Geoscience Reference
In-Depth Information
- it is also important to remember the annual and diurnal cycles, as well as the
turbulent and chaotic nature of the Earth's atmosphere.
The role that the Earth's orbital parameters play (very well described by the
Milankovitch theory) have been fully demonstrated with ice core data drilled into
the 3,000m thick Antarctic ice sheets at Vostok station. It dated back 750,000 years
and revealed the 7 past glacial cycles, and the close correlation between atmospheric
greenhouse gases concentration (carbon dioxide, methane), and temperature.
According to Milankovitch theory, the expected future evolution of orbital
parameters should have led a slow return to a small ice-age period six millennium
later. This was consistent with the observed trend of global temperatures until the
beginning of the industrial era, but is no longer the case, with an average increase
over the 20
th
century of about 0.7°C, and current rate of approximately 0.2°C per
decade, due to the impact of human activities.
Today, the total average amount of solar energy received as short wavelengths
radiation by each square meter at the top of the Earth's atmosphere is 342 Watts per
square meter (W/m
2
), i.e. one quarter of the solar constant.
Figure 4.2 shows the Earth's radiation budget:
- 31% of the incoming energy received at the top of the atmosphere (107 W/m
2
)
is reflected back to space by the atmosphere (6%), clouds and aerosols (21%) and
the Earth's surface (4%);
- the remaining 69% (235 W/m
2
) are partially absorbed by the atmosphere and
clouds (20% of these 69%, in other words 67 W/m
2
), whilst the 49% complement
(168 W/m
2
) is absorbed by the Earth's surface.
The Earth's surface also emits energy as infrared radiation to the atmosphere as
follows:
- 390 W/m
2
, of which 40 W/m
2
are radiated directly to space;
- as sensible heat flux (7% of the incoming 342 W/m
2
, i.e. 24 W/m
2
);
- as latent heat flux (23% of the incoming 342 W/m
2
, i.e. 78 W/m
2
),
corresponding to latent heat released when water vapor condenses in the
atmosphere.
The radiative losses at the Earth's surface are compensated by a back radiation
amount of 324 W/m
2
radiated by the atmosphere, especially due to water vapor and
other greenhouse gases.
The condition for a stable climate is:
- a balance between the incoming solar energy (342 W/m
2
), and the sum of the
reflected solar radiation (107 W/m
2
) and the outgoing to space long-wave radiation
(235 W/m
2
) emitted by the climate system;
- balanced short- and long-wave radiative budgets:
