Climate change is one of the most important global environmental challenges facing humanity with implications for food production, natural ecosystems, fresh water supply, health, etc. According to the recent IPCC scientific assessment, the earth's climate system has demonstrably changed on both global and regional scales since the pre-industrial era (IPCC, 2001). Further, there is evidence to show that most of the warming (of 0.1°C per decade) observed over the last 50 years is attributable to human activities.
The ten warmest years since the recording of global surface temperature began 140 years ago have all occurred in the last 15 years. Of these, 1998 was the warmest year on record, followed by 1997, 1995, 1990 and 2000 (Figure 1.1). The global surface temperature in 2000 was 15.3 degrees Celsius (°C) or six-tenths of a degree above the long-term average. The warming of the earth in the 20th century is likely to have been more than in any century in the past 1000 years for the Northern Hemisphere. A higher global temperature is an important indicator that the earth's climate is changing. But, the cooling between 1998 and 1999 illustrates that even when there is a strong long-term warming trend, significant year-to-year changes in either direction can occur as a result of natural processes within the climate system.