UAH satellite temperature dataset
Encyclopedia
The UAH satellite temperature dataset, developed at the University of Alabama in Huntsville
, attempts to infer the temperature of the atmosphere at various levels from satellite
measurements of radiance
.
It was one of the first global temperature datasets developed from satellite information and has been used as a tool for research since the 1990s. It has proved controversial as, previously, there were inconsistencies between the UAH dataset and surface measurements. Significant corrections during the late 1990s and the first decade of the 2000s, have resolved many of those differences.
The dataset is published by John Christy
et al. and formerly jointly with Roy Spencer
.
s do not measure temperature
directly. They measure radiance
s in various wavelength
bands, from which temperature may be inferred. 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 obtained different temperature data. Among these groups are Remote Sensing Systems
(RSS) and the University of Alabama in Huntsville
(UAH). 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 satellite drift and orbital 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.
Data are provided as temperature anomalies against the seasonal average over a past basis period, as well as in absolute temperature values.
All the data products can be downloaded from the UAH server.
Monthly averages are available in gridded format as well as by hemisphere and globally.
Each set has data back to December 1978.
) 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).
A detailed analysis produced 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.
The CCSP SAP 1.1 Executive Summary states:
The IPCC Fourth Assessment Report Summary for Policymakers states:
However, as detailed in CCSP SAP 5.1 Understanding and Reconciling Differences, neither Regression models or other related techniques were reconcilable with observed data. The use of fingerprinting techniques on data yielded that "Volcanic and human-caused fingerprints were not consistently identifiable in observed patterns of lapse rate change." As such, issues with reconciling data and models remain.
A potentially serious inconsistency has been identified in the tropics, the area in which tropospheric amplification should be seen. Section 1.1 of the CCSP report says:
The lower troposphere trend derived from UAH satellites (+0.128 °C/decade) is currently lower than both the GISS and Hadley Centre surface station network trends (+0.161 and +0.160 °C/decade respectively), while the RSS trend (+0.158 °C/decade) is similar. However, the expected trend in the lower troposphere, given the surface data, would be around 0.194 °C/decade, making the UAH and RSS trends 66% and 81% of the expected value respectively.
For some time, the UAH satellite data's chief significance was that they appeared to contradict a wide range of surface temperature data measurements and analyses showing warming. In 1998 the UAH data showed a cooling of 0.05K per decade (at 3.5 km - mid to low troposphere). Wentz & Schabel at RSS in their 1998 paper showed this (along with other discrepancies) was due to the orbital decay of the NOAA satellites. 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 'trend correction' refers to the change in global mean decadal temperature trend in degrees Celsius as a result of the correction.
The UAH TLT dataset was a source of controversy in the 1990's as, at that time, it showed little increase in global mean temperature, at odds with surface measurements. Since then a number of errors in the way the atmospheric temperatures were derived from the raw radiance data have been discovered and corrections made by Christy et al. at UAH.
The largest of these errors was demonstrated in a 1998 paper by Frank Wentz
and Matthias Schabel of RSS
. In that paper they showed that the data needed to be corrected for orbital decay of the MSU satellites. As the satellites' orbits gradually decayed towards the earth the area from which they received radiances was reduced, introducing a false cooling trend.
Even after the correction for satellite decay UAH continued to infer lower TLT temperatures than RSS based on the same raw data. For example Mears et al. at RSS found 0.193 °C/decade for lower troposphere up to July 2005, compared to +0.123 °C/decade found by UAH for the same period.
Much of the remaining disparity was resolved by the three papers in Science, 11 August 2005, which pointed out errors in the UAH 5.1 record and the radiosonde record in the tropics.
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.
Christy et al. asserted in a 2007 paper that the tropical temperature trends from radiosondes matches more closely with their v5.2 UAH-TLT dataset than with RSS v2.1.
Much of the difference, at least in the Lower troposphere global average decadal trend between UAH and RSS, has been removed with the release of RSS version 3.3 in January 2011. RSS and UAH TLT are now within 0.003K/decade of one another. Significant differences remain, however, in the Mid Troposphere (TMT) decadal trends.
University of Alabama in Huntsville
The University of Alabama in Huntsville is a state-supported, public, coeducational research university, located in Huntsville, Alabama, United States, is accredited by the Southern Association of Colleges and Schools to award baccalaureate, master's and doctoral degrees, and is organized in five...
, attempts to infer the temperature of the atmosphere at various levels from satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
measurements of radiance
Radiance
Radiance and spectral radiance are radiometric measures that describe the amount of radiation such as light or radiant heat that passes through or is emitted from a particular area, and falls within a given solid angle in a specified direction. They are used to characterize both emission from...
.
It was one of the first global temperature datasets developed from satellite information and has been used as a tool for research since the 1990s. It has proved controversial as, previously, there were inconsistencies between the UAH dataset and surface measurements. Significant corrections during the late 1990s and the first decade of the 2000s, have resolved many of those differences.
The dataset is published by John Christy
John Christy
John R. Christy is a climate scientist at the University of Alabama in Huntsville whose chief interests are satellite remote sensing of global climate and global climate change. He is best known, jointly with Roy Spencer, for the first successful development of a satellite temperature...
et al. and formerly jointly with Roy Spencer
Roy Spencer
Roy W. Spencer is a climatologist and a Principal Research Scientist for the University of Alabama in Huntsville, as well as the U.S. Science Team Leader for the Advanced Microwave Scanning Radiometer on NASA’s Aqua satellite...
.
Satellite temperature measurements
SatelliteSatellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
s do not measure temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
directly. They measure radiance
Radiance
Radiance and spectral radiance are radiometric measures that describe the amount of radiation such as light or radiant heat that passes through or is emitted from a particular area, and falls within a given solid angle in a specified direction. They are used to characterize both emission from...
s in various wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
bands, from which temperature may be inferred. 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 obtained different temperature data. Among these groups are Remote Sensing Systems
Remote Sensing Systems
Remote Sensing Systems is a private research company founded in 1974 by Frank Wentz. It processes microwave data from a variety of NASA satellites. Most of their research is supported by the Earth Science Enterprise program. The company is based in Santa Rosa, California.They are a widely cited...
(RSS) and the University of Alabama in Huntsville
University of Alabama in Huntsville
The University of Alabama in Huntsville is a state-supported, public, coeducational research university, located in Huntsville, Alabama, United States, is accredited by the Southern Association of Colleges and Schools to award baccalaureate, master's and doctoral degrees, and is organized in five...
(UAH). 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 satellite drift and orbital 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.
Description of the data
UAH provide data on three broad levels of the atmosphere.- The Lower troposphereTroposphereThe troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....
- TLT (originally called T2LT). - The mid troposphere - TMT
- The lower stratosphere - TLS
Data are provided as temperature anomalies against the seasonal average over a past basis period, as well as in absolute temperature values.
All the data products can be downloaded from the UAH server.
Geographic coverage
Data are available as global, hemispheric, zonal, and gridded averages. The global average covers 97-98% of the earth's surface, excluding only latitudes above +85 degrees, below -85 degrees and, in the cases of TLT and TMT, some areas with land above 1500m altitude. The hemispheric averages are over the northern and southern hemispheres 0 to +/-85 degrees. The gridded data, in effect, provide a global temperature map.Temporal coverage
Daily global, hemispheric and zonal data are available.Monthly averages are available in gridded format as well as by hemisphere and globally.
Each set has data back to December 1978.
Comparison with other data and models
Climate models predict that as the surface warms, so should the global troposphere. Globally, the troposphere should warm about 1.2 times more than the surface; in the tropics, the troposphere should warm about 1.5 times more than the surface. For some time the only available satellite record was the UAH version, which (with early versions of the processing algorithmAlgorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
) 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
Intergovernmental Panel on Climate Change
The Intergovernmental Panel on Climate Change is a scientific intergovernmental body which provides comprehensive assessments of current scientific, technical and socio-economic information worldwide about the risk of climate change caused by human activity, its potential environmental and...
(IPCC) (section 2.2.4).
A detailed analysis produced by dozens of scientists as part of the US Climate Change Science Program
Climate Change Science Program
The Climate Change Science Program was the program responsible for coordinating and integrating research on global warming by U.S. government agencies from February 2002 to June 2009...
(CCSP) identified and corrected errors in a variety of temperature observations, including the satellite data.
The CCSP SAP 1.1 Executive Summary states:
- "Previously reported discrepancies between the amount of warming near the surface and higher in the atmosphere have been used to challenge the reliability of climate models and the reality of human induced global warming. Specifically, surface data showed substantial global-average warming, while early versions of satellite and radiosonde data showed little or no warming above the surface. This significant discrepancy no longer exists because errors in the satellite and radiosonde data have been identified and corrected. New data sets have also been developed that do not show such discrepancies."
The IPCC Fourth Assessment Report Summary for Policymakers states:
- "New analyses of balloon-borne and satellite measurements of lower- and mid-tropospheric temperature show warming rates that are similar to those of the surface temperature record and are consistent within their respective uncertainties, largely reconciling a discrepancy noted in the TAR."
However, as detailed in CCSP SAP 5.1 Understanding and Reconciling Differences, neither Regression models or other related techniques were reconcilable with observed data. The use of fingerprinting techniques on data yielded that "Volcanic and human-caused fingerprints were not consistently identifiable in observed patterns of lapse rate change." As such, issues with reconciling data and models remain.
A potentially serious inconsistency has been identified in the tropics, the area in which tropospheric amplification should be seen. Section 1.1 of the CCSP report says:
- "In the tropics, the agreement between models and observations depends on the time scale considered. For month-to-month and year-to-year variations, models and observations both show amplification (i.e., the month-to-month and year-to-year variations are larger aloft than at the surface). This is a consequence of relatively simple physics, the effects of the release of latent heat as air rises and condenses in clouds. The magnitude of this amplification is very similar in models and observations. On decadal and longer time scales, however, while almost all model simulations show greater warming aloft (reflecting the same physical processes that operate on the monthly and annual time scales), most observations show greater warming at the surface.
- "These results could arise either because “real world” amplification effects on short and long time scales are controlled by different physical mechanisms, and models fail to capture such behavior; or because non-climatic influences remaining in some or all of the observed tropospheric data sets lead to biased long-term trends; or a combination of these factors. The new evidence in this Report favors the second explanation."
The lower troposphere trend derived from UAH satellites (+0.128 °C/decade) is currently lower than both the GISS and Hadley Centre surface station network trends (+0.161 and +0.160 °C/decade respectively), while the RSS trend (+0.158 °C/decade) is similar. However, the expected trend in the lower troposphere, given the surface data, would be around 0.194 °C/decade, making the UAH and RSS trends 66% and 81% of the expected value respectively.
For some time, the UAH satellite data's chief significance was that they appeared to contradict a wide range of surface temperature data measurements and analyses showing warming. In 1998 the UAH data showed a cooling of 0.05K per decade (at 3.5 km - mid to low troposphere). Wentz & Schabel at RSS in their 1998 paper showed this (along with other discrepancies) was due to the orbital decay of the NOAA satellites. 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.
Corrections made
The table below summarizes the adjustments that have been applied to the UAH TLT dataset.The 'trend correction' refers to the change in global mean decadal temperature trend in degrees Celsius as a result of the correction.
| UAH version | Main adjustment | Trend correction | Year |
|---|---|---|---|
| A | Simple bias correction | 1992 | |
| B | Linear diurnal drift correction | ||
| 1994 | |||
| C | Removal of residual annual cycle related to hot target variation |
0.03 | 1997 |
| D | Orbital decay | 0.10 | 1998 |
| D | Removal of dependence of time variations of hot target temperature |
||
| 1998 | |||
| 5.0 | Non-linear diurnal correction | 0.008 | 2003 |
| 5.1 | Tightened criteria for data acceptance | ||
| 2004 | |||
| 5.2 | Correction of diurnal drift adjustment | 0.035 | 2005 |
| 5.3 | Annual cycle correction | 0 | 2009 |
| 5.4 | New annual cycle | 0 | 2010 |
The UAH TLT dataset was a source of controversy in the 1990's as, at that time, it showed little increase in global mean temperature, at odds with surface measurements. Since then a number of errors in the way the atmospheric temperatures were derived from the raw radiance data have been discovered and corrections made by Christy et al. at UAH.
The largest of these errors was demonstrated in a 1998 paper by Frank Wentz
Frank Wentz
Frank Wentz is the director and founder of Remote Sensing Systems a company specializing in research in satellite microwave remote sensing in 1974. With Carl Mears he is best known for developing a satellite temperature record from MSU and AMSU...
and Matthias Schabel of RSS
Remote Sensing Systems
Remote Sensing Systems is a private research company founded in 1974 by Frank Wentz. It processes microwave data from a variety of NASA satellites. Most of their research is supported by the Earth Science Enterprise program. The company is based in Santa Rosa, California.They are a widely cited...
. In that paper they showed that the data needed to be corrected for orbital decay of the MSU satellites. As the satellites' orbits gradually decayed towards the earth the area from which they received radiances was reduced, introducing a false cooling trend.
Even after the correction for satellite decay UAH continued to infer lower TLT temperatures than RSS based on the same raw data. For example Mears et al. at RSS found 0.193 °C/decade for lower troposphere up to July 2005, compared to +0.123 °C/decade found by UAH for the same period.
Much of the remaining disparity was resolved by the three papers in Science, 11 August 2005, which pointed out errors in the UAH 5.1 record and the radiosonde record in the tropics.
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.
Christy et al. asserted in a 2007 paper that the tropical temperature trends from radiosondes matches more closely with their v5.2 UAH-TLT dataset than with RSS v2.1.
Much of the difference, at least in the Lower troposphere global average decadal trend between UAH and RSS, has been removed with the release of RSS version 3.3 in January 2011. RSS and UAH TLT are now within 0.003K/decade of one another. Significant differences remain, however, in the Mid Troposphere (TMT) decadal trends.
External links
- Temperature Trends in the Lower Atmosphere: Steps for Understanding and Reconciling Differences CCSP Synthesis and Assessment Product 1.1
- Measuring the Temperature of Earth From Space NASA news
- Scientists Present 1998 Earth-Temperature Trends NASA: Updated 20-year temperature record unveiled at 1999 AMS Meeting
- Global Hydrology and Climate Center at NASANASAThe National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
- What Microwaves Teach Us About the Atmosphere
- Globally Averaged Atmospheric Temperatures as assessed by NASA 1998 - 2007