Microwave Sounding Unit temperature measurements
Microwave measurements have been obtained from the troposphere since 1979, when they were included within NOAA weather satellites, starting with TIROS-N. By comparison, the usable balloon (radiosonde) record begins in 1958 but has less geographic coverage and is less uniform.
They measure radiances in various wavelength bands, which must then be mathematically inverted to obtain indirect inferences of temperature.
Particularly large differences between reconstructed temperature series occur at the few times when there is little temporal overlap between successive satellites, making intercalibration difficult.
However this process amplifies noise,[5] increases inter-satellite calibration biases and enhances surface contamination.
and an attenuation factor: where the secant theta term accounts for the dependence of optical path length on the vertical angle, and
TMT and TLT represent the altitude range computed lower troposphere temperature calculated using an atmospheric model as discussed below.
The T4 or TLS channel in representative of the temperature in the lower stratosphere with a peak weighting function at around 17 km above the Earth surface.
However, this process amplifies noise,[5] increases inter-satellite calibration biases and enhances surface contamination.
[6] Spencer and Christy developed the synthetic "2LT" (later renamed "TLT") product by subtracting signals at different view angles; this has a maximum at about 650 hPa.
Another such methodology has been developed by Fu and Johanson,[12] the TTT(Total Troposphere Temperature) channel is a linear combination of the TMT and TLS channel: TTT=1.156*TMT-0.153*TLS for the global average and TTT=1.12*TMT-0.11*TLS at tropical latitudes All the MSU instruments and to a lesser extent AMSU drift slowly from the Sun-synchronous equatorial crossing time changing the local time observed by the instrument, therefore the natural diurnal cycle may be aliased into the long term trend.
The orbital decay change the instrument view angle relative to the surface and thus the observed microwave emissivity, furthermore the long-term time-series is constructed by sequential merging of the inter-calibrated satellite data so that the error is summed up over time, the required correction is in the order of 0.1 °C/decade for TLT.
Once every Earth scan MSU instrument use the deep space (2.7K) and on-board warm targets to make calibration measures, however as the spacecraft drifts through the diurnal cycle the calibration target temperature may change due to varying solar shadowing effect, the correction is in the order of 0.1 °C/decade for TLT and TMT.
One widely reported satellite temperature record is that developed by Roy Spencer and John Christy at the University of Alabama in Huntsville (UAH).
[13] NOAA-11 played a significant role in a 2005 study by Mears et al. identifying an error in the diurnal correction that leads to the 40% jump in Spencer and Christy's trend from version 5.1 to 5.2.
[15] NOAA-11 played a significant role in a 2005 study by Mears et al. identifying an error in the diurnal correction that leads to the 40% jump in Spencer and Christy's trend from version 5.1 to 5.2.
Using the T2 or TMT channel (which include significant contributions from the stratosphere, which has cooled), Mears et al. of Remote Sensing Systems (RSS) find (through January 2017) a trend of +0.140 °C/decade.
They addressed this problem by combining the surface temperature measurements with satellite data to fill in the coverage.
Over the time period 1979-2016, combining the HadCRUT4 surface data with UAH satellite coverage, they show a global surface-warming trend of 0.188 °C/decade.
A lack of warming then seen in the UAH retrieval trends 1978-1998 was noted by Christy and Spencer[32] and commented on in a 2000 report by the National Research Council[33][34] and the 2001 IPCC Third Assessment Report[35] Christy et al. (2007) claimed that tropical temperature trends from radiosondes matches closest with his v5.2 UAH dataset.
Wentz & Schabel at RSS in their 1998 paper showed this (along with other discrepancies) was due to the orbital decay of the NOAA satellites.
[38] Once the orbital changes had been allowed for the data showed a 0.07K per decade increase in temperature at this level of the atmosphere.
The IPCC Fifth Assessment Report (2014) stated: "based on multiple independent analyses of measurements from radiosondes and satellite sensors it is virtually certain that globally the troposphere has warmed and the stratosphere has cooled since the mid-20th century.
Despite unanimous agreement on the sign of the trends, substantial disagreement exists among available estimates as to the rate of temperature changes, particularly outside the NH extratropical troposphere, which has been well sampled by radiosondes,[41] and concluded "Although there have been substantial methodological debates about the calculation of trends and their uncertainty, a 95% confidence interval of around ±0.1 °C per decade has been obtained consistently for both LT and MT (e.g., Section 2.4.4; McKitrick et al., 2010).
[42] As well as the correction by Wentz and Schabel,[38] doubts had been raised as early as 2000 about the UAH analysis by the work of Prabhakara et al.,[43] which minimised errors due to satellite drift.
Since the earliest release of results in the 1990s, a number of adjustments to the algorithm computing the UAH TLT dataset have been made.
Since then, a longer record and a number of corrections to the processing have revised this picture, with both UAH and RSS measurements showing a warming trend.
A detailed analysis produced in 2005 by dozens of scientists as part of the US Climate Change Science Program (CCSP) identified and corrected errors in a variety of temperature observations, including the satellite data.
They stated: The most recent climate model simulations give a range of results for changes in global average temperature.
