We know from weather station records that Earth's surface temperature has increased on average by 0.6 degrees C in the last 100 years. The 1990s have been the warmest decade on record. Over the same period, global sea level has increased by 10-20 cm. We know also that planet Earth has an atmosphere that creates a natural greenhouse effect, keeping our surface warmer than it would otherwise be. Human activities are substantially increasing the atmospheric concentrations of greenhouse gases, principally carbon dioxide and methane, to levels far above those that have existed for the past 200,000 years.
We do not know, on the other hand, all the details of our complex climate system sufficiently well to predict the exact consequence of greenhouse gas increases on global temperature. Should we wait for greater certainty about global warming or should we take steps immediately to stabilize possible climate change? Global trends suggest that allowing "business as usual" is a risky path. Much of the population growth will be in developing countries with a natural desire for an increased standard of living. That living standard increase, according to current examples, comes with increases in per capita energy consumption. Because 90% of the society’s energy presently is produced by burning fossil fuels, the inevitable population increase and drive towards higher standard of living simultaneously aggravates the enhanced greenhouse gas condition and, with it, global warming.
There is an alternate path. We could unleash our engineering, economic, and political entrepreneurs to improve energy conservation and efficiency and move us towards greater use of renewable energy sources. Technology and training in energy efficiency and use of non-polluting fuels could allow developing nations to skip the carbon intensive, energy -production stage of industrialization. Such a path would simultaneously reduce excessive consumption in developed countries and provide conditions that would bring worldwide population growth under control. Global warming may be the "smoke alarm" that pushes us to action.
What do we know? What do we not know? What can we do? What should we do?
Life has existed on planet Earth for about four billion years. In that time, climate has swung between ice ages and warm periods. But generally, the earth's atmosphere has been in chemical balance, its composition changing slowly in response to changes in geology and life on the planet's surface. Now, the growing population and the by-products of civilization are upsetting this balance.
In 1995, after years of study, the Intergovernmental Panel on Climate Change (IPCC) reached the cautious conclusion that is worth stating in full. "Our ability to quantify the human influence on global climate is currently limited because the expected signal is still emerging from the noise of natural climate variability, and because there are uncertainties in key factors. These include the magnitude and patterns of long-term natural variability and the time-evolving pattern of forcing by, and response to, changes in the concentrations of greenhouse gases and aerosols, and land surface changes. Nevertheless, the balance of evidence suggests a discernible human influence on global climate.
