The earth is warming up, I think theres pretty good evidence of that. Underground temperatures have risen a few degrees, polar ice caps are melting, all small signs. The cause is also probably a combination of greenhouse gases, the shifting equilibrium of the earth, and other small things. However it is a dangerous trend and we need to take steps to prevent it. Should it be our top priority, no.
The evidence is not as good as many people think. In the 1970s the trend was that temperatures were falling. It all depends on where you start counting from. An experiment with double the amount of carbon dioxide proved that plants with the increased carbon dioxide levels grew to double their normal size and could produce double the amount of oxygen, therefore sustaining a larger population. Even the satellite proof fails to take clouds into account, which lower he temperature a great deal.