Measuring Earth’s temperature is no easy task.

Four different groups produce temperature records that attempt to compile a single global mean surface temperature: NASA’s GISStemp, the Hadley Center’s HadCRU, Remote Sensing Systems’ RSS, and the University of Alabama, Huntsville’s UAH.

NASA and Hadley rely on an overlapping set of surface and ocean temperature measurement stations and span the period from 1880 to present. RSS and UAH use satellite monitoring and include only the period from 1979 to present.

Despite differences in calculation criteria and a host of technical problems that have plagued the satellite-based records in the past, all four temperature records now show a remarkable degree of agreement. No single temperature record exhibits a significant or consistent warming bias relative to the others.

Prior to the late 1970s, surface temperature measurements were primarily done via land stations located mostly in the Northern Hemisphere and with limited coverage outside the 20 to 90 degrees North latitudes. Since then, a growing number of surface temperature measurement stations worldwide, coupled with improved methods for correcting for biases induced through urban heat island effects and other station siting and operational issues, have allowed for the development of accurate global temperature estimates. Ocean temperature data prior to the 1980s had been taken from ship-based temperature measurements, and is now obtained via satellite measurements.

The two major surface temperature records are NASA’s GISStemp and the Hadley Center’s HadCRU. Both mostly use the same network of surface stations, but they differ in how they extrapolate temperature changes in areas with few measurement stations. GISS assumes that the correlation of temperature change is fairly strong for stations separated by up to 1,200 kilometers, especially at middle and high latitudes. GISS uses this assumption to extrapolate for almost the entire land area of the Arctic, despite a relative paucity of measurement stations.

HadCRU uses a different approach, and some areas of the Arctic and Antarctic are not covered in its temperature series. This difference in coverage may help explain the slight divergences in series over time, especially given that the Arctic has been warming faster than the rest of the globe.

The two satellite data sets, RSS and UAH, use the Microwave Sounding Units (MSU) of orbiting satellites to estimate lower tropospheric temperature. They estimate this temperature based on measurements of the microwave emissions of oxygen molecules in the atmosphere, which increase proportional to temperature. Lower tropospheric temperature is expected to be similar to, though slightly higher than, surface temperatures, and the surface temperature record produced using the lower tropospheric temperature measurements is adjusted accordingly. MSU-based measurements also provide little coverage of Arctic and Antarctic regions.

The UAH team pioneered the approach in 1979, combining temperature measurements from multiple satellites to produce an estimate for monthly global mean temperatures. UAH published data showing significantly lower tropospheric cooling from 1979 till 1998, contradicting the warming trend observed by the surface stations.

In 1998, RSS was formed to provide an alternative analysis of the MSU data. RSS pointed out a significant error in UAH’s temperature analysis caused by a failure to accurately correct for the effects of orbital decay on observations across multiple satellites. This correction, along with another one in 2005, brought UAH largely in line with the other temperature records, though it continues to show a slightly lower long-term warming trend. Scientists generally supporting the Intergovernmental Panel on Climate Change (IPCC) findings on climate change see this correction of the UAH temperature analyses as a significant vindication of their findings on this issue and, as such, as a major rebuttal to climate contrarians who long had pointed to the differences in surface and upper atmosphere warming trends as supporting their viewpoints.

As shown in the figure below, all four temperature series align remarkably well when normalized on the same baseline period. GISS and HadCRU both show a warming trend of 0.16 degrees C per decade from 1979 to February 2008. RSS shows a warming trend of 0.18 per decade over the same period, while UAH shows a warming trend of 0.14. The largest divergence between temperature series over the last 30 years occurred in 1998, when both satellite-based series showed almost half a degree higher temperatures than land-based series, though the cause of this is unclear.

Global surface and lower troposphere monthly mean anomalies relative to the 1979-1998 mean temperature. Data from GISS, HadCRU, RSS, and UAH ranging from January 1979 to February 2008.

Journalists dealing with temperature data should keep in mind that there are a number of different global mean temperature series available, and that advocates often tend to pick the one that will best reinforce their perspectives. Over longer time periods, however, the differences between different temperature series’ settle out, and all show comparable warming temperature trends.