Climate variability and change have gone on throughout time, and on geological time scales of millions of years the climate system has undergone large changes as the earth has evolved. During the last glacial maximum, approximately 20,000 yr ago global temperatures were 5 to 6 °C less than those at the beginning of the 21st century (IPCC, 2001a). Even though temperatures have increased by this amount, the rise occurred over thousands of years, and stabilized into the modern climate regime about 10,000 yr ago.

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. Hydrological variations may have been larger though. The projected global mean temperature trends for the 21st century are without precedent, with rates expected of between 2 and 4.5 °C for the century.

Underneath the strands of 21st century climate warming due to anthropogenic activities, interdecadal and interannual climate variability will continue and possibly increase. Decadal climate variability, as induced by the IPO, will continue throughout the Pacific basin. Both will impact on regional and global climate and hence temperature and rainfall.

Interannual climate variabilities, particularly that caused by ENSO and NAO are expected to continue and possibly increase throughout this century. In fact global warming has been identified to lead to greater extremes of drying and heavy rainfall and increases the risk of droughts and floods that occur with ENSO events in many different regions, thus increasing climate variability from these sources.

As well, extremes are expected to increase (Table I). More hot days are expected over nearly all land areas and more intense precipitation events over many Northern Hemisphere mid- to high-latitude land areas. Increased summer continental drying and associated drought risk is likely in a few areas and peak wind and precipitation intensities in tropical cyclones are assessed to increase in some areas.

Climates of the globe have always varied over the last millennium, because of natural phenomena such as ENSO and the IPO. However, as the century progresses the interannual and decadal phenomenon will be superimposed on an unprecedented global warming trend. Together these will produce rapid climate change, increasing climate variability and climate extremes. Thus agriculture and forestry will face unprecedented challenges in the 21st century.

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