Figure 2.9: (a) to (d) Annual surface temperature trends for the periods 1901 to 2000, 1910 to 1945, 1946 to 1975, and 1976 to 2000, respectively (°C/decade), calculated from combined land-surface air and sea surface temperatures adapted from Jones et al. (2001). The red, blue and green circles indicate areas with positive trends, negative trends and little or no trend respectively. The size of each circle reflects the size of the trend that it represents. Trends were calculated from annually averaged gridded anomalies with the requirement that annual anomalies include a minimum of 10 months of data. For the period 1901 to 2000, trends were calculated only for those grid boxes containing annual anomalies in at least 66 of the 100 years. The minimum number of years required for the shorter time periods (1910 to 1945, 1946 to 1975, and 1976 to 2000) was 24, 20, and 16 years, respectively.
Figure 2.10: (a) to (d) Seasonal surface temperature trends for the period 1976 to 2000 (°C/decade), calculated from combined land-surface air and sea surface temperatures adapted from Jones et al. (2001). The red, blue and green circles indicate areas with positive trends, negative trends and little or no trend respectively. The size of each circle reflects the size of the trend that it represents. Trends were calculated from seasonally averaged gridded anomalies with the requirement that the calculation of seasonal anomalies should include all three months. Trends were calculated only for those grid boxes containing seasonal anomalies in at least 16 of the 24 years.
Most of the warming in the 20th century occurred in two
distinct periods separated by several decades of little overall globally averaged
change, as objectively identified by Karl et al. (2000) and discussed in IPCC
(1990, 1992, 1996) and several references quoted therein. Figures
2.9 and 2.10 highlight the worldwide behaviour
of temperature change in the three periods. These linear trends have been calculated
from the Jones et al. (2001) gridded combination of UKMO SST and CRU land-surface
air temperature, from which the trends in Table 2.2
were calculated. Optimum averaging has not been used for Figures
2.9 and 2.10, and only trends for grid boxes
where reasonably complete time-series of data exist are shown. The periods chosen
are 1910 to 1945 (first warming period), 1946 to 1975 (period of little global
temperature change), 1976 to 2000 (second warming period, where all four seasons
are shown in Figure 2.10) and the 20th century,
1901 to 2000. It can be seen that there is a high degree of local consistency
between the SST and land air temperature across the land-ocean boundary, noting
that the corrections to SST (Folland and Parker, 1995) are independent of the
land data. The consistency with which this should be true locally is not known
physically, but is consistent with the similarity of larger-scale coastal land
and ocean surface temperature anomalies on decadal time-scales found by Parker
et al. (1995). The warming observed in the period from 1910 to 1945 was greatest
in the Northern Hemisphere high latitudes, as discussed in Parker et al. (1994).
By contrast, the period from 1946 to 1975 shows widespread cooling in the Northern
Hemisphere relative to much of the Southern, consistent with Tables
2.1 and 2.2 and Parker et al. (1994). Much of
the cooling was seen in the Northern Hemisphere regions that showed most warming
in 1910 to 1945 (Figure 2.9 and Parker et al., 1994).
In accord with the results in the SAR, recent warming (1976 to 2000) has been
greatest over the mid-latitude Northern Hemisphere continents in winter. However,
the updated data shows only very limited areas of year-round cooling in the
north-west North Atlantic and mid-latitude North Pacific. Over 1901 to 2000
as a whole, noting the strong consistency across the land-ocean boundary, most
warming is observed over mid- and high latitude Asia and parts of western Canada.
The only large areas of observed cooling are just south and east of Greenland
and in a few scattered continental regions in the tropics and sub-tropics.
Faster warming of the land-surface temperature than the ocean surface temperature in the last two decades, evident in Figure 2.6, could in part be a signal of anthropogenic warming (see Chapters 9 and 12). However, a component, at least in the Northern Hemisphere north of 40 to 45°N, may result from the sharp increase in the positive phase of the winter half year North Atlantic Oscillation (NAO)/Arctic Oscillation (AO) since about 1970 (Section 2.6.5), though this itself might have an anthropogenic component (Chapter 7). There has also been a strong bias to the warm phase of El Niño since about 1976 (Section 2.6.2). In particular, Hurrell and van Loon (1997) and Thompson et al. (2000a) show that the positive phase of the NAO advects additional warm air over extra-tropical Eurasia north of about 45°N. The positive phase of the NAO or AO is therefore likely to be a major cause of the winter half-year warming in Siberia and northern Europe in Figure 2.10, as also quantified by Hurrell (1996). Cooling over the western North Atlantic Ocean also occurs, partly due to advection of cold air in an enhanced north to north-west airflow. Hurrell (1996) also shows that the warm phase of El Niño is associated with widespread extra-tropical continental warming, particularly over North America and parts of Siberia, with cooling over the North Pacific Ocean. Both effects are consistent with the strong warming over Siberia in winter in 1976 to 2000 (Figure 2.10), warming over much of North America and cooling over the Davis Strait region. Note that some regional details of the seasonal trends for 1976 to 2000 in Figure 2.10 may be sensitive to small changes in record length. A test for the shorter period 1980 to 1997 showed the same general worldwide pattern of (generally somewhat reduced) seasonal warming trends as in Figure 2.10, but with some regional changes, particularly over North America, almost certainly related to atmospheric circulation fluctuations. However, Siberian trends were considerably more robust.
We conclude that in the 20th century we have seen a consistent large-scale
warming of the land and ocean surface. Some regional details can be explained
from accompanying atmospheric circulation changes.
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