The Evidence: Observing Climate Change
Why Use Temperature Anomalies? Temperature anomalies measure how the temperature at any location has changed relative to a base period that covers a much longer period of time at the same location (usually more than 15 and preferably more than 30 years). Selection of the most appropriate base period is very important. When chosen correctly, it can even out the impact of short-term cycles such as warm El Niño events and the impact of volcanic activity. For this reason, a minimum of 30 years of data is normally recommended. When chosen incorrectly, the base period can skew the data and suggest either greater or lesser warming than is actually occurring.