We are changing Earth more rapidly than we are understanding it. Our planet is Homo sapiens-dominated, and the momentum of population growth and industrial development will only increase that dominance. Most of us turn a blind or denying eye to the enormous scale of the human enterprise and its implications, immersing ourselves instead in personal profit and material distractions. It is past time to pay attention.

I am surely prejudiced in the matter, but it seems to me that science, with it emphasis on evidence, rational thought, and skeptical realism, can help. And so I offer, especially for those who still believe that our impact on the planet has been beneficial or at least innocuous, a few scientifically well-documented instances to the contrary. Most of the documentation can be found in the 25 July 1997 special issue of Science--the world's most prestigious, rigorous, and authoritative science journal--devoted to "Human Dominated Ecosystems." For more details, please email me.

For starters, an estimated 40% of Earth's total plant growth is now appropriated by humans. This photosynthetic growth provides food, directly or indirectly, for all animal life. It is probably unprecedented for a single species to appropriate so much of it.

Land use represents the most substantial human alteration of the planet. Estimates of the fraction of land transformed or degraded by humans fall in the range of 39 to 50%. Land transformation drives the worldwide loss of biological diversity. Its effects extend far beyond the transformed lands, contributing for example to human carbon dioxide emissions, to fires that alter our atmosphere's chemistry, and to runoff that causes flooding and aquatic ecosystem damage.

Alteration of the oceans is hard to measure but substantial. Coastal wetlands have been altered over large areas. As of 1995, 66% of marine fisheries were extinct, overexploited, or at their limit of exploitation. Harmful algal blooms (increases in harmful marine micro-organisms) have spread widely in the past two decades, and seem to be implicated in current local pollution of the Illinois River, and in the recent black tides observed in the Gulf of Mexico.

Humanity now uses more than half of Earth's reasonably accessible fresh runoff water, mostly for agriculture. Some 6% of Earth's river runoff is now evaporated as a result of human manipulation. Major rivers such as the Colorado, the Nile, and the Ganges barely reach the sea. The Caspian Sea, the world's largest lake, is drying and dying due to overfishing, overuse, and (coming soon) oil exploitation. The Aral Sea in Russia is drying up and becoming a major source of windblown dust. Lake Chad, a giant lake at the intersection of four African countries, has shrunk by 95% in the past 40 years. In coming years, water shortages coupled with population increase will exacerbate human conflicts--including conflicts in Northwest Arkansas.

Interference with the nitrogen cycle is enormous but little noticed. Although there is a vast atmospheric reservoir of nitrogen, only that small fraction that is "fixed" (combined with carbon, hydrogen, or oxygen) by natural or human processes can be used by most organisms. The human contribution to Earth's annual production of fixed nitrogen is now about equal to the natural contribution, and is expected to increase by another 50% by 2030. Consequences include increasing concentrations of the greenhouse gas nitrous oxide, substantial increases in nitric oxide air pollution, contributions to acid rain and photochemical smog, and the chicken litter that seeps into the Illinois River and creates toxic algal blooms.

Species extinction is a natural process, but extinction rates are now 100 to 1000 times those before Homo sapiens dominated Earth. Rates for some well-known groups are even greater: 25% of the planet's bird species have been driven to extinction, and 11% of the remaining birds are threatened. In addition, the rearrangement of biotic systems through human introduction of invading species is enormous. In many continental areas, 20% or more of all species are invasive, and on many islands the figure reaches 50%. Experts such as paleoanthropologist Richard Leakey refer to this as "the sixth extinction," comparable to the five previous massive natural extinctions. It has been 65 million years since an asteroid collision caused the fifth extinction, exterminating the dinosaurs. The sixth extinction, unlike the others, is caused by a biological species.

The modern increase in atmospheric carbon dioxide is the clearest signal of our impact. Carbon dioxide's concentration is important because it controls the greenhouse effect that warms our planet by trapping solar energy. For at least the past 450,000 years, concentrations have oscillated up and down with the recurrent ice ages, reaching their highest levels of about 280 parts per million during interglacial (warm) periods, including the present interglacial period. But things changed in 1750, at the beginning of the industrial age. Concentrations began an accelerating increase that today reaches 373 parts per million, 33% above any level in the past 450,000 years. There is no doubt that it is driven by human activity. Unless humans change their ways substantially, levels will continue to increase. The consensus of the climate research community is that this already affects climate detectably and will drive substantial climate change in this century.

Finally population, a driver of all the other impacts, continues to climb. Humankind took some 6 million years to reach its first one billion living individuals. We reached the second billion 105 years later, and the third billion 30 years after that. That was in 1960. Today, we number 6 billion and climb toward 10 to 12 billion during this century.

We cannot escape responsibility for managing the state of the planet, because we have created that state. Our harmful and largely irreversible impact stems from the uncontrolled growth of populations and economies. If future generations are to enjoy nature's abundant fruits, we must slow down, pay attention, educate ourselves, start using our brains for more than profit, and be unafraid to take meaningful political steps to reverse the negative effects of one species' overindulgence.

Art Hobson

Fayetteville

ahobson@uark.edu