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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

What the science says...

Satellites don't measure temperatures, and their uncertainty is five times as large as that in the global surface temperature record.

Satellites don't measure temperature. When people refer to the satellite temperature record, they're referring to microwave sounding unit (MSU) instruments on satellites. As Andrew Dessler describes in the video below by Peter Sinclair, MSUs measure voltages on detectors, which themselves are detecting microwave signals emitted by oxygen molecules in the Earth's atmosphere. To translate these voltages and microwave detections into estimates of the temperature of various layers of the Earth's atmosphere requires a model.

Satellite Temperature Record Challenges

Converting those MSU microwave detections into a reliable long-term atmospheric temperature record is a challenging proposition, made all the more difficult by a number of confounding factors. For example, the satellites have a limited life span. The overall satellite MSU record is comprised of numerous satellites, and each has a different calibration, orbit, etc. that must be accounted for. During that life span, the satellites also experience friction, which causes their orbits to drift. If not correctly taken into account, these factors can create a bias in the estimated temperature record.

Another issue is that the MSU detections can be influenced by factors besides just temperature-influenced oxygen microwave signals, for example, cloud liquid water. Weng et al. (2014) found that the MSU channel (Channel 3) that focuses on the lowest level of the atmosphere (the lower troposphere) is most influenced by the presence of cloud liquid water. Weng et al. suggest,

the global mean temperature in the low and middle troposphere has a larger warming rate (about 20–30% higher) when the cloud-affected radiances are removed from AMSU-A data.

Roy Spencer who, with John Christy, runs the University of Alabama at Huntsville (UAH) satellite temperature dataset disagrees, believing that the cloud-caused bias is insignificant. The magnitude of this bias in the satellite data remains an unresolved question.

Another issue related to changes in the satellites' orbits is called 'diurnal drift'. The satellites are in 'Sun Synchronous orbits' and are meant to stay aligned with the Sun so that they always cross the equator at the same time. If they don’t, then the normal daily temperature cycles below will start to add a false bias to the data. Po-Chedley et al. (2015) d

Updated on 2016-01-14 by dana1981.

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Satellite Temperature Record Challenges