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.
ΠΕΡΙΣΣΟΤΕΡΑ θέματα ΜΕΤΕΩΡΟΛΟΓΙΑΣ, ΕΔΩ.
ΛΕΞΕΙΣ: κλιμα, κλιματικη αλλαγη, Σελνχουμπερ
ΣΧΟΛΙΑ
ΣΧΟΛΙΑ ΜΕΣΩ Facebook