Satellite data

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From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change

The IPCC claims that data collected by satellite-mounted microwave sounding units (MSU) and advanced MSU measurements since 1979 reveal a warming trend of 0.12º C to 0.19º C per decade, which it says “is broadly consistent with surface temperature trends” (IPCC, 2007-I, p. 237). This would be surprising, since we indicated in the previous section that the surface-based temperature record is unreliable and biased toward a spurious warming trend. In this section we investigate the truth of the IPCC’s claim in this regard and report other findings based on satellite data.

Most climate models predict that the troposphere should warm about 1.2 times more than the surface globally, and about 1.5 times more in the tropics. Although the MSUs mounted on satellites sent into orbit by NASA for the National Oceanic and Atmospheric Administration (NOAA) were not originally intended to be used to measure temperatures in the troposphere, they have been used for this purpose since 1979 and, despite some ongoing debate, are acknowledged to be a reliable source of information about temperatures in the troposphere (Christy et al., 2003, 2007; Santer et al., 2005). As Wentz and Schabel observed in an article in Nature in 1998, “the detection and measurement of small changes in the Earth’s climate require extremely precise global observations of a broad spectrum of complementary physical variables. In this endeavour, satellite observations are playing an increasingly important role. As compared to conventional in situ observations, satellites provide daily near-global coverage with a very high statistical precision that results from averaging millions of individual observations” (Wentz and Schabel, 1998).

Four groups currently report MSU measurements: the University of Alabama in Huntsville (UAH), Remote Sensing System (RSS) (a small private weather forecasting firm led by the previously cited Frank Wentz), the University of Maryland (UMd), and a group from NOAA whose data series begins in 1987. RSS and UAH produce estimates of temperatures for the lower troposphere (LT), mid-troposphere (MT), and lower stratosphere (LS). UMd produces estimates only for MT (Christy and Norris, 2006). New data for the UAH series is posted every month on a Web site maintained at the University of Alabama at Huntsville.

The first satellite record was produced by Roy Spencer, then with NASA and now the U.S. Science Team leader for the Advanced Microwave Scanning Radiometer flying on NASA’s Aqua satellite, and John Christy, distinguished professor of atmospheric science and director of the Earth System Science Center at the University of Alabama in Huntsville. Published in Science in 1990 (Spencer and Christy, 1990), the article presented the first 10 years of satellite measurements of lower atmospheric temperature changes (from 1979 to 1988) and found “no obvious trend for the 10-year period.” Although this finding covered too short a period of time to prove a trend, it seemed to contradict claims by some scientists at the time that a warming trend was underway. It triggered a long-running debate, which continues to this day, over the accuracy of the satellite data.

In 1997, Kevin Trenberth, of the U.S. National Center for Atmospheric Research (NCAR) and later a lead author of the IPCC’s Third Assessment Report, along with coauthors challenged the reliability of the satellite data (Trenberth, 1997; Trenberth and Hurrell, 1997; Hurrell and Trenberth, 1997). Trenberth argued that Spencer and Christy had failed to properly calibrate the sensors on each new satellite as older satellites were retired and new ones launched into orbit, based on a surface-satellite comparison. Spencer and Christy, however, showed that the surface and tropospheric discrepancy was real, as it also was found in independent balloon comparisons (Christy et al., 1997).

Critics of the satellite data pointed to other possible and actual errors in the satellite record, and Spencer and Christy made two adjustments based on these external criticisms for such things as orbit decay and changes in technology. One of the larger changes was made to correct for drift in local crossing time (i.e., change in the time-of-day that the measures are taken), an error discovered by Mears and Wentz (2005) and subsequently corrected by Christy and Spencer (2005). Many of the adjustments made by Christy and Spencer resulted in the satellite record showing a small warming trend of 0.123 ºC between 1979 and 2005.

In 2006, a panel of the U.S. Climate Change Science Program (CCSP) attempted to reconcile differences between satellite and surface-station data. While the executive summary of the report claimed (as the IPCC does) that “this significant discrepancy [between surface station records and satellite records] no longer exists because errors in the satellite and radiosonde data have been identified and corrected,” in fact significant differences in some values (especially in the important region of the tropics, as discussed in Section 3.4. Fingerprints) remained unsolved (CCSP, 2006).

Satellite data allow us to check the accuracy of the warming trend during the last three decades reported by the three combined land-surface air temperature and sea-surface temperature (SST) records used by the IPCC: CRU (from the Climate Research Unit (CRU) at the University of East Anglia, in Norwich, England), NCDC (from the National Climatic Data Center), and GISS (from NASA’s Goddard Institute for Space Studies). The IPCC lists temperature trends (ºC /decade) for each record for the periods 1850-2005, 1901-2005, and 1979-2005 (IPCC, 2007-I, Table 3.3, p. 248). All three surface temperature records used by the IPCC show positive trends in global temperatures during the 1979-2005 period (the period that can be checked against satellite data) of between 0.163º C/decade and 0.174 ºC /decade, compared to the UAH record of only 0.123º C/decade. This means the IPCC’s estimates of warming are between 33 percent and 41.5 percent more rapid than the most scientifically accurate record we have of global temperatures during this 26-year period. The IPCC claims an even higher estimate of 0.177º C/decade (44 percent higher than UAH) in a graph on page 253, a variation of which appears in the Summary for Policymakers (p. 6). Finally, we note that none of the warming rates reported in the IPCC’s Table 3.3 reaches the 0.19º C that the IPCC claimed to be the upper end of the range of credible estimates, while the UAH record of 0.128º C/decade sits very close to the lowest estimate of 0.12º C.

Similarly, the IPCC’s temperature records for the Northern Hemisphere are CRU’s 0.234º C and NCDC’s 0.245º C, approximately 17 percent to 22.5 percent higher than the UAH’s record of 0.20º C. For the Southern Hemisphere, the IPCC’s estimates are 0.092º C (CRU) and 0.096 ºC (NCDC), or a very large 84 percent and 92 percent higher than the UAH’s 0.05º C. To say this is “broadly consistent,” as the IPCC does, is not accurate. In light of the large discrepancy between satellite and surface records for the Southern Hemisphere, it is notable that Christy has been using the UAH database to detect and correct errors in the Australian radiosondes record (Christy and Norris, 2009) and the surface station record in East Africa (Christy et al., 2009).

Satellite data also allow us to compare real-world temperatures to the predictions (or “scenarios”) offered by those who have been predicting warming since the 1980s. Figure 3.5.1. compares the UAH and RSS temperature records “adjusted to mimic surface temperature variations for an apples to apples comparison with the model projections (factor of 1.2, CCSP SAP 1.1)” to three model projections of global surface temperature presented by NASA’s James Hansen in Senate testimony in 1988 (Christy, 2009). “GISS-A 88” and “GISS-B 88,” at the top of the graph, are Hansen’s two “business-as-usual” model projections of temperature which assumed greenhouse gas emissions would be similar to what actually has happened. “GISS-C 88” is Hansen’s temperature forecast if drastic GHG reductions were made. Obviously, real-world temperatures have failed to rise as Hansen had predicted, and indeed, global temperatures in 2009 were no higher than when Hansen testified in 1988. As Christy comments, “Even the model projection for drastic CO2 cuts still overshot the observations. This would be considered a failed hypothesis test for the models from 1988” (Christy, 2009).

By 2008, the UAH data series indicated that global temperatures in the lower atmosphere had warmed at the slightly higher rate of about 0.14º C/decade from January 1979 through December 2007, while the RSS data series showed a warming rate of 0.17º C/decade. Recent research by Randall and Herman (2009) using data collected from a subset of weather balloon observations thought to be most reliable suggests the RSS data incorporate an improper handling of diurnal cycle effects that causes a small warming bias over global land areas, thus suggesting that the lower UAH estimate of 0.14º C/decade may have been closer to correct. Graphs showing both data sets and a third graph showing the difference between the two data sets appear in Figure 3.5.2.

Explanations exist for two of the biggest differences between the two datasets. The first is a sudden warming in RSS relative to UAH in January 1992. This feature has been found in comparison with indicating RSS contains a spurious warming shift at that time (Christy et al., 2007).

The second feature is the relative cooling of RSS vs. UAH since 2006. This can be explained by the fact UAH uses a spacecraft (NASA’s Aqua) that is not subject to orbital drifting, whereas RSS relies on NOAA-15, which is drifting into warmer diurnal times. The implication here is that RSS is overcorrecting for this spurious warming by reporting too much cooling. Overall, the shifts unique to RSS create a spurious warming in the record, which is being slowly mitigated by the more recent spurious cooling.

The graphs show that the temperature anomalies in the RSS dataset for November 2007 and December 2007 were below the 1979-1998 mean average for the first time since 2000. Both data series show the rate of warming has slowed dramatically during the past seven to 12 years. Between the end of 2007 and early 2009, global temperature anomalies fell even further, effectively returning the world to the temperatures that prevailed in the late 1980s and early 1990s. See Figure 3.5.3. below.

The new trend toward less warming has prompted some scientists to wonder if the world’s climate experienced a trend break in 2001-2002, similar to ones that occurred around 1910 (ending a cooling trend), the early 1940s (ending a warming trend), and the mid-1970s (ending a cooling trend). Swanson and Anastasios (2009), writing in Geophysical Research Letters, say “a break in the global mean temperature trend from the consistent warming over the 1976/77–2001/02 period may have occurred.” Moreover, the episodic nature of temperature changes during the past century is “difficult to reconcile with the presumed smooth evolution of anthropogenic greenhouse gas and aerosol radiative forcing with respect to time” and “suggests that an internal reorganization of the climate system may underlie such shifts.”

Noel Keenlyside, a scientist with Germany’s Leibniz Institute of Marine Science, writing with colleagues in a letter published in Nature, said “the climate of the North Atlantic region exhibits fluctuations on decadal timescales that have large societal consequences” and “these multidecadal variations are potentially predictable if the current state of the ocean is known” (Keenlyside et al., 2008). Using a database of sea-surface temperature (SST) observations, they “make the following forecast: over the next decade, the current Atlantic meridional overturning circulation will weaken to its long-term mean; moreover, North Atlantic SST and European and North American surface temperatures will cool slightly, whereas tropical Pacific SST will remain almost unchanged. Our results suggest that global surface temperature may not increase over the next decade, as natural climate variations in the North Atlantic and tropical Pacific temporarily offset the projected anthropogenic warming.”

We predict more predictions of this kind as more scientists recognize, first, that estimates of past warming have been exaggerated by reliance on surface-station data that have been discredited by physical observation and by testing against superior satellite data; second, that recent temperature trends contradict past and recent forecasts by the IPCC and other prominent advocates of the theory that temperatures will steadily rise in response to increasing forcing by rising CO2 levels in the atmosphere; and third, as attention turns to natural cycles like those modeled by Keenlyside et al., as most scientists have known all along are more influential than the small effects of rising CO2 in the atmosphere.


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Christy, J.R. 2009. Written testimony to House Ways and Means Committee. 25 February., last accessed May 10, 2009.

Christy, J.R. and Norris, W.B. 2006. Satellite and VIZ-radiosonde intercomparisons for diagnosis of nonclimatic influences. Journal of Atmospheric and Oceanic Technology 23: 1181-1194.

Christy, J.R. and Norris, W.B. 2009. Discontinuity issues with radiosondes and satellite temperatures in the Australia region 1979-2006. Journal of Atmospheric and Oceanic Technology 25: OI:10.1175/2008JTECHA1126.1.

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Climate Change Reconsidered: Website of the Nongovernmental International Panel on Climate Change.

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Randall, R.M. and Herman, B.M. 2008. Using limited time period trends as a means to determine attribution of discrepancies in microwave sounding unit derived tropospheric temperature time series. Journal of Geophysical Research: doi:10.1029/2007JD008864.

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Wentz, F.J. and Schabel, M. 1998. Effects of satellite orbital decay on MSU lower tropospheric temperature trends. Nature 394: 661-664.

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