We have been a nomadic species for tens of thousands of years but, starting from the 1970s, all this has changed at a speed rate like never seen before.
In 1970 only 30% of the world population lived in cities, while the remaining 70% lived in rural areas. In less than 50 years - a nil fraction of time in geological terms - the percentage has reversed.
We have become a specialist species: we thrive only in the city. Specialist species, however, need a stable environment whereas our planet is quickly changing.
For this reason we have to change the way we think and build cities. Those of the future cannot be the ecologically incompatible 'monsters' of today.
Think of London: the ecological footprint of the capital of the United Kingdom is larger than the whole of Great Britain. That of Berlin greater than half of Germany.
Since we will not be able to reduce CO₂ emissions within the next century or so, the only solution is to plant billions of trees in and around cities, as also established by the recent G20.
To date, the machinery to fix CO₂ in the ground is a thousand times less efficient than plants, and a thousand times more expensive. If the trees were called 'carbon dioxide fixing columns' we would have already planted billions