The climate system is a complex, interactive system consisting of the atmosphere, land surface, snow and ice, oceans and other bodies of water, and living things. The basic greenhouse gas phenomenon ie., warming up of the earth’s surface due to re-emittance of infrared energy that is trapped in the atmosphere by greenhouse gases and clouds is scientifically sound and well accepted.  As climate science and the understanding of the Earth’s climate have continued to evolve over recent decades, increasing evidence of anthropogenic influences on climate change has been found. Human activities—primarily burning of fossil fuels, deforestation and other land-use changes—are modifying the concentration of atmospheric constituents or properties of the Earth’s surface that absorb or scatter radiant energy. In particular, increases in the concentrations of greenhouse gases (GHGs) -- CO_2, H₂O, CH₄, N₂O and CFCs -- and aerosols are strongly implicated as contributors to climatic changes observed during the 20th century and are expected to contribute to further changes in climate in the 21st century and beyond.  According to the WMO Statement on the Status of the Global Climate in 2010, average global temperatures were estimated to be 0.53°C ± 0.09°C above the 1961–1990 annual average of 14°C. This makes 2010 tied for warmest year on record in records dating back to 1880.   The decade 2001–2010 was also the warmest on record. Recent warming has been especially strong in Africa. Temperatures for the 2001–2010 decade averaged 0.85°C above normal, 0.49°C warmer than any previous decade, and the five hottest years on record for the continent have all occurred since 2003.

In some regions, there are changes in the frequency and intensity of extreme events such as high intensity rainfall, tropical storms, high winds, extreme temperatures and droughts.  Agricultural productivity in tropical Asia is sensitive not only to temperature increases, but also to changes in the nature and characteristics of monsoon.  In the semi-arid tropics of Africa, which are already having difficulty coping with environmental stress, climate change resulting in increased frequencies of drought poses the greatest risk to agriculture.  In Latin America, agriculture and water resources are most affected through the impact of extreme temperatures and changes in rainfall. 

WMO’s Commission for Agricultural Meteorology (CAgM) as well as WMO’s Regional Associations have been actively addressing the issue of climate variability, climate change and agriculture over the past three decades through the establishment of working groups and rapporteurs and production of reports by experts from different regions of the world.  The Agricultural Meteorology Programme of WMO has been organizing international workshops and seminars on climate change and agriculture in different regions of the world such as South Asia, West Africa and has been promoting the development of regional adaptation frameworks to climate change in agriculture.  The Global Atmosphere Watch (GAW) programme of WMO, a partnership involving 80 countries, provides reliable scientific data and information on the chemical composition of the atmosphere, its natural and anthropogenic change, and helps to improve the understanding of interactions between the atmosphere, the oceans and the biosphere.   Understanding and predicting climate variability and change requires comprehensive investigation of all major components of the climate system (the atmosphere, hydrosphere, oceans, land and cryosphere). The World Climate Research Programme, which is co-sponsored by WMO, studies these components and their interactions through the activities of its Core Projects.  These are illustrated with suitable examples.