Geoscience Reference
In-Depth Information
example, conditions the circulation of the atmosphere and ocean. The
chemical composition of the atmosphere also reflects the slow
evolution of the lithosphere and biosphere and contributes to
characterizing the climate. On a shorter scale of time, dust emissions
due to violent volcanic eruptions that can reach the stratosphere lead
to a cooling of the Earth's surface for a few months to a few years. On
the scale of the last million years, extensive information taken from
polar ice cores and from oceanic sediments demonstrates the
importance of the parameters controlling the Earth's orbit around the
Sun. If the origin of these oscillations between a dominant glacial
climate state and hot interglacial periods is easily identifiable in the
difference in distribution of the solar energy received by the Earth's
surface, the amplitude of these oscillations can only be explained by
implying feedback between greenhouse gases and the climate,
variations in solar energy being much too weak to explain the size of
the observed variations.
These strong variations in the land climate have been accompanied
by changes in polar caps and in sea level. The analysis of air bubbles
trapped in polar cores enable the reconstruction of atmospheric
concentrations of trace gases; they show important variations with an
incidence of elevated concentration rates of carbon dioxide (CO
2
),
methane (CH
4
) and nitrogen protoxide (N
2
O) during interglacial
periods and low levels in glacial periods. Moreover, these records
indicate that current levels of CH
4
(1,774 ppb
5
in 2006) and CO
2
(379 ppm
6
in 2006) had never been reached during the last million
years, which suggests that the current climate has not experienced a
parallel during this period.
2.2.3.
The Earth's radiative equilibrium and greenhouse gases
The Earth is in a delicate equilibrium between the energy received
from the Sun and the energy that it emits into space. It is the small
variations in this distribution of energy on the Earth's surface that
paleoclimate specialists held responsible for oscillations between
5 ppb: part per billion in volume.
6 ppm: part per million in volume.
