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10 July 2019The climate of the Earth is a complex system, characterized by many components (atmosphere, hydrosphere, geosphere, cryosphere, biosphere) that interact with each other on all scales of time and space. The planetary climate is subject to time-varying forces, such as solar energy (without which there would be no "climate" to discuss), the characteristics of the earth's orbit, large volcanic eruptions, the impact of asteroids, convection in the mantle, plate tectonics. Furthermore, the climate components interact with each other in a non-linear way, ie without a direct proportionality between changes in forcing and the response of the climate system, and are able to generate climatic variability even in the absence of changes in external forcing.For the continuous game between the variations of the forcing and the dynamic response of the internal components, the terrestrial climate system has shown throughout its history great variations, such as the alternation between glacial and interglacial periods in the last three million years, the very hot period of the Cretaceous between 90 and 120 million years ago, or, even farther back in time, the episodes of "Snowball Earth" with almost total ice cover. Despite these variations, the Earth's climate has always been characterized by a temperature range that has allowed the uninterrupted presence of life for at least 3 and a half billion years ago. Nevertheless, over time, many dominant species have become extinct, often because they have not been able to adapt to changing environmental conditions. It is important to underline that all this information was obtained from data and measurements, from paleoclimatic and environmental reconstructions, from geochemical and isotopic analyzes, which provided a fascinating, though still incomplete, picture of the history of the Earth's climate. In particular, the ocean ecosystem is the oldest and largest of our planet and provides much essential information to understand what happened in the past and what is happening in these years.
One of the important parameters in the control of global temperature, and of the climate in general, is the quantity of greenhouse gases in the atmosphere: carbon dioxide (CO2), methane and, indirectly, in response to the variations of the first two, the vapor d ' water. Without the greenhouse effect, the Earth would be much colder and substantially unsuitable for the complex and luxuriant life we know, as on Mars. With an excessive greenhouse effect, generated by a concentration of CO2 in the atmosphere that exceeds a critical value, the temperatures at the surface of the planet would become much higher, up to extreme situations, as happened on Venus (fortunately, even in the longer periods) hot our planet has never reached this critical threshold of atmospheric CO2). The recent volume on Geochemical Perspectives by Wally Broecker, unfortunately recently deceased, illustrates with great effectiveness the role of CO2 in the dynamics of the earth's climate, from the Archean to the present. Although certainly not the only determinant of climate, the composition of the atmosphere - and in particular the concentration of greenhouse gases - emerges from all the data, observations and measurements as a fundamental element of the planetary climate. In this context, the ocean has always played a fundamental role, both as a climate mitigator and for its ability to sequester CO2 from the atmosphere.
In addition to the natural variability, and to that induced by the changes of external forcing, in the last 150 years a new factor has been added, linked to human activities. The use of fossil fuels has strongly increased the concentration of CO2 and other greenhouse gases in the atmosphere, as documented by the measures and confirmed by the isotopic analyzes that have clearly demonstrated since the 1950s and 1960s the "fossil" origin of CO2 accumulated in the atmosphere (Suess effect). In particular, the average global CO2 concentration value in the atmosphere in December 2018 was around 410 ppm, with a 46% increase compared to the pre-industrial value (280 ppm) of the end of the eighteenth century. It is important to note that concentrations like the current ones have never been detected in the last 800 thousand years. Also the speed of the increase is a new element: a CO2 increase of 98 ppm was measured in 60 years, compared with an increase of 80 ppm in about 7000 years, during the transition from the last glacial peak to the current interglacial . The rate of CO2 increase in the atmosphere increased from 0.85 ppm / year in the first decade of measurements (1960-1969) to over 2 in the last decade (2009-2018): in particular, in 2017 and 2018 it was measured an increase of 2.5 ppm of CO2 per year.
Following the increase in the concentration of CO2 and other greenhouse gases, the "radiative forcing" (ie the positive difference between the incoming solar radiative flow and the outgoing terrestrial flow) in the last 40 years has increased by 77%, exceeding 3 W / m2. In the transition from the last glaciation to the present interglacial the increase in radiative forcing is believed to have been around 7 W / m2. In addition to the dominant factor associated with greenhouse gases, there are also changes in land use, deforestation in some geographical areas and reforestation in others, the spread of intensive farming practices, soil erosion, desertification and, last but not least, l extended urbanization have contributed to changing the earth's climate.
These factors, of which the most important is the increase in CO2 and the relative intensification of the greenhouse effect, have led from 1900 to today to the increase in the global average temperature to the surface of about one degree Celsius. The magnitude of this change is particularly significant, if we compare the increase in the global average surface temperature of about 1 ° C in just over a century with the increase of about 3.5 ° C in 8000 years during the last deglaciation .
As a result, especially in some regions, effects have been measured such as the intensification of the hydrological cycle, the increase of summer heat waves and drought, a greater instability of the Arctic polar vortex, the increase in the global average level of the seas, the extension of forest fires in northern Europe, as well as changes in ecosystems.
Globally, all the large groups of mountain glaciers and the large polar ice caps of Antarctica and Greenland are currently, in their entirety and simultaneously, in negative balance, that is, they return water to the oceans. The average global level of the oceans, after substantial stability in the last 2000 years, from the mid-19th century is rising, with increasing speed, both due to the increase in water volume following their heating and to the increasing and synchronous fusion of glaciers. Furthermore, a significant part of the excess heat absorbed by the Earth's surface is stored in the oceans, as shown by the increase in the heat content of the oceans. When this heat is returned from the oceans to the atmosphere, a further rise in temperature will be inevitable.
The increase in temperature, especially when combined with the fragmentation of natural habitats, the introduction of invasive species, the loss of biodiversity, the increase in population and the relative demands for water and energy resources, can lead to considerable stress for social and economic systems of our civilization. The planet and life on Earth are not in danger, because they have gone through catastrophes and changes far greater than we see today. But it is not said that individual species can survive change, as happened many times in the past of the Earth. The biosphere, in fact, responds to even extreme climate changes in a very complex way, with some opportunist species that manage to take advantage and other species that are unable to survive the new conditions.
Probably not even today the human species is in danger, but surely the populations with less economic resources have less ability to adapt to the changed climatic conditions and therefore the only solution will be emigration, with the consequent social and economic instability in all countries . The changed availability of resources will also increase conflicts due to reduced water availability or sea level rise. A complex society like ours, full of critical infrastructures in vulnerable areas such as coastal areas, is therefore particularly exposed to the social, economic and environmental damage brought about by climate change.
Many aspects are naturally still to be understood, as happens in almost every branch of science and in particular in the science of planetary climates. The climate system is complex, climate science is a relatively "young" research field, the problem is intrinsically difficult and there is still room for generations of geologists, biologists, chemists and physicists to devote themselves to understanding the theme of how the climate of a planet works over geological time. But the general picture is now clear and both the data collected over the last fifty years and the laws of chemistry and physics have defined the field of action and indicated where new research is needed. The persistence of some uncertainties, above all for the processes of interaction and for the quantitative estimate of the future evolution of the climate, must not be an excuse for not taking note of the need to limit the emission of anthropogenic greenhouse gases, even with all the difficulties that this conversion entails. At the same time, the need to change the energy route is a powerful engine of scientific, technological and economic innovation, which many countries (including China) are traveling through and which would be suicidal, on the European and Italian side, to neglect.
In conclusion, all the available data, measurements, observations and analyzes conducted by tens of thousands of researchers around the world indicate that the increase in temperatures observed in the last century is largely due to the increase in concentration of the CO2 and other atmospheric greenhouse gases, generated by human activities: it is not shown by any measure, neither astronomical nor terrestrial, that the other natural forcing is currently responsible for this heating. The denial of quantitative evidence, based on data, of the current climate change is a prejudicial and unscientific attitude. As citizens, we believe that it is necessary to take avenues of technological and scientific innovation that will allow our country to remain competitive, rather than pursuing obsolete and misleading ideas. As scientists, we believe that there is still a lot of research that can and must work on, while at the same time profoundly respecting the results obtained so far, following the data and the reality that the data revealed to us. The rest is a myth with no foundation.