The meaning of 'climate change' is fairly straightforward—a clear, sustained change (over several decades or longer) in the components of climate, such as temperature, precipitation, atmospheric pressure, or winds. Such changes must constitute a clear trend, and be clearly distinguished from the small random variation in these parameters that takes place all the time. That is why climate change can only be determined after careful analysis of several decades of observations. In this context, it is important to understand clearly the difference between climate and weather. See Weather and climate—what’s the difference
Climate may change in a single region or across the whole planet. Throughout earth's history, climates have changed. The causes are various (see Why climates change). Change can be brought about by a variety of factors. These include natural external factors, such as changes in solar emission or slow changes in the earth's orbit; or natural internal processes of the climate or earth system such as volcanic activity; or, as has occurred recently, human-induced (anthropogenic) factors.
To help separate out the difference between human-induced and natural factors, the United Nations Framework Convention on Climate Change (UNFCCC) uses the term 'climate change' to refer to changes that can be attributed to human activity that has changed the composition of the atmosphere and, thereby, the functioning of the earth's climate system. The UNFCCC uses the term 'climate variability' to refer to natural alterations in the earth's climate.
Global warming (which is not considered a technical term) refers to an increase in the average temperature at the surface of the earth, or the lower part of the atmosphere. Most climatologists consider that the global warming that we are now experiencing is mainly the result of human actions changing the composition of the atmosphere. However, global warming and cooling have occurred naturally throughout the history of the earth, as a result of natural climate variability. Such changes in the past were usually much slower than the rate of warming that has occurred in the last few decades.
The increase in global temperatures measured over recent decades, if it continues, has the potential to seriously disrupt many of the environmental, economic and urban structures upon which human society depends. Whilst it is possible that some of this warming may have a natural cause, there is mounting evidence that human activity is responsible for most of the measured warming. The principal contributor to the present phase of global warming is considered to be the enhancement of the natural greenhouse effect.
Global surface warming is just one consequence of the changes to the climate being caused by human activity. The various components of the climate and earth system are inextricably linked through complex feedback mechanisms, and a change in one component such as temperature will induce changes and adjustments in other components. Other changes that have either already been observed or are projected to occur as a result of human activity include sea level rise; changes in rainfall patterns; increases in extreme weather events; decreases in ice mass of glaciers, ice sheets and sea ice; ocean warming and acidification; changes in ocean circulation; and drying of the land.
Department of Climate Change, Climate change science—questions answered.