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Many observations indicate that global warming has taken place during the 20th century. Global warming has resulted in an increase of average surface temperature, a decrease in snow cover and ice extent and a rise of the sea level. Moreover, global warming affects precipitation, cloud cover and extreme temperatures. Human activities are increasing significantly & hence the concentrations of greenhouse gases, mainly CO₂ in the atmosphere, resulting in global warming.

How is climate changing and how has it changed in the past?

The warming of global climate is now undeniable. There are many observations of increasing air and ocean temperatures, widespread melting of snow and ice, and rising sea levels. More specifically, eleven of the last twelve years (1995-2006) rank among the 12 warmest years ever recorded since global surface temperatures are measured (1850). Over the last 100 years (1906–2005), global temperature has increased by 0.74°C. Global sea level has risen by 17 cm during the 20^th century, in part because of the melting of snow and ice from many mountains and in the polar regions. More regional changes have also been observed, including changes in Arctic temperatures and ice, ocean salinity, wind patterns, droughts, precipitations, frequency of heat waves and intensity of tropical cyclones.

What is causing the present-day changes in climate? 

It is very likely that a significant part of temperature variability in the Northern hemisphere during the past seven centuries prior to 1950 is due to volcanic eruptions and changes in the intensity of solar radiation. However, most of the observed increase in global temperature since then is very likely due to the observed increase in atmospheric greenhouse gas concentrations due to human activities. Human activities now clearly affect other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns.It is likely that these increases in greenhouse gas concentrations alone would have caused more warming than observed, had volcanic and human-induced aerosols not offset some of the warming that would otherwise have taken place

 What impacts are expected in the future?

Over the course of the 21^st century, many impacts are expected to occur in natural systems. For instance, changes in precipitation and the melting of ice and snow are expected to increase flood risks in some areas while causing droughts in others. If there is significant warming the capacity of ecosystems to adapt will be exceeded, with negative consequences such as an increased risk of extinction of species.

The most vulnerable people are in general the poor, since they have less capacity to adapt, and their livelihoods are often dependent on resources that are linked to climate. Africa is found to be particularly vulnerable to climate change, because of existing pressures on its ecosystems and its low capacity to adapt. On all continents, water supply and the threat to coastal areas will be an issue. Overall future impacts are expected to be negative, although some positive effects are also expected initially, such as an increase in agricultural productivity at high latitudes accompanying a moderate warming, or decreased heating needs in cold regions.

Impacts will depend on the magnitude of the temperature increase. For instance, some crops at mid- to high latitudes will have higher productivity if local temperature increases by 1-3 °C, but will be negatively affected beyond that.

If higher temperatures persist after the 21^st century it could result in very large impacts. For instance, the large sea-level rise that would result from the melting of the Greenland and Antarctic ice sheets would have major repercussions on coastal areas. The cost associated with the effects of climate change is projected to increase over time with rising temperatures.