One faction has denied the existence of global warming caused by man's activities, claiming that the climate and temperature trends are simply part of the "natural" variations.  They have cited the variations of the past, including some changes that may have happened on time scales even shorter than the predictions for Global Warming.  They do have a valid point, which has made it necessary for researchers to examine all the factors to determine conclusively that the present rate of change cannot be explained except by the accumulation of manmade "greenhouse gases" in the atmosphere.

The argument why we should be very concerned about Global Warming induced by man's activities can be summarized by ten well documented statements of facts that are accepted by almost all scientists.

1. The climate of the Earth has been found to be highly variable and possibly unstable.  Some of the extremes have been catastrophic to life.  In the past 4 billion years there have been several climate episodes when life was almost extinguished.
   
   The worst climate cataclysm occured in the Precambrian Era, sometime prior to 550 million years ago.  The Earth is believed to have been caught in a run-away cooling event, and became largely covered by ice and snow, perhaps extending far into the tropical regions.  This phase of Earth's history is often called the "Snowball Earth."  If the Earth became entirely covered by ice and snow, it is difficult to see how life could have survived, or how the Earth could ever have recovered.
   
   The second most ominous event occurred about 250 million years ago, at the end of the Permian Period. The climate deteriorated catastrophically, perhaps as a consequence of massive volcanic eruptions.  As many as 99% of all living beings were annihilated.  Though other causes such as cometary impacts have not been ruled out, it appears that the greatest extinction event in Earth's history was a result of extreme climate and environmental deterioration.
   
   
2. Within the past million years the natural instability of the climate has resulted in several episodes of severe cooling at high latitudes.  These so-called Ice Ages were accompanied by worldwide temperature decreases of many degrees, and the growth of gigantic ice caps covering much of Europe and North America.  They have occurred both relatively recently and in the past, as far back in time as we have a geological record.  The Ice Ages were caused by seemingly tiny variations in the Earth's motion around the Sun; those tiny variations were amplified by feedback effects in which the growth of ice caps increased the reflection of the Sun's radiation back into space.
   
   The Ice Ages resulted in great environmental stress throughout the world.  Many species disappeared with the onset of the Ice Ages, and even more disappeared in the rapid warming at the end of the Ice Ages, about 10,000 to 12,000 years ago.
   
   
3. Human activities, particularly agriculture are especially sensitive to climate fluctuations.  A modest global cooling event lasting from the 16th century to about 1840 caused widespread failure of harvests, famine, and intense suffering.  Iceland suffered a severe population decline, when advancing sea ice made it difficult to transport food and essentials.  Advancing glaciers threatened many villages and towns in the Alps.  During this period, called the "Little Ice Age," average temperatures in northern regions dropped by no more than 1 to 2 degrees—far less than the increase expected over the 21st century.
   
   
4. Some climate variations have been attributed to changes in the Earth's orbit, or to fluctuations in the output of the Sun; but most climate changes have been accompanied by changes in the atmospheric concentration of carbon dioxide.  This correlation is not fully understood, but it suggests that there are feedback processes that release ever more carbon dioxide as the Earth warms.
   
   
5. Carbon dioxide and several other atmospheric constituents, particularly water vapor and methane, cause retention of solar heat, by blocking the infrared emissions that would otherwise be radiated back into space.  These gases are responsible for maintaining a habitable planet, with temperatures suitable for life; for that reason they are generally called "greenhouse gases."  Because carbon dioxide and methane are present in such low concentrations, the temperatures in the lower atmosphere are very sensitive to the atmospheric concentrations of those substances.
   
   
6. Extensive measurements have shown that the concentration of carbon dioxide in the Earth's atmosphere has been gradually increasing throughout the 20th and 21st centuries.  The measurements are very precise, and show an oscillating pattern in which the amount of carbon dioxide decreases during the Northern Hemispher summer, and increases again in the winter.  However, every year the maximum concentration exceeds the previous maximum of the preceding year.
   
   
7. The average temperature of the Earth's surface and lower atmosphere has been increasing gradually over the past several hundred years.  A part of the increase is attributed to the end of the Little Ice Age, which ended quite abruptly about 1840.  The last years of the 20th century saw an acceleration of the temperature rise, which is not consistent with any residual effect of the Little Ice Age.  That acceleration has continued in recent years, with several years in the past decade registering the highest global temperatures ever measured.
   
   
8. The recent temperature rises are correlated very well with the increases in atmospheric carbon dioxide.  Computer models of the atmosphere confirm that the temperature increases are exactly what should be expected from the measured concentrations of greenhouse gases.
   
   
9. Man's activities, particularly the burning of fossil fuels are the only possible source of carbon dioxide that could acount for the huge increase in recent centuries.  While there are other potential sources of greenhouse gases, they have all been ruled out by extensive measurements.  Some potential sources, such as the oceans, could be either sources or sinks of greenhouse gases.  These have been found to act mainly as sinks, which will eventually become saturated.  All the evidence points to man-made emissions of greenhouse gases as the cause of the observed Global Warming.
   
   
10. Computer models of the Earth's warming suggest that average worldwide temperatures could increase by at least several degrees by the end of the 21st century.  All the predictions indicate that the amount of warming will exceed the cooling event of the Little Ice Age. 

There are a lot of niggling details to be worked out, and occasionally researchers do get some of those details wrong—that's the way science works—but the basic facts are undeniable and highly disturbing.  Even if we aren't responsible, we have a moral imperative to do everything within our capabilities to slow or halt the present warming.