When an inhabitant of one of the 22 megacities or nearly 13,000 large metropolises on Earth begins their day, their actions aren’t neutral in caring for the environment. Very small daily events such as taking a bicycle and pedaling to get to a local store where you can make daily purchases instead of using a car, powered by fossil fuels, will make a difference in the (context of the) general awareness of the world population regarding the danger of global warming that could grow by 4ºC by the year 2100.

Of course, there is a consensus reached by most scientists, although not all, about the implications of the productive activity of the human species and, also the major responsibilities for which profound changes in the climate that are being recorded can be defined.

But before getting to the current state of the situation, its projection, if nothing changes, and the metrics that will serve to raise awareness and bring about an improvement in conditions, it’s useful to review the reasons why UN officials formulated serious warnings.

Earth has encountered, throughout its history, a multitude of climatic changes with the common denominator that they have all been natural. The climate change that is observed and felt today is a product of anthropic origin, that is, it has been generated by human activity.

The model of production and consumption that has brought enormous material benefits in the past is producing a variation in the temperature level of surfaces and water masses, generating impacts on living organisms.
On average, the temperature increased 0.6ºC during the 20th century and the sea rose between 10 and 12 cm. due to melting.
 

The planet is getting warmer due to the action of greenhouse gases (GHG).
Life on Earth, impossible without the sun´s energy, is protected by a filter that is the atmosphere that allows about half of the radiation emitted by the star to enter. This energy, which heats the earth's surface, emits radiation in the form of hot, infrared waves, of which 90% is retained by the greenhouse gases that allow life as we know it.
However, when the concentration of these GHGs increases significantly and in a short time, we are in the presence of a climate change event.
Carbon dioxide (CO2), made up of carbon and oxygen, isn’t the only GHG, but it’s the most famous and the one with the worst press. For example, nitrous oxide or fluorinated gases absorb infrared radiation much more effectively, as does methane, which has a global warming potential 23 times greater than CO2.
However, the big cities of the world are the ones with the greatest presence of carbon, and they have become the main sources of emission of this gas. One hundred capitals in the world generate 18% of the total that has been measured in the last five years.