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Oceans and Global Warming
Dr. Mae-Wan Ho
explains how oceans determine climate and influence climate change. Urgent
need to shift away from fossil fuels to renewable options (Which
Energy?)
A fully
referenced version of this article is posted on ISIS members’ website.
Membership details here
Oceans
of life
Oceans are vast, deep, and
mysterious. They cover 71 percent of the earth’s surface and contain approximately
97 percent of the earth’s water [1]. The average depth of the ocean is almost
4 000 metres [2], and nearly half the area is over 3 000 metres deep.
The oceans are home
to the majority of plant and animal life on earth, accounting for 90 percent
of the world’s living biomass, and many new species are being discovered from
the depths. The Census of Marine Life (COML), an international alliance of
scientists from 70 countries, discovered some 13 000 new species in 2003 alone
[3]. COML is amassing data to create a map of the distribution of 38 000 marine
species from plankton to whales.
According to one
theory, life on earth originated around hydrothermal vents in the deep seafloor
that heat up the water to 450 C [2]. Rare creatures living there today are
still able to obtain energy from chemical reactions in the same way as the
primordial life forms, and do not need to depend directly or indirectly on
photosynthesis for food like 99.97 percent of the biosphere.
Oceans
and climate
Oceans affect climate in many ways [4]. As the major
reservoir of water, oceans dominate the movement of water, supplying most
of the water vapour in the atmosphere by evaporation. Of this evaporated water,
91 percent is returned to the oceans as precipitation, the remainder is transported
and precipitated over landmasses. Runoff and groundwater from land flow back
to the oceans [5].
The oceans and the
atmosphere are tightly linked, and together form the most dynamic component
of the earth’s climate system. They bring moisture to coastal areas that may
be carried inland by the wind. Typhoons and hurricanes form over the oceans,
and as oceans get warmer these will be more frequent.
Oceans store heat.
When the earth’s surface cools or is heated up by the sun, the temperature
change is greater and faster over land than over the oceans. One consequence
of the ocean’s ability to absorb more heat is that it cools the surrounding
when it is hot and warms it when it is cold, which is why maritime climates
tend to be less extreme than continental.
Winds and currents are constantly moving the ocean’s waters. Surface currents
flowing north or south transport heat, and carry warmed or cooled water several
thousand kilometres, thereby ameliorating the extremes of heat or cold. Deep
ocean currents also flow across the globe [6] (Why
Gaia needs rainforests, SiS 20). The Gulf Stream Drift, for example,
is powered by cold, dense, salt-laden water sinking off the north polar coastal
regions and moving south in the depths, pushing the surface warm water from
the tropical and subtropical Atlantic (including some from the Gulf of Mexico)
up north to bathe the shores of Western Europe, producing a climate that is
surprisingly mild for that latitude.
Global warming and melting of the polar icecaps freshens the surface water,
reducing its density and preventing it from sinking (Global
warming is happening, this issue). As a result, the Gulf Stream slows down,
or may even reverse, bringing severe winters to northern Europe while the rest
of the earth heats up [7] (Global
warming and then the big freeze, SiS 20). In places where cold deep
waters come up to the surface, as near San Francisco in California USA, the
climate is as cool as Dublin in Ireland 1 600 km further north.
Oceans
and the carbon cycle
Oceans play a fundamental role in climate change through the Carbon Cycle (Box
1). The major driver of the Carbon Cycle is the biosphere, which depends entirely
on the ability of green plants, algae and certain bacteria to produce food for
themselves and other organisms through photosynthesis. For that reason they
are called primary producers.
Box 1
The Carbon Cycle
The carbon cycle supports life on earth and keeps its climate stable.
Carbon is the fourth most abundant element on earth, and makes up 50 percent
of the dry weight of living organisms [4]. The global carbon cycle involves
the flow of carbon between the major carbon reservoirs: the atmosphere,
the oceans, the vegetation and soils of terrestrial ecosystems, and fossil
fuels deposits (Fig. 1).
As can be seen, huge amounts of carbon are stored in the oceans (especially
the deep oceans), in the fossil fuels reserves, and the soils, compared
with what’s in the atmosphere. There is no returning arrow to the fossil
fuels to balance the outflow, at least not over timescales shorter than
millions of years, which means that the carbon released into the atmosphere
cannot be reabsorbed. In addition, change in land use also releases the
carbon stored in old forests over thousands of years into the atmosphere.
Figure 1. The Carbon Cycle
Standing carbon stocks in Gt C, carbon entering and leaving the atmosphere
in Gt C per year
The major driver of the carbon cycle is the biosphere, especially
on time scales of hours, days, to thousands of years. Carbon is assimilated
into the biosphere through photosynthesis, which turns carbon dioxide
from the atmosphere into sugars that provide the building blocks and energy
for all life forms. Carbon dioxide is returned to the atmosphere when
complex molecules are broken down in respiration to provide energy for
living activities.
The amount of carbon taken up by photosynthesis and released back
to the atmosphere by respiration each year is 1 000 times greater than
the amount of carbon flowing through the geological carbon cycle, which
operates over times scales of millions of years. Carbon dioxide dissolved
in water forms a weak carbonic acid, which slowly combines with calcium
and magnesium in the earth’s crust to form insoluble carbonates, a process
called weathering. Then through erosion, the carbonates are washed into
the oceans and eventually settle to the bottom. The cycle continues as
these materials are folded into the deep layers of the earth and the carbon
is the returned to the atmosphere as carbon dioxide during volcanic eruptions.
The biosphere also plays a role in the geological cycle. Land vegetation
enhances weathering of soil and the slow uptake of carbon dioxide from
the atmosphere to form carbonates. In the oceans, some of the carbon taken
up by phytoplankton (microscopic marine plants that form the basis of
the marine food web), zooplankton, and other organisms to make calcium
carbonate shells settles to the bottom of the ocean to form sediments
(see Box 2). Over millions of years when photosynthesis exceeded respiration,
organic matter accumulated to form coal and oil deposits, which removed
carbon dioxide from the atmosphere to carbon stores in geologic sediments.
Burning fossil fuels thereby destroys carbon stores that took millions
of years to build up.
There is no doubt that human activities have thrown the carbon cycle
out of balance, and carbon dioxide levels have risen 30 percent since
the industrial revolution around the middle of the 19th century.
Carbon dioxide is the predominant greenhouse gas that raises the temperature
of the earth, causing a general disruption of the climate.
Most of the increase in atmospheric carbon dioxide concentrations came
from the use of fossil fuels, but about 25 percent came from changes in
land use, as for example, through clearing forests for intensive agriculture.
Scientists at Woods Hole Research Center showed that between 1859 and
2000, about 155 Gt of C were released to the atmosphere from changes in
land use worldwide, the amount released each year generally increased
over the period, and by the 1990s, the rate of release averaged about
2 Gt C per year [8].
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On land, the primary
producers are almost all higher plants: trees, shrubs, grasses, ferns, mosses.
In the oceans, however, the primary producers are algae, especially microscopic
phytoplankton floating in the surface layers of water [9] (see Box
2).
Phytoplankton
the green fuse of life in the oceans
Although comprising only 0.2 percent of the earth’s total biomass, phytoplankton
is responsible for nearly 50 percent of the earth’s primary production on account
of its prodigious rate of growth, which is why it can support the incredibly
rich and diverse marine community from zooplankton to whales.
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Box
2
Phytoplankton
Phytoplankton consists
of single-celled algae of three main groups, diatoms, coccolithophores,
and dinoflagellates. Diatoms
have a pillbox-like outer shell made of silica. As silica is a relatively
insoluble in seawater, diatoms form a major component of the sediments in
areas of high productivity, most notably in a belt around the Antarctic
and in the North Pacific. If the sediment is at least 30 percent diatom
shells, it is known as diatomaceous ooze. The silica shells are very ornamental.
Centric diatoms are round, and pennates are elongated. Some species form
chains. Many have spines, increasing the surface area of the cell and retarding
predation. Coccolithophores are coated with calcium
carbonate plates or discs called coccoliths. When the cell dies, the coccoliths
sink to the bottom and contribute to the sediments. They are usually quite
small. Dinoflagellates typically
have an outer covering of cellulose plates that fit together like a jigsaw
puzzle. Cellulose is organic, and does not contribute to the sediments.
Dinoflagellates have flagella, whip-like
structures used for motility. Some dinoflagellates
are bioluminescent, producing “living light” while others are responsible
for ‘red tides’ - toxic algal blooms produced in polluted water that’s over-rich
in nutrients.
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Phytoplankton inhabits the top 100 m or so of seawater,
limited by the availability of light. Photosynthesis can still take place
at 1 percent of the light level and phytoplankton grows best not right at
the surface, but further down at about 100 m where the water is richer in
nutrients such as nitrogen, phosphorous and iron.
Most of the carbon in the upper ocean is recycled, but some of it
sinks into the deeper waters to feed the other organisms or to become buried
in the seafloor. And as organisms die, their bodies also sink to the seafloor
where decomposers feed on them to release carbon dioxide and other nutrients.
As a result the deep waters are rich in nutrients.
In certain regions of the ocean, currents interacting with the coast
or with other currents or both bring the cold deep nutrient-rich ocean water
to the surface. This ‘upwelling’ is important for keeping the phytoplankton
productive.
The latest evidence, obtained by towing a video camera submerged
in the surface layers of the ocean, suggests that nitrogen-fixing cyanobacteria
growing there may be supplying up to 50 percent of the nitrogen required for
the phytoplankton [10].
The enormous food web in the oceans that depend on phytoplankton makes phytoplankton
the most important primary producer. It is unique among primary producers
for growing at the fastest rates, with doubling times from hours to days depending
on the conditions [11]. Because of its prodigious rate of growth, phytoplankton
population also determines carbon balance in the oceans, which will impact on
climate change (Oceans
carbon sink or source? this series).
For phytoplankton to thrive there must be an appropriate supply of
nutrients. Too little, and the phytoplankton will fail to grow, too much all
at once from agricultural runoffs sewage or other industrial pollutants, and
algal blooms will result that could become toxic to fish, livestock and humans
eating shellfish that feed on the phytoplankton and accumulate the toxins
[12]. Moreover, the water must not be too warm, or too acid. All these conditions
are deteriorating on account of global warming, bringing the prospect of a
collapse in the marine biota, and turning the oceans into a massive carbon
source that would further aggravate global warming (see other articles in
this series).
We need urgent action to protect our oceans from pollution and over-exploitation.
At the same time, we need to replace fossil fuels with the many renewable energy
options that already exist (Which
Energy?).
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