Introduction

The climate of our planet is continually changing. At certain periods in its history, the Earth was hot and lush with forests, and at others, it was almost entirely covered by a thick layer of ice. At times, there have been crocodiles at the North Pole, and at others, ice at the equator. Over the course of millions of years, sudden rises in temperature caused by massive volcanic eruptions have been followed by slow cooling periods associated with the dance of the continents, moved by convective motions in the Earth's mantle. And all of this has been punctuated, every so often, by monstrous catastrophes that have swept away most of that epoch's living beings.

Over the last 3 million years, the climate has been dominated by the almost regular alternation between prolonged cold and dry glacial periods and shorter, hot and rainy interglacials, in response to slow variations in the Earth's orbit. The time in which we are currently living, the Holocene, began around 12,000 years ago when the ice retreated after the last glacial peak. Unlike many of the periods that preceded it, the Holocene has been characterized up until now by a stable climate with modest variations in temperature, something that has facilitated the birth of agriculture and the shift from nomadic hunter-gatherer populations to sedentary human societies.

The mechanisms that determine the continuous modifications in our climate are multiple and depend on both external forces - such as variations in solar luminosity, Earth's orbital characteristics and volcanic activity - and, most of all, on the many internal processes connecting the atmosphere, the oceans, the ice, the biosphere, the crust and the mantle. These are extremely complex phenomena in which the response is often disproportionate to the stimulus, and it can activate a myriad of feedback processes. They are associated to changes across all scales of time and space, with the amplification or reduction of the effects of external forcing factors. On top of these, the bond between organisms and environment is what makes the Earth a living planet whose evolution is inextricably linked to the action of the biosphere.

Among the many players involved, two play a central role. One is the composition of the atmosphere, in particular the concentration of greenhouse gases such as carbon dioxide, methane and water vapour. The other is the albedo of terrestrial surfaces and the clouds, or, rather, the quantity of solar energy that is reflected into space without entering into the planet's thermal machine. Alongside these principal actors, we also have the transport of heat and matter in the atmosphere and the ocean; the water cycle with its transformation of water into liquid, solid and vapour; and the biogeochemical cycles that remix and redistribute crucial elements such as carbon, phosphorus, nitrogen, iron and many others among the various components of the Earth System.

Over the last two centuries, a new factor has been added to these mechanisms of the climate's natural variability: human activity. This has reached planetary dimensions, emitting enormous quantities of carbon dioxide into the atmosphere and profoundly modifying the land, while eliminating an incredibly large number of natural ecosystems, deforesting the tropics, dramatically reducing terrestrial and marine biodiversity, and moving unimaginable quantities of sediment. All of these changes, in particular the growth in the concentration of carbon dioxide in the atmosphere, have led to a rapid increase in the average global temperature, which has risen by more than 1 degree centigrade over the last century. In some areas, such as the Arctic, the warming has been double that. And these changes in temperature have an impact on all of the climate's components - leading, for example, to an intensification of drought conditions in the Mediterranean basin and higher rainfall in Northern Europe.

Of course, an increase in temperature of almost 1 degree is negligible when compared to the much more conspicuous variations of the past. Some 50 million years ago, the average global temperature was probably 10 degrees higher than today. So, what are we worrying about?

Essentially, two aspects. First, there is the fact that the temperature increase of the last fifty years has been extremely fast, probably faster than has ever occurred in the past, making it more difficult for the environment to adapt to the new conditions. And there is the not-insignificant fact that, as of 2022, almost 8 billion people inhabit the Earth within fixed infrastructures and megalopolises that grow ever larger, often along densely populated coastlines that are the site of intense production activity. This is a population that needs water, food, energy. We are a highly technological society, but one that is very vulnerable to environmental changes. If the mercury continues to climb, there will be more droughts and consequent famines but also, paradoxically, increasingly violent floods, more extensive fires, a rise in sea level and more intense coastal erosion events. Some ecosystems could even collapse, and there will be difficulties in agricultural production. And, presumably, there will also be mass migration, social and economic instability and an even greater risk of war. If, by the end of the century, the temperature has increased by 4 °C compared to 100 years ago, as is expected if we continue to behave in the way we have done so far, the world will not be an easy place to live.

In order to understand fully what is happening, to put it in perspective and attempt to resolve the problems we ourselves have created, we must not deny the factual evidence - global warming is caused by human activity - and nor should we scream that this is the end of the world. Instead, it is necessary to understand how the climate functions, how it has changed and why, what is happening now, what the risks are, and what the past, even the remote past, can teach us. And how we can attempt to predict the future, and what strategies we can use to tackle the climate crisis we are living in. In short, we need to apply our intelligence and capacity for analysis. This book aims to make a small contribution in this regard, discussing what we have understood about the fascinating and complex system that is the Earth's climate, and what remains unclear. It does so by following a path that begins in the darkness of time, in the era when our planet was formed, through conditions so very different from those we see today that they bring to mind alien worlds - all the while, trying to understand why things have happened in a particular way.

We will journey through the history of the climate of the Earth, investigating the mechanisms by which it functions, exploring the most extreme conditions of the past and the great instabilities that have changed the world, finally arriving at the last century, at the cumbersome presence of humanity and the effects caused by the rise in the concentration of carbon dioxide in our atmosphere and oceans. We will do this in order to compare what is happening today with what we have learned from the Earth's past, and to use our knowledge of previous disasters to avoid (where possible) new ones.

The knowledge and capacity to forecast that come from this cannot, however, remain an end in themselves. These things must inform those taking decisions and be used to resolve real problems. At this moment, one of the issues we must face up to is how to combat anthropogenic climate change, as well as the need to stop the loss of biodiversity and fight poverty the world over. These three aspects are strongly connected to one another and involve the health of human beings and ecosystems alike, as well as the availability of water of sufficient quantity and quality for both human beings and the natural environment. In the final chapters of this book, we will consider some of these aspects with the knowledge that the solution is not a return to a golden age that never actually existed, but by taking as a starting point those technologies and that development which created the problem and using them to resolve it. We need to imagine and implement innovative approaches, even more advanced technologies, societies that are less delusional and more sustainable, working with nature and not against it. Preferring utopia to dystopia and trying to create it in reality, at least as far as possible. Not so much to save the planet, which will do just fine on its own and probably better without us, but to save ourselves.

I would like to thank everyone who has helped me to write this book. I am grateful to Giò and Maria for their support during the months of writing. Thank you to Marco Ferrari, Silvia Giamberini, Elisa Palazzi and Maddalena Pennisi, who had the patience to read everything I wrote and have always given me encouragement, advice and relevant suggestions. Thank you to Chiara Boschi, Andrea Dini, Gianfranco Di Vincenzo and Eleonora Regattieri, who provided me with precious guidance on those chapters closest to their own scientific expertise. A special thank you goes to Laura Grandi, who pushed me to take on this task and followed it throughout each of its phases, putting up with my delays and providing me with important advice. Thank you also to the Festival della Mente in Sarzana, Italy, where the idea for this book was born. Finally, sincere thanks to Elise Heslinga of Polity Press, who gently helped me throughout the preparation of the English version of this book and firmly reminded me of deadlines; to Alice Kilgarriff, who translated the book from Italian and with whom I worked...