Reconciling Observations of Global Temperature Change (2000) / Chapter Skim
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2 Background
2 Background
Pages 9-20
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From page 9... ...
The exact amount of estimated warming depends upon which of the existing compilations of the data is used as a basis for the calculation, the method used to estimate global means on the basis of irregularly spaced station observations, and the way in which the data are smoothed in time. Such globally averaged time series are not necessarily representative of local conditions: for example, Canada and Siberia have warmed much more rapidly during the past 20 years than indicated in Figure 2.1, while parts of the high latitude North Atlantic and North Pacific regions have cooled slightly.
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From page 10... ...
, and by satellite measurements of microwave radiation emitted by oxygen gas in the lower to mid-troposphere, taken with an instrument known as the Microwave Sounding Unit (MSU) .5 The balloon measurements are taken at the same Greenwich mean times each day, whereas the times of day of the satellite measurements for a given location drift slowly with changes in the satellite orbits.
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From page 11... ...
Figure 2.3 shows three time series of gIobal-mean temperature anomalies. The black curve represents surface temperature, and the colored curves represent the temperature of the lower to mid-troposphere as inferred from MSU measurements (red)
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From page 13... ...
The first season is Mareh-May 1979, and the last season is September-November 1999 for the MSU data set and December 1998-February 1999 for the other two data sets.
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From page 14... ...
Since this phenomenon first became apparent in the early 1990s, the research community has been seeking to identify and quantify possible sources of errors in the surface and upper air temperature measurements, and it has been trying to understand the physical processes that may have caused surface and upper air temperatures to change relative to one another. A number of biases in the data sets have been identified and corrected, and the process of refining the data sets is continuing.
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From page 15... ...
As growing metropolitan areas encroach into the surroundings of formerly rural observing stations, the temperatures at these stations rise, particularly at night, in response to the welldocumented "urban heat island effect." Some have suggested that much of the observed rise in global surface temperature during the twentieth century might be merely the expression of such local environmental transformations that are real, but not necessarily a signature of the global warming predicted to be associated with an increase in atmospheric greenhouse gas concentrations. These concerns have been addressed in numerous studies over the years that have sought to quantify the effect of land use changes and adjust the estimated global surface temperature
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From page 16... ...
to sample the strongest local temperature anomalies over the subtropical eastern Pacific shown in Figure 2.2. Even in the absence of any real temperature variation, the global-mean temperature anomaly computed from radiosonde data could conceivably change Dom one season or decade to the next, merely as a result of stations in one of these poorly sampled regions going into or out of operation.
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From page 17... ...
Global Observing System upper air network. Fewer than half of these 905 stations report monthly climatic data, and only about two-thirds have reported regularly over the past few decades.
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From page 18... ...
sea surface temperature data set (very small dots)
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From page 19... ...
simulations suggest that such natural variability can still amount to an appreciable fraction of the observed disparity between the gIobal-mean temperature trends at the earth's surface and in the lower to mid troposphere. Because 20-year trends can be substantially influenced by just a few single or multi-year "warm" or "cold" events, they are not necessarily representative of the true response of the climate system to the more gradual changes .
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From page 20... ...
Despite the many unresolved issues touched on in this chapter and discussed in more detail in chapters 5-9, the progress that has been achieved over the past few years provides a basis for drawing some tentative conclusions concerning the nature of the observed differences between surface and upper air temperature trends, and their implications for the detection and attribution of global climate change.
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