Firstly, it is important to distinguish between weather (when we talk about conditions in a specific place at a specific time) and climate (which refers to long-term averages that can be regional or global). 

The climate is not something static. It varies over time. It has undergone major changes throughout the Earth’s history due to natural causes, but the current climate change is the result of human activity and the emission of greenhouse gases, which increase the atmosphere’s ability to retain heat.

The United Nations Framework Convention on Climate Change (UNFCCC), in its Article 1, defines climate change as a “change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods”. The UNFCCC therefore distinguishes between climate change attributable to human activities that alter atmospheric composition and climate variability attributable to natural causes.

• Because this is the first climate change caused by human activity, whereas previous ones were due to natural causes.
• Because it is the fastest one ever experienced. Previous climate changes took thousands of years to become evident, while the one we are currently facing has occurred at an unprecedented speed. Since the pre-industrial era, the Earth’s average global temperature has already risen by more than one degree, making it extremely difficult for natural systems to adapt.
• Because it is causing an increase in average temperature that has clear and serious effects on ecosystems and people.
• Because it is unfair and creates inequalities. Not all countries share the same responsibility for generating greenhouse gases, and not all have the same capacity to adapt. In fact, the countries most affected and with the greatest difficulties adapting to the changes are those that have contributed least to the problem. It therefore becomes an issue that goes beyond strictly environmental aspects, turning into a matter of ethics and responsibility.

According to the United Nations Framework Convention on Climate Change, climate change is directly or indirectly attributed to human activity through greenhouse gas emissions caused by the use of fossil fuels and changes in land use.

Greenhouse gases are naturally present in the atmosphere, and thanks to them, the Earth maintains a suitable temperature for life, around 15ºC (a phenomenon known as the greenhouse effect). However, human activity has increased their concentration to levels that endanger both natural systems and human societies. These gases include:

• Carbon dioxide (CO2): naturally produced by forest fires and volcanic eruptions. The current excess is due to the use of fossil fuels (oil, coal and natural gas) for energy and their use in vehicles, heating, thermal power stations, industry, etc. 
• Methane (CH4): generated as a result of agricultural and livestock activity. It is also emitted from landfills, wetlands and swamps. 
• Nitrous oxide (N2O): released from crop fertilisers, manure and livestock waste. 
• Gases directly originating from human activity: Chlorofluorocarbons (CFCs) from industry, used in air conditioning systems, aerosols, extinguishers, etc. and sulphur hexafluoride.
• Water vapour.