m harmful ultra-violet radiation. Most scientists agree that human activities are contributing to global warming, raising global temperatures and sea levels.
             These processes affect the habitats and environmental pressures under which all species exist. As a result, we have had an incalculable effect on the Earth's biodiversity. The 484 animal and 654 plant species recorded as extinct since 1600 are only the tip of a massive iceberg.
             We have become a major force of evolution, not just for the "new" species we breed and genetically engineer, but for the thousands of species whose habitats we modify, consigning many to extinction; compelling others to evolve and adapt to our pressures. We have become a force of nature comparable to volcanoes or to cyclical variations in the Earth's orbit (Harrison 121.)
             When people think of the human impact on the environment, they often think in terms of total population numbers and population growth. These elements are important, but they are only two of the demographic factors that have an environmental impact. Population density and distribution, determined by migration and urbanization, are also important, as is population composition in terms of age and household size. All of these impact on consumption levels and trends.
             Along with consumption and technology, our sheer numbers affect the total burden we place on the environment. Rough estimates suggest that the population of the entire world 2 000 years ago may have been around 300 million. Over the whole of the next millennium, the period of the Dark Ages in the West, this rose by as little as 10 million.
             The acceleration of human expansion can be seen dramatically in the time it took for each milestone of a billion to be reached. Our first billion, passed around 1804, took perhaps 200 000 years to reach. The second billion took only 123 years and the third, reached in 1960, a mere 33 years. Since then we have been i...