During the late 1700s and 1800s, the world entered the industrial era. Technological innovations such as engines powered by steam and later by gasoline transformed our ability to produce and make use of inexpensive power. The invention of the reaper and other devices transformed agriculture, enabling farmers to turn the deep roots of prairie grasses and plow the fertile soils. The population of the planet exploded from 1 billion in 1850 to 6 billion by 2000.
The environmental consequences of this era of innovation have been startling. By some accounts, humans now move more rock and soil than all of nature's forces (water, ice, wind, and landslides) combined. Croplands now occupy some 10 to 15 percent of the world's land surface and pastures take up another 6 to 8 percent, for a total of roughly 20 percent. Much of the useable land—those parts that are neither desert nor mountain nor tundra nor ice sheets—is now used for agriculture. More than half of the freshwater drainage is now in use by humans, most of it for irrigation. In most industrialized countries, nearly every acre of forest has been cut at one time or another. The natural flow of most of the world's rivers has been checked by dams, in many cases several times along each river. In all, some 30 to 50 percent of the land surface has now been significantly transformed by human action, with smaller changes in other regions.
Humans have now become the major environmental force on Earth. Nature can wrest back control on a regional scale and for a short interval, with a hurricane, a flood, an earthquake, a tornado, or a drought. But once the natural threat has receded, out come the bulldozers and the earthmovers to put things back the way humans want them.
In part 3 of this book, I made the case that humans took control of greenhouse-gas trends thousands of years ago, and that the gradual increases in gas concentrations prevented most of a natural cooling that would otherwise have occurred. In this contest, nature still exerted a slightly stronger control on climate than did humans. During the last two centuries, however, the human influence on greenhouse gases has increased markedly, with gas concentrations soaring to levels well beyond the natural range of the last 400,000 years, and the human influence on climate has become considerably larger than it was during the preindustrial era. But has it exceeded that of nature?
Most scientists agree about two observations: (1) greenhouse-gas concentrations have quickly risen above their natural levels during the last 200 years; and (2) global temperature has increased by about 0.6° to 0.7°C in the last 125 years at an unusually rapid rate. The immediate temptation is to draw the conclusion that an unprecedented warming caused by humans is under way.
Yet this conclusion is not so easily justified as it initially sounds. Any credible climate scientist will agree that at least some of the observed warming must be due to the measured rise in greenhouse-gas concentrations, but almost none would say that the entire warming has been caused only by greenhouse gases. Other factors also affect climate, and it is necessary to disentangle them before coming to a conclusion about the size of the industrial-era impact of humans on climate.
One seemingly anomalous observation is that the climatic warming of the last 200 years has been no larger than that caused by humans during the preindustrial era, even though the man-made increases of CO2 and methane have been larger (chapter 15). The primary explanation is that the climate system takes decades to adjust fully to rapidly introduced greenhouse gases, and global temperature has not caught up to the explosive increase in these gases during the last half-century. Another likely factor is a partial cancellation of greenhouse-gas warming by other industrial emissions to the atmosphere.
The likely size of the future greenhouse warming depends on how much fossil carbon is economically accessible, how much of it we actually put into the atmosphere (and how fast), and how sensitive the climate system is to that input (chapter 16). A major uncertainty in such projections is whether or not future technological advances will reduce our greenhouse-gas emissions.
For millions of years, our human ancestors had no effect on climate. Then for several thousand years, we had a small but growing impact. In the last century, our impact has exceeded that of nature, and it will likely do so for many centuries to come. A millennium or so from now, the fossil-fuel era will largely have ended, and the climate system will be slowly returning toward its natural (cooler) state (chapter 17).
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