British meteorologist George Hadley reached another fundamental
understanding of the factors that influence global climate
in the 18th century. Hadley proposed a simple,
convective type of circulation in the atmosphere, in which
heating by the Sun causes the air to rise near the equators and
move poleward, where the air sinks back to the near surface,
then returning to the equatorial regions. We now recognize a
slightly more complex situation, in that there are three main
convecting atmospheric cells in each hemisphere, named
Hadley, Ferrel, and Polar cells. These play very important
roles in the distribution of different climate zones, as moist or
rainy regions are located, in the Tropics and at temperate latitudes,
where the atmospheric cells are upwelling and release
water. Deserts and dry areas are located around zones where
the convecting cells downwell, bringing descending dry air
into these regions.
The rotation of the Earth sets up systems of prevailing
winds that modify the global convective atmospheric (and
oceanic) circulation patterns. The spinning of the Earth sets
up latitude-dependent airflow patterns, including the trade
winds and westerlies. In addition, uneven heating of the
Earth over land and ocean regions causes regional airflow
patterns such as rising air over hot continents that must be
replenished by air flowing in from the sides. The Coriolis
force is a result of the rotation of the Earth, and it causes any
moving air mass in the Northern Hemisphere to be deflected
to the right, and masses in the Southern Hemisphere to be
deflected to the left. These types of patterns tend to persist
for long periods of time and move large masses of air around
the planet, redistributing heat and moisture and regulating
the climate of any region.
Temperature is a major factor in the climate of any area,
and this is largely determined by latitude. Polar regions see
huge changes in temperature between winter and summer
months, largely a function of the wide variations in amount
of incoming solar radiation and length of days. The proximity
to large bodies of water such as oceans influences temperature,
as water heats up and cools down much slower than
land surfaces. Proximity to water therefore moderates temperature
fluctuations. Altitude also influences temperature,
with temperature decreasing with height.
Climate may change in cyclical or long-term trends, as
influenced by changes in solar radiation, orbital variations of
the Earth, amount of greenhouse gases in the atmosphere, or
through other phenomena such as the El Niño or La Niña.
See also ATMOSPHERE; CLIMATE CHANGE; EL NIÑO;
PLATE TECTONICS.
climate change Earth’s climate changes on many different
timescales, ranging from tens of millions of years to decadal
and even shorter timescale variations. In the last 2.5 billion
years, several periods of glaciation have been identified, separated
by periods of mild climate similar to that of today.
Other periods are marked by global hothouse type conditions,
when the Earth had a very hot and wet climate,
approaching that of Venus. These dramatic climate changes
are caused by a number of different factors that exert their
influence on different timescales. One of the variables is the
amount of incoming solar radiation, and this changes in
response to several astronomical effects such as orbital tilt,
eccentricity, and wobble. Changes in the incoming solar radiation
in response to changes in orbital variations produce
cyclical variations known as Milankovitch cycles. Another
variable is the amount of heat that is retained by the atmosphere
and ocean, or the balance between the incoming and
outgoing heat. A third variable is the distribution of landmasses
on the planet. Shifting continents can influence the
patterns of ocean circulation and heat distribution, and placing
a large continent on one of the poles can cause ice to
build up on that continent, increasing the amount of heat
reflected back to space and lowering global temperatures in a
positive feedback mechanism.
Shorter term climate variations include those that operate
on periods of thousands of years, and shorter, less regular
decadal scale variations. Both of these relatively short-period
variations are of most concern to humans, and considerable
effort is being expended to understand their causes and to
estimate the consequences of the current climate changes the
planet is experiencing. Great research efforts are being
expended to understand the climate history of the last million
years and to help predict the future.
Variations in formation and circulation of ocean currents
may be traced some thousands of years to decadal scale
variations in climate. Cold water forms in the Arctic and
Weddell Seas. This cold salty water is denser than other
water in the ocean, so it sinks to the bottom and gets ponded
behind seafloor topographic ridges, periodically spilling over
into other parts of the oceans. The formation and redistribution
of North Atlantic cold bottom water accounts for about
30 percent of the solar energy budget input to the Arctic
Ocean every year. Eventually, this cold bottom water works
its way to the Indian and Pacific Oceans where it upwells,
gets heated, and returns to the North Atlantic. This cycle of
water circulation on the globe is known as thermohaline circulation.
Recent research on the thermohaline circulation
system has shown a correlation between changes in this system
and climate change. Presently, the age of bottom water
in the equatorial Pacific is 1,600 years, and in the Atlantic it
is 350 years. Glacial stages in the North Atlantic have been
correlated with the presence of older cold bottom waters,
approximately twice the age of the water today. This suggests
that the thermohaline circulation system was only half
as effective at recycling water during recent glacial stages,
with less cold bottom water being produced during the
glacial periods. These changes in production of cold bottom
water may in turn be driven by changes in the North Ameri-
80 climate change
can ice sheet, perhaps itself driven by 23,000-year orbital
(Milankovitch) cycles. It is thought that a growth in the ice
sheet would cause the polar front to shift southward,
decreasing the inflow of cold saline surface water into the
system required for efficient thermohaline circulation. Several
periods of glaciation in the past 14,500 years (known
as the Dryas) are thought to have been caused by sudden,
even catastrophic injections of glacial meltwater into the
North Atlantic, which would decrease the salinity and hence
density of the surface water. This in turn would prohibit the
surface water from sinking to the deep ocean, inducing
another glacial interval.
Shorter term decadal variations in climate in the past
million years are indicated by so-called Heinrich Events,
defined as specific intervals in the sedimentary record showing
ice-rafted debris in the North Atlantic. These periods of
exceptionally large iceberg discharges reflect decadal scale sea
surface and atmospheric cooling. They are related to thickening
of the North American ice sheet, followed by ice stream
surges, associated with the discharge of the icebergs. These
events flood the surface waters with low-salinity freshwater,
leading to a decrease in flux to the cold bottom waters, and
hence a short period global cooling.
Changes in the thermohaline circulation rigor have also
been related to other global climate changes. Droughts in the
Sahel and elsewhere are correlated with periods of ineffective
or reduced thermohaline circulation, because this reduces the
amount of water drawn into the North Atlantic, in turn cooling
surface waters and reducing the amount of evaporation.
Reduced thermohaline circulation also reduces the amount of
water that upwells in the equatorial regions, in turn decreasing
the amount of moisture transferred to the atmosphere,
reducing precipitation at high latitudes.
Atmospheric levels of greenhouse gases such as CO2 and
atmospheric temperatures show a correlation to variations in
the thermohaline circulation patterns and production of cold
bottom waters. CO2 is dissolved in warm surface water and
transported to cold surface water,which acts as a sink for the
" Coping with Sea-Level Rise in Coastal Cities
People have built villages, towns, cities, and industrial sites near
the sea for thousands of years. The coastal setting offers beauty
and convenience but also may bring disaster with coastal storms,
tsunami, and invading armies. Coastal communities are currently
experiencing the early stages of a new incursion, that of the sea
itself, as global sea levels slowly and inexorably rise.
Sea-level rises and falls by hundreds of feet over periods of
millions of years have forced the position of the coastline to move
inland and seaward by many tens of miles over long time periods.
The causes of sea-level rise and fall are complex, including
growth and melting of glaciers with global warming, changes in
the volume of the mid-ocean ridges, thermal expansion of water,
and other complex interactions of the distribution of the continental
landmass in mountains and plains during periods of orogenic
and anorogenic activity. Most people do not think that changes
over these time frames will affect their lives, but a sea-level rise
of even a foot or two, which is possible over periods of tens of
years, can cause extensive flooding, increased severity of storms,
and landward retreat of the shoreline. Sea-level rise is rapidly
becoming one of the major global hazards that the human race is
going to have to deal with in the next century, since most of the
world’s population lives near the coast in the reach of the rising
waters. Cities may become submerged and farmlands covered by
shallow, salty seas. An enormous amount of planning is needed,
as soon as possible, to begin to deal with this growing threat. The
current rate of rise of an inch or so every 10 years seems insignificant,
but it will have truly enormous consequences. When sealevel
rises, beaches try to maintain their equilibrium profile,
moving each beach element landward. A sea-level rise of one
inch is generally equated with a landward shift of beach elements
of more than four feet. Most sandy beaches worldwide are
retreating landward at rates of 20 inches–3 feet per year, consistent
with sea-level rise of an inch every 10 years. If the glacial ice
caps on Antarctica begin to melt faster, the sea-level rise will be
much more dramatic.
What effect will rising sea levels have on the world’s cities
and low-lying areas? Many of the world’s large cities, including
New York, London, Houston, Los Angeles, Washington D.C., Cairo,
Shanghai, Brussels, and Calcutta have large areas located within a
few feet of sea level. If sea levels rise a few feet, many of the
streets in these cities will be underwater, not to mention basements,
subway lines, and other underground facilities. Imagine
Venice-like conditions in New York! If sea levels rise much more,
many of the farmlands of the midwest United States, North Africa,
Mesopotamia, northern Europe, Siberia, and eastern China will be
submerged in shallow seas. These areas are not only populated but
serve as some of the most fertile farmlands in the world. Thus,
large sea-level rise will at best displace or more likely simply eliminate
the world’s best agricultural lands, necessary for sustaining
global population levels.
What can be done to prepare for sea-level rise? Some
lessons can be learned from the Netherlands, where the Dutch
have built numerous dikes to keep the sea out of low-lying areas, at
costs of billions of dollars. If the United States had to build such
barriers around the coastlines of low-lying areas, the cost would be
unbearable and would amount to one of the largest construction
projects ever undertaken. Humans are contributing to global warming,
which in turn is probably contributing to enhanced melting of
the glaciers and ice caps. Although it is too late to stop much of the
warming and melting, it may not be too late to stop the warming
before it is catastrophic and the ice caps melt, raising sea levels by
hundreds of feet. In any case, it is time that governments, planners,
and scientists begin to make more sophisticated plans for action
during times of rising sea levels.
Gaia Hypothesis
For billions of years the Earth has maintained its temperature and
atmospheric composition in a narrow range that has permitted life
to exist on the surface. Many scientists have suggested that this
remarkable trait of the planet is a result of life adapting to conditions
that happen to exist and evolve on the planet. An alternative
idea has emerged that the planet behaves as some kind of self-regulating
organism that invokes a series of positive and negative
feedback mechanisms to maintain conditions within the narrow
window in which life can exist. In this scenario, organisms and
their environment evolve together as a single coupled system, regulating
the atmospheric chemistry and composition to the need of
the system. Dr. James Lovelock, an atmospheric chemist at Green
College in Oxford, U.K., pioneered this second idea, known as the
Gaia hypothesis. However, the idea of a living planet dates back at
least to Sir Isaac Newton.
How does the Gaia hypothesis work? The atmosphere is
chemically unstable, yet it has maintained conditions conducive to
life for billions of years even despite a 30 percent increase in solar
luminosity since the Early Precambrian. The basic tenet of the
hypothesis is that organisms, particularly microorganisms, are
able to regulate the atmospheric chemistry and hence temperature
to keep conditions suitable for their development. Although
this tenet has been widely criticized, some of the regulating mechanisms
have been found to exist, lending credence to the possibility
that Gaia may work. Biogeochemical cycles of nutrients
including iodine and sulfur have been identified, with increases in
the nutrient supply from land to ocean leading to increased biological
production and increased emissions to the atmosphere.
Increased production decreases the flux of nutrients from the
oceans to the land, in turn decreasing the nutrient supply, biological
production, and emissions to the atmosphere.
As climate warms, rainfall increases, and the weathering of
calcium-silicate rocks increases. The free calcium ions released
during weathering combine with atmospheric carbon dioxide to
produce carbonate sediments, effectively removing the greenhouse
gas carbon dioxide from the atmosphere. This reduces global
temperatures in another self-regulating process. An additional
feedback mechanism was discovered between ocean algae and
climate. Ocean algae produce dimethyl sulfide gas, which oxidizes
in the atmosphere to produce nuclei for cloud condensation. The
more dimethyl sulfide that algae produce, the more clouds form,
lowering temperatures and lowering algal production of dimethyl
sulfide in a self-regulating process.
That the Earth and its organisms have maintained conditions
conducive for life for 4 billion years is clear. However, at times the
Earth has experienced global icehouse and global hothouse conditions,
where the conditions extend beyond the normal range. Lovelock
relates these brief intervals of Earth history to fevers in an
organism, and he notes that the planet has always recovered. Life
has evolved dramatically on Earth in the past 4 billion years, but this
is compatible with Gaia. Living organisms can both evolve with and
adapt to their environment, responding to changing climates by
regulating or buffering changes to keep conditions within limits that
are tolerable to life on the planet as a whole. However, there are
certainly limits, and the planet has never experienced organisms
such as humans that continually emit huge quantities of harmful
industrial gases into the atmosphere. It is possible that the planet,
or Gaia, will respond by making conditions on Earth uninhabitable
for humans, saving the other species on the planet. As time goes
on, in about a billion years the Sun will expand and eventually burn
all the water and atmosphere off the planet, making it virtually uninhabitable.
By then humans may have solved the problem of where
to move to and developed the means to move global populations to
a new planet.
CO2. During times of decreased flow from cold, high-latitude
surface water to the deep ocean reservoir, CO2 can build up
in the cold polar waters, removing it from the atmosphere
and decreasing global temperatures. In contrast, when the
thermohaline circulation is vigorous, cold oxygen-rich surface
waters downwell and dissolve buried CO2 and even carbonates,
releasing this CO2 to the atmosphere and increasing
global temperatures.
The present-day ice sheet in Antarctica grew in the Middle
Miocene, related to active thermohaline circulation that
caused prolific upwelling of warm water that put more moisture
in the atmosphere, falling as snow on the cold southern
continent. The growth of the southern ice sheet increased the
global atmospheric temperature gradients, which in turn
increased the desertification of midlatitude continental
regions. The increased temperature gradient also induced
stronger oceanic circulation, including upwelling, and
removal of CO2 from the atmosphere, lowering global temperatures,
and bringing on late Neogene glaciations.
Major volcanic eruptions inject huge amounts of dust
into the troposphere and stratosphere, where it may remain
for several years, reducing incoming solar radiation and
resulting in short-term global cooling. For instance, the eruption
of Tambora volcano in Indonesia in 1815 resulted in
global cooling and the year without a summer in Europe. The
location of the eruption is important, as equatorial eruptions
may result in global cooling, whereas high-latitude eruptions
may only cool one hemisphere.
It is clear that human activities are changing the global
climate, primarily through the introduction of greenhouse
gases such as CO2 into the atmosphere, while cutting down
tropical rain forests that act as sinks for the CO2 and put
oxygen back into the atmosphere. The time scale of observation
of these human, also called anthropogenic, changes is
short but the effect is clear, with a nearly one degree change
in global temperature measured for the past few decades. The
increase in temperature will lead to more water vapor in the
atmosphere, and since water vapor is also a greenhouse gas
this will lead to a further increase in temperature. Many computer-
based climate models are attempting to predict how
much global temperatures will rise as a consequence of our
anthropogenic influences, and what effects this temperature
rise will have on melting of the ice sheets (which could be
catastrophic), sea-level rise (perhaps tens of meters or more),
and runaway greenhouse temperature rise (which is possible).
Climate changes are difficult to measure, partly because
the instrumental and observational records go back only a
couple of hundred years in Europe. From these records, global
temperatures have risen by about one degree since 1890, most
notably in 1890–1940, and again since 1970. This variation,
however, is small compared with some of the other variations
induced by natural causes, and some scientists argue that it is
difficult to separate anthropogenic effects from the background
natural variations. Rainfall patterns have also changed
in the past 50 years, with declining rainfall totals over low latitudes
in the Northern Hemisphere, especially in the Sahel,
which has experienced major droughts and famine. However,
high-latitude precipitation has increased in the same time period.
These patterns all relate to a general warming and shifting
of the global climate zones to the north.
understanding of the factors that influence global climate
in the 18th century. Hadley proposed a simple,
convective type of circulation in the atmosphere, in which
heating by the Sun causes the air to rise near the equators and
move poleward, where the air sinks back to the near surface,
then returning to the equatorial regions. We now recognize a
slightly more complex situation, in that there are three main
convecting atmospheric cells in each hemisphere, named
Hadley, Ferrel, and Polar cells. These play very important
roles in the distribution of different climate zones, as moist or
rainy regions are located, in the Tropics and at temperate latitudes,
where the atmospheric cells are upwelling and release
water. Deserts and dry areas are located around zones where
the convecting cells downwell, bringing descending dry air
into these regions.
The rotation of the Earth sets up systems of prevailing
winds that modify the global convective atmospheric (and
oceanic) circulation patterns. The spinning of the Earth sets
up latitude-dependent airflow patterns, including the trade
winds and westerlies. In addition, uneven heating of the
Earth over land and ocean regions causes regional airflow
patterns such as rising air over hot continents that must be
replenished by air flowing in from the sides. The Coriolis
force is a result of the rotation of the Earth, and it causes any
moving air mass in the Northern Hemisphere to be deflected
to the right, and masses in the Southern Hemisphere to be
deflected to the left. These types of patterns tend to persist
for long periods of time and move large masses of air around
the planet, redistributing heat and moisture and regulating
the climate of any region.
Temperature is a major factor in the climate of any area,
and this is largely determined by latitude. Polar regions see
huge changes in temperature between winter and summer
months, largely a function of the wide variations in amount
of incoming solar radiation and length of days. The proximity
to large bodies of water such as oceans influences temperature,
as water heats up and cools down much slower than
land surfaces. Proximity to water therefore moderates temperature
fluctuations. Altitude also influences temperature,
with temperature decreasing with height.
Climate may change in cyclical or long-term trends, as
influenced by changes in solar radiation, orbital variations of
the Earth, amount of greenhouse gases in the atmosphere, or
through other phenomena such as the El Niño or La Niña.
See also ATMOSPHERE; CLIMATE CHANGE; EL NIÑO;
PLATE TECTONICS.
climate change Earth’s climate changes on many different
timescales, ranging from tens of millions of years to decadal
and even shorter timescale variations. In the last 2.5 billion
years, several periods of glaciation have been identified, separated
by periods of mild climate similar to that of today.
Other periods are marked by global hothouse type conditions,
when the Earth had a very hot and wet climate,
approaching that of Venus. These dramatic climate changes
are caused by a number of different factors that exert their
influence on different timescales. One of the variables is the
amount of incoming solar radiation, and this changes in
response to several astronomical effects such as orbital tilt,
eccentricity, and wobble. Changes in the incoming solar radiation
in response to changes in orbital variations produce
cyclical variations known as Milankovitch cycles. Another
variable is the amount of heat that is retained by the atmosphere
and ocean, or the balance between the incoming and
outgoing heat. A third variable is the distribution of landmasses
on the planet. Shifting continents can influence the
patterns of ocean circulation and heat distribution, and placing
a large continent on one of the poles can cause ice to
build up on that continent, increasing the amount of heat
reflected back to space and lowering global temperatures in a
positive feedback mechanism.
Shorter term climate variations include those that operate
on periods of thousands of years, and shorter, less regular
decadal scale variations. Both of these relatively short-period
variations are of most concern to humans, and considerable
effort is being expended to understand their causes and to
estimate the consequences of the current climate changes the
planet is experiencing. Great research efforts are being
expended to understand the climate history of the last million
years and to help predict the future.
Variations in formation and circulation of ocean currents
may be traced some thousands of years to decadal scale
variations in climate. Cold water forms in the Arctic and
Weddell Seas. This cold salty water is denser than other
water in the ocean, so it sinks to the bottom and gets ponded
behind seafloor topographic ridges, periodically spilling over
into other parts of the oceans. The formation and redistribution
of North Atlantic cold bottom water accounts for about
30 percent of the solar energy budget input to the Arctic
Ocean every year. Eventually, this cold bottom water works
its way to the Indian and Pacific Oceans where it upwells,
gets heated, and returns to the North Atlantic. This cycle of
water circulation on the globe is known as thermohaline circulation.
Recent research on the thermohaline circulation
system has shown a correlation between changes in this system
and climate change. Presently, the age of bottom water
in the equatorial Pacific is 1,600 years, and in the Atlantic it
is 350 years. Glacial stages in the North Atlantic have been
correlated with the presence of older cold bottom waters,
approximately twice the age of the water today. This suggests
that the thermohaline circulation system was only half
as effective at recycling water during recent glacial stages,
with less cold bottom water being produced during the
glacial periods. These changes in production of cold bottom
water may in turn be driven by changes in the North Ameri-
80 climate change
can ice sheet, perhaps itself driven by 23,000-year orbital
(Milankovitch) cycles. It is thought that a growth in the ice
sheet would cause the polar front to shift southward,
decreasing the inflow of cold saline surface water into the
system required for efficient thermohaline circulation. Several
periods of glaciation in the past 14,500 years (known
as the Dryas) are thought to have been caused by sudden,
even catastrophic injections of glacial meltwater into the
North Atlantic, which would decrease the salinity and hence
density of the surface water. This in turn would prohibit the
surface water from sinking to the deep ocean, inducing
another glacial interval.
Shorter term decadal variations in climate in the past
million years are indicated by so-called Heinrich Events,
defined as specific intervals in the sedimentary record showing
ice-rafted debris in the North Atlantic. These periods of
exceptionally large iceberg discharges reflect decadal scale sea
surface and atmospheric cooling. They are related to thickening
of the North American ice sheet, followed by ice stream
surges, associated with the discharge of the icebergs. These
events flood the surface waters with low-salinity freshwater,
leading to a decrease in flux to the cold bottom waters, and
hence a short period global cooling.
Changes in the thermohaline circulation rigor have also
been related to other global climate changes. Droughts in the
Sahel and elsewhere are correlated with periods of ineffective
or reduced thermohaline circulation, because this reduces the
amount of water drawn into the North Atlantic, in turn cooling
surface waters and reducing the amount of evaporation.
Reduced thermohaline circulation also reduces the amount of
water that upwells in the equatorial regions, in turn decreasing
the amount of moisture transferred to the atmosphere,
reducing precipitation at high latitudes.
Atmospheric levels of greenhouse gases such as CO2 and
atmospheric temperatures show a correlation to variations in
the thermohaline circulation patterns and production of cold
bottom waters. CO2 is dissolved in warm surface water and
transported to cold surface water,which acts as a sink for the
" Coping with Sea-Level Rise in Coastal Cities
People have built villages, towns, cities, and industrial sites near
the sea for thousands of years. The coastal setting offers beauty
and convenience but also may bring disaster with coastal storms,
tsunami, and invading armies. Coastal communities are currently
experiencing the early stages of a new incursion, that of the sea
itself, as global sea levels slowly and inexorably rise.
Sea-level rises and falls by hundreds of feet over periods of
millions of years have forced the position of the coastline to move
inland and seaward by many tens of miles over long time periods.
The causes of sea-level rise and fall are complex, including
growth and melting of glaciers with global warming, changes in
the volume of the mid-ocean ridges, thermal expansion of water,
and other complex interactions of the distribution of the continental
landmass in mountains and plains during periods of orogenic
and anorogenic activity. Most people do not think that changes
over these time frames will affect their lives, but a sea-level rise
of even a foot or two, which is possible over periods of tens of
years, can cause extensive flooding, increased severity of storms,
and landward retreat of the shoreline. Sea-level rise is rapidly
becoming one of the major global hazards that the human race is
going to have to deal with in the next century, since most of the
world’s population lives near the coast in the reach of the rising
waters. Cities may become submerged and farmlands covered by
shallow, salty seas. An enormous amount of planning is needed,
as soon as possible, to begin to deal with this growing threat. The
current rate of rise of an inch or so every 10 years seems insignificant,
but it will have truly enormous consequences. When sealevel
rises, beaches try to maintain their equilibrium profile,
moving each beach element landward. A sea-level rise of one
inch is generally equated with a landward shift of beach elements
of more than four feet. Most sandy beaches worldwide are
retreating landward at rates of 20 inches–3 feet per year, consistent
with sea-level rise of an inch every 10 years. If the glacial ice
caps on Antarctica begin to melt faster, the sea-level rise will be
much more dramatic.
What effect will rising sea levels have on the world’s cities
and low-lying areas? Many of the world’s large cities, including
New York, London, Houston, Los Angeles, Washington D.C., Cairo,
Shanghai, Brussels, and Calcutta have large areas located within a
few feet of sea level. If sea levels rise a few feet, many of the
streets in these cities will be underwater, not to mention basements,
subway lines, and other underground facilities. Imagine
Venice-like conditions in New York! If sea levels rise much more,
many of the farmlands of the midwest United States, North Africa,
Mesopotamia, northern Europe, Siberia, and eastern China will be
submerged in shallow seas. These areas are not only populated but
serve as some of the most fertile farmlands in the world. Thus,
large sea-level rise will at best displace or more likely simply eliminate
the world’s best agricultural lands, necessary for sustaining
global population levels.
What can be done to prepare for sea-level rise? Some
lessons can be learned from the Netherlands, where the Dutch
have built numerous dikes to keep the sea out of low-lying areas, at
costs of billions of dollars. If the United States had to build such
barriers around the coastlines of low-lying areas, the cost would be
unbearable and would amount to one of the largest construction
projects ever undertaken. Humans are contributing to global warming,
which in turn is probably contributing to enhanced melting of
the glaciers and ice caps. Although it is too late to stop much of the
warming and melting, it may not be too late to stop the warming
before it is catastrophic and the ice caps melt, raising sea levels by
hundreds of feet. In any case, it is time that governments, planners,
and scientists begin to make more sophisticated plans for action
during times of rising sea levels.
Gaia Hypothesis
For billions of years the Earth has maintained its temperature and
atmospheric composition in a narrow range that has permitted life
to exist on the surface. Many scientists have suggested that this
remarkable trait of the planet is a result of life adapting to conditions
that happen to exist and evolve on the planet. An alternative
idea has emerged that the planet behaves as some kind of self-regulating
organism that invokes a series of positive and negative
feedback mechanisms to maintain conditions within the narrow
window in which life can exist. In this scenario, organisms and
their environment evolve together as a single coupled system, regulating
the atmospheric chemistry and composition to the need of
the system. Dr. James Lovelock, an atmospheric chemist at Green
College in Oxford, U.K., pioneered this second idea, known as the
Gaia hypothesis. However, the idea of a living planet dates back at
least to Sir Isaac Newton.
How does the Gaia hypothesis work? The atmosphere is
chemically unstable, yet it has maintained conditions conducive to
life for billions of years even despite a 30 percent increase in solar
luminosity since the Early Precambrian. The basic tenet of the
hypothesis is that organisms, particularly microorganisms, are
able to regulate the atmospheric chemistry and hence temperature
to keep conditions suitable for their development. Although
this tenet has been widely criticized, some of the regulating mechanisms
have been found to exist, lending credence to the possibility
that Gaia may work. Biogeochemical cycles of nutrients
including iodine and sulfur have been identified, with increases in
the nutrient supply from land to ocean leading to increased biological
production and increased emissions to the atmosphere.
Increased production decreases the flux of nutrients from the
oceans to the land, in turn decreasing the nutrient supply, biological
production, and emissions to the atmosphere.
As climate warms, rainfall increases, and the weathering of
calcium-silicate rocks increases. The free calcium ions released
during weathering combine with atmospheric carbon dioxide to
produce carbonate sediments, effectively removing the greenhouse
gas carbon dioxide from the atmosphere. This reduces global
temperatures in another self-regulating process. An additional
feedback mechanism was discovered between ocean algae and
climate. Ocean algae produce dimethyl sulfide gas, which oxidizes
in the atmosphere to produce nuclei for cloud condensation. The
more dimethyl sulfide that algae produce, the more clouds form,
lowering temperatures and lowering algal production of dimethyl
sulfide in a self-regulating process.
That the Earth and its organisms have maintained conditions
conducive for life for 4 billion years is clear. However, at times the
Earth has experienced global icehouse and global hothouse conditions,
where the conditions extend beyond the normal range. Lovelock
relates these brief intervals of Earth history to fevers in an
organism, and he notes that the planet has always recovered. Life
has evolved dramatically on Earth in the past 4 billion years, but this
is compatible with Gaia. Living organisms can both evolve with and
adapt to their environment, responding to changing climates by
regulating or buffering changes to keep conditions within limits that
are tolerable to life on the planet as a whole. However, there are
certainly limits, and the planet has never experienced organisms
such as humans that continually emit huge quantities of harmful
industrial gases into the atmosphere. It is possible that the planet,
or Gaia, will respond by making conditions on Earth uninhabitable
for humans, saving the other species on the planet. As time goes
on, in about a billion years the Sun will expand and eventually burn
all the water and atmosphere off the planet, making it virtually uninhabitable.
By then humans may have solved the problem of where
to move to and developed the means to move global populations to
a new planet.
CO2. During times of decreased flow from cold, high-latitude
surface water to the deep ocean reservoir, CO2 can build up
in the cold polar waters, removing it from the atmosphere
and decreasing global temperatures. In contrast, when the
thermohaline circulation is vigorous, cold oxygen-rich surface
waters downwell and dissolve buried CO2 and even carbonates,
releasing this CO2 to the atmosphere and increasing
global temperatures.
The present-day ice sheet in Antarctica grew in the Middle
Miocene, related to active thermohaline circulation that
caused prolific upwelling of warm water that put more moisture
in the atmosphere, falling as snow on the cold southern
continent. The growth of the southern ice sheet increased the
global atmospheric temperature gradients, which in turn
increased the desertification of midlatitude continental
regions. The increased temperature gradient also induced
stronger oceanic circulation, including upwelling, and
removal of CO2 from the atmosphere, lowering global temperatures,
and bringing on late Neogene glaciations.
Major volcanic eruptions inject huge amounts of dust
into the troposphere and stratosphere, where it may remain
for several years, reducing incoming solar radiation and
resulting in short-term global cooling. For instance, the eruption
of Tambora volcano in Indonesia in 1815 resulted in
global cooling and the year without a summer in Europe. The
location of the eruption is important, as equatorial eruptions
may result in global cooling, whereas high-latitude eruptions
may only cool one hemisphere.
It is clear that human activities are changing the global
climate, primarily through the introduction of greenhouse
gases such as CO2 into the atmosphere, while cutting down
tropical rain forests that act as sinks for the CO2 and put
oxygen back into the atmosphere. The time scale of observation
of these human, also called anthropogenic, changes is
short but the effect is clear, with a nearly one degree change
in global temperature measured for the past few decades. The
increase in temperature will lead to more water vapor in the
atmosphere, and since water vapor is also a greenhouse gas
this will lead to a further increase in temperature. Many computer-
based climate models are attempting to predict how
much global temperatures will rise as a consequence of our
anthropogenic influences, and what effects this temperature
rise will have on melting of the ice sheets (which could be
catastrophic), sea-level rise (perhaps tens of meters or more),
and runaway greenhouse temperature rise (which is possible).
Climate changes are difficult to measure, partly because
the instrumental and observational records go back only a
couple of hundred years in Europe. From these records, global
temperatures have risen by about one degree since 1890, most
notably in 1890–1940, and again since 1970. This variation,
however, is small compared with some of the other variations
induced by natural causes, and some scientists argue that it is
difficult to separate anthropogenic effects from the background
natural variations. Rainfall patterns have also changed
in the past 50 years, with declining rainfall totals over low latitudes
in the Northern Hemisphere, especially in the Sahel,
which has experienced major droughts and famine. However,
high-latitude precipitation has increased in the same time period.
These patterns all relate to a general warming and shifting
of the global climate zones to the north.
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