Oh yes, one more question. Is carbon dioxide the problem? But isn't that what we exchange for plants for oxygen, or am I thinking of a different term? How can us and plants sharing breath with each other cause Global Warming?
The breathing is fine! We should definitely continue breathing.
However, the burning of fossil fuels also spits out lots of carbon dioxide. (about 29,000,000,000 tons per year from man-made sources) Nature can't absorb this much extra carbon.
Carbon dioxide absorbs a particular spectrum of energy - longwave infrared radiation. This is the same "heat" energy that the earth radiates. (think like a hot sidewalk or an electric stove) Without carbon dioxide and other greenhouse gases, this heat would escape the earth out into space. Instead, it gets absorbed on the way out and warms the earth. For the most part, this is actually a good thing. If ALL of that heat escaped, the earth would be a giant ball of ice. So what's the problem?
Well, we're rapidly adding to the carbon dioxide content of the atmosphere, which is causing a warming trend much faster than the earth usually goes through. Plants and animals can only adapt to changes so fast. Several mass-extinction events are linked to periods where the earth rapidly changed its temperature. So, if the earth warms too fast we can expect to see animals start dying off, and our crops will have trouble adapting so growing food will probably become increasingly difficult.
We're not going to all die off, not by a long shot. Food prices will go up, and countries that have trouble feeding themselves will have that problem become even worse. Ocean levels will rise, both by melting glaciers and polar ice caps as well as thermal expansion of the ocean. (liquids increase in volume somewhat when they are warmed up. this is how a thermometer works actually!) A higher ocean level will decrease usable land area, something that will be problematic in island nations or low-lying areas like Florida or the south coast of India.
Also worth noting is that carbon dioxide is not the only greenhouse gas. Methane, CFCs, and nitrous oxide are all greenhouse gases which mankind spits out into the atmosphere. Water vapor is also a greenhouse gas, but we can't really add extra water vapor to the atmosphere, it all just falls back out as rain. There are also different reasons that the earth will warm or cool, which is why the earth has changed temperature gradually throughout all of its history. Large volcanic eruptions can cool the earth, because the ash blocks out part of the sunlight. The sun itself changes its output on a fairly regular ~11-year up and down cycle, and it also has long-term trends to account for. (we measure this) The earth's orbit is also not completely stable, there's a regular, gradual shift in various orbital mechanics that work in a ~20,000 year cycle. (this effects the amount of sunlight that hits us) Even the position of the continents can affect temperature, because land and ocean absorb heat differently.