recent climate changes

recent climate changes A climatic trend can be distinguished from a fluctuation, since trends are unidirectional and usually have a minimum duration, such as twenty years. The recent past is usually regarded as being within the period of instrumental observations, from which detailed information about climatic trends may be derived.

Long-term instrumental records of 200–300 years' duration are mostly restricted to Europe. Knowledge of the climate of the eighteenth century, when pioneer instrumental observations were being introduced, can be augmented by scrutiny of surviving weather diaries. For Britain, a data-series giving mean monthly temperatures for central England back to 1659 was calculated by Professor Gordon Manley in the 1970s. The series starts with several decades of warming associated with the end of the Little Ice Age, a trend that has resumed since the 1970s.

The construction of an aerial data-set as the central England temperature (CET) series involves reconciling an uneven spatial and temporal distribution of observations with the additional value of a long-period series. At the global scale, this is resolved by calculating temperatures at a regular pattern of grid-points. This requires the analysis of temperature observations made by shipping, from which sea surface temperature estimates can be derived, It is essential to make careful allowance for changes in observational practice over time, to avoid introducing non-climatic ‘noise’ into the series.

Global temperatures show an increasing trend from the mid-nineteenth century; the warmest year on record from 1854 to 1999 was 1998 (see Fig. 1 and global warming). This was punctuated by cooling in the 1950s and 1960s (mostly in the northern hemisphere), but warming has resumed at an unprecedented rate since the late 1970s in most (but not all) parts of the world. While this period has witnessed an undoubted strengthening of the greenhouse effect, the 0.5°C warming observed over the past hundred years is regarded as being just within the range of natural variation. A small part of this warming (estimated at 0.05 °C) has been attributed to an increased urban warming effect, but it is thought that the urban effect on the CET since the 1940s is roughly 0.1 °C.

An important diverging trend since the 1960s is the cooling of the North Atlantic and adjacent coastal land masses, such as Greenland, Iceland, and Scandinavia. This has resulted in increased temperature gradients between 50° and 65°N, linked with a recent upturn in the vigour of the westerly airstream over Europe, especially in winter. This has amplified the warming of winters in parts of Europe by encouraging the advection of mild, maritime air.

Variations in solar output have the potential to exert a more widespread influence upon temperature. While the Little Ice Age of the sixteenth to eighteenth centuries coincided with a reduction in solar activity (as revealed by the Maunder Minimum of sunspot observations), it is thought that any future diminution of solar irradiance would be insignificant in comparison with the radiative effects of the enhanced greenhouse effect.

It has been noted that night minimum temperatures are rising faster than day maximum temperatures, especially in the northern hemisphere. This is believed to be caused by the cooling effect of the backscattering of solar radiation in the troposphere by sulphate aerosols emitted from industrial sources. It is possible that this sulphate aerosol cooling has offset up to one-third of the greenhouse warming. Consequently, action to control sulphur emissions (in combating acid rain) might inadvertently hasten further global warming.

Sulphate particles can also affect temperature trends in the stratosphere as a result of volcanic eruptions. Major eruptions—such as El Chichón in Mexico in 1982 and Mount Pinatubo in the Philippines in 1991—can produce a sulphur-rich stratospheric dust cloud that can affect global temperatures for one to two years following an eruption. This can lead to a temporary reversal of warming, as in 1992 and 1993. Conversely, the periodic El Niño events of the equatorial Pacific can provide temporary warming. This arises from the weakening of the easterly trade winds, interrupting the upwelling of relatively cool, nutrient-rich water off the South American coast. Anomalous low pressure over South America (giving floods) and high pressure over the western Pacific can initiate drought in Australia and South-East Asia (the Southern Oscillation). An unusually persistent El Niño event began in 1991, raising speculation of a relationship with global warming.

Trends in rainfall generally have greater spatial and temporal variability than those of temperature. Since the end of the mid-twentieth century, rainfall over the low latitudes of the northern hemisphere has declined, most notably in the Sahel region of Africa. By contrast, high-latitude rainfall has increased (see Fig. 1). In the mid-high latitudes, rainfall is influenced by the vigour of the westerly circulation and depression tracks. Increased rainfall over north-west Britain since the late 1970s, especially in winter, is in marked contrast to a reduction in summer rainfall that has affected much of Europe over the same period. These changes are linked to a northerly shift of depression tracks, believed to be in response to warming, illustrating the links between temperature, rainfall, and the atmospheric circulation.

Julian Mayes

Bibliography

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