The greenhouse effect is easily explained, though most textbooks on weather and climate do not present the whole story.  The temperature of the Sun's radiating surface is about 6000 Kelvin (a Kelvin degree is the same as a Celsius, or Centigrade degree, or 9/5 times a Fahrenheit degree, the Kelvin scale begins at absolute zero = -273 C = -459 F).  The peak radiation is at a wavelength of about 5.5 micron, which corresponds to yellow-green light in the middle of the visible spectrum.  The total radiant energy emitted by a hot body like the Sun is proportional to the 4th power of the temperature.  A tiny change in the temperature would result therefore in a large increase in the total radiated energy.  Fortunately the Sun's output is quite constant; though a tiny change in the average temperature might have caused the Little Ice Age.

If the Earth were completely surrounded by a radiating body at a temperature of 6000 K it would attain a radiative equilibrium temperature of 6000 K.  This is equivalent to being in the center of an intensely hot oven.  Of course the Earth is not fully surrounded by the Sun; in fact the Sun fills only 4.76 millionths of the entire sky (including both the night and day hemispheres).  The radiant energy received at the Earth's surface is therefore "diluted" by the same proportion. 

In radiative equilibrium the Earth must radiate back into space the same amount of energy it absorbs from the Sun.  The laws of radiant energy balance tell us that the average radiant energy emitted by the Earth is proportional to the Earth's effective temperature to the 4th power.  Thus the emission temperature of the Earth must be T_e = (T_sun times (0.00000476)) to the 1/4 power.  If there were no atmosphere, and the Earth radiated energy like a theoretical black body, the average temperature of the Earth would thus be about 280 K.  This is much too cold to sustain abundant life, and is less than the average at the worst of the ice ages.

Beginning sometime between the late 19th century and the mid 20th century the Earth has entered a period of gradual warming.  A suspected major contributor to this warming trend is the accumulation of so-called greenhouse gases in the atmosphere: primarily carbon dioxide, methane, and chloroflurocarbons (such as Freon).  These are all produced by man's activities.  There are also other greenhouse gases such as water vapor on which man has little effect, and which have not shown rapid rises in concentration.  The evidence that these gases have actually contributed to the warming trend has been controversial, but in the past several years the evidence has become incontrovertable.

The present warming trend began about the year 1840, with the end of the Little Ice Age.  The cause of the Little Ice Age is not understood, but it may related to fluctuations in the output of the sun.  The warming trend was rather weak until the mid 20th century.  The general trend was difficult to discern because the total change in average temperature was far less than the daily and annual variations; and several times the trend was briefly reversed by natural year-to-year variations.  There were some cold years in the 1950's and 1960's when it seemed that the Earth was in for a new round of cooling.  Occasionally warnings were uttered that the Earth might soon be subject to another Ice Age.  Then, in the late 1960's the warming trend resumed.  That trend accelerated at the end of the century.

If current trends continue, and if scientists are right in their predictions about the effects of accumulating greenhouse gases, future climates will be even more complex and more variable than present climates.  The temperature increases that have been reported for mid and high latitudes have generally pointed to warmer winters and relatively unchanged summers.  While trees and many other plants are extremely sensitive to the climatic conditions, the distribution of heat and moisture over the year is more important than averages.  As yet we have no simple way of predicting how plants will be affected by a temperature increase as great as several degrees.

Though significant warming has been observed in the past 50 years, the rate of warming has not yet been too great for plants to make appropriate compensations.  That situation may not hold in the future if the warming rate increases.  Another compensating factor is that the twentieth century has seen increased rainfall and moisture in those places where large amounts of the world's food is grown.  The warming over much of North America, especially on the Plains, has been accompanied by increased moisture—contrary to what might have been deduced from the association of previous warming events and droughts.

While single species may adapt to Global Warming, the response of symbiotic species and entire ecosystems may not keep up with the changes.  The problem is complicated by the fact that many species have adapted to the timing of another species; and those interrelated species may fall out of syncronism.  What happens, for instance, if a bird has to time the laying and hatching of eggs to take advantage of the flowering of a particular plant or the emergence of particular insects?  Evidence has been accumulating in recent years that such asyncronizations are indeed occuring—though no species have yet been shown to be in immediate danger.

There is no doubt that Global Warming will cause changes in the oceanic and atmospheric circulation.  As a consequence, it has been suggested that the 21st century will see more storms and violent weather.  The effects of circulation changes will especially affect the mountain regions of western North America, where there are great geographical variations in rainfall.  The northern Great Plains could also be strongly affected, since the weather there depends so critically on how the atmospheric currents make it over (or around) one of the longest north-south oriented mountain chain in the world.  The North American Cordillera has a tremendous influence on the global climate, since they are largely responsible for turning the winds and establishing the high and low pressure systems that dominate northern hemisphere weather.

In North America the effects of Global Warming will vary considerably by region.  The desert Southwest may see warmer, drier summers and winters.  The Southern Plains may experience elevated temperatures, though no one can predict with confidence how the rainfall will be affected.  In regions that already have warm climates a relatively large increase in moisture may be needed to compensate for hotter summers.  The cooler Northern Plains may actually become more favorable for agriculture.  A longer growing season could be beneficial there, if it is accompanied by increased moisture.  Since the warming there may most noticeable in the winter, more of the winter precipitation may be absorbed by the soil, which could lead to increased moisture available to growing crops.

One of the most important concerns which has arisen in recent years is the possibility that much of the Greenland and Antarctic Ice Caps could melt, causing a rise in the sea level.  The rate of melting in Greenland has increased alarmingly in recent years, but it is not yet possible to assess the likely severity.  The total rise in sea level since 1900 is still measured in millimeters; if that should become meters the results would be devastating.  It does seem likely that there will be a noticeable rise in the sea level long before the end of the twenty-first century, leading to the need for large projects to protect coastal cities.  A sea level rise of several meters or more would flood many huge coastal cities, whose total population exceeds 100 million.  The problem would only be exacerbated by the expected increase in the frequency of large cyclonic storms.  The expense of protecting cities like London or New York from such a sea level rise is unimaginable.

The lack of progress in achieving world-wide reductions in the production of greenhouse gases has led some scientists and engineers to begin thinking about whether there are ways to remove the greenhouse gases from the atmosphere.  There are many ways that this might be achieved, though none of them are yet practical on a large scale.

The most obvious way to eliminate greenhouse gases would be to scrub them from the emissions at the source.  Scrubbing the gases seems relatively easy; the big problem is what to do with the waste carbon dioxide.  Carbon dioxide could be converted to relatively inert limestone and buried; the logistics of this kind of operation are quite staggering to contemplate.  Or, the carbon dioxide might be injected deep underground. 

Some favor replacing coal and petroleum with renewable combustion energy sources.  The objective would be to grow plants which absorb carbon dioxide from the air.  Then, when the plant material is burned, the net contribution to atmospheric carbon dioxide is negligible.  The idea is sound, but logistically and economically impractical at the present.

Another major deterioration of the global environment is the depletion of the stratospheric ozone layers.  The stratospheric ozone provides a beneficial shield against solar ultraviolet radiation.  Ultraviolet radiation causes sunburn and skin damage to humans; it can also have detrimental effects on vegetation and the food crops on which we all depend.  There are, however, no significant direct effects on the climate from depletion of stratospheric ozone.

Unfortunately the stratospheric ozone layer is easily destroyed by reactions with certain compounds containing chlorine and fluorine, such as freon.  The reactions destroy ozone without affecting the amounts of the destructive agents.  Each molecule of the destructive agent can catalytically destroy hundreds of ozone molecules.  Because of their great efficiency, relatively small amounts of freon and related compounds have caused a nearly total depletion of the ozone layer at the south pole, and a considerable depletion of the ozone layer around the north pole.  The effect extends to relatively low latitudes, so Canada and northern Europe are especially susceptible to excessive ultraviolet exposure.

The problem was recognized several decades ago, and international agreements now limit the production and release of freon and similar compounds.  It will take many years before the destructive agents are fully removed from the atmosphere by natural processes, so the depletion of stratospheric ozone will be a serious concern for many decades.  In the meantime Canada and several European countries were the leaders in establishing ultraviolet exposure standards, and setting up networks of ultraviolet monitors.  Such monitors are in place in the United States, and ultraviolet exposure indices are routinely issued along with weather summaries.

There has been much controversy over the stratospheric ozone; some scientists suspect that other phenomena, such as volcanic eruptions, can also release ozone destroying gases.  Up to now, however the principal culprit appears to be chlorofluorocarbons.