What determines the Earth's Climate?

1. How much heat is received from the Sun?

* nuclear reactions in the Sun.

2. How much heat is reflected back into space ?

* character (color) of the Earth's surface - albedo

* composition of the atmosphere - greenhouse effect.

3. How is heat distributed over the Earth? Currents of air (winds) and water (oceans)

* difference in temperature between high-low latitudes

* location and elevation of the continents

What are the parts of the Earth System?

The outer Earth System (i.e., the exosphere) consists of

• atmosphere,
• hydrosphere (oceans, fresh water, ice caps), daisy.htm
• biosphere (all life)
• lithosphere (solid earth).

How is CO₂ important in the long-term earth's climate?

CO₂ in the atmosphere acts as a greenhouse gas and thus influences climate. If CO₂ is taken out of the atmosphere, and becomes part of the lithosphere (in limestone skeletons), or part of the biosphere (taken up by photosynthesis and made into organic matter), or becomes dissolved in the oceans (mainly dissolved bicarbonate, HCO₃^-, by dissolution of limestone) it can not influence climate. Organic matter can become part of the lithosphere (oil, coal, organic matter in soils).

What is positive feedback? Negative feedback?

A process with positive feedback has effects that reinforce the process itself. For example, cooling of the Antarctic continent make the ice cap grow larger which causes more heat to be reflected back into space which causes the continent to cool even more, and so on. A process with positive feedback is thus highly unstable. A process with negative feedback has effects that counteract the process itself. Processes with negative feedback thus lead to stability (e.g., thermostat).

What were differences between Earth in the early Archean (say, 4 billion years ago) and today's world?

• The Sun was not as hot.
• The Moon was formed by the collision of a Mars-size asteroid with early Earth, probably at some time during the big meteor bombardment that ended about 4 billion years ago.
• The Earth's rotation rate was faster (more days in a year) because tidal friction resulting from the presence of the Moon had not yet slowed it down.
• The continents were probably smaller.
• The atmosphere had a very different composition.

What was the composition of the atmosphere early in Earth history?

The primary atmosphere (from the solar nebula), dominated by hydrogen gas, H₂ (with ammonia, NH₃, and methane, CH₄ ) was lost. A secondary atmosphere was supplied by outgassing (dominantly CO₂, with some N₂, H₂O, minor CO, SO₂, H₂S).

How did the early atmosphere change into our present atmosphere, of dominantly N₂ (80%), 20% O₂, with trace constituents of other gases ?

After prokaryotic organisms (bacteria) invented photosynthesis, CO₂ was taken out of the atmosphere and put into biomass, O₂ was put into the atmosphere. Part of the biomass rotted again (using O₂), but part was buried in the lithosphere and thus did not use O2 in rotting. In addition, large amounts of CO₂ were stored in limestones (CaCO₃) by organisms making shells.

When did the atmosphere gain free oxygen gas?

Between about 2.2 and 1.9 billion years ago (Ga) the oxygen levels in the atmopshere increased from less than 1% of present atmospheric level to more than 15% of present atmospheric level.

What evidence do we have for oxygenation of the atmosphere?

• Paleosols (fossil soil horizons); minerals in the soils vary according to oxygen levels in the atmosphere.
• Continental Red Beds are red because grains are covered with iron oxide, and form only when there is enough oxygen in the atmosphere (form only after oxygenation).
• The mineral uraninite occurs in pebbly sediments that formed by the accumulation of gravel in ancient rivers. This mineral can not survive prolonged contact with free oxygen gas in the atmosphere (forms only before oxygenation).
• Banded Iron Formation (BIF) formed when oceans and atmosphere contained very little free oxygen, so that the waters could become very rich in iron (which is insoluble when much iron is around), for instance, coming out of hydrothermal vents. The iron have precipitated ONLY where it came in contact with free oxygen, produced by floating bacteria during their photosynthesis (form only before oxygenation).

What are stromatolites? What do they tell us about the evolution of life?

Stromatolites are thinly-layered limestones formed by as a result of the photosynthetic activity of bacteria. The occurrence of stromatolites thus tells us that fairly complex bacteria (able to photosynthesize) were around at the time that the stromatolites formed (at least by 3.5 billion years ago, maybe by 3.85 billion years).

How did long-term climate remain stable?

As a result of the 'Earth's thermostat'. Volcanoes release CO₂, which together with waters is used in weathering of minerals in the earth crust. The weathered material (including the CO₂ incorporated in dissolved bicarbonate) arrives in the oceans. In the oceans unicellular eukaryotic organisms secrete shells of silicon dioxide (opal) and calcium carbonate limestone; these shells accumulate on the ocean floor. When the ocean floor is subducted below a continent, the limestone and opal react under high temperature and pressure, and form silicate minerals as well as CO₂, which migrates out into the atmosphere, coming out of volcanoes. This thermostat have negative feedbacks. For example. More CO₂ leads to more weathering in which CO₂ is used up.

What is Gaia theory?

Gaia theory holds that life and non-life on Earth form a combined system, and have developed in very close connection over Earth's history. The close links between life and non-life are based on the chemical interaction of life (biosphere) and non-living matter in the atmosphere, hydrosphere and lithosphere: elements move through biogeochemical cycles between these four 'spheres'.

What is Daisyworld?

A very simple model which is set up to demonstrate by a simplistic example, how interactions between living organisms and global climate could work. In the example, life and climate are inter-related by the effect on the climate by changing albedo of the biota.

What are the most important greenhouse gases emitted by humans?

• CO₂
• methane (CH₄)
• nitrous oxide (N₂O)
• chlorofluorocarbons (CFCs)

Over the past few decades the combined warming effects of the last 3 other greenhouse gases together were comparable to that from CO₂.

Why is the pattern in increase of anthropogenic CO₂ emissions different from that of the other greenhouse gases?

Methane and nitrous oxide emissions are related to agricultural activity. Their increase is thus linked to the increase in human population. CFCs are produced by humans only (no natural source), and their production is phased out because of the problems with stratospheric ozone levels. CO₂ emissions are linked not only to population growth, but also to energy use. They thus increase with population growth as well as with improving standards of living in developing countries.

How long do greenhouse gases stay in the atmosphere?

Methane about 12 years; the others centuries or more.

About how large are the various carbon reservoirs in atmosphere, lithosphere, hydrosphere and biosphere?

You are not required to know these numbers, but you should have an approximate idea of their relative sizes; for example, you should know that the deep ocean reservoir is many times larger than the atmospheric reservoir, and that the biosphere reservoir on land is somewhat smaller than the atmospheric reservoir.

The World's Carbon Reservoirs

(after J. Kasting, 1998, www.gcrio.org/CONSEQUENCES/vol4no1/carbcycle.html

Reservoir	Size (Gigaton Carbon)
Atmosphere	750
Forests	610
Soils	1580
Surface ocean	1020
Deep ocean	38,100
Fossil fuels
Coal	4,000
Oil	500
Natural gas	500
Total fossils fuel	5,000

How much CO₂ is there in the atmosphere now? How does that compare to pre-industrial levels?

At the start of 1999, the air will contain roughly 365 ppm, corresponding to about 765 Gt C. Before the industrial revolution atmospheric concentrations were about 280 ppm. During the last ice age they were about 190 ppm.

Where does the CO₂ generated by fossil fuel burning go to?

Humans produce about 7.1 GtC (GtC = Giga Ton Carbon) per year by fossil fuel burning and deforestation. About two third of that accumulates in the atmosphere. We are not sure where the remaining third goes. A possible sink is the ocean; another is the biosphere (e.g., in places such as Connecticut, where there has been considerable re-growth of forests since the 1930s).

What are the ecosystems with highest diversity on Earth? What do these ecosystems have in common?

Tropical rainforests, coral reefs, deep ocean. Climatic stability.

What is the pacemaker of the ice ages?

Ice ages (periods of large ice caps at the poles) during the last 2.5 Ma occurred cyclically. The cycles in ice volume at the poles correspond in duration with cyclical variations in the amount of solar energy received at high northern latitudes as a result of irregularities in the pattern of the Earth's motion around the Sun, caused by the gravitational effects of the other planets circling the Sun. The cyclical variations are also called orbital variations, because they are variations in the orbit of the earth around the Sun.

What is the periodicity of the variabilities in solar energy received?

There are three periodicities (Milankovitch periodicities):

• Periodicity of about 100,000 years; caused by the variation in excentricity, or ellipticity of the Earth's orbit.
• Periodicity of 41,000 years, caused by variation in obliquity, i.e., the tilt of the Earth's axis away from the orbital plane.
• Periodicities of about 23,000 and 19,000 years, as a result of the precession of the equinoxes, a wobble of the Earth's axis affecting the direction of the earth's axis, not its tilt.

Do orbital variations in solar energy received (insolation) cause ice ages?

The orbital variations trigger ice ages, but are almost certainly not their sole cause: orbital fluctuations have always existed during earth history, but there were not always ice caps at the poles. The variations in insolation are too small to cause the large variations in ice volume. Their must be positive feedbacks within the earth system that enlarge the effects of the orbital variations (e.g., involving albedo, ocean productivity or ocean circulation).

What isotopes are studied most commonly in paleoclimate studies?

Hydrogen isotopes and oxygen isotopes in ice cores; oxygen and carbon isotopes in carbonate shells in samples from cores taken from the sea floor. Carbon isotopes are also studied in organic matter to find out which photosynthetic process occurred (C3 or C4 photosynthesis), or what organisms ate (you are what you eat).

Which type of photosynthesis is more effective at higher CO₂ level? What type of photosynthesis do we see in most crop plants?

C3 plants must keep openings in their leaves (stomata) open during photosynthesis, thus lose water by evaporation. They also may lose part of the CO₂ they are trying to use in photosynthesis. Therefore C3 plants outcompete C4 plants at higher CO2 levels. But C4 plants outcompete C3 plants at places where water loss through the open stomata is a problem (i.e., where it is dry). Most crop plants are C3 (except corn which is C4).

What climate parameters are studied by using these isotopes?

Hydrogen and oxygen isotopes tell us about the temperature at which the snow fell that made the ice in the cores. Oxygen isotopes in carbonates tell us about the temperature of formation of carbonate, but in combination with the size of polar ice caps. Carbon isotopes in carbonates do not primarily tell us about climate, but they tell us about productivity in the oceans, and about the total mass of the biosphere (and thus the occurrence of mass extinctions).

Why are ice sheets isotopically light (in hydrogen as well as oxygen isotopes)?

As a result of Rayleigh fractionation. Water evaporates in large amounts in the tropics, and there is net transport of water from low to high latitudes (where the ice caps are). During evaporation, the vapor phase becomes enriched in the lighter isotope. During condensation of rain, the liquid phase (water) becomes enriched in the heavier isotope. All rain is isotopically heavier than the vapor that if formed from, and as water vapor travels from low to high latitude it thus becomes more and more enriched in the heavy isotope (with every time part of the vapor falls as rain or snow).

What is isotope fractionation?

The (partial) separation of isotopes of an element as result of chemical or physical processes.

What methods do we have to calculate high latitude temperatures from ice core data?

Oxygen and/or hydrogen isotopes in ice; direct temperature measurements in the ice caps ('frozen turkey' model), nitrogen isotopes in ice.

What have oxygen isotope studies studies told us about the development of climate in the Cenozoic?

Climate cooled during the studies, but not gradually. Major steps in cooling occurred in the earliest Oligocene (about 33.5 Ma), when the east Antarctic ice sheet formed in about 100,000 years, in the middle Miocene (about 14.6 Ma) when the West Antarctic ice sheet may have formed, and at 3.0-2.5 Ma, when extensive northern hemispheric ice sheets formed, which have waxed and waned ever since.

What are foraminifera?

Foraminifera are unicellular eukarotic organisms (protists) which live in the oceans and make a shell (test) of calcium carbonate. They are heterotrophs (eat other organisms and do not photosynthesize.

What protists contribute shells to oceanic sediments (oozes)?

The heterotrophic foraminifera and the autotrophic calcareous nannoplankton contribute calcium carbonate shells, the hetrotrophic radiolarians and the autotrophic diatoms contribute opal (SiO2) shells.

In what regions of the present oceans do we find anoxic waters?

• In marginal basins where water stagnate (such as the Black Sea).
• Along continental margins in places where oceanic productivity is high: all the organic matter rots, using up oxygen.

How were the oceans during Greenhouse Periods on earth different from the present oceans?

1. The deep waters sinking could take up much less dissolved oxygen from the atmosphere, because oxygen is less soluble at higher temperatures. The deep oceans thus could become anoxic when the little oxygen was used up by rotting of organic matter on the bottom.
2. Sea level was higher because there were no ice sheets, so that the continental shelves were flooded and there was less land above sea level, leaving less land surface to be weathered and fewer nutrients reaching the oceans so that oceanic productivity was low.
3. The average winds were probably less vigorous than they are today because the strength of surface winds depends upon the temperature differences between low and high latitudes. Less winds means active surface currents, meaning that deep waters can not well up to the surface very actively. These upwelling deep ocean waters carry the nutrients from rotting dead organisms back to the surface where primary productivity can take place (sun light only penetrates a little bit into ocean waters. If this upwelling occurs less vigorously, fewer nutrients reach the primary producers (algae) in the surface waters, and productivity in the oceans declines.

Points 2 and 3 suggest that the overall productivity of the oceans was lower during warm periods.

What is the scenario for the period of rapid global warming and extinction of deep-sea benthic foraminifera at the end of the Paleocene (55.5 million years ago)?

• The world warmed gradually, possibly as a result of emission of lots of CO₂ from large flood basalts in the north Atlantic Ocean ,which opened at the time.
• This warming was most pronounced at high latitudes, as is always the case with CO₂-induced warming.
• The surface waters at high latitudes, close to the poles, became very warm and had such little density that they could no longer sink to the bottom of the oceans.
• The deep oceans became filled with waters coming from lower latitudes, which were warm and salty, coming from areas with lots of evaporation. They were dense because of their high salt content.
• The warmer waters caused dissociation of methane hydrates; this caused more warming.
• The foraminifera on the bottom of the oceans died because oxidation of methane in the oceans used up quite a lot of the oxygen in the oceans.

How could an Oceanic Anoxic Event have started and ended?.

• Release of gas (methane) hydrates caused extreme warming (shown in the oxygen isotopic records).
• Oxidation of the methane used up what little oxygen there was in the oceans.
• Organic matter was stored in the oceanic sediments, because it could not rot in the absence of oxygen in the ocean waters.
• Storage of organic matter also meant that carbon dioxide was once again taken out of the atmosphere, and the world cooled once again. And the cooling of the ocean waters brought oxygen back into the deep oceans.

How could evolution of the biosphere have influenced climate?

• Evolution of photosynthesis caused decrease of CO₂ in the atmosphere because organic matter formation uses up CO₂, produces oxygen. Burial of the organic matter prevents its re-oxidation.
• Evolution of burrowing animals at the beginning of the Cambrian period (550 million years ago) influenced the recycling of carbon from the ocean floor into the ocean (Snowball Earth).
• Evolution of land plants and establishment of forests in the late Devonian (about 415 million years ago) resulted in the formation of coal deposits, lowering of carbon dioixde levels in the atmosphere.
• Evolution of Angiosperm plants (flowering plants) caused storage of carbon in soils (taking more CO₂ from the atmosphere because deciduous trees produce more leaves seasonally than gymnosperm plants (such as evergreens).

How can we judge whether ice sheets occurred on Earth?

From the rock record. In the oceans, we use the occurrence of ice-rafted material, including drop stones. On land, we use various types of rocks, including rocks that were scratched by glaciers, and mixtures of coarse boulders, sand and clay that were dropped at the end of glaciers by the melting ice. We also can recognize sediments deposited in lakes close to glaciers (as seen during the excursion in the sand pit).

How could plate tectonics have played a role in the Cenozoic cooling?

• Continental position: ice caps can form only when there are continents in polar position (Antarctica) or marginal seas surrounded by continents (Arctic Ocean).
• Location of ocean gateways. The gateway between the Antarctic peninsula and South America: its opening allowed the formation of the Antarctic Circumpolar Current and the thermal isolation of Antarctica, thus possible causing formation of the Antarctic ice cap. Panamanian gateway: its closure may have caused sinking of water in the North Atlantic, keeping these warmer waters out of the Arctic Ocean, thus contributing to formation of northern hemisphere ice sheets.
• Large flood basalt volcanism can put much carbon dioxide into the atmosphere which causes greenhouse warming. There were no large flood basalt episodes in the Cenozoic after about 55 Ma, although there was a smaller episode at about 17 Ma.
• The collision of the Indian subcontinent and the Asian continent resulted in the formation of the Himalayas. The Himalayas may have influenced climate directly (location of jet stream), and indirectly, because large mountain ranges offer much opportunity for weathering rocks, and weathering rocks takes up carbon dioxide (see 'thermostat of the earth).
• Intensive weathering of rocks could have added more elements such a nitrogen and phosphorus to the oceans, which acted as fertilizer and caused increased productivity in the oceans, which would have resulted in removal of even more CO₂.

What is the pattern of present deep-sea circulation?

In the present day oceans, the waters at the bottom of all oceans close to freezing, because the deep waters fill the oceans at the poles only: at these high latitudes the waters are very cold. If sea-ice forms (which is not salty - the salt remains behind when sea water freezes), the remaining water is very cold as well as very salty. This means that it has a high density, and sinks to the ocean floors. Deep waters stream from the North Atlantic (close to Greenland) to the South Atlantic, where they mix with waters streaming out of the Weddell Sea (Antarctica). This cold mix streams around the Antarctic continent, with currents flowing over the bottom into the Indian and Pacific Oceans. It takes water that sinks in the northern Atlantic about 1000 years to reach the northern Pacific; the upwelling occurs much more diffuse than the sinking.

How could global warming as the result of anthropogenic CO₂ emissions cause a change in deep sea circulation?

CO₂induced warming is more pronounced at high than at low latitudes. High latitude warming can cause melting of the northern hemispheric ice sheets. The melt water is cold but has a very low salinity, and thus also has a low density. Such low density waters can not sink to the bottom of the oceans. The presence of such melt waters could thus inhibit the formation of North Atlantic Deep water.