Current Atmospheric Temperature Trends

In the following, we will concentrate on the primary, though by far not the only, climate parameter of importance: surface temperature. Direct observations, which will be central to our discussion here, date back to the 1860s, that is, the dawn of the so-called industrialized revolution. It was at this time that humans first started to expand their sphere of activity, mainly through the extensive use of natural resources. Among them were fossil fuels (oil, gas, and coal) that were needed to drive industrial production at a previously unknown magnitude. The observed trends in surface temperature change correspond to the increased utilization of fossil fuel for energy production (Intergovernmental Panel on Climate Change, 2001).

Despite the significant interannual variation in global mean yearly temperatures, a few general observations are clearly pertinent: the global average surface temperature has increased by 0.6±0.2°C since the late 19th century; the trends are somewhat unclear for the 1940s to the 1980s, but have remained increasing ever since; the rate of temperature increase for the periods 1910-1945 and 1976-2000 amounts to 0.15°C per decade; and the 1990s have been the warmest years on record for the last 140 years (see Figure 3).

An analysis of the spatial distribution of temperature changes over the period 1860-2000 reveals that the regional manifestation of change was different in the early parts of the 20th century compared to the last few decades (Intergovernmental Panel on Climate Change, 2001). The latter period is characterized by almost unanimous warming (except for year-round cooling in the northwestern North Atlantic and the central North Pacific Ocean) and reveals the largest increases in temperature for the mid- and high latitudes of the Northern Hemisphere. It has been shown that the recently observed patterns of temperature change can be—at least in part—related to various phases of atmosphere-ocean oscillations, such as the Atlantic-Arctic Oscillation (Baldwin et al., 2001; see Climate Oscillations)

The Arctic Oscillation phenomenon refers to opposing atmospheric pressure patterns in northern middle and high latitudes. The oscillation exhibits a

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Figure 3: Measured surface temperatures for the period 1860-2000 and combined for land and ocean surfaces; seasonal deviations relative to the average temperatures for 1961-1990 as well as two-standard error uncertainties (Intergovernmental Panel on Climate Change, 2001) are shown.

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Figure 3: Measured surface temperatures for the period 1860-2000 and combined for land and ocean surfaces; seasonal deviations relative to the average temperatures for 1961-1990 as well as two-standard error uncertainties (Intergovernmental Panel on Climate Change, 2001) are shown.

"negative phase" with relatively high pressure over the polar region and low pressure at midlatitudes (at about 45° N), and a "positive phase" in which the pattern is reversed. During the positive phase, higher pressures at midlatitudes drive ocean storms farther north, and changes in the circulation pattern bring wetter weather to Alaska, Scotland, and Scandinavia, as well as drier conditions to the western United States and the Mediterranean. In the positive phase, frigid winter air does not extend as far into the middle of North America as it would during the negative phase of the oscillation. This keeps much of the United States east of the Rocky Mountains warmer than normal, but leaves Greenland and Newfoundland colder than usual. Weather patterns in the negative phase are in general "opposite" to those of the positive phase.

An important observation relates to the heat content of the global ocean. Characterized by a larger thermal inertia, oceans react much slower to shifts in climate compared to the atmosphere. Thus, they exert a damping effect on climate shifts whether they are positive or negative. On the other hand, once the oceans start to change, it will help to maintain an initial shift in climate. The observations show that there has been a significant increase in oceanic heat content over the last 100-140 years, and that more than half of this increase has occurred in the upper 300 m of the ocean. This increase amounts to an equivalent rate of average temperature increase in the upper ocean of 0.04°C per decade. While significantly smaller than the atmospheric warming trend, it is of the same sign and thus supports the conclusion of a generally warmer Earth since the late 19th century.

Again, the question arises of whether this observation reflects primarily natural variations or an influence of human disturbance of the natural climate system. This will be addressed below.

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