This record shows that the Earth's temperature was highly variable even before humans started affecting the global atmosphere, and there has been an upward trend in temperature since well before the beginning of the Industrial Revolution (age of industrialization is highlighted in red). Even though the increase in carbon dioxide levels is unprecedented, the increase in global temperature seems to be modest and not out of line with the increase in temperature preceding the most recent build-up of carbon dioxide. So this leads one to question whether the increase in global temperature in the last century is a natural occurrence, or is being enhanced by anthropogenic greenhouse gases and by how much.

In addition to the problem of determining if the recent upward trend in global temperature is natural or anthropogenic is that the year to year temperatures are highly variable. We have been recording some of the highest temperatures in history, but then the next year or next few years might be much cooler and thus bring the average back down (see the temperature trend in the last twenty years on this chart). People have short memories about climate, so this roller coaster temperature ride causes interest in global climate change to wax and wane annually, making it difficult to enact effective policy changes to regulate greenhouse gas emissions or mitigate effects from climate change.

These charts show trends for the last 1000 or so years. The leftmost chart shows that global temperatures have been running below "normal" for the most of the period, so it may not be unusual for the temperature to be on the upswing at this moment. However, the other charts show the concentration trends for carbon dioxide, methane, and nitrous oxide, and they have increased dramatically during industrialization, with the global temperature seeming to increase at about the same time. It is this correlation between sudden increase in global temperature and the sudden increases in the major anthropogenic greenhouse gases that has many scientists convinced that we are in the midst of an anthropogenically enhanced global warming event.