Carbon dioxide photosynthetic limit

A plant needs four major things to survive and grow- water, sunlight, warmth, and atmospheric CO2. Of these four things, CO2 is the most rare on Earth and the most difficult for the plant to obtain.

Carbon dioxide (CO₂) is a vitally-important trace gas in the Earth's atmosphere and an integral part of the geo-biological carbon cycle. It currently constitutes only 0.04% (400 parts per million) of the atmosphere. Despite its relatively small concentration, atmospheric CO₂ provides the crucial, scarce element- Carbon- that terrestrial plants need in order to grow and to create the biosphere. Plants and other living things use solar energy to make their carbohydrates, and all of their parts, from atmospheric carbon dioxide and water by photosynthesis. These carbohydrates derived from plants are food, the primary source of energy and carbon compounds for animals and almost all other organisms.

The rate of plant growth depends on several factors- the amount of sunlight and water, the temperature, and the amount of carbon dioxide in the air. All else equal, plants are more vigorous when there is more carbon dioxide available for them. At current atmospheric pressures, the rate of photosynthesis, and so plant growth rates, slows down considerably when atmospheric CO₂ concentrations fall below perhaps 200 ppm. This is a prime driver of global ecological shifts as different types of plants become more or less competitive at different concentration levels. The CO₂ concentration dropped so low (though still higher than today's) a few million years ago that many different kinds of plants independently invented a new method of extracting it from the atmosphere. At even lower concentrations, terrestrial plants would no longer be able to grow at all. A mass extinction would result.

Atmospheric CO₂ The text in this featured NASA [[1]] says "For 650,000 years..." but the graph only goes back perhaps 425,000 years. The years before today are not aligned with the vertical chart lines, and so it is not quite clear how many years this represents. The dips to about 180ppm were during the coldest parts of four separate Ice Ages.

Reconstructions show that concentrations of CO₂ in the atmosphere have varied considerably, ranging from as high as perhaps 7,000 parts per million during the Cambrian period about 500 million years ago to as low as 180 parts per million during several Ice Ages of the past two million years. The most recent of these events was only a few thousand years ago. CO₂ levels nearly doubled in the intervening years from that Ice Age to the Renaissance.

Although the reconstructions differ from each other in the details, all of them show a striking, overall trend: over the ages, the level of CO₂ in the atmosphere has been decreasing, and biologically-available Carbon has been slowly removed from the biosphere. This Lost carbon is being irretrievably locked away, sequestered, in massive, geological formations. The Photosynthetic Limit, the point where there is not enough CO₂ left in the atmosphere to support life on land, has been approaching.

Atmospheric carbon dioxide and photosynthesis

Main article: Terrestrial biological carbon cycle

See also: photosynthesis, Carbon fixation, C4 carbon fixation, and C3 carbon fixation

Photosynthesis changes sunlight into chemical energy, splits water to liberate O₂, and fixes CO₂ into sugar.

Carbon dioxide in the Earth's atmosphere is essential to life and to the present planetary biosphere. Over the course of Earth's geologic history, variations in CO₂ concentrations have played an important part in biological evolution. The first photosynthetic organisms probably evolved early in the evolutionary history of life and most likely used reducing agents such as hydrogen or hydrogen sulfide as sources of electrons, rather than water. Cyanobacteria appeared later, and the excess oxygen they produced contributed to the oxygen catastrophe, which rendered the evolution of complex life possible. In recent geologic times, low CO₂ concentrations below 600 parts per million might have been the stimulus that favored the evolution of C4 plants which increased greatly in abundance between 7 and 5 million years ago over plants that use the less efficient C3 metabolic pathway.

Today, the average rate of energy capture by photosynthesis globally is approximately 130 terawatts, which is about six times larger than the current power consumption of human civilization. Photosynthetic organisms also convert around 100–115 thousand million metric tonnes of carbon into biomass per year.

Photosynthetic organisms- plants- are photoautotrophs, which means that they are able to create themselves directly from CO₂ and water using energy from light. In plants, algae and cyanobacteria, photosynthesis releases oxygen. This is called oxygenic photosynthesis. Although there are some differences between oxygenic photosynthesis in plants, algae, and cyanobacteria, the overall process is quite similar in these different organisms.

Carbon dioxide is converted into sugars in a process called carbon fixation. Carbon fixation is an endothermic redox reaction, so photosynthesis needs to supply both a source of energy to drive this process, and the electrons needed to convert CO₂ into a carbohydrate. This addition of the electrons is a reduction reaction. In general outline and in effect, photosynthesis is the opposite of cellular respiration, in which glucose and other compounds are oxidized to produce CO₂ and water, and to release exothermic chemical energy to drive the organism's metabolism. However, the two processes take place through a different sequence of chemical reactions and in different cellular compartments.

Impact on plant growth rates

A 1993 review of the scientific literature found that a doubling of CO₂ concentration would stimulate the growth of 156 different plant species by an average of 37%. The amount of gain varied significantly by species, with some showing much greater gains, and a small number showing a loss. For example, a 1979 greenhouse study compared the dry weights of cotton and maize plants grown in different glass houses, one with double the CO₂ concentration of the other. In the enriched CO₂ air, the dry weight of 40-day-old cotton plants doubled, but the dry weight of 30-day-old maize plants, which use the C₄ photosynthesis pathway and so require more energy, increased by only 20%.

For this reason, commercial greenhouse operations employ carbon dioxide generators for CO₂ enrichment to boost plant yields in a closed environment.

The competitive advantages between C₃ and C₄ plants under different conditions (CO₂ concentration, humidity, etc.) has apparently already caused a noticeable change in the metabolism of some plants. The study analysed several different C3 plants, i.e. plants which collectively account for the majority of global photosynthesis, and of calories for human nutrition. In historic beet sugar samples that grew at different times between 1890 and 2012, the researchers observed a change in metabolic fluxes, which can fully be explained as CO2-driven shift

In general, as CO₂ concentration increases, environmental conditions tend to favor the older, less efficient C₃ type of plants, which evolved at much higher levels of atmospheric CO₂. Decadal-scale metabolic responses of plants to environmental changes, including the magnitude of the “CO₂ fertilization” effect, are a major knowledge gap in Earth system models... Trends in a deuterium isotopomer ratio allow quantification of a biogeochemically relevant shift in the metabolism of C₃ plants toward photosynthesis, driven by increasing atmospheric CO₂ since industrialization.

[It is a] well-established fact that CO₂ is a powerful aerial fertilizer, which when added to the air can substantially increase the vegetative productivity of nearly all plants…numerous studies have demonstrated that the percent increase in growth produced by an increase in the air’s CO₂ content typically rises with an increase in air temperature. In addition, at the species-specific upper-limiting air temperature at which plants typically die from thermal stress under current atmospheric CO2 concentrations, higher CO₂ concentrations have been shown to protect plants and help them stave off thermal death…[and] increase the species-specific temperature at which plants grow best. Indeed it has been experimentally demonstrated that the typical CO₂-induced increase in plant optimum temperature is as great as, if not greater than, the CO₂-induced global warming typically predicted…Hence, [with] an increase in the air’s CO₂ concentration – even if it did have a tendency to warm the earth (which is hotly debated) – …[plants] …would grow equally well, if not better, in a warmer and CO₂-enriched environment.

At current atmospheric pressures photosynthesis slows down when atmospheric CO₂ concentrations fall below 200 ppm although some microbes can extract carbon from the air at much lower concentrations.

The effect of CO₂ concentrations on plant growth rates has been systematically underestimated in climate models.

The rise of C₄-fixation plants

Further information: Evolutionary history of plants § Evolution of photosynthetic pathways

Photosynthetic limits of C3 and C4 pathways. Vertical axis is the net photosynthetic uptake of CO2 in micro-moles per second per square meter of leaf area. C₃ plant growth stops entirely at 100 ppm CO₂ concentration. The newer C₄ plants can extract atmospheric CO₂ down to perhaps 20 ppm. The rise in atmospheric CO₂ from 300 ppm to 400 ppm has greatly benefited the older, C₃ plants. C₄ plant cannot take advantage of any further rise in CO₂.

C₄ plants have a competitive advantage over plants possessing the more common C₃ carbon fixation pathway under conditions of drought, high temperatures, and nitrogen or CO₂ limitation. When grown in the same environment, at 30 °C, C₃ grasses lose approximately 833 molecules of water per CO₂ molecule that is fixed, whereas C₄ grasses lose only 277. This increased water use efficiency of C₄ grasses means that soil moisture is conserved, allowing them to grow for longer in arid environments.

C₄ carbon fixation has evolved on perhaps 40 independent occasions in different families of plants, making it a prime example of convergent evolution. This convergence may have been facilitated by the fact that many potential evolutionary pathways to a C₄ phenotype exist, many of which involve initial evolutionary steps not directly related to photosynthesis. C₄ plants arose around 25 to 32 million years ago during the Oligocene (precisely when is difficult to determine) and did not become ecologically significant until around 6 to 7 million years ago, in the Miocene Period. C₄ metabolism originated when grasses migrated from the shady forest undercanopy to more open environments, where the high sunlight gave it an advantage over the C₃ pathway. Drought was not necessary for its innovation; rather, the increased resistance to water stress was a by-product of the pathway and allowed C₄ plants to more readily colonise arid environments.

Today, C₄ plants represent about 5% of Earth's plant biomass and 3% of its known plant species. Despite this relative scarcity, they account for about 30% of terrestrial carbon fixation. Increasing the proportion of C₄ plants on earth could remove biosequestration CO₂. This is proposed as a climate change (meaning global warming due to greenhouse gas) avoidance strategy. Present-day C₄ plants are concentrated in the tropics and subtropics (below latitudes of 45°) where the high air temperature contributes to higher possible levels of oxygenase activity by RuBisCO, which increases rates of photorespiration in C₃ plants.

Plants that use C₄ carbon fixation

About 7,600 plant species use C₄ carbon fixation, which represents about 3% of all terrestrial species of plants. All these 7,600 species are angiosperms. C₄ carbon fixation is less common in dicots than in monocots, with only 4.5% of dicots using the C₄ pathway, compared to 40% of monocots. Despite this, only three families of monocots utilise C₄ carbon fixation compared to 15 dicot families. Of the monocot clades containing C₄ plants, the grass (Poaceae) species use the C₄ photosynthetic pathway most. Forty-six percent of grasses are C₄ and together account for 61% of C₄ species. These include the food crops maize, sugar cane, millet, and sorghum.

Converting C₃ plants to C₄

Given the advantages of C₄, a group of scientists from institutions around the world are working on the C₄ Rice Project to turn rice, a C₃ plant, into a C₄ plant. As rice is the world's most important human food—it is the staple food for more than half the planet—having rice that is more efficient at converting sunlight into grain could have significant global benefits towards improving food security. The team claims that C₄ rice could produce up to 50% more grain—and be able to do it with less water and nutrients.

Lost carbon

Main article: Lost carbon

Most of the Carbon on Earth has never been available to make life. Almost all of what once was available has been lost, turned to stone. There are several ways that carbon is lost to the biosphere. In the most important method, carbon dioxide or carbonate is dissolved in oceans and lakes. Life forms make it into shells and skeletons. Some of these fall to the bottom to become carbon-bearing sediment. The sediment is turned into rocks of different kinds. Most of the carbon in these new rocks can't ever again be accessed by life forms. It is "Lost Carbon". Another important method of locking up carbon is the formation of coral reefs. Some of these became so big they are now known by the name of the country that sits on top of them.

It is estimated that the solid earth as a whole contains 730 ppm of carbon, with 2000 ppm in the core and 120 ppm in the combined mantle and crust. If the mass of the earth is 5.972 × 10²⁴ kg, this implies a mass of 4360 million gigatonnes of carbon (abbreviated as GtC) inside the Earth. This is thousands of times more than the amount of carbon actually available in the biosphere. It is, for the most part, Lost Carbon that has always been unavailable.

Carbon is a major component of carbonate rock (limestone, dolomite, marble and so on), which are formed into massive geological features. Coal is the largest commercial source of mineral carbon, accounting for 4,000 gigatonnes or 80% of fossil carbon fuel. It is also rich in carbon – the highest grades of anthracite contain 92–98%.

In combination with oxygen in carbon dioxide, carbon is found in the Earth's atmosphere (approximately 810 GtC) and dissolved in all water bodies (approximately 36,000 GtC). Around 1,900 GtC are present in the biosphere. Hydrocarbons (such as coal, petroleum, and natural gas) contain carbon as well. Coal usable "reserves", not "resources") amount to around 900 gigatonnes with perhaps 18,000 Gt of potential resources. Oil reserves are around 150 gigatonnes. Proven sources of natural gas are about 175 10¹² cubic metres (representing about 105 GtC), but it is estimated that there are also about 900 10¹² cubic metres of "unconventional" gas such as shale gas, containing about 540 GtC. Carbon is also locked up as methane hydrates in polar regions and under the seas. Estimates of the amount of carbon in methane hydrates range from 500 to 2500 GtC, and up to 3000 GtC.

Limestone makes up about 10% of the total volume of all sedimentary rocks on Earth. Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO₃). Most limestone is composed of skeletal fragments of marine organisms such as coral, forams and molluscs. By various estimates there are at least 5,000,000 Gt, and perhaps more than 100 million Gt, of various kinds of limestone in the crust of the Earth. Most of it was formed by marine-biological processes over the course of life on Earth. Carbon is a small percentage of the total mass. Calcium carbonate (CaCO₃), the chief ingredient in limestone, is 12% Carbon by weight, but there are other kinds of minerals that generally reduce that percentage. So the total Lost Carbon locked up in the Earth's limestone may be somewhere between a few hundred thousand GtC and a few million GtC.

Carbon cycles

Atmospheric carbon dioxide and the carbon cycle

Main articles: Carbon cycle and Atmospheric carbon cycle

This diagram of the fast carbon cycle shows the movement of carbon between land, atmosphere, and oceans in billions of tons of carbon per year. Yellow numbers are natural fluxes, red are human contributions in billions of tons of carbon per year. White numbers indicate stored carbon.

The term “carbon cycle” is used to describe the movement, sequestering, release, and recycling of elemental Carbon throughout the Earth’s land, sea and air. Atmospheric carbon dioxide plays an integral role in the Earth's carbon cycle, as carbon dioxide is removed from the atmosphere by some natural processes and added back to the atmosphere by other natural processes. There are two broad carbon cycles on earth, called the ”fast carbon cycle” and the “slow carbon cycle”. The fast carbon cycle refers to movements of carbon between the environment and living things in the biosphere; the slow carbon cycle involves the movement of carbon between the atmosphere, oceans, soil, rocks and volcanism. Both carbon cycles are interconnected.

Plants, algae and cyanobacteria convert carbon dioxide to carbohydrates by a process called photosynthesis. They gain the energy needed for this reaction from absorption of sunlight by chlorophyll and other pigments. Oxygen, produced as a by-product of photosynthesis, is released into the atmosphere and subsequently used for respiration by heterotrophic organisms and other plants, forming a cycle.

Most sources of CO₂ emissions are natural, and are balanced to various degrees by natural CO₂ sinks. For example, the natural decay of organic material in forests and grasslands and the action of forest fires results in the release of an estimated 439 gigatonnes of carbon dioxide every year, while new growth entirely counteracts this effect, absorbing 450 gigatonnes per year. Volcanic activity releases only 130 to 230 megatonnes of carbon dioxide each year. These natural sources are nearly balanced by natural sinks, physical and biological processes which remove carbon dioxide from the atmosphere. There is a large natural flux of CO₂ into and out of the biosphere and oceans. Currently, an estimated 57% of human-emitted CO₂ is either taken up by the biosphere or dissolved in the oceans. From pre-industrial era to 2010, the terrestrial biosphere represented a net source of atmospheric CO₂ prior to 1940, switching subsequently to a net sink. The ratio of the increase in atmospheric CO₂ to emitted CO₂ is known as the airborne fraction (Keeling et al., 1995); this varies for short-term averages and is typically about 45% over longer (5 year) periods. Estimated carbon in global terrestrial vegetation increased from approximately 740 billion tons in 1910 to 780 billion tons in 1990. This may be an early example of the beneficial effects of artificial atmospheric carbon dioxide enrichment.

Atmospheric carbon dioxide and the oceanic carbon cycle

Main article: Oceanic carbon cycle

Air-sea exchange of CO₂. The two sedimentation down-arrows are the main exit paths to Lost Carbon.

The Earth's oceans contain a large amount of reacted CO₂ in the form of bicarbonate and carbonate ions — much more than the amount in the atmosphere. Although most of the oceanic CO₂ remains in solution, a small fraction of it reacts to form carbonic acid. The bicarbonate is produced in reactions between rocks, water, and carbon dioxide. One example is the dissolution of calcium carbonate:

CaCO₃ + CO₂ + H₂O ⇌ Ca²⁺ + 2 HCO₃^-

Reactions like this tend to buffer changes in atmospheric CO₂. Since the right side of the reaction produces an acidic compound, adding CO₂ on the left side decreases the pH of sea water, a process which has been unfortunately termed ocean acidification (the pH of the ocean becomes less alkaline although the pH value remains in the alkaline range). Reactions between CO₂ and non-carbonate rocks also add bicarbonate to the seas. This can later undergo the reverse of the above reaction to form carbonate rocks, releasing half of the bicarbonate as CO₂. Over hundreds of millions of years, this has produced huge quantities of carbonate rock formations, such as limestone beds, marble, and chalk. All of this carbon, which is most of the carbon near the surface of the Earth, has been removed from the biosphere and is no longer available to support life. As this process continues, there will come a time, perhaps in a few thousand or millions of years, where the remaining bio-available carbon can no longer supply enough CO₂ to keep the atmospheric concentration above the minimum needed for photosynthesis.

Ultimately, most of the CO₂ sent into the air by human activities will dissolve in the ocean; however, the rate at which the ocean will take it up in the future is less certain. Even if equilibrium is reached, including dissolution of carbonate minerals, the increased concentration of bicarbonate and decreased or unchanged concentration of carbonate ion will give rise to a higher concentration of un-ionized carbonic acid and dissolved CO₂. This, along with higher temperatures, would mean a higher equilibrium concentration of CO₂ in the air, temporarily countering the sequestration effects of carbonate rock formation. (Needs a fact check)

Current concentration

See also: Global warming controversy, Climate change, Atmospheric methane, Holocene climate, and Quaternary climate

CO₂ concentrations over the last 400,000 years

Over the past 400,000 years, CO₂ concentrations have shown several cycles of variation from about 180 parts per million (near the Photosynthesis Limit) during the deep glaciations of the Holocene and Pleistocene to 280 parts per million during the interglacial periods. Following the start of the Industrial Revolution, atmospheric CO₂ concentration has increased to 400 parts per million and continues to increase.

420,000 years of Atmospheric CO₂ (grey line) plus Atmospheric methane (black line) compared with global temperature variations (red line).

The global average concentration of CO₂ in Earth's atmosphere is currently about 0.04%, or 400 parts per million by volume (ppm). There is an annual fluctuation of about 3–9 ppm which is negatively correlated with the Northern Hemisphere's growing season. The Northern Hemisphere dominates the annual cycle of CO₂ concentration because it has much greater land area and plant biomass than the Southern Hemisphere. Concentrations reach a peak in May as the Northern Hemisphere spring greenup begins and decline to a minimum in October when the quantity of biomass undergoing photosynthesis is greatest.

Workers closely monitor atmospheric CO₂ concentrations and their impact on the present-day biosphere. At the recording station in Mauna Loa, the concentration reached 400 ppm in May 2013, although this concentration had already been reached in the Arctic in June 2012.

Past concentration

See also: Paleoclimatology and Great Oxygenation Event

Changes in carbon dioxide during the Phanerozoic (the last 542 million years). The graph is drawn backwards, with the recent period on the left side of the plot. "0" is today. This figure illustrates a range of events over the last 550 million years during which CO₂ played a role in global climate. The graph begins (on the right) with an era predating terrestrial plant life. Land plants only became widespread after 400Ma, during the Devonian (D) period, and their diversification (along with the evolution of leaves) may have been partially driven by a decrease in CO₂ concentration. Toward the left side of the graph the sun gradually approaches modern levels of solar output, while vegetation spreads, removing large amounts of CO₂ from the atmosphere. The last 200 million years includes periods of extreme warmth, and sea levels so high that 200 metre-deep shallow seas formed on continental land masses (for example, at 100Ma during the Cretaceous (K) Greenhouse).

This is the same graph as the one above it, flipped left-to-right so that time proceeds in the conventional way. Today is on the right at "0", with the distant past on the left. The gray horizontal line near the bottom, marked "Photosynthesis limit", represents the minimum amount of atmospheric CO₂ needed to support terrestrial plant growth. The two red trend lines cross the Photosynthesis Limit line some time in the future. Those intersections are marked by the two black vertical bars. These are only rough trends. CO₂ reduction during glaciation events might reach the Photosynthesis Limit much sooner.

Carbon dioxide concentrations have varied widely over the Earth's history. Carbon dioxide is believed to have been present in Earth's first atmosphere, shortly after Earth's formation. In this theory, Earth's second atmosphere emerged after the lighter gases, hydrogen and helium, escaped to space or like oxygen were bound up in molecules and is thought to have consisted largely of nitrogen, carbon dioxide and inert gases produced by outgassing from volcanism, supplemented by gases produced during the late heavy bombardment of Earth by asteroids. The production of free oxygen by cyanobacterial photosynthesis eventually led to the oxygen catastrophe that ended Earth's second atmosphere and brought about the Earth's third atmosphere (the modern atmosphere) long ago. Carbon dioxide concentrations dropped from perhaps 7,000 parts per million (about twenty times the concentration in the 20th C.) during the Cambrian period about 500 million years ago to as low as 180 parts per million during the Quaternary glaciation of the last two million years.

Drivers of ancient-Earth carbon dioxide concentration

On long timescales, atmospheric CO₂ concentration is determined by the balance among geochemical processes including organic carbon burial in sediments, silicate rock weathering, and volcanism. The net effect of slight imbalances in the carbon cycle over tens to hundreds of millions of years has been to reduce atmospheric CO₂. On a timescale of billions of years, such downward trend appears bound to continue indefinitely as occasional massive historical releases of buried carbon due to volcanism will become less frequent (as earth mantle cooling and progressive exhaustion of internal radioactive heat proceeds further). The rates of these processes are extremely slow; hence they are of no relevance to the atmospheric CO₂ concentration over the next hundreds or thousands of years.

In million-year timescales, it is predicted that plant, and therefore animal, life on land will die off altogether, since by that time most of the remaining carbon in the atmosphere will be sequestered underground, and natural releases of CO₂ by radioactivity-driven tectonic activity will have continued to slow down. The loss of plant life would also result in the eventual loss of oxygen. Some microbes are capable of photosynthesis at concentrations of CO₂ of a few parts per million and so the last life forms would probably disappear finally due to the rising temperatures and loss of the atmosphere when the sun becomes a red giant some four billion years from now.

Measuring ancient-Earth carbon dioxide concentration

See also: climate reconstruction proxies

Graph of CO₂ (green), reconstructed temperature (blue) and dust (red) from the Vostok ice core for the past 420,000 years

The most direct method for measuring atmospheric carbon dioxide concentrations for periods before instrumental sampling is to measure bubbles of air (fluid or gas inclusions) trapped in the Antarctic or Greenland ice sheets. Some such studies come from a variety of Antarctic cores and indicate that atmospheric CO₂ concentrations were about 260–280 ppmv immediately before industrial emissions began and did not vary much from this level during the preceding 10,000 years. The longest ice core record comes from East Antarctica, where ice has been sampled to an estimated age of 800,000 years, though this figure is in dispute. During this time, the atmospheric carbon dioxide concentration has varied between 180–210 ppm during ice ages, increasing to 280–300 ppm during warmer interglacials. The beginning of human agriculture during the current Holocene epoch may have been strongly connected to the atmospheric CO₂ increase after the last ice age ended, a fertilization effect raising plant biomass growth and reducing stomatal conductance requirements for CO₂ intake, consequently reducing transpiration water losses and increasing water usage efficiency.

Various proxy measurements have been used to attempt to determine atmospheric carbon dioxide concentrations millions of years in the past. These include boron and carbon isotope ratios in certain types of marine sediments, and the number of stomata observed on fossil plant leaves. While these measurements give much less precise estimates of carbon dioxide concentration than ice cores, there is evidence for very high CO₂ volume concentrations between 200 and 150 million years ago of over 3,000 ppm, and between 600 and 400 million years ago of over 6,000 ppm. In more recent times, atmospheric CO₂ concentration continued to fall after about 60 million years ago. About 34 million years ago, the time of the Eocene–Oligocene extinction event and when the Antarctic ice sheet started to take its current form, CO₂ is found to have been about 760 ppm, and there is geochemical evidence that concentrations were less than 300 ppm by about 20 million years ago. Carbon dioxide decrease, with a tipping point of 600 ppm, is theorized to have been the primary agent forcing Antarctic glaciation. Low CO₂ concentrations may have been the stimulus that forced the evolution of C4 plants, which are able to extract from a lower atmospheric concentration. These increased greatly in abundance between 7 and 5 million years ago.

Ancient-Earth climate reconstruction is a large field with numerous studies and reconstructions that sometimes reinforce one another and sometimes disagree with each other. One study disputed the claim of stable CO₂ concentrations during the present interglacial of the last 10,000 years. Based on an analysis of fossil leaves, Wagner et al. argued that CO₂ levels during the last 7,000–10,000 year period were significantly higher (~300 ppm) and contained substantial variations that may be correlated to climate variations. Others have disputed such claims, suggesting they are more likely to reflect calibration problems than actual changes in CO₂. Relevant to this dispute is the observation that Greenland ice cores often report higher and more variable CO₂ values than similar measurements in Antarctica. However, the groups responsible for such measurements (e.g. H. J Smith et al.) believe the variations in Greenland cores result from in situ decomposition of calcium carbonate dust found in the ice. When dust concentrations in Greenland cores are low, as they nearly always are in Antarctic cores, the researchers report good agreement between measurements of Antarctic and Greenland CO₂ concentrations.

File:Biosphere CO2 Flux 08072006.gif|Biosphere CO₂ flux in the northern hemisphere summer (NOAA Carbon Tracker). File:Biosphere CO2 Flux 23122006.gif|Biosphere CO₂ flux in the northern hemisphere winter (NOAA Carbon Tracker). </gallery>

See also

• Atmospheric carbon cycle
• Carbon cycle
• Climate change
• Global warming
• Holocene climatic optimum
• Paleoclimatology
• Photosynthesis