Satellite temperature measurements
The temperature of the atmosphere at various altitudes as well as sea and land surface temperatures can be inferred from satellite measurements. These measurements can be used to locate weather fronts, monitor the El Niño-Southern Oscillation, determine the strength of tropical cyclones, study urban heat islands and monitor the global climate. Wildfires, volcanos, and industrial hot spots can also be found via thermal imaging from weather satellites.
Weather satellites do not measure temperature directly but measure radiances in various wavelength bands. Since 1978 Microwave sounding units (MSUs) on National Oceanic and Atmospheric Administration polar orbiting satellites have measured the intensity of upwelling microwave radiation from atmospheric oxygen, which is proportional to the temperature of broad vertical layers of the atmosphere. Measurements of infrared radiation pertaining to sea surface temperature have been collected since 1967.
Satellite datasets show that over the past four decades the troposphere has warmed and the stratosphere has cooled. Both of these trends are consistent with the influence of increasing atmospheric concentrations of greenhouse gases.
Satellites do not measure temperature. They measure radiances in various wavelength bands, which must then be mathematically inverted to obtain indirect inferences of temperature. The resulting temperature profiles depend on details of the methods that are used to obtain temperatures from radiances. As a result, different groups that have analyzed the satellite data have produced differing temperature datasets. Among these are the UAH dataset prepared at the University of Alabama in Huntsville and the RSS dataset prepared by Remote Sensing Systems. The satellite series is not fully homogeneous – it is constructed from a series of satellites with similar but not identical instrumentation. The sensors deteriorate over time, and corrections are necessary for orbital drift and decay. Particularly large differences between reconstructed temperature series occur at the few times when there is little temporal overlap between successive satellites, making intercalibration difficult.
Satellites may also be used to retrieve surface temperatures in cloud-free conditions, generally via measurement of thermal infrared from AVHRR. Weather satellites have been available to infer sea surface temperature (SST) information since 1967, with the first global composites occurring during 1970. Since 1982, satellites have been increasingly utilized to measure SST and have allowed its spatial and temporal variation to be viewed more fully. For example, changes in SST monitored via satellite have been used to document the progression of the El Niño-Southern Oscillation since the 1970s. Over the land the retrieval of temperature from radiances is harder, because of the inhomogeneities in the surface. Studies have been conducted on the urban heat island effect via satellite imagery. Use of advanced very high resolution infrared satellite imagery can be used, in the absence of cloudiness, to detect density discontinuities (weather fronts) such as cold fronts at ground level. Using the Dvorak technique, infrared satellite imagery can used to determine the temperature difference between the eye and the cloud top temperature of the central dense overcast of mature tropical cyclones to estimate their maximum sustained winds and their minimum central pressures. Along Track Scanning Radiometers aboard weather satellites are able to detect wildfires, which show up at night as pixels with a greater temperature than 308 K (95 °F). The Moderate-Resolution Imaging Spectroradiometer aboard the Terra satellite can detect thermal hot spots associated with wildfires, volcanoes, and industrial hot spots.
Tropospheric and stratospheric measurements
From 1979 to 2005 the microwave sounding units (MSUs) and since 1998 the Advanced Microwave Sounding Units on NOAA polar orbiting satellites have measured the intensity of upwelling microwave radiation from atmospheric oxygen. The intensity is proportional to the temperature of broad vertical layers of the atmosphere, as demonstrated by theory and direct comparisons with atmospheric temperatures from radiosonde (balloon) profiles. Upwelling radiance is measured at different frequencies; these different frequency bands sample a different weighted range of the atmosphere. The brightness temperature (TB) measured by satellite is given by:
where W(S) is the surface weight,T(0) and T(Z) are the temperatures at the surface and at the atmospheric level z and W(Z) is the atmospheric weighting function.
Both the surface and atmospheric weights are dependent on the surface emissivity eS, the absorption coefficient κ(z) and the earth incidence angle θ; the surface weight is the product of eS and an attenuation factor:
The atmospheric weighting functions W(Z) can be written as:
The first term in this equation is related to the radiation emitted upward from the level z and attenuated along the path to the top of the atmosphere (∞), the second include the radiation emitted downward from the level z to the surface (0) and the radiation reflected back by the surface (proportional to eS) to the top of the atmosphere, the exact form of W(Z) is dependent upon the temperature, water vapor and liquid water content of the atmosphere.
MSU Channel 1 is not used to monitor atmospheric temperature because it's too much sensitive to the emission from the surface, furthermore it is heavily contaminated by water vapor/liquid water in the lowermost troposphere.
Channel 2 or TMT is broadly representative of the troposphere, albeit with a significant overlap with the lower stratosphere (the weighting function has its maximum at 350 hPa and half-power at about 40 and 800 hPa). In an attempt to remove the stratospheric influence, Spencer and Christy developed the synthetic "2LT or TLT" product by subtracting signals at different view angles; this has a maximum at about 650 hPa. However, this amplifies noise, increases inter-satellite calibration biases and enhances surface contamination. The 2LT product has gone through numerous versions as various corrections have been applied.
Another methodology to reduce the influence of the stratosphere has been developed by Fu and Johanson, 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.
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.
Since 1979 the Stratospheric sounding units (SSUs) on the NOAA operational satellites provided near global stratospheric temperature data above the lower stratosphere. The SSU is a far-infrared spectrometer employing a pressure modulation technique to make measurement in three channels in the 15 μm carbon dioxide absorption band. The three channels use the same frequency but different carbon dioxide cell pressure, the corresponding weighting functions peaks at 29 km for channel1, 37 km for channel2 and 45 km for channel3.
Trends from the record
Records have been created by merging data from nine different MSUs, each with peculiarities (e.g., time drift of the spacecraft relative to the local solar time) that must be calculated and removed because they can have substantial impacts on the resulting trend. The satellite record is short, which means adding a few years on to the record or picking a particular time frame can change the trends considerably. The problems with the length of the MSU record is shown by the table to the right, which shows the UAH TLT (lower tropospheric) global trend (°C/decade) beginning with December 1978 and ending with December of the year shown.
The process of constructing a temperature record from a radiance record is difficult. The satellite temperature record comes from a succession of different satellites and problems with inter-calibration between the satellites are important, especially NOAA-9, which accounts for most of the difference between various analyses. 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. There are ongoing efforts to resolve differences in satellite temperature datasets.
Christy et al. (2007) find that the tropical temperature trends from radiosondes matches closest with his v5.2 UAH dataset. Furthermore, they assert there is a growing discrepancy between RSS and sonde trends beginning in 1992, when the NOAA-12 satellite was launched. This research found that the tropics were warming, from the balloon data, +0.09 (corrected to UAH) or +0.12 (corrected to RSS) or 0.05 K (from UAH MSU; ±0.07 K room for error) a decade.
Using the T2 channel (which include significant contributions from the stratosphere, which has cooled), Mears et al. of Remote Sensing Systems (RSS) find (through December 2013) a trend of +0.078 °C/decade. Spencer and Christy of the University of Alabama in Huntsville (UAH), find a smaller trend of +0.045 °C/decade.
A no longer updated analysis of Vinnikov and Grody found +0.20 °C per decade (1978–2005). Another satellite temperature analysis is provided by NOAA/NESDIS STAR Center for Satellite Application and Research and use simultaneous nadir overpasses (SNO) to remove satellite intercalibration biases yielding more accurate temperature trends. The SNO analysis finds a 1979-2013 trend of +0.105 °C/decade for T2 channel.
Lower stratospheric cooling is mainly caused by the effects of ozone depletion with a possible contribution from increased stratospheric water vapor and greenhouse gases increase. There is a decline in stratospheric temperatures, interspersed by warmings related to volcanic eruptions. Global Warming theory suggests that the stratosphere should cool while the troposphere warms 
The long term cooling in the lower stratosphere occurred in two downward steps in temperature both after the transient warming related to explosive volcanic eruptions of El Chichón and Mount Pinatubo, this behavior of the global stratospheric temperature has been attributed to global ozone concentration variation in the two years following volcanic eruptions.
The process of deriving trends from SSUs measurement has proved particularly difficult because of satellites drift, inter-calibration between different satellite with scant overlap and gas leak in the instrument carbon dioxide pressure cell, furthermore since the radiances measured by SSUs are due to emission by carbon dioxide the weighting functions move to higher altitudes as the carbon dioxide concentration in the stratosphere increase. Mid to upper stratosphere temperature show strong negative trend interspersed by transient volcanic warming after the explosive volcanic eruptions of El Chichón and Mount Pinatubo, little temperature trend has been observed since 1995. The greatest cooling occurred in the tropical stratosphere consistent with enhanced Brewer-Dobson circulation under greenhouse gas concentrations increase.
|Channel||Start||End Date||RSS Global Trend
|UAH Global Trend
|STAR v2.0 Global Trend
Comparison to instrumental record
The satellite records have the advantage of global coverage, whereas the radiosonde record is longer. There have been complaints of data problems with both records.
To compare to the trend from the surface temperature record (approximately +0.07 °C/decade over the past century and +0.17 °C/decade since 1979) it is most appropriate to derive trends for the part of the atmosphere nearest the surface, i.e., the lower troposphere. Doing this, through December 2013:
An alternative adjustment to remove the stratospheric contamination has been introduced by Fu et al. (2004), after the correction the vertical weighting function is nearly the same of the T2(TMT) channel in the troposhere, the University of Washington analysis finds 1979-2012 trends of +0.13 °C/decade when applied to the RSS data set and +0.10 °C/decade when applied to the UAH data set.
Reconciliation with climate models
Climate model results summarized by the IPCC in their third assessment show overall good agreement with the satellite temperature record. In particular both models and satellite record show a global average warming trend for the troposphere (models range for TLT/T2LT 0.6 - 0.39 °C/decade; avg 0.2 °C/decade) and a cooling of the stratosphere (models range for TLS/T4 -0.7 - 0.08 °C/decade; avg -0.25 °C/decade).
There remain, however, differences in detail between the satellite data and the climate models used.
Globally, the troposphere is predicted by models to warm about 1.2 times more than the surface; in the tropics, the troposphere should warm about 1.5 times more than the surface. Most climate models used by the IPCC in preparation of their third assessment show a slightly greater warming at the TLT level than at the surface (0.03 °C/decade difference) for 1979-1999 while GISS and Hadley Centre surface station network trends are +0.161 and +0.160 °C/decade respectively, the lower troposphere trends calculated from satellite data by UAH and RSS are +0.140 °C/decade and +0.148 °C/decade. The expected trend in the lower troposphere, given the surface data, would be around 0.194 °C/decade.
This greater global average warming in the troposphere compared to the surface (present in the models but not observed data) is most marked in the tropics.
"In the tropics, surface temperature changes are amplified in the free troposphere. Models and observations show similar amplification behavior for monthly and interannual temperature variations, but not for decadal temperature changes. Tropospheric amplification of surface temperature anomalies is due to the release of latent heat by moist, rising air in regions experiencing convection."
Although all the datasets show the expected tropospheric amplification at seasonal and annual timescales it is still debated whether or not the long term trends are consistent with the expected moist adiabatic lapse rate amplification due to difficulty of producing homogenized datasets, some satellite temperature reconstruction are consistent with the expected amplification while others are not.
For some time the only available satellite record was the UAH version, which (with early versions of the processing algorithm) showed a global cooling trend for its first decade. Since then, a longer record and a number of corrections to the processing have revised this picture: the UAH dataset has shown an overall warming trend since 1998, though less than the RSS version. In 2001, an extensive comparison and discussion of trends from different data sources and periods was given in the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) (section 2.2.4).
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