FINDING THE SIGNAL IN CLIMATE DATA

Any trend or anomaly is measured relative to a norm or baseline. Climate change is commonly generalized as change or deviation in one or a few variables, such as temperature or rainfall, over time. Teachers and students may have difficulty assessing claims that climate is changing and by how much. What is the norm or baseline, and what is the basis for choosing it? How much of a deviation is considered real or significant? How can prehistoric climate be directly compared to measurements taken by instruments in the last 100 years? Can the magnitude of an anomaly or trend be assessed by means other than visual impression of a plot, which is subject to manipulation?

We present examples of climate data to illustrate how teachers and students can consider factors such as varying time scales, human measurements of climate vs. other indicators, local climate vs. global averages, and judging the choice of baseline for comparison. An awareness of how these factors influence presentation of data help in judging their validity and credibility.