General · 24th March 2015
In an exercise that gives some perspective to the impact of humans on the ecosystems of Earth, scientists in anthropology, biology and geophysics have considered what would have happened if, 125,000 years ago, a viral disease or some other lethal event had destroyed the less than 100,000 Homo sapiens that were still confined to their nascent continent of Africa (Christopher Kemp, “Rewind, Erase, Rerun”, NewScientist, Nov. 16/13).
First and obviously, only a minute portion of the approximately 108 billion people who have now lived on the planet would ever have existed. Of the 135 million square kilometres of land surface, 15% would not be occupied by crops, 30% would not be pasture, and another 5% or more would not be roads, towns or megacities. Without us, the terrestrial environment would be one continuous succession of forests, savannahs, jungles, grasslands and tundra. The trillion barrels of oil we have burned since 1870 would still be in the ground, as would countless tonnes of coal and minerals. Landscapes would not have been flattened or contoured to grow rice and grain, to construct highways and buildings, to control rivers or to extract resources — more of the planet's surface is now being reshaped by us than by natural erosion processes. We would not be consuming most of the world's fresh water supplies. The global cattle population of 1.4 billion would not exist, nor would the 1 billion pigs and sheep, nor the 20 billion chickens — nearly three for every person now alive.
With atmospheric carbon dioxide levels at about 275 parts per million rather than the present 400 ppm, the planet would be cooler by an average of about 1°C. Given the rhythms of Ice Ages, Earth would now be moving from the warming portion of the Holocene epoch into a cooling phase for the next 11,000 years. Arctic ice would be slowly freezing rather than rapidly melting. Of the world's 130,000 glaciers, most would be growing rather than shrinking — 10 times as many are now melting.
A normal cycling of carbon, sulphur, phosphorous and nitrogen would mean cleaner skies, oceans and watercourses — no smog, acid rain, oceanic dead zones or algae-clogged lakes. All the major rivers of the world, unobstructed by dams, would be flowing freely to the seas. Fish stocks would be immensely larger than today — recall BC salmon populations of the 1800s and the seemingly infinite supply of Grand Banks cod of the 1600s. The seas would be teeming with whales. Humpback populations of 1.5 million would be at least 15 times their present numbers, with equally large numbers of minke, sperm and bowhead — in the Pacific Northwest, prior to whaling, the Salish Sea sustained an estimated 500 resident humpbacks and greys.
Without humans, woolly mammoths and mastodons may have survived the Ice Ages to roam the Arctic tundra. The plains of North America would have contained an estimated 50 million bison, with large herds of other herbivores and generous populations of wolves, bears and big cats — a display of megafauna abundance that would have rivalled the Serengeti. The vast deciduous forests of the giant elms that covered much of the eastern portion of the continent would still exist, helping to support the billions of passenger pigeons that would not be extinct.
Most of the Middle East deserts would still be covered in great trees and productive grasslands. The fertile soils and lush forests of the Tigris-Euphrates Valley — probably the biblical Garden of Eden — would not have been converted to barren land by salinization and the results of other destructive agricultural practices. The entire Mediterranean Basin would be cooler, greener, wetter, and biologically richer, the hills and valleys still covered in soils that would not have been depleted by overuse or eroded by the inappropriate planting of olive orchards. Without the repeated abuse by Mediterranean civilizations, most of coastal North Africa would be fertile grassland. A forested Europe would be alive with wild animals, its rivers teeming with fish.
Rates of extinction have risen noticeably in those places we have populated; the recent explosion in our numbers has made us the planet's most invasive and disruptive species. Without us, the extinction rate would be somewhere between 1,000 and 10,000 times lower than today — we can only estimate our current impact because we do not have a complete inventory of all plants and animal nor can we monitor all those we have identified. The dodo, great auk, sea mink and monk seal are just the surface layer of the conspicuous extinctions attributed to our presence — at least 322 known animal species in the last 500 years.
The exercise by scientists that gives some perspective to the impact of humans on the ecosystems of Earth has also considered what might happen if we suddenly disappeared today. But we have already initiated such widespread disturbance in the last 125,000 years that the outcome is largely unpredictable — many species are irretrievably launched on a course to extinction and the unfolding effects of global climate change are just being felt.
Without us, however, woodlands the size of 10 football fields would begin restoring themselves every minute. In about 150 years, most of the forested areas we have destroyed would have returned. Perhaps the grassland, too, would recover so the vast herds of bison could once again roam the Great Plains. Condors would thrive on the carrion. And our primate relatives would probably continue on their slow evolutionary course in the jungle trees, contented with their one-night sleeping nests and simple stick tools.