Global warming is exactly that. The planet is getting hotter, and nothing we can do right now is going to stop that for 50 years. So hold onto your water wings, things are going to get INTERESTING.
To maintain a steady temperature, the Earth must be in a state of thermal equilibrium with its environment. In other words, the amount of heat the Earth absorbs from the Sun must equal the amount of heat that the Earth radiates away to space.
The theory of global warming is that "greenhouse gases" -- carbon dioxide, methane, and water vapor being examples -- insulate the Earth, reducing the amount of heat radiated to space. The temperature of the Earth therefore rises. As the temperature rises, the rate at which heat escapes through the "greenhouse gas blanket" increases until the planet achieves equilibrium again.
Human activity produces large amounts of carbon dioxide, one of the major greenhouse gases. Billions of tons of carbon that used to be buried in the form of oil or coal are now back in the atmosphere in the form of carbon dioxide. The carbon dioxide content of the atmosphere now matches that of some of the highest-temperature periods in Earth's past, creating concern that the average temperature of the Earth will rise sharply in the near future. An increase of 1-2 degrees C has already been noted.
The exact effects global warming will have aren't known. General shifts in weather patterns are likely, which could affect agriculture around the world. Glaciers around the world are likely to melt (many, in fact, have already shrunk substantially in recent decades), causing a rise in sea level. Either of these consequences could affect human civilization.