Climate has always been changing as a result of changes in solar activity, Earth's orbit around the Sun, atmospheric composition and volcanic activity.
Humans have over time exerted an increasingly important influence on the climate system. Early human activities affected the climate on a local to regional scale only.
With the industrial revolution, however, human activities began to alter the composition of the atmosphere thereby changing the Earth's climate on a global scale.

The atmospheric concentrations of so called long-lived GHGs are now far higher than at any time during the last 800 000 years. This is a result of the burning of fossil fuels, deforestation, and to a lesser extent the raising of cattle and the use of synthetic fertilisers.

The IPCC AR4 (IPCC, 2007) concludes with very high confidence that human activities have contributed to the warming of the global climate since at least 1750. It estimates that the total warming effect of human activities is at least 10 times larger than that of natural factors, in particular changes in solar activity.
The AR4 further concludes that the warmth since the mid 20th century is exceptional in at least the last 1 300 years, and that the observed rapid increase in global average temperatures since the mid-20th century is very likely due to the observed increase in GHG concentrations due to human activities.

In other words, humans have now become the dominating cause of changes in global climate on decadal and centennial time scales.

Providing the best available scientific information to decision-makers helps narrowing the range of possible future conditions that policies need to address.