The role of humans in Earth's climatic history falls into four phases (fig. 17.1).
Phase 1 (before 8,000 Years Ago)
Until 8,000 years ago, nature was in control. Even though our remote prehuman precursors had been present on Earth for several million years, nature alone drove climate change. Even when our fully human ancestors appeared sometime after 150,000 years ago, our impact on the global landscape was still trivial. People used "firesticks" to burn grasslands or forested areas in order to drive game or provide open areas to attract game and permit the growth of berries and other natural sources of food. Some of these early cultures may have pushed small branches into moist soils in wet tropical regions where trees bearing fruit or nuts would naturally sprout.
Yet both the number of humans and our footprint on the landscape remained small and highly localized in scale. The impacts of human fires were largely inseparable from the effects of natural lightning strikes, which cause random fires of varying intensity. Humans who set fires in one region and then moved on to another as part of their hunter-gatherer lives mimicked natural events. The number of humans setting fires was not yet large enough to rise above the background level of fires occurring as a part of nature.
Nearer the present, in the millennia just before 11,000 years ago, large changes in Earth's climate were under way in response to natural causes. High summer solar radiation in the Northern Hemisphere was melting the huge ice sheet on North America and the smaller ones on Scandinavia and in far-northern Eurasia. The strong summer Sun that provided the basic driving force for this immense effort was aided by high (natural) values of two key greenhouse gases: carbon dioxide and methane. Together, the Sun and these two greenhouse gases managed to melt the great ice sheets of the Northern Hemisphere over an interval of 10,000 years. But these were the same natural processes that had been going on for the several million years of Northern Hemisphere ice-age cycles. Nature was still in full control of climate.
Years Ago Years in the Future
17.1. Greenhouse-gas emissions from farming have offset part of a natural cooling since 8,000, years ago and probably prevented a new glaciation. The rapid changes caused by modern industrialization will eventually reach levels of warmth not attained on Earth for many millions of years. Once the supply of fossil fuels is depleted in a few centuries, climate will gradually cool toward natural levels.
Ice sheets naturally respond very sluggishly to any push toward change: the word "glacial" is synonymous with "slow." As a result, even though the maximum push to melt the ice sheets occurred nearly 11,000 years ago, the ice sheets did not actually finish melting until 6,000 years ago. During the intervening 5,000 years, control of climate in the Northern Hemisphere became a contest between two great opposing forces: (1) the ice sheets, which had previously kept climate cold but were now rapidly melting, and (2) solar radiation and the greenhouse gases, which had reached peak (warm) values nearly11,000 years ago but were now beginning to decline toward lower modern values. The result of this contest of contending forces was a compromise: maximum temperatures were reached nearly8,000 years ago, when the ice sheets had been sufficiently reduced in size to have little cooling impact, but when solar radiation values were still high enough to make climate warm.
Phase 2 (8,000 to 200 Years Ago)
As the last remains of the great ice sheet in North America melted, humans began to clear forests in southern Europe and northern China to make way for agriculture roughly 8,000 years ago, and deforestation gradually spread to other parts of southern Eurasia over the next several thousand years. Burning of trees added CO2 to the atmosphere at rates that initially were small but increased steadily through time. These carbon emissions reversed the natural decline in CO2 levels that had occurred in the early parts of previous interglaciations. By clearing forests for farms, humans had begun to take control of Earth's greenhouse gases and in this way to exert a small but growing impact on Earth's climate.
Humans also began to irrigate lowland areas in Southeast Asia 5,000 years ago. The lowland areas flooded for rice crops added methane, another important greenhouse gas, to the atmosphere. As a result, a natural drop in methane values was reversed, and the atmospheric CH4 trend began to rise instead of fall because it too had come under human control.
Forest clearance and irrigation for rice increased steadily through both the Bronze Age (which started nearly 6,000 years ago) and the Iron Age (which started nearly 3,000 years ago). When horses and oxen were domesticated and hitched to metal plows, each farmer could clear and cultivate larger amounts of land. Long before the start of the industrial era, most of the arable land along the southern tier of Eurasia had been cleared of forests, and most of the lowland delta regions of Asia were planted in rice.
By the start of the industrial era, human emissions of CO2 and CH4 had caused increases in the atmospheric concentrations of both gases equivalent to about half of the natural range of variation that had occurred previously. Greenhouse-gas concentrations in the atmosphere remained within their natural range of variation, but the levels were approaching the top of that range. These increases caused by humans produced a greenhouse warming effect that canceled a large portion of a natural cooling under way at high latitudes. This warming effect probably kept an ice sheet from beginning to grow in northeastern Canada during the last several thousand years. Humans had not yet taken control of climate (global temperature), but our effect on global climate was now very nearly equal to that of nature.
Phase 3 (200 Years Ago until 200-300 Years in the Future)
The industrial era arrived in the late 1700s and the middle 1800s, and it ushered in a third phase of human effects on climate. Rates of deforestation increased to provide fuel for mills and mining operations and to open up new farmland for rapidly increasing populations in tropical areas. Beginning in the late 1800s, use of fossil fuels (first coal, and later oil and natural gas) rapidly increased, eventually replacing deforestation as the primary source of CO2 emissions by humans.
Methane emissions also accelerated, in part because of continuing increases in irrigated areas, but more importantly from the spread of methane-emitting landfills, releases of natural gas, and other human activities.
Since the 1800s, atmospheric concentrations of carbon dioxide and methane have risen exponentially. Late in the 1900s, concentrations of both gases reached levels not recorded in ice cores for the last several hundred thousand years, and thought to have last existed on Earth many millions of years ago. At the start of the current millennium, CO2 concentrations were about 30% higher than the levels typical of previous natural interglaciations and were increasing at a rate of about one-half percent per year. Methane concentrations have now reached a level almost 2.5 times higher than those during previous interglaciations, with increases of about 2% per year. Each new increment of CO2 and methane carries us farther into uncharted greenhouse territory.
Global climate has warmed by 0.6° to 0.7°C during the last 125 years, the interval for which enough ground stations exist to make reasonably valid estimates of global temperature. Even though the greenhouse-gas levels have risen well beyond natural levels, the global temperature value reached by the start of the current millennium has not yet exceeded those reached during previous interglaciations of the last several hundred thousand years.
The explanation for this seeming discrepancy lies in several factors covered earlier:
1. Solar radiation values in the Northern Hemisphere during the last several thousand years have fallen well toward their minimum natural levels as changes in Earth's orbit have become favorable for renewed glaciation (chapter 10). As a result, a large part of the unprecedented rise in greenhouse gases caused by humans has been used to offset an overdue glaciation.
2. The decades-long response of the climate system has delayed a substantial part of the warming that will eventually result from the current level of greenhouse gases (chapter 15). When the climate system comes to its full response to the current greenhouse-gas levels, Earth will likely enter an era of warmth unprecedented during the previous millions of years of glacial-interglacial cycles. Of course, by that time, greenhouse-gas levels in the atmosphere will have risen even higher, and Earth's temperature will be moving toward an even higher value.
3. Industrial-era emissions of sulfate aerosols have probably canceled part of the warming that greenhouse-gas emissions would otherwise have caused (chapter 15).
At the current rates of increase of greenhouse-gas concentrations, future temperatures are likely to be pushed beyond the natural glacial-interglacial range of variation in the very near future, probably within a decade or two. Over subsequent decades, as greenhouse-gas concentrations rise at unprecedented rates to levels unprecedented in the last several million years, increases in global temperature will follow as the climate system responds. Deforestation in the tropics will play a significant role in the CO2 increase, but by far the largest factor will be the burning of the world's remaining oil, gas, and (especially) coal reserves. Predictions of gas concentrations and global temperatures more than a century from now are impossible, but what we know now indicates that the increases will be large. The major hope for preventing high CO2 levels lies in developing new technologies for trapping carbon before it leaves smokestacks or exhaust pipes. But for now, this is only a hope; no technology that would be economically feasible on the massive scale required is now in sight.
Phase 4: After 200-300 Years from Now
A few centuries from now, well after the economically recoverable reserves of oil and natural gas are largely depleted, and as the supply of accessible coal is also in decline, the diminishing rate of CO2 emissions to the atmosphere from human consumption of fossil fuels will eventually fall below the rate at which the ocean takes up the excess carbon we have produced. At that point, we will enter a new phase in the relationship between humans and climate: the CO2 concentration in the atmosphere will begin to fall, and eventually much of the industrial-era CO2 excess we have caused will be removed.
In this longer run, the CO2 taken up by the ocean will take part in a chemistry experiment far beyond anything attempted in any laboratory. The extra CO2 will make the ocean slightly more acidic than it is today, and this acidity will dissolve some of the soft, chalky sediments lying on the sea floor. These chalky "oozes" are formed from the calcium carbonate (CaCO3) shells of plankton that live in shallow ocean waters and settle to the sea floor after they die. The deepest ocean waters, below about 4,000 meters, are corrosive and dissolve most of the shells raining down from above. The deeper ocean basins are left with a brownish (CaCO3-poor) residue of debris blown out to sea from the continents, while whiteish (CaCO3-rich) sediments accumulate only on higher elevation features like snow on mountain peaks. As the excess CO2 generated by humans makes ocean waters more corrosive, they will begin to attack existing deposits of CaCO3 ooze on the higher sea-floor topography, and the oceanic CaCO3 "snowline" will retreat to shallower levels. In this way, the CO2 generated by humans will slowly be consumed in a gigantic chemistry experiment—dissolving CaCO3 on the sea floor.
Meanwhile, as the ocean slowly takes up the CO2 excess, global temperature will gradually cool back toward natural levels. If the greenhouse-gas concentrations fell all the way back to their natural level, Earth would probably cool enough to allow an ice sheet to begin accumulating in northeastern Canada, a return to an overdue glaciation. But this will not happen, or at least not for many millennia.
A small fraction—perhaps 15 percent—of the CO2 generated during the preindustrial and industrial eras will remain in the atmosphere for several thousand years and keep global temperatures warmer than the natural value. In addition, methane concentrations will remain high as long as we continue irrigating for rice farming and storing our waste products in large, methane-emitting landfills. These activities will produce large amounts of methane every year and keep atmospheric concentrations well above natural levels. It is even possible that we will never return to that now-overdue glaciation.
But of course, no one can see centuries into the future.
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