Temperature Anomalies and Graphing Data

Globally Averaged Temperature Anomaly

One statistic that is used to understand climate is the annual globally averaged temperature anomaly.  It is not the only measure of global warming; there are a great many others, but it is one that the media tend to focus on because it is a convenient way of explaining what is happening to surface temperatures as a function of time.

The data here are taken from Global Land-Ocean Temperature Index (C) (Anomaly with Base: 1951-1980), which includes data from 1882-2007.  There are updated numbers available for more recent years, but I am using these data to respond to an argument made by a friend.

The Claim of a Warming Pause

Introduction

There are many claims that global warming has stopped. Such claims look at the average of the surface temperature record over the past few years and conclude that ever since 1998 temperatures have cooled.

The trick is that 1998 was a particularly warm year. In one of the commonly used data sets, 1998 was the hottest year ever recorded. If 1998 was so hot and all of the years since 1998 have been cooler, does that mean that the earth has cooled since 1998?

The fact is that one cannot demonstrate a trend by starting with one year and comparing each year since that year individually. There are two problems with such an analysis. First, if the analysis always starts with 1998, the data are being cherry-picked.

 Why start with 1998? Why not start with 2000, or 1995? If someone starts with 1998, it is because he or she is trying to use the data to support a pre-determined conclusion. If one is interested in understanding what information is really embedded in the data, one cannot pick and choose where to start. Of course one has to work with the data that are available. It would be nice to go back to the Medieval Warm Period and plant sensors all over the earth, but one cannot do such a thing.