Encyclopedia of Evolution.pdf - Online Reading Center

In the 20th century, many creationists proclaimed that
evolutionary science is incompatible with belief in God. Asa
Gray, and many other scientists and theologians of the 19th
century, show that this is not true. Gray died January 30,
1888.
Further Reading
Livingstone, David N. Darwin’s Forgotten Defenders. Grand Rapids,
Mich.: Eerdmans; Edinburgh, U.K.: Scottish Academic Press,
1987.
greenhouse effect The greenhouse effect is the absorption of
infrared radiation by certain atmospheric gases, which causes
the atmosphere to become warmer. The concept was proposed
by Swedish chemist Svante Arrhenius at the end of the 19th
century. When sunlight shines upon the Earth, most of the
visible light penetrates the atmosphere. Some of the sunlight
(albedo) reflects off of light-colored surfaces such as ice and
snow, directly back into outer space. Reflected sunlight does
not contribute to warming the Earth. The rest of the sunlight
is absorbed by things like rocks, oceans, plants, and animals
and causes them to become warmer. They conduct heat into
the air, causing the air to become warmer. This is the reason
that air is warmest near the average surface level of the ground
(low altitudes and elevations) rather than at high altitudes or
high elevations. Rocks, oceans, plants, and animals also glow
with invisible photons known as infrared radiation, at wavelengths
just beyond the red end of the visible light spectrum.
Most of these infrared photons radiate into outer space. Certain
atmospheric gases, such as carbon dioxide, methane, and
water, absorb these photons on their way out. This causes the
gases to become warmer, and they impart their warmth to
the other gas molecules in the atmosphere. In this way they
act metaphorically like the glass roof of a greenhouse which
holds in the heat of the sunlight. This is how the presence of
greenhouse gases causes the atmosphere to become warmer.
The greenhouse effect is essential to the survival of life on this
planet. Without the greenhouse effect, the Earth would have
an average temperature similar to that of Mars.
Three atmospheric gases are important in the greenhouse
effect:
• Water is the most abundant greenhouse gas. Clouds block
sunlight but also absorb infrared radiation; the net effect,
according to climatological calculations, is that clouds have
a slight cooling effect on the Earth.
• The most potent major naturally occurring greenhouse gas
is methane. Methane is produced mainly by bacterial fermentation
but becomes carbon dioxide when it reacts with
oxygen gas. Although methane is continually produced, it
does not accumulate in the atmosphere.
• Most atmospheric scientists consider carbon dioxide to
be the most important greenhouse gas because it absorbs
more infrared radiation than water and is more stable than
methane.
The importance of atmospheric carbon dioxide to the
greenhouse effect is demonstrated by studies of ice cores. In
greenhouse effect
Greenland and in Antarctica, annual layers of ice have accumulated
for over many thousands of years. For example, ice
cores from Vostok, Antarctica, contain ice layers from the
recent past to over 400,000 years ago. By counting back from
the present, the age of each layer of ice can be determined.
The oxygen and deuturium isotope ratios (see isotopes) of
each layer of ice is an estimate of the global average temperature.
In addition, each layer of ice contains dissolved atmospheric
gases. When these gases are released in a laboratory
and analyzed, scientists can directly measure the concentration
of methane and carbon dioxide that was in the air at
that time. There is a close correlation between atmospheric
carbon dioxide and global temperature over the past 400,000
years. Both temperature and carbon dioxide have fluctuated
dramatically during that time, but always together, at least in
the past 400,000 years.
The greenhouse effect is just one influence upon global
temperatures. Another major influence is the arrangement of
continents. Continents have shifted position drastically during
the history of the Earth (see continental drift). Before the
Pleistocene epoch, the average Earth temperature was warmer
than it is today. During much of the Cenozoic era, forests
grew up to the North Pole itself. The trees were deciduous but
may have been so in response to the six months of darkness
rather than to particularly cold temperatures. The reason that
the high northern latitudes were warm was because ocean
currents brought warm water from tropical areas. Ocean
currents have always been a major factor in distributing the
heat on planet Earth. Today, the Gulf Stream brings warmth
from the Caribbean to Western Europe, so that England’s climate
is much milder than that of Labrador, which is at the
same latitude. Before the Pleistocene, a global ocean current
(a very large version of the Gulf Stream) carried warm water
into the north polar regions. When the islands between North
and South America coalesced into the isthmus of Panama this
global ocean current was interrupted. It was at this time that
the cycle of ice ages began. There have been periods of cooling
in the past history of the Earth; there is evidence for glaciation
around the South Pole during the Paleozoic era. At
other times, forests grew at the South Pole.
The principal process by which carbon dioxide is
removed from the atmosphere is by photosynthesis (see photosynthesis,
evolution of). Cyanobacteria (see bacteria,
evolution of) and the chloroplasts of eukaryotic cells
(mostly of plants; see symbiogenesis) absorb carbon dioxide
and make it into carbohydrates, using sunlight energy. Not
only is this the source of all the food in the world, but it also
removes carbon dioxide from the air. Cellular respiration of
organisms releases carbon dioxide into the air. At many times
in the past history of the Earth, photosynthesis absorbed
more carbon dioxide than respiration released. Especially
during the Carboniferous period, huge forests of plants
were buried and became deposits of coal and oil, with the
result that the carbon atoms were stored in the Earth rather
than returning to the atmosphere. Also, many marine protists
and invertebrates (see eukaryotes, evolution of; invertebrates,
evolution of) consumed single-celled plants, and