The number of tornadoes that we have every year depends on how big an area you are looking at. It's also difficult to answer because some tornadoes are a lot more powerful than others, but the powerful ones are also rare. The weak, common tornadoes may in fact be so weak that we don't even realize that they're tornadoes, which makes counting or guessing at the true number of tornadoes even harder.

The National Oceanographic and Atmospheric Administration (NOAA) has published a graph of tornadoes per year estimated around the world. You can see it here, please ask your teacher to see it with you, so you can receive help to read the graph:

In this graph, notice that the number of tornadoes per year is getting larger as you move to more recent time to the right. Is this because we're looking harder and seeing more, or is it because of global warming?