2. Global Warming
The second effect is one you have probably heard of, that burning fossil fuels contributes to global warming, or "the green house effect" due to the release of large amounts of CO2. Global warming is a very complex problem, since it is caused by increases in a variety of "greenhouse gases," many of which, like CO2 itself, are naturally occurring. Greenhouse gases are gases which cause the atmosphere to retain more heat when they are present in larger amounts. They act like the glass in a greenhouse, which reflects some of the sunlight which enters back inside, causing the structure to retain heat, but on a global level.
The most important greenhouse gas is carbon dioxide, which is released by the combustion of all of the fossil fuels. It is also released by the destruction of tropical and other forests globally, where the carbon released combines with atmospheric oxygen to form CO2. In fact, forests are carbon sinks: plants take in CO2 and use it to build their cells, giving off oxygen. Thus, clearing forests is a double whammy, since it not only releases stored carbon, but also reduces the biosphere's ability to process carbon dioxide at the same time.
A look at the melting glaciers in the North Pole.
Other green house gases include methane, water vapor (especially in the upper atmosphere), and CFCs (also involved in ozone depletion, a separate problem). In combination, burning fossil fuels and deforestation both promote an increase in the carbon dioxide content of the atmosphere above the range required in order to maintain global average temperatures within their optimum range. What is happening is that we are releasing carbon dioxide much faster than the biosphere can naturally process it, since fossil fuels are stored organic energy/carbon which have been buried for millions of years; the remains of prehistoric life.