This is currently a scientific "hot" topic (ha :). There are many projections of what will happen, and all of them depend on what we do today and in the next 5-10 years. The farther we go int he future the harder it is to predict what's going to happen. Politically, there is currently very little motivation to make change to how we get energy, and with developing countries increasing how much fuel they use, some of the predictions can be severe. It's very hard to say what exactly will happen, so there are a range of results. The three main changes will be in overall temperature, rainfall, and sea level. All resources used in this are from the EPA unless otherwise noted.

Carbon dioxide:
Carbon dioxide itself is harmless, but it is being released in high amounts by cars and factories and any time we burn fuels like gasoline, oil, or coal. Although it is at too low a level for humans to be harmed by it, it causes the earth to heat up. Normally, the sun heats the Earth, and the Earth releases this heat back into space. Carbon dioxide has the ability to also absorb heat from the Earth, so instead of going to space, it goes into the carbon dioxide in the atmosphere. Since we have greatly increased the amount of carbon dioxide in the atmosphere, this is like putting a blanket on the earth and keeping the heat in. That's how it causes global warming. Here: in this link are different predictions of how much carbon dioxide we release. This part depends a lot on how we act in the future. Scientists are trying to lower the amount or carbon dioxide 2 ways: alternative non-burning energy (solar, wind, water, nuclear), and making materials that filter carbon dioxide out of the air. Some of these solutions are expensive and many people don't actually believe in global warming, so nobody really has invested too much in these fixes yet. But, they are hopefully on the way. Now that you've seen what we are releasing and the range of projections, let's go on to the effects.

Temperature:
In general, temperatures will rise. Temperatures will rise the most at the north and south pole regions. This will melt ice and raise sea levels. It will also change what crops can be grown where. This is probably my biggest concern, as the global food supply should shift somewhat unpredictably. With the advances we are making in genetic modification, we should be OK. Globally, here is how temperatures would rise for the different predictions: click here. This would result in some significant changes in climate, and an increase in the desert sizes nearby. Here's an image showing how the poles will be the hottest: hot poles

Rainfall:
Temperature and ocean changes will lead to shifts in where rain falls. This will be largely a mixed bag globally. Parts of Africa that are nearly a desert will have forests, and parts of the US where we grow food will become deserts. Many scientists think our current drought is only a small sample of what will happen. It's very hard to predict these things, but they will change. The change will be slow and take many years, so it may be hard to see without looking at lots of data. Here is a map of the world and how rainfall may change: rainfall "DJF" means December, Jan, and Feb so winter. JJA means June July August so summer. Blues and greens are wetter, reds and yellows are drier. The concern is that the US will have deserts where we currently grow food. This would be bad.

Sea level:
When temperatures rise, ice melts (especially at the poles). If an iceberg melts it won't make the seas rise (think ice cubes melting in a cup of water don't make it overflow). But, lots of ice on the land such as glaciers will melt into the ocean. This could put some land near the oceans underwater, including New York City and many island countries. This is more in the distant future (but still perhaps within your lifetime) so nothing to panic about yet. Many island countries are preparing for this possibly happening. Again, there are a wide range of possible scenarios and this will happen slowly. For an idea, here is what would change on the US east coast: changes

Keep in mind that models predicting the future are only as good as the people who make them. We have time to turn this warming around, and as it affects more and more people, we will be able to fix it. Still, it is perhaps the biggest challenge of the 21st century. We will have to wait and see how much of these predictions are true.