Climate changes happen at all times.
Climate change refers to any significant change in measures of climate (such as temperature, precipitation or wind) lasting for an extended period (decades or longer). Past changes are not unusual as they were of natural origin. But most of the changes within the last 50 years were very unusual, as they were caused by human activities which discharge carbon dioxide and other greenhouse gases at an alarmingly faster rate.
The rise is abnormal - it is the sum of both natural and man-made contributions.
 

Scientists warned recently that climate change is happening more rapidly than previously calculated and said the Earth could be in danger of major climatic changes that would trigger widespread social disruption.

Under a business as usual scenario (BAU), that is, if we do not take extra mitigating efforts, the greenhouse gas concentration could be trebled by end of the century, causing a possible temperature rise of 5^oC. The impacts then would possibly be beyond human control.

Scientific evidences are becoming more concerning. Emissions growth rates are at the upper boundaries of the IPCC's worst case scenarios, Arctic sea ice loss is faster than forecast, melting of the ice sheet in Greenland and Antarctica is accelerating, sea level rise is at the upper ends of IPCC's projections, and additional non-linear risks have been revealed that could make certain climate changes irreversible.

The risks from climate change are large and growing. We need to contain warming to below 2^oC. Predicted warming in excess of 2 degrees Celsius on pre-industrial levels would constitute dangerous climate change and is likely to have disastrous impacts on ecological and human systems. Without immediate action, mankind may loose the ability or methodology to contain warming to below 2^oC.

Even if warming is contained to below 2^oC, significant climate change is already unavoidable. We are locked into at least 50 years of climate change which we cannot now prevent because of historic greenhouse gas emissions and the time these gases will remain in the atmosphere.

It is clear that to prevent the most serious and irreversible effects of climate change, the world must come to a common consensus and act swiftly to substantially cut emissions.
The costs of strong and urgent action will be less than the costs of inaction.