The Beginning

Our planet has an history of 4.5 billion years during which he changed appearance and evolutionary conditions.
The so-called geological ages we briefly describe the characteristics of different stages of his change in the passage of time.
The first phase was incandescent and characterized by continuous volcanic eruptions.
In those primitive conditions the atmosphere prevailed in carbon dioxide and, in the absence of oxygen, was prohibitive for life.
Hundreds of millions of years after its cooling and the consequent changes in physical conditions - chemical environment, the Earth has allowed the development of the first forms of life that over time, with photosynthesis, have produced oxygen necessary for the development of the living complex bodies.
This gradual change, spacing with cataclysms from internal and external to the planet (glacial eras, catastrophic earthquakes caused by continental drift, volcanic eruptions, acid rain, fallen asteroids etc.) was instrumental in creating those special conditions for the selection and growth of living species that will colonize water and land.
Approximately 1.5 billion years ago a cyclical climate, that gave rise to major glaciations interspersed by periods warmer, was accentuated.
These various climatic conditions were due to periodic variations of the slope of the rotation axis of the Earth, to the change of several physical parameters such astronomical intrinsic eccentricity of the earth and the precession of perihelion.
Although modest, the variation in these values over time caused temperature changes due to a significant change in the solar radiation coming to the Earth.
Since the last glaciation, which ended about 10,000 years ago, began a strong climate change that led to the melting of ice that covered much of the northern hemisphere and that designed, with the lands released from ice, the current map of the planet.
In effect, the chemistry composition of the Earth has always been linked to climate, and in particular to the switch between glacial periods and tempered periods.
During the warmer periods this composition is changed, recording a quantity of greenhouse gases significantly higher than the cooler glacial eras.
These variations, in so far as they are naturally defined, have allowed the development of biology as we know it today.
The richness and diversity of life and its uniqueness in terms of every single living form are part of our daily experience.
The presence of greenhouse gases into the atmosphere, in the warmer periods that have followed over time, has always been limited, not exceeding never certain values.
In particular carbon dioxide is not never gone beyond 280 parts per million (280 molecules of carbon dioxide every million air molecules) and methane has never exceeded the value of 0.7 parts per million by volume.
The percentage of these gases in the atmosphere is very small compared to that of other gases such as nitrogen and oxygen, but also a modest variation of their percentage greatly affect the climate.
The hot stage that the Earth is experiencing is characterized by the prevalence of the “homo sapiens" species on all the other living species.
The life of this species, our life, is changing definitively the future history of the Earth planet.
If in fact we consider the values of carbon dioxide and methane being nowadays in the atmosphere, we see an increase of 30% of carbon dioxide and about twice as methane compared to natural emissions.
At a so short warmer period we have never recorded such high values (especially in such a "small" amount of time).
In the past, the growth of greenhouse gases in the warmer periods spacing glacial ages, increased over tens of thousands of years. Minimum disruptions of the general conditions of climate stability, referring to a specific period, generated climatic changes, even major, after several centuries.
In our age human activities during only a few generations have produced serious effects on atmospheric chemistry and composition, then on the climate, with an acceleration never occurred before.
The effects, the first small, very small effects we are already seeing them every year that passes and they are increasingly strong and violent. Unlike the past, they not wait thousands of years to manifest itself. But how has started all this kind of things?
Until about 10 thousand years ago our ancestors lived what nature gave them.
Hunting, fishing, fruit trees, roots, berries: all that was enough for them to live.
Then he began to develop agriculture that guaranteed certainty of food and with it the first organised families of several individuals who cooperated each other to ensure the whole group survival food.
To cultivate edible plant species our ancestors settled on the most favourable and, through irrigation, they increased productivity of land, ensuring, with the introduction of farming animals, stocks of food and remarkable workforce.
Human society began to organize and expand.
It took millions of years so that the human population to reach five million individuals, but only 10 thousand years to arrive, at the end of 1800, to a billion individuals and just over two hundred years to reach nowadays 6.5 billion.
If unpredictable dramatic episodes will not occur in the future, the human population should stabilize at around 10 billion individuals between the middle and the end of the century just begun.