Carbon, Climate and Civilization - H. J. Schellnhuber

Carbon, Climate and Civilization

By Hans Joachim Schellnhuber

The chemical element carbon (C) has been quintessential for the evolution of the global environment and of life on Earth. Without the abundance of C in our world, there would be no greenhouse effect as caused by air-borne carbon dioxide and no liquid water on the planetary surface. There would be no organic matter allowing for the eventual emergence of homo sapiens. And there would be no fossil fuels allowing for the rise of our technical civilization in the Industrial Revolution that started in the late 18th century in Lancashire, UK.

Beyond these general insights, modern science is currently developing a deep understanding of how C-dynamics in the Earth’s crust, oceans, atmosphere and biosphere determined the development and behaviour of the climate system over time scales ranging from billions of years to centuries. In particular, variations in the atmospheric carbon-dioxide and methane concentrations as generated by astrophysical and geophysical mechanisms have pushed our planet on a roller-coaster journey that included snowball, hothouse and chaos episodes. About 3 My ago, the natural atmospheric CO2 levels became low enough to usher the epic glacial cycle through quasi-periodic Milankovitch forcing. The so-generated waxing and waning of massive ice sheets on the Northern Hemisphere paved the way towards human civilization in at least two manners: Firstly, the huge variations in environmental conditions accelerated, through challenge and response, the Hominid evolution that culminated in the advent of modern humans about 250 ky ago. Secondly, the ice dynamics created tremendous amounts of fertile soils through abrasion of rocks in temperate and sub-tropical zones, thus supporting vast grasslands where the wild precursors of contemporary cereal varieties thrived. During the current inter-glacial called Holocene, which began about 11 ky ago and provided exceptionally stable climate conditions, the two factors merged to bring about agriculture and the rapid growth of the homo-sapiens population.
The latter virtually exploded in the modern age due to pervasive mechanization and electrification on the basis of coal, oil and gas. As an unintended side effect, CO2 has gradually been accumulating in the Earth’s atmosphere, reinforcing the natural greenhouse effect. As a consequence, the glacial cycle is already suppressed by human interference, i.e. there will be no new ice ages in the foreseeable future. While this is a distant impact, the anthropogenic increase of the global mean surface temperature by currently more than 1°C can already be felt through shifting climatic zones and altered extreme-weather regimes. The Paris Agreement struck in 2015 aims at confining global warming to “well below 2°C” and even calls for efforts to hold the 1.5°C line. These goals are clearly justified by scientific risk analysis that maps out the severe ecological, economic and social damages expected to materialize beyond the Paris corridor. In particular, so-called tipping points would most likely be transgressed where vital components of the Earth system (such as rain forests, monsoon patterns and ice sheets) could enter a pathway towards disruption or even destruction.
Even more worrying is the prospect that these events might conspire to set into motion something like a “runaway greenhouse dynamics” that could propel our planet into a hothouse state associated with conditions that prevailed about 15 My ago (during the Miocene epoch) or about 56 My ago (during the Eocene epoch), featuring 8-12°C higher temperatures and up to 70 m higher sea levels.
Whether this is possible at all is the subject of intense topical research. Recent hints that the so-called climate sensitivity (expressing the long-term temperature increase in response to a doubling of atmospheric CO2) is substantially higher than previously estimated indicate that the “Hothouse Earth” scenario may not be completely speculative.
It is clearly justified to ask whether civilization as we know it could be sustained in such a dramatically changed environment. Therefore, the Paris corridor can be perceived as a fire wall that needs to be respected by all means. But is it a realistic endeavour to keep global warming well below 2°C? In purely physical terms, the answer is yes, but two transformative socioeconomic developments are required:
On the one hand, the global emissions of greenhouse gases from anthropogenic sources need to be reduced to net zero by 2050 at the latest. The rich industrialized countries have to take the lead in this transformation and should decarbonize their economies approximately one decade earlier, i.e. by 2040. The scientific literature already provides explicit strategies and roadmaps for accomplishing these tasks, inspecting the relevant sectors (power production, mobility, heavy industry, construction, agriculture etc.) and their coupling. The rapid switch to renewable energies is imperative in this context and could offer crucial socioeconomic opportunities especially for Mediterranean countries like Greece.
On the other hand, the destruction and conversion of ecosystems worldwide has to be stopped immediately and actually reversed in the next couple of decades. Without the absorption of huge amounts of anthropogenic carbon by the biosphere (such as tropical and boreal forests), the atmospheric concentration of CO2 and the concomitant global warming would be already much higher today. Instead of appreciating these free ecosystem services to humanity, we keep on “killing our best friends” as illustrated by the ongoing slash-and-burn invasion of primary forests in Latin America, Central Africa and South-East Asia. Rather than annihilating natural carbon sinks, we have to generate new ones by reforesting degraded areas and creating additional wetlands.
Our civilization is at a crossroads now as aptly described by the term “climate emergency”. The chance of overcoming this.


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