17
370
360
350
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330
320
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60
65
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Figure 11: Monthly averaged C02 concentrations in parts per million of dry air at the
Mauna Loa Observatory, Hawaii.
Hence, the recovery of periodic or quasi-periodic modes in singular spectrum analysis, which are the EOF's derived from an ensemble of segments
of an extended time series, does not necessarily imply that periodicities or
quasi-periodicities are present. It might merely be an indication that the
time series under investigation resembles 'red noise'.
2.4 Trends
Decadal-to-century scale trends in the climate record constitute a particularly important class of variability. Figure 11 shows a familiar example:
the upward trend in carbon dioxide concentrations at Mauna Loa. Also
evident in this record is a robust annual cycle believed to be associated
with the uptake of carbon dioxide by the terrestrial biosphere during the
Northern Hemisphere growing season. What makes the trend so obvious
in this particular example is the clear indication of an enormous 'spectral
gap' between the annual cycle and century scale variability.
The record of Northern Hemispheric-mean surface air temperature anomalies shown in the bottom panel Fig. 2 of chapter 2 , exhibits distinctive
variability on the interdecadal-to-century time scale. The spectral gap is
not as obvious as in the previous example, but the evidence of warming in