The period since the mid-nineteenth century is a period of instrumental meteorological observations. Since this time, these instrumental observations become routine and the number of stations steadily increases until the late twentieth century. In addition, measurements from commercial ships and from the island stations cab are used for the oceanic regions.
In the last 150 years, humankind has exerted an unprecedented influence on the environment. During this period, emissions of the main anthropogenic greenhouse gas, CO2, amount to about 420 GtC (in carbon units). These emissions have led to the growth of the concentration of carbon dioxide in the atmosphere from about 275285 ppmv (this value was quite stable since the beginning of the Holocene) to about 370 ppmv in year 2000 (and about 380 ppmv in 2005). Concentrations of other greenhouse gases have increased as well, for example, for CH4 from about 830 to 1760 ppbv in year 2000. Atmospheric burdens of the sulfate aerosol also have increased several folds (to 0.52 MtS in year 2000). In addition, natural forcing has contributed significantly to the total external forcing of the climate system. In particular, linear increase of the total solar irradiance in 1900-2000 has amounted 1.6 W m- , and volcanic forcing was temporarily inhomo-geneous during the century.
According to the analysis of the blended land air/ marine surface records, on the whole, temperature has increased during the course of the twentieth century by about 0.6 °C. Generally, warming is more manifest in winter than in summer, over land than over ocean, and in the higher latitudes than in the tropics. In addition, decreases of the surface air temperature diurnal range have been noted to be typical over land. For the period 1961-90, the annual mean in globally averaged surface air temperature is estimated to be 14.0 °C (14.6 and 13.4 °C in the Northern and Southern Hemispheres, respectively).
The twentieth-century temperature change was not monotonic. Three main periods may be distinguished. During the 1900s to the early 1940s, global blended temperature has increased by 0.4-0.5 °C. Since this period, climate was cooling until the second half of the 1970s, with a total temperature decrease about 0.1-0.2 °C. Afterward, global temperature rose almost monotonically until the end of the twentieth century by about 0.3-0.4 °C. The last decade of the twentieth century was the warmest during the period of the instrumental meteorological measurements, and this warming continues in the early twenty-first century. Linear trend of warming increases steadily during the last decades.
Possible causes of coldness in the middle of the twentieth century are related to the enhanced volcanism experienced by the Earth during this period. In some studies, the mid-twentieth century cooling is related to the dynamical mechanisms associated with interdecadal climate variability.
The most prominent mode of internal interannual variability, the El Nino/Southern Oscillation (ENSO), is manifested by the recurrent warm and cold events experienced by the eastern equatorial Pacific with strong impacts on the climate in remote regions. This mode has intensified during the second part of the century, with the strongest warm events occurring in 1982-83 and 1997-98. Short-term external forcing has also contributed to the corresponding interannual variability. In particular, after the strong eruption of the Mt. Pinatubo in Indonesia in 1991, the estimated annual mean cooling in the lower troposphere was about 0.7-0.8 °C.
Along with the trends in the mean characteristics of the climate, changes in other climate statistics are noted for the twentieth century and for the beginning of the twenty-first century, for example, in those related to the climate extremes. The latter are defined as unusual climate or weather events (strong cold spells in winter and heat waves in summer, heavy rains, strong droughts, floods, etc.). While the data for these extreme statistics are inherently limited, it can be inferred that heavy rains become more frequent in different parts of the world. The number of extremely cold nights has diminished, and the number of extremely warm days has increased. One of the strongest heat waves occurred in the summer of 2003 in Europe.
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