The debate of recent decades concerning climate change has necessitated a need to measure global warming precisely. But with many different climate zones, whose temperatures range from freezing cold to extremely hot, the global average temperature will always be subject to some degree of uncertainty. Also, there are many more land-based observation stations — roughly 3,000 total — than ocean-based, and more are located in temperate zones than in the tropics or the Arctic.

To address the geographical gaps in measurement, scientists have divided the globe into a grid. Depending on the region, a station’s grid box will span between 155 and 745 miles. Closer to the poles, the grid boxes are smaller, which scientists compensate for mathematically when establishing an average temperature for a grid zone. By operating with grid boxes that have one calculated temperature per box, the number of measurement stations in the individual boxes becomes irrelevant. It’s also easy to add or eliminate individual measurement stations.

As a supplement to ground surface measurements, scientists have also begun to use satellite data in recent decades. By analyzing the spectrum of infrared radiation reflected from an area, scientists can reconstruct the conditions under which the radiation was emitted. Compared to surface measurements, it is possible to make much more detailed calculations than those provided by Earth stations. The drawback is that there is no satellite data from before 1979, so making historical comparisons that reach further back is impossible.

These sophisticated measurement methods have provided scientists (and politicians) with a good idea of how the climate has changed in recent decades: The average temperature of the usual base period (1961 to 1990) is 57.2 degrees F, but the past 20 years have seen a global estimated rise of 0.36 to 0.62 degrees F on average. As its consequences become clearer, and possibly more severe, scientists and politicians both will need to address global warming in a meaningful way.

THE GRID METHOD ELIMINATES ERRORS

1. Measurements from thousands of land- and ocean-based weather stations (red dots) are compiled.

2. Measurements are corrected for errors. For example, urban-area measurements are compared with nearby rural areas to keep the heat given off by cities from distorting the results.

3. A monthly average per station is calculated.

4. The results of the individual stations are plotted onto a grid covering the globe.

5. An average for each grid box is calculated.

6. The average of all grid boxes in the world is calculated. In this way, the average temperature of the Earth is determined.