Sunday, December 6, 2009

Trends and events

Last week before leaving Columbia I read a newspaper article about drought in the southwestern U.S. One of the points made in the article was that the drought, lack of rainfall and depletion of soil moisture, people had been experiencing over the last few months was caused by global warming. While forecasts of future conditions in the American southwest certainly indicate warming temperatures and a greater propensity for periodic drought, it is a bit misleading to attribute conditions people experience over a short time period to global warming. It is the difference between trends and events.

Climatology is the systematic examination of long-term trends or patterns that develop in atmospheric conditions over time. It is not exactly the same as the instantaneous or short-term mechanics of atmospheric behavior which are the focus of meteorology. Weather is event driven while climate is trend driven. Because of this distinction, there is a certain level of statistical uncertainty (noise) built into the description of climate and climate changes.

We are all familiar with similar everyday phenomenon based on probability. In baseball a batting average represents the statistical probability that a player will get a hit when he (or she) steps into the batters box to face a pitcher. Batting average does not determine what the outcome of any single event (each at-bat) will be; it simply describes the likelihood of what will happen based on previous at-bats. Each at-bat contributes information that is then used to calculate a new average. Each at-bat is a single event and the average is a statistical means of describing the group as a whole.

Patterns and trends can also be detected in these data. Over the course of a single season a typical player experiences slumps and hot streaks, each one pulling the batting average in one direction or another. If the slump or hot streak is short-lived, over a few games perhaps, then the resulting influence it will have on the players’ batting average will be minor. A prolonged hitting streak however, can generate a significant upwards trend in the average.

People unconsciously engage in complex forms of pattern detection all the time without realizing what it is we are doing. Georges Seurat painted “A Sunday Afternoon on the Island of La Grande Jatte” in the pointillist tradition. Up close, the painting looks like a random series of colored dots scattered all over. As you begin to step backwards in order to see the entire canvas the dots coalesce into recognizable shapes and figures. Up close you see dots – singular events. From a distance you see the patterns that develop from the totality of the dots taken together – spatial trends.

As the weather turns cold, listen for people scoffing at the idea that the climate can be warming when it feels so cold. But this misunderstands the statistical basis for climate. Like batting average, climate is based on examining the cumulative record of individual weather events. A predicted change in climate means that certain weather events are expected to become more likely, more probable, more frequent. It is a change in patterns, not individual events.

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