The ocean breathes

The ocean carbon reservoir is larger than either the land surface or the atmosphere. The ocean's carbon is not only dead, but it is also oxidized: energetically dead as well as biologically dead. The carbon is in the forms CO2, H2CO3, HCO-, and CO=, all of which we will get to know better in Chapter 10. The sum of all of them is called dissolved inorganic carbon. The ocean dissolved inorganic carbon pool is larger than the atmospheric pool by a factor of about 50. There is also dissolved organic carbon in the ocean: dead, scrambled molecules similar to soil organic carbon. And of course there are fish and dolphins and plankton, but the total amount of carbon in the ocean that lives is small, only about a Gton.

Carbon is released from the water to the air in some parts of the world ocean and dissolves into the water in other places. The total rates of exchange are large, comparable with the breathing rates of the terrestrial biosphere and much greater than the rate of anthropogenic carbon release (Chapter 9). The oceans would seem to have the

1965

1975 1985

Year

1995

Fig. 8.2 CO2 concentrations in the atmosphere over the past decades.

1955

1965

1975 1985

Year

1995

2005

Fig. 8.2 CO2 concentrations in the atmosphere over the past decades.

clout to affect the CO2 in the atmosphere as strongly as the terrestrial biosphere does, and yet the year-to-year breathing variations in the ocean are much less obvious in variations in atmospheric CO2. The ocean simply breathes more slowly than the land does.

The atmosphere absorbs CO2 from the ocean until the rates of CO2 into and out of the ocean balance each other. Computer model experiments show that it takes hundreds of years for the atmospheric CO2 concentration to approach this equilibrium value. The reason it takes so long is that the ocean takes about this long to circulate, for all the waters of the ocean to come to the sea surface somewhere to exchange carbon with the atmosphere.

The clearest example of the power of the ocean to affect CO2 in the atmosphere is the glacial/interglacial cycles. The geologic record of the last 500 million years shows Earth going into an ice age every 150 million years or so. We are in an ice age now, and have been for about 2 million years. During an ice age, the amount of ice, and the climate of the Earth, fluctuate rhythmically between glacial states and interglacial states (Fig. 8.3). We are currently in an interglacial state, having been for 10,000 years. During the Last Glacial Maximum, global temperature was 5-6 K colder than today

Age (kyear BP)

Fig. 8.3 CO2 and methane concentrations in the atmosphere over the past 400,000 years, along with temperature in Antarctica.

Age (kyear BP)

Fig. 8.3 CO2 and methane concentrations in the atmosphere over the past 400,000 years, along with temperature in Antarctica.

Fig. 8.4 The precession orbital cycle.

and much of North America and Northern Europe were covered with a massive ice dome, like what currently exists in Greenland.

The beat of the ice age rhythm apparently derives from variations in the Earth's orbit around the Sun. The orbit varies through three main cycles, each ringing on its own characteristic frequency. The first cycle is called the precession of the seasons or sometimes precession of the equinoxes (Fig. 8.4). The axis of rotation of the Earth spins around like a wobbling top, completing the entire circle in 20,000 years. Most of the solar heat influx variability at high latitudes derives from precession, and nearly all of the variability in the tropics comes from precession.

North pole

North pole

Equator

Orbit

South pole

Fig. 8.5 The obliquity orbital cycle.

Precession has an effect on climate because the Earth's orbit is not circular but elliptical. Where we are in the precession cycle at present, the Earth is closest to the Sun during winter in the northern hemisphere (Fig. 8.4). The seasonal cycle of solar heat flux in the northern hemisphere is weakened by this orientation, because the Earth is close to the Sun when the northern hemisphere is tilted away from the Sun. The seasonal cycle of solar heating is stronger in the southern hemisphere now because the Earth is close to the Sun and tilted toward the Sun at the same time. Remember, it is the tilt of the Earth that causes the Earth's seasons (Chapter 6). The precession cycle merely modifies the seasons somewhat. The history of precession cycle over the past 400,000 years is shown in Fig. 8.6.

Another cycle involves the obliquity of the angle of the pole of rotation, relative to the plane of Earth's orbit (Fig. 8.5). The Earth rotates, making day and night, on a rotation axis of the north and south poles. This rotational axis is not perpendicular to the plane of Earth's orbit around the Sun, but is tilted somewhat. The angle of tilt is currently 23.5°, but it varies between 22° and about 25.5°, on a cycle time of about 41,000 years (Fig. 8.6). The impact of obliquity on the solar heating flux is stronger in high latitudes.

The third orbital cycle involves how elliptical the orbit of the Earth is, also called its eccentricity. At present, the orbit of the Earth is nearly circular. The eccentricity of the orbit has cycles of 100,000 and 400,000 years (Fig. 8.6). The strongest climate impact of eccentricity is to determine the strength of the precessional forcing. If the Earth's orbit were circular (as it is nearly now), it would make no difference where the Earth was in its precession cycle because the Earth is equally far from the Sun at all parts of the orbit. When eccentricity is low, the orbit is circular, and the 20,000 year waves in the precession cycle vanish (shaded area in Fig. 8.6).

When you average over the entire surface of the Earth and over the entire year, the orbital cycles only have a tiny effect on the amount of heat the Earth gets from the Sun. The orbital cycles affect climate by rearranging the intensity of sunlight from one place to another, and from one season to another. It turns out that the climate of the Earth is especially sensitive to the solar heat flux at about 65° latitude in the northern hemisphere summer. This is like the solar plexus of global climate; a sucker

Brain Waves

Thousands of years ago

Fig. 8.6 History of orbital forcing of Earth's climate, since 400,000 years ago.

Thousands of years ago

Fig. 8.6 History of orbital forcing of Earth's climate, since 400,000 years ago.

punch there and the whole climate keels over. This is because the northern hemisphere is where the ice sheets are that come and go with glacial climate. The ice sheets in Antarctica and Greenland persisted through recent interglacial climate stages, although they do change their sizes. The production of an ice sheet in the northern high latitudes drags the whole world into a glacial climate. The intensity of summertime sunlight seems to be important, rather than wintertime, because it is always cold enough to snow in winter at these latitudes. The question is whether the summer is warm enough to melt the snow in the summer. Variations in sunlight intensity in June, 65° north latitude, calculated from models of the Earth's orbit, correlate well with the history of the amount of ice on Earth, inferred from ice core and deep sea sediment records.

The link from glacial cycles to the CO2 in the atmosphere and the ocean comes from bubbles of ancient atmosphere trapped in the ice sheet of Antarctica. The trapped air bubbles from glacial times have a lower proportion of CO2 in them, relative to the other gases in the bubble. The CO2 concentration during glacial intervals is 180-200 ppm, rising to 260-280 ppm during interglacial intervals before the industrial era. The decreased CO2 concentration during glacial time is responsible for about half of the cooling relative to the interglacial time. The increase in the albedo of the Earth, from the large ice sheets, is responsible for the other half. The climate during glacial time is an important test of the climate models we use to forecast global warming (Chapter 11).

No one is sure exactly why CO2 in the atmosphere cycles up and down along with the ice sheets, but the ocean must be the major player. The land carbon reservoirs were if anything smaller during the glacial time, which by itself would have left atmospheric CO2 higher, not lower as we see. The carbon in rocks, discussed in the next section, cycles too slowly to explain the large fast changes. The ocean is the only pool that is large enough and potentially reactive enough to explain the data. We will not answer the question of how exactly the ocean did this, but we will discuss ocean carbon chemistry a bit further in Chapter 10.

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Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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