The temperature record explained: what the series actually measures

The chart on this site’s data page has a single line for each year from 1880 onwards. It is one of the most reproduced graphs in existence, and it is routinely misread, because the number on its axis is not a temperature.

It measures a departure, not a reading

Absolute surface temperature varies by tens of degrees between a polar station and an equatorial one, and it varies with altitude, with local geography and with how the station is sited. Averaging raw readings across the globe produces a figure that depends mostly on which stations happened to report that year.

So the record does something else. For each location, it establishes a long-term normal over a fixed reference period, then reports how far each year departed from that normal. Departures are comparable in a way absolute temperatures are not: a station in Svalbard and a station in Singapore can both be half a degree above their own normal, and those two halves can be meaningfully averaged.

The consequence is that the y-axis is a difference. A value of plus one degree does not mean anywhere was one degree; it means the surface, taken as a whole, sat a degree above the reference period.

The baseline is a choice

The series used here references 1951 to 1980. Another analysis may reference a different period, and its numbers for the same year will differ by a constant. Nothing about the trend changes; the shape of the line is identical, shifted up or down.

This is worth knowing mainly because it explains apparent disagreements between records that are in fact reporting the same thing.

Land and sea are both in it

The version charted here combines land air temperature with sea-surface temperature. That combination is not a technicality. The ocean is most of the planet’s surface, it has far greater heat capacity than land, and it warms more slowly, so a land-only record runs warmer and noisier than a combined one. Any statement about global surface temperature has to say which it is using.

Why it appears on a Singapore site

Two reasons, and neither is decoration.

The first is that the record is the leading indicator for sea level. Water expands as it warms and land ice adds volume as it melts; both are downstream of the surface temperature series. For a country whose coastline is substantially reclaimed and whose water supply depends on managing rainfall against saltwater intrusion, that is a planning input.

The second is that the series is a good example of a general problem in reading data: the axis label carries most of the meaning, and it is the part people skip.

The figure on the data page is drawn directly from the published annual means, not redrawn from an image, so the shape on the page is the shape in the dataset.