There are always local variations. Globally temperatures have been getting warmer over a few hundred years, but not over the last 10-20 years.

Firstly, let's look at CO2. We currently produce about 30 billion tonnes of CO2 per year and pump that into the atmosphere. About 50% of that CO2 is absorbed on relatively short timescales by the planet's oceans and plants. The other 50% mixes in the atmosphere where it can remain for up to a century before it is broken down or absorbed. Since this CO2 is produced by us digging up fossil fuels and burning them, the CO2 we add to the atmosphere is *additional* CO2 that would not have been present had we not burnt the fuels. Since the CO2 we produce can linger for up to a century, every year that we pump CO2 into the sky will add to the total amount. Put simply, we are pumping it faster than it is removed.

By looking at CO2 concentrations in ice cores, we can establish that the CO2 concentration in our atmosphere is now higher than at any point in the last 800,000 years. We can measure the isotopes (or 'types' of carbon) in atmospheric CO2 and in plants that absorb CO2 and we find that the ratios of those isotopes matches what we'd expect if the carbon had been locked away underground (as in fossil fuels) for millions of years. We've measured an increase in atmospheric CO2 concentrations since the 1950s and we've also measured a decrease in oxygen concentration (which we'd expect, since burning carbon to produce CO2 involves binding oxygen molecules from the air with that carbon). There is virtually no doubt that the rise in CO2 levels is caused by us burning fossil fuels.

Now, we know CO2 is a greenhouse gas and traps heat that would otherwise escape from our planet. When we build computer models of the climate and compare the results of those models to the temperatures we have measured over the past 160 years, we find that the models match what we observed up until about the 1970s. After that, we have been unable to find a 'natural' cause that we can put into the models and still have them give us the temperatures we measured. When you include the effect of the CO2 we've added, we suddenly find the models pretty much match what we observed. The conclusion is that the CO2 we've produced is trapping additional heat, thereby warming our planet.

The trend towards rising temperatures has been observed in global temperature records stretching back to the 1800s. More recently, the rise in temperature has been confirmed by land, sea, air, and space based measurement systems. It has been verified by changes in Arctic ice thickness, rates of glacier melts, and rising sea levels. So there is no question that our planet is undergoing a rather rapid temperature increase.

What level of science class is it? If it is an advanced class, look at Bart Verheggen's blog Our Changing Climate, and search for unit root, I think it was June July 2010. Posts by VS, say there is no global trend, as temperatures have a unit root.

Maxx is right but if you go with that answer, you will probably fail, because it is generally accepted that there is a warming global trend (even though temperatures have not risen for the last 16yrs)

Take an hour and watch this very interesting documentary and you will have your answers.

Top climate scientists say there is no man-made Global Warming.

The Great Global Warming Swindle