The balance of evidence suggests a discernable human influence on global climate. However, any human induced effects on climate will take place on top of natural climatic variability, which may result from internal fluctuations and from external causes such as solar variability or volcanic eruptions. Scientific studies attempt to distinguish between human and natural influences on climate. In general, two types of scientific studies are carried out:
Detection studies – these set out to detect change and to demonstrate whether a change in climate is highly unusual in a statistical sense. These studies do not provide a reason for the change.
Attribution Studies – these studies set out to identify cause and effect relationships. i.e. they try to identify the reasons for climate change.

Since 1990, considerable progress has been made in attempts to distinguish between natural and anthropogenic influences on climate. The most important results include:

• Using indicators, the 20th century global mean temperature would appear to be at least as warm as any other century since 1400 AD. Data prior to 1400 AD is too sparse to allow the reliable estimation of global mean temperature.
  
• Assessments of the statistical significance of the observed global mean surface air temperature trend over the last century have used a variety of new estimates of internally and externally forced variability. Most of these studies have detected a significant change and show that the observed warming trend is unlikely to be entirely natural in origin.
  
• Recent and convincing evidence for the human effect on climate is emerging from ‘pattern based studies’. These studies model the climate response to forcing by greenhouse gases and human generated sulphate aerosols – and compares the results of the models with observed geographical, seasonal and vertical patterns of atmospheric temperature change. The results show that patterns of temperature change increase with time, as the anthropogenic ‘signals’ in the atmosphere increase. Statistically, it is very unlikely that these relationships could occur by chance as a result of natural internal variability only. The vertical patterns of change observed are also inconsistent with those expected for solar or volcanic forcing alone.
  
• The ability of scientists to quantify the human influence on global climate is still limited because of the difficulties of identifying changes which result from human climate influences from the background of natural climate variability. There are also significant uncertainties in key factors. These include: the magnitude and patterns of long-term natural variability; the patterns over time of forcing by changes in concentrations of greenhouse gases and aerosols (and the impacts these changes will have over time); and the impacts of land surface changes. Despite these uncertainties, the balance of evidence suggests that there is a discernable human influence on global climate.
  
  For more information on human impacts on the climate and the evidence arising from climate models - see the following UNFCCC Climate Change Tool Kit fact sheets:
  
  Has Climate Change already begun?
  How will the climate change?
  Evidence from Climate Models