To quantify how much observed warming is due to natural factors and how much to human influence, we plot observed temperatures against human-induced and natural temperature changes at the same time.

Drag this plot around to get a feel for this way of presenting the data. Each dot still corresponds to one year.

Click on the graph and press R. Then rotate this graph slowly to the left to line up the points as best you can along a diagonal line. You are trying to find a flat surface that lies close to all the points.

If you press this button the computer will find the flat surface for you. How did you get on by hand? The fit is never perfect, because of random year-to-year temperature fluctuations.

The slope of this surface means both human and natural factors have contributed to observed temperature changes. The black line shows the most likely "attributable warming".