The Varying Climate

The Intergovernmental Panel on Climate Change (IPCC, 2001a) has shown that prior to the 20th century Northern Hemisphere average surface air temperatures have varied in the order of 0.5 °C back to AD 1000. Various climate reconstructions indicate that slow cooling took place until the beginning of the 20th century. Subsequently, global-average surface air temperature increased by about 0.6 °C, with the 1990s being the warmest decade on record. The pattern of warming has been greatest over mid-latitude northern continents in the latter part of the century. At the same time the frequency of air frosts has decreased over many land areas, and there has been a drying in the tropics and subtropics. The late 20th century changes have been attributed to global warming because of increases in atmospheric greenhouse gas concentrations due to human activities.

Salinger (2005) has shown that beneath the longer-term trends there is decadal scale variability in the Pacific basin at least induced by the Interdecadal Pacific Oscillation (IPO). On interannual timescales ENSO causes much variability throughout many tropical and subtropical regions and some mid-latitude areas. The North Atlantic Oscillation (NAO) provides climate perturbations over Europe and northern Africa.

Projections from the IPCC (2001a, 2001b) indicate that in the course of the 21st century global-average surface temperatures are very likely to increase by 2 to 4.5 °C as greenhouse gas concentrations in the atmosphere increase. At the same time there will be changes in precipitation, and climate extremes such as hot days, heavy rainfall (causing floods) and drought are expected to increase in many areas.

The combinations of global warming, with decadal climate variability (IPO) and interannual fluctuations (ENSO, NAO) are expected to lead to a century of increasing climate variability and change. This could bring with it unknown climate surprises and their impacts, such as flooding from unmanageable catchments and inundation of land areas due to storm surges and sea level rise. In the perspective of human settlement and agriculture and forestry activities over the last 10,000 yr at least climate changes have been quite small, and certainly in the documented record for the last 1,000 yr small variations in global temperature have occurred compared with the glacial/interglacial changes. Although the changes of the past and present have stressed food and fibre production at times, the 21st century changes will be extremely challenging to agriculture and forestry. As the century progresses, the interannual and decadal phenomena will be superimposed on an unprecedented global warming trend. Together these will produce rapid climate change, increasing climate variability and more climate extremes. Thus agriculture and forestry will face unprecedented stresses in the 21st century. Some of these concerns were addressed in an earlier international agrometeorological workshop (Stigeter et al., 2000).

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