Atmospheric Temperatures

In addition to surface-based thermometer measurements, regular and widespread measurements of the vertical profile of atmospheric temperatures are available from both satellites and weather balloons for the last several decades. Weather balloons, which are launched twice per day from over 800 sites around the world, carry instruments known as radiosondes that directly measure atmospheric conditions and radio these data back to receiving stations. Although these measurements are taken primar-

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New York City daily average temperature for the first 6 months of 2009. Red and green arrows denote the beginning and end of two periods when temperatures declined on average, despite an overall warming trend due to the seasonal cycle. SOURCE: NCDC (2006).

a It should be noted, however, that there are some aspects of the climate system—such as global sea level rise due to the slow thermal expansion of the oceans (see Chapter 7)—that naturally tend to reflect longer-term changes in radiative forcing, and that short-term (e.g., decadal-scale) trends are important for identifying and studying the potential for "abrupt" climate changes, which are discussed later in the chapter.

ily to support weather prediction, researchers have developed methods for aggregating the data, removing a variety of systematic biases (including changes in instrumentation, the fact that all of the balloons are launched at the same two times each day, which means they are launched at different local times, and a recently identified bias associated with the sun heating the instruments) to yield a record of three-dimensional changes in atmospheric temperature over the last 50 years (McCarthy et al., 2008).

Annual J-D 2000-2009 L-0TI(°C) Anomaly vs 1951-1980 .51
FIGURE 6.13 Average surface temperature trends (degrees per decade) for the decade 2000-2009 relative to the 1950-1979 average. Warming was more pronounced at high latitudes, especially in the Northern Hemisphere, and over land areas. SOURCES: NASA GISS (2010; Hansen et al., 2006, with 2009 update).

Regular satellite-based observations of temperature and other atmospheric properties began in the late 1970s. Rather than directly sampling atmospheric conditions, satellites measure the upwelling radiation from the Earth at specific wavelengths, and this information can be used to infer the average temperature of different layers in the atmosphere underneath. As with surface temperature records, the raw satellite data are analyzed by several different research teams, each using its own techniques and assumptions, to produce estimates of inferred temperature changes (Christy et al., 2000, 2003; Mears and Wentz, 2005). While satellite-derived data offer the advantage of excellent global coverage, they still require corrections to remove artificial biases, such as the slow decay of satellite orbits and changes in instrumentation when satellites are replaced. The fact that satellite-inferred temperatures represent layers of the atmosphere rather than specific points in space also leads to some uncertainties in the analysis and interpretation of the data—for example, it was recently demonstrated that previous estimates of lower-atmosphere warming from satellites were biased slightly downward due to the inclusion of some data from the stratosphere, which has cooled (see next paragraph; also Fu et al., 2004). As discussed in Chapter 4 and in some of the other chapters in Part II, satellite data also offer a wealth of information about other changes in the Earth system.

Radiosonde and satellite-derived data both show that the troposphere (the lowest layer of the atmosphere, extending up to roughly 10 miles [16 km] in the tropics and 6 miles [10 km] near the poles) has warmed substantially over the past several decades (Figure 6.14). The most recent analyses of satellite data from 1979 through the end of 2009 estimate a tropospheric warming of +0.23°F (+0.13°C) per decade (Christy et al., 2000, 2003) to +0.28°F (+0.15°C) per decade (Mears and Wentz, 2005; RSS,2009), while radiosonde-derived temperature estimates yield +0.30°F (+0.17°C) per decade for the same time period and +0.29°F (+0.16°C) for the full radiosonde record starting in 1948 (HadAT2; McCarthy et al., 2008). For comparison, surface temperatures increased +0.29°F (+0.16°C) per decade since 1979 and +0.23°F (0.13°C) per decade since 1948.

Additionally, radiosondes and satellites both indicate that the stratosphere has cooled even more strongly than the troposphere has warmed (Figure 6.14, top panel).This

Global lower stratospheric anomalies from Jan 1958 to Mar 2010

Global lower stratospheric anomalies from Jan 1958 to Mar 2010

Year Jan 1958 Dec 1978

Global lower tropospheric and surface anomalies from Jan 1958 to Mar 2010

Global lower tropospheric and surface anomalies from Jan 1958 to Mar 2010

1960 1970 1980 1990 2000 2010 from from

Year Jan 1958 Dec 1978

HadAT2 radiosonde data and HadCRUT3 surface data are produced by the Hadley Centre and are available at www.hadobs.org

UAH MSU satellite data are produced by the University of Alabama in Huntsvllle and are available at www.nsstc.uah.edu/publlc/msu courtesy of John Christy and Roy Spencer RSS MSU satellite data are produced by Remote Sensing Systems and are available at www.remss.com courtesy of Carl Mears

1960 1970 1980 1990 2000 2010 from from

Year Jan 1958 Dec 1978

HadAT2 radiosonde data and HadCRUT3 surface data are produced by the Hadley Centre and are available at www.hadobs.org

UAH MSU satellite data are produced by the University of Alabama in Huntsvllle and are available at www.nsstc.uah.edu/publlc/msu courtesy of John Christy and Roy Spencer RSS MSU satellite data are produced by Remote Sensing Systems and are available at www.remss.com courtesy of Carl Mears

FIGURE 6.14 Radiosonde- (black) and satellite-based (blue and red) estimates of temperature anomalies for 1958-2009 in the (top) stratosphere and (bottom) troposphere. The squares on the right-hand side of the figure indicate the trends in each data series from two different start dates. SOURCE: Hadley Center (data available at http://hadobs.metofftce.com/hadat/images.html).

1970 1S60 1990 2Ü00 2010

FIGURE 6.15 Satellite-based trend of September (end of summer) Arctic sea ice extent for the period 1979 to 2009, expressed as percentage difference from 1979-2000 average sea ice extent (which was 7.0 million square miles). These data show substantial year-to-year variability, but a long-term decline in sea ice extent is clearly evident, as highlighted by the dashed linear trend line. As discussed in the text, the average thickness of Arctic sea ice has also declined markedly over the last 50 years. SOURCE: NSIDC (2010).

1970 1S60 1990 2Ü00 2010

FIGURE 6.15 Satellite-based trend of September (end of summer) Arctic sea ice extent for the period 1979 to 2009, expressed as percentage difference from 1979-2000 average sea ice extent (which was 7.0 million square miles). These data show substantial year-to-year variability, but a long-term decline in sea ice extent is clearly evident, as highlighted by the dashed linear trend line. As discussed in the text, the average thickness of Arctic sea ice has also declined markedly over the last 50 years. SOURCE: NSIDC (2010).

vertical pattern of temperature change, with warming in the troposphere and cooling in the upper atmosphere, is consistent with the pattern expected due to increasing GHG concentrations (Roble and Dickinson, 1989). Current research on temperature trends focuses on, among other issues, regional, seasonal, and day-night differences in temperature trends, especially in the tropics, where climate models predict a stronger warming in the upper troposphere than has been observed to date (e.g., Fu and Johanson, 2005).

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