced by Earth at the end of the Paleozoic Era some 250 million years ago. A WORLDWIDE BLACK SEA For the next few million years, plate motions should continue in the current directions. The Atlantic Ocean will continue to widen, and the Pacific Ocean will continue to close. But then we can expect major changes. By 50 million years from now a world map would show fantastic differences from the present-day, and these continental positions

OCR for page 229
Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere are more completely described in my 2003 book coauthored with Don Brownlee, The Life and Death of Planet Earth. Perhaps most noticeable would be the loss of the Mediterranean Sea, with its space taken up by an enormous mountain range extending from what is now Europe to the Persian Gulf region. Australia will have moved northward, closing the regions that are now composed of Papua New Guinea and Indonesia, while Baja California will have slid northward along the Pacific Coast of North America. Far more important than these new continental positions will be the formation of new subduction zones, the regions where Earth’s crust dives beneath the continents. Today we know that subduction is initiating in the central Indian Ocean and in the ocean off Puerto Rico. These events suggest that new subduction zones will be in place off both eastern North and South America. As this happens, mountain building will be initiated once again in the Appalachian regions and along the eastern coastline of South America as well. These regions will become home to gigantic active volcanoes and rising mountain chains. It is not just the positions of the mountains that will change. As Antarctica drifts northwards, its vast ice sheets will melt and the level of the sea will rise. As the sun continues to increase its energy output, it will also cause temperatures to rise, melting other continental ice sheets, with Greenland’s being the most important. When both Greenland and Antarctica have seen all of their ice cover melt, the oceans will rise to a sea level nearly 300 feet higher than it is today. Tectonic forces, including the formation of new mid-ocean spreading centers, will also exacerbate sea-level rise. As these form, they will cause the oceans to spill out of their basins and onto low land surfaces. Flooding of the continental margins brought about by the rise of the sea will cause our planet to undergo a radical climate change. Will it cause there to be a change in oxygen levels as well? As we saw in previous chapters, the two most radical changes in oxygen spanned the interval from the Carboniferous to the end of the Triassic. From the start of the Carboniferous oxygen levels rose to a maximum of over 30 percent by the early Permian. They then began to drop, reaching minimal values of 10 percent some 210 million years ago. Why was there this one-two punch, and could it be repeated? We

OCR for page 229
Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere know that the interval of time examined included major environmental and biological changes, both of which affected oxygen. The most notable of the former was the coalescing of the continents into Pangea. This amalgamation was finished about the time that oxygen reached its maximal values. Then the continents broke apart at the same time that oxygen levels dropped. Could it simply be that merging continents raise oxygen levels and splitting continents draw down oxygen? That answer may be partially correct. We know why oxygen went up for the first time on land—plants grew into great trees that filled out into planet-spanning forests. As the trees toppled, and topple they did with great regularity, since roots had not yet modernized and all plants of the Carboniferous were less securely rooted, large quantities of organic material were quickly buried before decomposing. In our world, microbes quickly attack felled trees, and the process of decomposition uses oxygen. But 300 million years ago, lignin, the hard parts of trees, was only newly evolved, and microbes of the time had not yet evolved the trick of breaking down this new material. The consequence was the Coal Age. It was the burial of all this organic matter before it could oxidize that caused the rise in temperature, not the merging of continents. What about the drop in oxygen? Here there may have been more of a connection to continental position. The fused continent caused aridity, and the breakup may have instigated the formation of the Siberian Traps—the biggest flood basalts in Earth’s history. The latter caused immense volumes of carbon dioxide to enter the atmosphere, making an already hot planet even hotter. The deep oceans became devoid of oxygen, while on land the rate of forest burial and even the rate of forest growth, dropped markedly. As a consequence, so did oxygen. We must conclude that the breakup of a supercontinent is not good for our planet’s biology, as it seems to cause a drop in oxygen. Can we foresee the formation and then split up of another supercontinent in the far future? The answer seems to be yes. The pattern of smaller continents assembling into a large land mass, or supercontinent, and then breaking apart again over hundreds of million of years of Earth history has been dubbed the Wilson Cycle, in honor of one of the pioneering discoverers of plate tectonics, J. Tuzo

OCR for page 229
Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere Wilson. The entire cycle seems to take about 500 million years, and there is no reason to believe that plate movement in the near (and far) future will alter this trend. The last supercontinent formation, Pangea, occurred some 300 million years ago, allowing a rough prediction that the next will occur in about 200 million years. By about 100 million years from now, the continents will have reached their maximum separation and begin to coalesce. By 150 million years from now, the Atlantic will have become far smaller. Subduction will continue along the entire eastern seaboard of both North and South America as the Atlantic Ocean floor is subducted beneath the coastline along gigantic, linear subduction zones. This process will, in turn, create a series of high mountains along the eastern coastlines of those two continents. The Pacific Ocean will increase in size as the continents all begin a mad rush at mutual collision. By 250 million years into the future, the process of continental amalgamation will have been completed. Europe, Africa, North and South America, and Asia will have formed the supercontinent; however, Antarctica and Australia will not be amalgamated. A curious aspect of this projection is the existence of a large, central equatorial sea. Because of the presence of subduction zones virtually encircling this giant supercontinent, a wall of mountains will enclose much of the land surface, walling off the interiors of the continent, just as the Andes, Sierra Nevada, Cascade, and Coast Range mountains—all high mountains with active volcanism—wall off the interiors of the western parts of North and South America. But like the last time, this supercontinent will not last. When it breaks apart, it may be that Earth will once again experience a major drop in oxygen, perhaps the equivalent of the Permian mass extinction. FUTURE WORK It will be up to scientists to see how many of the new hypotheses offered in this radical revision of Earth’s history are accepted. If even a few are ultimately accepted, it will mean that we will have to revise our understanding of the whys in the history of life. If oxygen has varied

OCR for page 229
Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere through time along the lines that Robert Berner and others suggest, it seems highly likely that organisms would adapt in varied ways to these different conditions. How to test these various ideas? John VandenBrooks—a Ph.D. student of Robert Berner at Yale, as I write this in 2006—is carefully stealing American alligator eggs and raising them in higher-oxygen conditions. He is going to start rearing them in lower temperature and begin the same sort of experiments with parchment eggs, to try to answer questions about why and when these two different kinds of reproduction came about and were subsequently used. Equally important will be studies that measure the rates of oxygen uptake by invertebrates and vertebrates with different kinds of gills. Most important of all, though, is that new generations of scientists spread out across the world, exhuming from the fossil record new clues to better understand the history of life. Oxygen is to be loved and hated—but also respected. Respiration has been the most important driver of evolution. Will it be an epitaph for our planet?

OCR for page 229
Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere This page intially left blank