Satellite temperature measurements
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Satellites have been sensing the temperature of the troposphere since 1979; the usable balloon (radiosonde) record begins in 1958.
Several groups have analyzed the satellite data to calculate temperature trends in the atmosphere, and a range of values have been obtained. To compare to the increase from the surface record (of approximately +0.06 °C/decade over the past century and +0.17 °C/decade since 1979) it is more appropriate to derive trends for the lower troposphere in which the stratospheric cooling is removed. After doing this, RSS find a trend of +0.192 °C/decade [1] while the UAH analysis finds only +0.133 °C/decade [2] with a 95% confidence interval of +/- 0.05 C/decade. (UAH v5.2 corrects an error recently identified in the analysis of the dataset, and the new trend is 0.035 °C/decade higher than in v5.1 [3]). An alternative adjustment introduced by Fu et al [4] finds trends (up to the end of 2004) of +0.19 °C/decade when applied to the RSS dataset [5]. A less regularly updated analysis is that of Vinnikov and Grody with +0.22°C to +0.26°C per decade (Oct. 03) [6], [7].
Using just the T2 channel (which include significant contributions from the stratosphere, which has cooled), Mears et al of Remote Sensing Systems (RSS) find (through February 2006) a trend of +0.135 °C/decade [8]. Spencer and Christy of the University of Alabama in Huntsville, find a smaller trend of +0.054 °C/decade [9].
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. Climate models predict that the troposphere should warm faster than the surface, so all but the Spencer and Christy version of the satellite record are compatible with this and the surface records. An extensive, but now somewhat dated, comparison and discussion of trends from different data sources and periods is given in the IPCC TAR section 2.2.4.
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[edit] The satellite temperature record
Since 1979, Microwave Sounding Units (MSUs) 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 [10]. Channel 2 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" product by subtracting signals at different view angles; this has a maximum at about 650 hPa. However this amplifies noise (Christy et al., J. Clim., 1998), increases inter-satellite calibration biases and enhances surface contamination (Fu and Johanson, GRL, 2005). The 2LT product has gone through numerous versions as various corrections have been applied.
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 [11] [12].
The process of constructing a temperature record from a radiance record is difficult. The best-known, though controversial, record, from Roy Spencer and John Christy at the University of Alabama in Huntsville (UAH), is currently version 5.2, which corrects previous errors in their analysis for orbital drift and other factors. The 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 the RSS and UAH analyses [13]. NOAA-11 played a significant role in the August 2005 Mears et al paper [14] 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.
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 line with that estimated by climate models. Global warming skeptics used this contradiction to argue that warming was not occurring, or was occurring at rates far below model forecasts. In April 2002, for example, their analysis of the satellite temperature data showed warming of only 0.04 °C / decade, compared with surface measurements showing 0.17 +/- 0.06 °C / decade. The correction of errors in the analysis of the satellite data, as noted above, have brought the two datasets more closely in line with each other, supporting general circulation modeling results and the conclusions of the IPCC.
[edit] Discussion of the satellite temperature records
In the late 1990s the disagreement between the surface temperature record and the satellite records was a subject of research and debate. The lack of warming then seen in the records was noted, e.g. [15]. A report by the National Research Council that reviewed upper air temperature trends stated:
- "Data collected by satellites and balloon-borne instruments since 1979 indicate little if any warming of the low- to mid- troposphere - the atmospheric layer extending up to about 5 miles from the Earth's surface. Climate models generally predict that temperatures should increase in the upper air as well as at the surface if increased concentrations of greenhouse gases are causing the warming." [16]
However, the same panel then concluded that
- "the warming trend in global-mean surface temperature observations during the past 20 years is undoubtedly real and is substantially greater than the average rate of warming during the twentieth century. The disparity between surface and upper air trends in no way invalidates the conclusion that surface temperature has been rising."[17][18]
As noted earlier, these temperature data, misinterpreted from the satellite data, are now known to have been too low.
An important critique of the satellite record is its shortness - 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 below, which shows the UAH TLT (lower tropospheric) global trend (°C/decade) beginning with Dec 1978 and ending with December of the year shown.
1992 0.014 1993 -0.020 1994 -0.013 1995 0.019 1996 0.023 1997 0.031 1998 0.101 1999 0.091 2000 0.081 2001 0.091 2002 0.109 2003 0.118 2004 0.116 2005 0.13
Likewise, even though they began with the same data, each of the major research groups has interpreted it with different results. Most notably, Mears et al. at RSS find 0.193 °C/decade for lower troposphere up to July 2005, compared to +0.123 °C/decade found by UAH for the same period. There are ongoing efforts to resolve these differences, however, much of the disparity may have been 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.
[edit] Satellite measurements of the stratospheric temperature
The satellites also measure the lower stratospheric temperature [19] and show 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. However, the lower stratospheric record is mostly explained by the effects ozone depletion, which has caused a cooling of the stratosphere [20].
[edit] Weather balloons (radiosondes)
The longest data sets of upper air temperature are derived from instruments carried aloft by balloons (radiosondes). The radiosonde data set becomes usably global in about 1958. Changes in balloon instrumentation and data processing over the years have been pervasive, however, resulting in discontinuities in these temperature records [21]. The Sherwood et al. study [22], published August 2005, looked at solar heating issues and found a spurious trend of about -.16K per decade had been introduced into the record, masking the true warming, with particularly large spurious trends in the tropical regions [23]. This is enough to make the trend compatible with surface warming.
The radiosondes and the MSU were designed to detect short term changes in temperatures and not long term trends so it would be inappropriate to criticize them for being poor for long term trend detection. Other problems with the radiosondes in addition to the recently discovered solar heating issue could remain in the data.
A graph comparing of the surface, balloon and satellite records.
[edit] External references
- 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 NASA
- What Microwaves Teach Us About the Atmosphere
- Globally-Averaged Atmospheric Temperatures
- Angell radiosonde record
![[10]](http://www.ssmi.com/msu/img/msu_weighting_functions.png){kind=link}
