The author discusses a small number of possible global catastrophes in a popular scientific way, namely global warming, ice ages, the impacts of debris from outer space, and the workings of the Earth itself.
To give you an idea of how this all fits together:
Due to the action of the earth, there are tectonic plates where volcanoes can occur with more probability. But now Humanity also has a strong share in its own demise: due to the ever-increasing industrialization, greenhouse gases accumulate in the upper layers of the atmosphere. In a nutshell, this is a major contributor to global warming. This can continue until the earth is a furnace where life dies out en masse.
However, this causes the polar ice caps to melt, causing the seawater to rise by up to 60m, depending on the climatic model. This happens due to a massive influx of water. This in itself can cause two things: the Gulf Stream changes or stops, causing today’s temperate climate to stop being temperate. Less pressure from ice on the polar ice caps causes changes in the Earth’s functioning with more force of the tectonic plates on each other elsewhere on Earth.
This creates substantially more volcanic action, which drives massive amounts of sulfur-containing gases into the upper layers of the atmosphere. These gases then reflect sunlight, which can trigger a new ice age.
In addition, the author also discusses the effects, forces, and probabilities of impacts of debris from space, such as asteroids and comets. These can also trigger tsunamis and earthquakes, indirectly causing a mass extinction of life.
All this shows that the earth is a dangerous place to live.
Is this a reason to be doom-mongering?
Not directly, according to the epilogue. In it, the author argues that before all this takes place, humanity may reduce itself to a minimum through the wars it will wage for the fertile lands, which will become scarcer as the climate becomes more unsustainable. Each nation will fight for the survival of its own people. As a result, the world’s population will probably be thrown back to a level of coexistence on a scale of villages. One could even argue whether these global catastrophes aren’t nature’s way of restoring the balance that has been upset throughout human evolution.
My personal positive note here is that if these wars for raw materials and food come to pass, they may not be nuclear wars. After all, that would make conquering the land unusable.