Presently, there is growing and compelling evidence that the Earth's surface is getting warmer. In particular, it is warmer today than it was a century ago. Sea surface temperatures, for example, are running about 1 to 2 degrees Centigrade (approximately 1.8 to 3.6 degrees Fahrenheit) above normal. There have been two relatively rapid periods of temperature increases, one between 1910 and 1940, and the other between 1960 and today.

Many scientists believe the current trend of global warming is one of the greatest environmental threats facing humanity, next to a nuclear cataclysm. For instance, climate warming could accelerate the melting of the West Antarctic Ice Sheet (anchored in the ocean below sea level) and of the Greenland Ice Cap. The resulting rise in sea level over the next century could threaten major coastal communities. The implications of the current phase of global warming for modern civilization could be very serious, even though all its damaging effects remain to be fully ascertained.

Because so many forces influence the Earth's climate, there is still some uncertainty about the relative importance of each set of causes behind the current observed climate warming. For example, some scientists estimate that part of the trend in Earth's warming could be caused by natural factors acting within a very long cycle, such as a recurring closer proximity of the Earth to the Sun, the star that supplies our planet with heatand energy. The remaining observed warming is attributed to human-made pollution, such as the release of carbon dioxide (CO2) gas into the atmosphere.

It appears that human-made pollution of the atmosphere is already having a serious affect on the Earth's climate and on the Earth's ecological system. On the one hand, human activities over the past 100 years, such as the burning of fossil fuel (coal, oil, natural gas, etc.) have contributed to creating 'global warming'. It is because the greenhouse gases that result from fossil fuel consumption insulate the Earth's atmosphere and prevent the Earth's heat from escaping into space, causing surface air temperatures and sub-surface ocean temperatures to rise. Greenhouse gases in the atmosphere absorb infrared radiation, thereby preventing some of the outgoing energy from returning to space. As a result, the Earth's atmosphere gets warmer.

On the other hand, the same human-made air pollutants caused by fossil fuel use, and which result in the visible layers of smog that can be seen in the atmosphere, make clouds that reflect more of the sun's rays back into space. This leads to a contrary effect known as 'global dimming', whereby less heat and energy from the Sun reaches the Earth. As a result of the Earth receiving less sunlight, there is a global cooling effect on land and the oceans. This has influenced the occurence and patterns of rainfalls in some regions of the globe, causing droughts and famine in these areas, especially in the Northern Hemisphere mid-latitudes. It has been postulated that the failure of the monsoon in sub-Saharan Africa during the 1970s and 1980s may have been caused by 'global dimming'.

The Earth's climate seems to be caught in a sort of tug-of-war between two competing atmospheric effects: One kind of pollution prevents sunlight from reaching the Earth, while another one prevents the radiation of heat from the Earth from escaping into space. The global cooling effects of air pollution with visible particles thus tend to mask somewhat the global warming effects of invisible greenhouse gases. However, scientists have estimated that over the last few decades, the global warming effects of pollution have been stronger than its cooling effects. Without the cooling effects of pollution, indeed, the Earth's surface temperature would have risen by about 1.8 degree Centigrade (about 3 degrees Fahrenheit), over the last few decades. But, because of the cooling effect of "global dimming', estimated at around 1 degree Centigrade (less than 1.5 degrees Fahrenheit), the Earth's average temperature only rose between 0.6 and 0.8 degrees Centigrade.

This poses a potentially difficult dilemma regarding the fight against atmospheric pollution. Indeed, if there is less visible pollution in the air at the same time that greenhouse gas emissions keep augmenting, a reduction in global dimming effects would tend to exacerbate global warming. Thus, paradoxically, a rapid solution to global dimming may lead to more pronounced increases in the Earth's temperature. The ice caps at the Earth's poles would risk melting at a faster rate and the level of oceans could rise faster, with all the consequences that such occurrences could entail for low sea-level inhabited regions and for the global ecological system. The obvious but difficult solution would consist in reducing simultaneously both visible (particles) pollution and invisible (greenhouse gases) pollution. This will be the mounting challenge facing humanity in the coming decades.

Whether we like it or not, humans are now a significant part of the Earth climate system and this means that they can do something to influence it in the right direction, or, at least, slow its advance in the wrong direction. Humans have to pay attention to the environment and to the global life support system. We need more scientific understanding of the Earth as a complex system and more enlightened international collaboration to face the new challenges that global pollution presents. The current generation of humans has no right to leave a damaged and depreciated environment to future generations.

Meanwhile, let us deplore that American scientists are being prevented by the Bush administration from speaking forthrightly to the public about global warming and other pollution topics. We need more debate on this issue, not less. In a democracy, intimidation and censorship are inherently dangerous. As one scientist put it, this "seems more like Nazi Germany or the Soviet Union than the United States." The more so, knowing full well that the United States is the world's biggest polluter.