Page 16 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

8 Fundamentals of Ocean Renewable Energy

as Milankovitch cycles, which have a period of around 21,000 years. Although
these Milankovitch cycles have minimal influence on the global annual mean
solar radiation received by the Earth, they alter the solar radiation that is received
at each latitude. It has been suggested that when the summer sunshine on the
northern continents drops below a critical threshold, snow from the previous
winter does not melt, and this triggers an ice age [e.g. 4]. In addition to
Milankovitch cycles, incoming radiation varies because the energy that is output
from the Sun is not constant. In particular, changes in sunspot activity have
been linked to prolonged changes in winter temperatures [5], leading to periods
such as the Maunder Minimum (1645–1715), when sunspots were rare, and
temperatures across Europe were below average.
Of the Sun’s energy that reaches the top of the atmosphere, around one
4
quarter of this energy is reflected by clouds and ‘aerosols’, and a smaller
amount is reflected by the Earth’s surface, mainly by light coloured surfaces
such as snow and ice. Together, this reflected solar radiation is known as
albedo. The energy that has not been reflected is absorbed by the atmosphere
and, particularly, the surface of the Earth. To balance this incoming solar
energy, the Earth emits long-wave radiation. Greenhouse gases (GHGs) in the
atmosphere act as a partial ‘blanket’ for this emitted long-wave radiation, and
the most important GHGs are water vapour and carbon dioxide (CO 2 ). The
amount of CO 2 in the atmosphere has increased dramatically since the industrial
revolution. Since 1960, for example, observations show that the concentration
of CO 2 in the atmosphere has increased from around 320 ppm to over 400 ppm
(2016)—an increase in concentration of almost 30% in 56 years (Fig. 1.8). This
increased concentration of CO 2 in the atmosphere has led to the phenomenon of
global warming. Feedbacks in the climate system exacerbate global warming.
For example, increased concentrations of GHGs in the atmosphere warm the
Earth’s climate, melting snow and ice. This melting reduces the Earth’s albedo,
and so these darker surfaces that are revealed absorb more of the Sun’s heat in a
feedback cycle known as the ‘ice-albedo feedback’.
The evidence that GHGs in the atmosphere have drastically increased is
incontrovertible (e.g. Fig. 1.8). Much scientific research has been invested in
trying to determine the causes and consequences of these increases [e.g. 6].
The cause has primarily been identified as the combustion of fossil fuels, but
deforestation also has a role. Looking at Fig. 1.9A, global CO 2 emissions have
increased dramatically from close to zero (in 1880) to 9449 million metric
tonnes in 2011. This has contributed to an increase in global temperature of
◦
over 1 C between 1880 and 2016 (Fig. 1.9B), with consequences, amongst
others, of a global rise in sea level of around 200 mm over the same time period
(Fig. 1.9C) due to thermal expansion caused by warming of the ocean, and
increased melting of land-based ice.

4. Small particles in the atmosphere.