Geoscience Reference
In-Depth Information
gases, some are very volatile but others have a long life and their long
residence time in the atmosphere enables homogenization on a global
scale. The residence time of CH
4
, for example, is in the order of 12
years, N
2
O more than a hundred years and CO
2
, chemically inert, only
disappears from the atmosphere through absorption in the land
biosphere and in the ocean. Well mixed in the atmosphere, these gases
affect its radiative capacity in a durable way. Their signature is
therefore visible on a global scale, even in isolated sites such as the
Austral Islands in the Indian Ocean or in Antarctica. “Radiative
forcing” measures how the energy present in the global system is
altered by the presence of a gas which interacts with solar radiation
and the radiation emitted by the Earth and atmosphere: it is defined as
the net change in radiation at the summit of the lower atmosphere after
having allowed for the upper atmosphere to readjust its radiative
equilibrium, the temperatures of the surface and the troposphere
remaining fixed at their nondisturbed value. This idea of
radiative forcing allows us to evaluate and compare potential players
in climate change in the same unit: CO
2
emerges as the most
important player with a radiative forcing of 1.66 Wm
-2
, CH
4,
which
has a stronger radiative forcing potential but whose concentration and
life span are weaker is second at 0.48 Wm
-2
, then follow the other
gases, like N
2
O at 0.16 Wm
-2
.
The concentration of CO
2
has increased by 36% from 280 ppm in the
pre-industrial era to 379 ppm in 2006, and in 2013 approached
400 ppm at the site of Mauna Loa in Hawaii (www.esrl.noaa.gov/
gmd/ccgg/trends/), a level of concentration that had never been
observed in the previous 600,000 years recorded in the polar cores of
Dome C in Antarctica. The increase in the level of concentration
however only represents half of the level of emissions: almost half the
emissions resulting from human activity are indeed absorbed by the
natural cycle of CO
2
in the land and marine biosphere through
photosynthesis and by their solubility. The physiology of plants allows
the fixing of some of the CO
2
excess in the environment. However,
this stimulation is not sustainable in the long term for many
ecosystems and it is to be feared that some of them cannot adapt to
