(last updated: Feb 24, 2009)

The Global Carbon Cycle
Global Energy Balance
The Human Contribution to Climate Change
Climate Sensitivity and Feedbacks
Warming and Cooling Forces
Water Vapor
Clouds, Aerosols and Albedo
Variations in Solar Energy Reaching the Earth
Carbon Dioxide and Methane
The Effect of the Oceans on Climate

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A Quick Summary of Climate Trends

This table presents a simplified view of  how climate is changing. The time periods chosen are the last century and the last decade. While this shows that the changes are happening at an increasing rate, conclusions about accelerating change should not be drawn from this minimal amount of data.

Changes over the last Century 30 Years 10 Years
Fossil Fuel Emissions 0.6 to 8.4 Gt per year 4.6 to 8.4 Gt / yr 6.4 to 8.4 Gt /yr, growing at 3.3%
Carbon Dioxide in the Atmosphere 280 to 380 ppm  (1 ppm / yr) 330 ppm, 1.6 ppm / yr 1.9 ppm / year
Methane in the Atmosphere 715 ppb to 1774 ppb 1580 ppm, 14 ppb / yr The increase has stopped.
Global Average Temperature 0.74 C  (0.07 per decade)   0.17 C per decade
Sea Level Rise 17 cm  (1.7 mm / year) 1.8 0.5 mm  / year 3.1 0.7 mm / year

The Global Carbon Cycle - Where is the Carbon?

The global carbon cycle is examined in order to compare human inputs with the natural background. The flows described are effective on the timescale of one year.

Making sense of measurement units:
  • When describing the carbon cycle, only the weight of the carbon is measured. The rest of the compound (such as the oxygen part of carbon dioxide) is only "going along for the ride", so it is ignored. We follow the carbon as it moves through its cycle of different compounds.
  • Quantities are measured in gigatonnes. One Gigatonne (Gt) = One Petagram (Pg) = 1015 grams = 1 cubic kilometer of water.
  • Carbon dioxide levels in the atmosphere are often measured in parts per million (ppm). One ppm of CO2 in the air is equivalent to 2.12 Gt of carbon.
  • Carbon dioxide is also measured as a partial pressure of the atmosphere. Today's level of 380 ppm is equivalent to 380 Atm (micro atmospheres) or 0.38 millibars.
  • Given that CO2 has risen by 100 ppm, that means we have added about half a kilogram of carbon to the atmosphere over every square meter.
  • Each barrel of oil releases 120 kg of carbon. 8.5 billion barrels of oil contain one Gt of carbon. 112 Gt of oil has been extracted so far.

The picture below illustrates the flow of carbon compounds between various components of the ecosphere. The arrows show the annual flow, in Gigatones per year, while the boxes indicate "sinks" where carbon is stored. For example, reading from the top right of the chart, 6.2 Gigatonnes per year of carbon (as part of carbon dioxide) from fossil fuels flow into the atmosphere, which contains 775 Gigatonnes of carbon. This chart is rather out of date, today nearly 8 Gt of carbon is emitted, into an atmospheric sink of 830 Gt.

Global Carbon Cycle

The geological carbon cycle

The natural regulation of atmospheric CO2 implies different carbon reservoirs playing roles on different time scales. Ocean and biomass reservoirs play a role at time scales lower than a couple of thousand years. At longer (geological) time scales, carbonate weathering is balanced by carbonate precipitation in the ocean, but the volcanic input of CO2 to the atmosphere is only compensated by the weathering of Ca-Mg silicate minerals in soils (CaSiO3 + CO2 = CaCO3 + SiO2). For each mol of C precipitated into carbonate, a mol of C is released to the atmosphere, and the net sequestration of atmospheric carbon is 0.07 PgC/year. The organic sequestration in the form of fossil organic matter buried in sediments is thought to be compensated by the oxidation of ancient organic matter on land (not shown). New findings (4, 5) in the Himalaya lead to a new flux of CO2 degassing in mountain ranges and make mountains a locus of CO2 production instead of CO2 consumption. Fluxes in PgC/year or PtC/My. [Science July 2008]

Other human sources of carbon dioxide (not shown in the chart)

Human activity emits around 8.5 Gigatonnes of carbon per year (or 3.3 ppm) into the atmosphere (in 2006), which is about 3% of the natural exchange involving the ocean and land vegetation. The observed increase of CO2 in the atmosphere from about 280 ppm in the preindustrial era to 315 ppm when accurate measurement began in 1958, to 378 ppm in 2004 (see How do we know), now averaging about 1.9 ppm (or 3.3 Gt) / year, or 212 Gt total. About 45% of humanity’s carbon production has remained in the atmosphere, with a less certain division between the terrestrial biosphere and oceans (2.0  0.8 Gt).

The ratio of the annual CO2 increase in the atmosphere to the annual CO2 emissions, the airborne fraction, varies a lot from year to year, but it has averaged about 58% for half a century with no obvious trend. The other 42% must be taken up by the ocean, the vegetation, and the soils. The ocean is thought to take up about 20-35%, leaving 5-20% as the net sink in vegetation and soil. Vegetation and soils are also a source of CO2 via deforestation and biomass burning, so this refers to their net effect. [Hansen 2005]

Other facts about the carbon cycle:


Forests cover 42 million km2 in tropical, temperate, and boreal lands, about 30% of the world's land surface. Forests influence climate through exchanges of energy, water, carbon dioxide, and other chemical species with the atmosphere. Forests store 45% of terrestrial carbon. Carbon uptake by forests contributed to a "residual" 2.6 GtC/yr terrestrial carbon sink in the 1990s, about one third of anthropogenic carbon emission from fossil fuel and land-use change. [Science June 2008]  The average absorption of a U.S. commercial forest is 0.8 tons of carbon/hectare/year [ref].

Boreal forest fires add to warming initially, as greenhouse gases are released, but the increased exposure of snow in burned areas produces a delayed reflection that induces cooling. This result implies that future increases in boreal fire may not accelerate climate warming. [Science Nov 2006]. But because global warming will reduce the area covered by snow, forest fires may have more effect in the future.

Global forest fires burn an estimated 3 to 4.5 million km2 per year - about 4% of the vegetated land surface - and emit 2 to 3 Gt of carbon into the atmosphere annually, equivalent to 30% of fossil fuel emissions. [Science Aug 2008]

Coal Seam Fires

Wild coal fires are a global catastrophe burning hundreds of millions of tonnes of coal every year and contributing to climate change and damaging human health. These fires can rage both above and below ground and may contribute more than three per cent of the world's annual carbon dioxide emissions, which are thought to be causing global warming. [ref]

Coal fires occur wherever there is coal, but major fires blaze in Indonesia, China, India and the US. Alfred Whitehouse, of the Office of Surface Mining in Jakarta, Indonesia, says there may be up to 1000 fires blazing underground in that country alone. 63 fires are currently being monitored in the US. [ref]

Dr Glenn Stracher, of East Georgia College, Swainsboro, Georgia, said coal field fires accounted for an estimated one to two per cent of world emission of carbon dioxide from burning fossil fuels. "This is equivalent to the carbon dioxide emitted each year from all the vehicles in the United States," he said. [ref]

Andries Rosema of the Environmental Analysis and Remote Sensing (EARS) firm in the Netherlands worked with colleagues using satellite and airplane data to analyze the damage uncontrolled fires caused in China. The team found that the fires released up to 360 million metric tons of carbon dioxide — 2 to 3 percent of worldwide production per year from burning fossil fuels, an amount equivalent to that emitted per year from all automobiles and light trucks in the United States. [ref]

Carbon Dioxide and Acid Rain

When water is exposed to the air, substances in the air dissolve in the water.  Carbon dioxide is always present in air.  It dissolves by a series of reactions to form carbonic acid , H2CO3 (the acid that makes soda fizzy).  The acid which is naturally present in rain is carbonic acid.

CO2(g) + H2O(l) <===> H2CO3(aq) + H2O<===> HCO3-(aq)  +  H3O+

Carbonic acid is a weak acid, and produces few hydrogen ions in water.  Only about 0.3% of this acid is dissociated, so it is about 10 times weaker than acetic acid.  Therefore, even large concentrations of carbonic acid would not produce rain of very low pH.  With the current levels of carbon dioxide in the atmosphere (about 360 ppm in 1997), water naturally has a pH of about 5.6. This slight acidity is a natural part of the transport of carbon in the environment.  Based on ice core data, we know that the concentration of carbon dioxide in the atmosphere has varied from a low of 280 ppm to the present high of 360 ppm over the past 100,000 years.  During this time, the natural pH of rain has remained between 5.7 and 5.6. Therefore rising carbon dioxide levels in the atmosphere will have only a small effect on the pH of rain. [ref]

Other illustrations of the carbon cycle:

The Carbon Cycle
Good illustration of Carbon Cycle
The Natural Carbon Cycle, from the 2001 IPCC report.

The Distribution of Carbon on the Earth

The table below shows the relative amounts of each form of carbon on the Earth. Observe that the biosphere contains about 1% of the world's organic carbon, most of which is fossil fuels or dissolved in the ocean. The total amount of carbon dioxide in the ocean is about 50 times greater than the amount in the atmosphere, and is exchanged with the atmosphere on a time-scale of several hundred years.

Carbon Distribution on Earth
(All values in Gigatonnes)
 All Carbon on Earth  Organic Carbon  The Biosphere
Organic Carbon
6.4 x 106
Non-organic Carbonates
29.2 x 106
Biosphere: 5.5 x 104
Metamorphised Sediment
1.4 x 106
5 x 106
CO2 in the Air:  830
Plant and Animal Life:   610
Permafrost:  900
Soil Organics, Peat:  1,500
Fossil Fuel Deposits: 5,000
Shales:  7,000
Methane Hydrates
500 - 3000
Ocean Surface Layer:  600
Intermediate Ocean:  7,000
Deep Ocean - dissolved CO2 and carbonates

In the 1990s it was assumed that carbon quantities on the order of 10,000Gt C were stored in the form of methane hydrates (that equates to around twice the entire fossil energy resource: Rogner, 1997), but current estimates suggest a much lower value (500–3000Gt C: Buffett and Archer, 2004; Milkov, 2004). [ref]

The amount of buried reduced carbon exceeds the amount of O2 in the atmosphere by about a factor of 10. [Archer, ch 7]

Global Energy Balance

It is a remarkable fact that, averaged over the planet, the surface receives more radiation from the atmosphere than directly from the sun! To balance this extra input of radiation—the radiation emitted by atmospheric greenhouse gases and clouds—the earth’s surface must warm up and thereby emit more radiation itself. This is the essence of the greenhouse effect.

If air were not in motion, the observed concentration of greenhouse gases and clouds would succeed in raising the average temperature of the earth’s surface to around 85F, much warmer than observed. In reality, hot air from near the surface rises upward and is continually replaced by cold air moving down from aloft; these convection currents lower the surface temperature to an average of 60F while warming the upper reaches of the atmosphere. So the emission of radiation by greenhouse gases keeps the earth’s surface warmer than it would otherwise be; at the same time, the movement of air dampens the warming effect and keeps the surface temperature bearable.

Below is the standard chart  from the Intergovernmental Panel on Climate Change (IPCC) of how energy from the sun is absorbed by the Earth and/or radiated back into space. Energy flow is measured in watts per square meter (W/m2 or Wm-2).

An increase of 1 W/m2 over the course of a year is the equivalent to an increase in the energy content of the ocean of 2x1022 Joules. Regionally, ocean temperatures can vary by the equivalent of 50 W/m2 over the course of two years. [ref]  It would take 8.5x1023 Joules to raise average temperature of the upper layer of the ocean (down to 700 meters) by one degree Celsius.

The IPCC Global Energy Balance Chart
IPCC Energy Balance Chart

Satellite measurements show that 235 W/m2 of incoming solar radiation is absorbed by the Earth, but the latest models and measurements suggest that the atmosphere is responsible for just 67 W/m2 of this amount. The rest is absorbed by the ground and by the oceans, which play a key role in the energy budget due to their large heat capacity and their ability to store carbon dioxide, and, of course, water vapour. The greenhouse effect is precisely the difference between the long-wave radiation that is emitted by the Earth’s surface and the upward thermal radiation that leaves the tropopause – the upper boundary of the turbulent portion of the atmosphere that we all inhabit. The greenhouse effect is about 146 W/m2 in clear skies and some 30 W/m2 higher under cloud cover. [ref]

The sun bathes the Earth in energy, continuously at a rate of about 173,000 terawatts. Photosynthesis captures about 100 TW per year. Mankind is consuming about 13 TW of energy per year.

Earth's Internal Heat

83% of present surface heat flow is due to decay of radioactive isotopes; 17% due to cooling of the Earth fom its initial molten state. The mantle is cooling at 36C/Ga; 3 billion years ago it was 150 C hotter than present. Average heat loss through the surface is 87 milli-watts per square meter (mW/m2), with 101 mW/m2 in the ocean, and 65 mW/m2 under continents);  total 44x1012 W (from here). Therefore at about 0.1 W/m2 the Earth's internal heat is an insignificant part of the global energy balance.

The Human Contribution to Climate Change

The chart below, from the IPCC 2007 Summary for Policy Makers, shows the warming or cooling effect (radiative forcing) of greenhouse gases and other influences on climate at the present time. Obseve that most of the uncertainty comes from the cooling effects of aerosols and clouds.

[Figure SPM-2] Global-average radiative forcing (RF) estimates and ranges in 2005 for anthropogenic carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and other important agents and mechanisms, together with the typical geographical extent (spatial scale) of the forcing and the assessed level of scientific understanding (LOSU). The net anthropogenic radiative forcing and its range are also shown. These require summing asymmetric uncertainty estimates from the component terms, and cannot be obtained by simple addition. Additional forcing factors not included here are considered to have a very low LOSU. Volcanic aerosols contribute an additional natural forcing but are not included in this figure due to their episodic nature. Range for linear contrails does not include other possible effects of aviation on cloudiness.

These figures use an "abundance based" approach to measuring forcings, meaning they are based on what is acually in the atmosphere now. An alternative approach is called "emissions based", from Shindell et al. They point out that part of the methane emissions get converted into tropospheric ozone and carbon dioxide, and the real impact of adding methane is nearly twice the IPCC value (0.79 W/m2 compared to 0.48 W/m2). Also, emissions of NOx have had an overall negative forcing, as the induced reduction in methane outweighs the increased ozone. Therefore one should not use the values in the above table alone to determine which emisisons we should target to reduce global warming.

Human Sources of Greenhouse Gas Emissions

This chart shows how human activity gets coverted into greenhouse gas emissions. It can be seen there is no single end use that is responsible from most of the emissions. Reductions will need to take place in all sectors.


Climate Sensitivity and Feedbacks

Climate Sensitivity is the expected change in temperature caused by a given change in energy retained by the Earth. It makes little difference whether the change is caused by in increase in solar intensity or by increasing greenhouse gas levels. This energy, called radiative forcing, measured in watts per square meter (W/m2). The direct climate sensitivity (before any feedbacks) is 0.3 C per W/m2.

However, any change in radiative forcing, from greenhouse gases or anything else, is amplified by feedbacks. The main positive feedback is an increase in water vapor, which itself is a greenhouse gas that causes further warming. This is estimated to be between 1.5 and 2.0 W/m2 per degree K of warming, roughly doubling the the amount of warming that would otherwise occur [Science Feb 2009].  Other positive and negative feedbacks are described below.

A common defintion of climate sensitivity is the equilibrium global warming expected to result from doubling the amount of CO2 in the air. The radiative forcing caused by a doubling of CO2 is well known from atmospheric physics, which is 3.7 W/m2. See The Greenhouse Effect for more details. The actual (after feedbacks) climate sensitivity can be determined from climate models, and from evidence of past climate changes. There is much more uncertainty than for the direct effects. The most common estimate is around 3C for a doubling of carbon dioxide. 3 / 3.7 W/m2 gives a sensitivity of 0.8 C per W/m2.


A Summary of Positive and Negative Feedbacks

Positive feedbacks:

Negative feedbacks:
Arctic Amplification of Greenhouse warming (see also Ice Sheets and Sea Level)
There are both positive and negative feedback processes in the Arctic, occurring over a range of timescales. Positive feedbacks include snow and ice albedo feedback;  reduction in the duration of time that sea ice insulates the atmosphere from the Arctic Ocean; and permafrost–methane hydrate feedbacks. Negative feedbacks can result from increased freshwater input from arctic watersheds, which makes the upper ocean more stably stratified and hence reduces temperature increases near the air–sea interface; reductions in the intensity of the thermohaline circulation that brings heat to the Arctic; and a possible vegetation–carbon dioxide (CO2) feedback that has the potential to promote vegetation growth, resulting in a reduced albedo due to more vegetation covering the tundra.

Probably as a result of natural variations, the Arctic may have been as warm in the 1930s as in the 1990s, although the spatial pattern of the warming was quite different and may have been primarily an artifact of the station distribution.

One might at first suppose that since the amount of water ascending into clouds increases, the amount of rain that falls out of them must increase in proportion. But condensing water vapor heats the atmosphere, and in the grand scheme of things, this must be compensated by radiative heat loss. On the other hand, simple calculations show that the amount of radiative heat loss increases only very slowly with temperature, so that the total heating by condensation must increase slowly as well. Models resolve this conundrum by making it rain harder in places that are already wet and at the same time increasing the intensity, duration, or geographical extent of droughts. Thus, the twin perils of flood and drought actually both increase substantially in a warmer world. [ref]

Water Vapor

Water vapor is the main major player in the Earth’s energy budget, but its concentration in the atmosphere is buffered on a time scale of weeks by the huge oceanic reservoir of water, which can rapidly evaporate into the atmosphere and equally rapidly rain out. Water vapor thus adjusts in response to other changes in climate [ref]


The uneven distribution of water vapor increases its greenhouse effect.

Having some very dry air and some very moist air allows more infrared cooling than would the same amount of water spread uniformly over the atmosphere. To provide some idea of the magnitude of this effect, consider an atmosphere with a horizontally uniform relative humidity of 50%. Next, increase the humidity in one half of the atmosphere to 87.5%, while keeping the total water in the system constant. This requires a reduction of the rest of the atmosphere to 12.5% relative humidity. One not-quite doubles the humidity in half of the atmosphere, while reducing the humidity in the rest by a factor of 1/4, yielding a net increase of OLR of 3.6 W/m2, based on the sensitivity factor given above. If we make the dry air still drier, reducing it to 6.25% while increasing the moist air to 93.75%, the OLR increase relative to the uniform state becomes 11 W/m2. [ref]
Greenhouse gases high in the atmosphere contribute more to the greenhouse effect.

Any greenhouse gas, placed near the surface has little effect on OLR, because the low level air temperature is not much lower than the surface temperature. To get a significant greenhouse effect, one must increase the infrared opacity of a portion of the atmosphere that is significantly colder than the surface. Because the radiative effect of water vapor is logarithmic in its concentration, small quantities of water vapor can accomplish this task aloft. This leads to the concept of ”Free Tropospheric Humidity,” (FTH) or ”Upper Tropospheric Humidity” (UTH) which may be loosely defined as the water content of the portion of the atmosphere where water vapor has a considerable effect on the radiation budget. Diversion of a tiny proportion of the atmosphere’s net water vapor content would be sufficient to saturate the mid to upper troposphere and radically warm the climate. For example, consider a 50mb thick layer of saturated air near the surface of the tropical ocean, having a temperature of 295K. Less than 3% of the water content of this layer would suffice to completely saturate a layer of equal thickness having a temperature of 250K, such as would be encountered in the tropical mid-troposphere, or at lower altitudes in the extratropics. Clearly, the magnitude of the boundary layer water vapor reservoir is not the limiting factor in determining the free tropospheric humidity.

Water vapor feedback does not contribute to polar amplification.

It is important to note that water vapor feedback does not contribute to polar amplification. In fact, for the present pole to equator temperature range, water vapor feedback makes the slope larger at cold temperatures than at warm temperatures [Pierrehumbert (2002)], and hence would lead to tropical rather than polar amplification. Certainly, ice albedo feedback plays a role in polar amplification, but dynamical heat transport and clouds may also contribute. These feedbacks must be sufficiently strong to overcome the tendency of water vapor feedback to put the greatest warming in the tropics.

The greenhouse effect is stronger in the tropics.

As compared to the calculation with no atmospheric greenhouse effect whatsoever, CO2 by itself brings the tropical OLR down by 60 W/m2. The OLR reduction decreases with latitude in the extratropics, falling to 25 W/m2 at 60S in the summer hemisphere, and even smaller values in the winter extratropics. The latitudinal variation in the CO2 greenhouse effect derives from the vertical structure of the atmosphere: In the tropics there is more contrast between surface and tropopause temperature than there is in the extratropics, and the summer extratropics has more contrast than the winter extratropics. When OLR is recalculated with the observed humidity content of the atmosphere (based on NCEP) in addition to the CO2 from the previous case, OLR drops by an additional 100 W/m2 in the tropics, and a lesser amount in the extratropics. In fact, at each latitude the greenhouse effect of water vapor is approximately twice that of CO2.

Clouds, Aerosols and Albedo

Clouds are the least understood part of the climate system. Cloud cover reduces ground-level radiation by a global average of about 30 W/m2 [ref]. They have a cooling effect by reflecting visible light from the sun, and a warming effect by absorbing infrared energy from the Earth's surface. High altitude cirrus clouds are largely transparent to light, but are more effective at absorbing infrared energy because they are cooler. Low altitude clouds have less of an infrared effect because they are warm, but a large change in albedo because they are thick and opaque. Clouds are believed to have a net cooling effect. As cloud levels increase with higher water vapor levels, they are expected to act as a negative feedback to greenhouse warming.

Notes on cloud effects on climate:<


Aerosols are solid or liquid particles suspended in the atmosphere, consisting of (in rough order of abundance): sea salt, mineral dust, inorganic salts such as ammonium sulfate (which has natural as well as anthropogenic sources from e.g. coal burning), and carbonaceous aerosol such as soot, plant emissions, and incompletely combusted fossil fuel. As should be apparent from this list, there are many natural sources of aerosol, but changes have been observed in particular, in the atmospheric loading of carbonaceous aerosol and sulphates, which originate in part from fossil fuel burning. While a relatively minor part of the overall aerosol mass, changes in the anthropogenic portion of aerosols since 1750 have resulted in a globally averaged net radiative forcing of roughly -1.2 W/m2, in comparison to the overall average CO2 forcing of +1.66 W/m2. [ref]

Fossil fuel burning generates both aerosols and carbon dioxide, which act as cooling and warming forcings respectively. The amount of aerosols depends greatly on what fuel is being used. However, the cooling effect of the aerosols can be a substantial fraction of the warming effect of the carbon dioxide. If we were to remove, for example, a coal fired generating station, the aerosol cooling effect would end in a week, while it would take some time before effects of the carbon dioxide to be felt. That means closing the coal plant actually causes global warming for a while, a period that could be as much as twenty years.

Let me attempt a crude calculation of how long it takes before a reduction in fossil fuel use lead to cooling. I will start by assuming all aerosols and carbon dioxide come from fossil fuel burning (in reality it is only part of both, so maybe the errors roughly cancel). According to figure SPM-2 (shown above), stopping fossil fuel use would result in an immediate increase in forcing of 1.2 watts per square meter (W/m2), the sum of the two aerosol forcings. The present forcing from carbon dioxide is 1.7 W/m2. Assuming the relationship between CO2 concentration and forcing is linear (not true: it is logarithmic, each unit of decrease will have more effect, but that is good enough for this crude calculation) we need to get rid of 70% of the CO2 to remove 1.2 W/m2 of forcing.

Reading from the CO2 decay rate from this page (also shown below), it will take one hundred years to reduce CO2 by 70%. However the result is very sensitive - if we assume we only need a 50% CO2 reduction (because of the large uncertainty in aerosol forcing or other problems with the figures I use), the time is reduced to 20 years.

Even the more optimistic result indicates we get net warming for 20 years after reducing fossil fuel use. I wonder how many people are really aware of the long time scales involved in this issue. There are no quick fixes. This is not a justification for doing nothing, but we must be realistic.


Albedo is a measure of how much light is reflected from the Earth. The greater the albedo, the more light is reflected, and the cooler the planet. The average albedo of the Earth is about 30%, but varies seasonally: Winter: 0.309, Spring: 0.2906, Summer: 0.2878, Autumn: 0.310. Changing albedo through land use changes can affect the climate.

The Earth's Albedo in March 2005

Albedo Notes:

Variations in Solar Energy Reaching the Earth

The climate system is almost entirely driven by energy from the sun. The sun's influence on climate change is determined by variations in solar output. This has been accurately measured by satellites over the past few decades, as shown below. The dominant feature is the 11 year sunspot  cycle, which causes a change of 0.08%.

The total insolation above the Earth is about 1,365 W/m2. Because the Earth is round, most of the radiation reaches the Earth's surface obliquely, and half of the Earth is in total darkness, so this figure must be divided by four, which gives 341 W/m2. Then multiply by 0.7 to take into account the portion that is reflected back into space (albedo), which gives 239 W/m2.

Therefore the sunspot cycle variation amounts to about 0.17 W/m2. This amount of change is hard to detect in the climate record (but see [ref]) because it is smaller than the annual temperature variation from other causes. This is the equivalent direct forcing as adding seven years worth of carbon dioxide to that atmosphere at the current rate of 2 ppm per year.

Solar_Irradiance_20_years_AR4.png Percentage change in monthly values of the total solar irradiance composites of Willson and Mordvinov (2003; WM2003, violet symbols and line) and Frhlich and Lean (2004; FL2004, green solid line).

Source: IPCC AR4 WG1, ch 02, fig. 2.16

Measuring change in solar irradiance over the past few centuries requires the use of indirect records. There is a good record of the change in sunspot number. Low numbers of sunspots correspond to lower insolation. Knowledge of solar activity is also inferred indirectly from the 14C and 10Be cosmogenic isotope records in tree rings and ice cores. These measures clearly show the sunspot cycle, but determining if there is a change in the overall magnitude of solar output over this period is more difficult. This can be determined by observing changes in stars that a similar to the Sun. Recent information shows that solar variation is smaller than was once thought. The IPCC 2007 report claims the change in solar irradiance since 1750 is estimated to cause a radiative forcing of +0.12 [+0.06 to +0.30] W/m2, which is less than half the estimate given in IPCC 2001.

IPCC 2007 states that the increase in total irradiance from the Maunder Minimum (a period in the 17th century with low sunspot activity) to current cycle minima is 0.04% (an irradiance increase of roughly 0.5 W/m2 in 1,365 W/m2), corresponding to a forcing of +0.1 W/m2.

Solar_Irradiance_AR4.png Reconstructions of the total solar irradiance time series starting as early as 1600. The upper envelope of the shaded regions shows irradiance variations arising from the 11-year activity cycle. The lower envelope is the total irradiance reconstructed by Lean (2000), in which the long-term trend was inferred from brightness changes in Sun-like stars. In comparison, the recent reconstruction of Y. Wang et al. (2005) is based on solar considerations alone, using a flux transport model to simulate the long-term evolution of the closed flux that generates bright faculae.

Source: IPCC AR4 WG1, ch 02, fig. 2.17

Compare this to the paleoclimate data for the past 1000 years.

Carbon Dioxide and Methane

Carbon dioxide levels in the atmosphere have risen from 270 ppm to 380 ppm, or by roughly 25%. We know that almost all of that increase is human caused for several reasons. Humans have produced twice as much carbon dioxide than the amount that has been added to the atmosphere. We are currently adding 2.8 ppm of CO2 to the atmosphere each year, but the amount that remains in the air is only increasing by nearly 2 ppm per year. The rest is going into land vegetation and the ocean. The only possible natural source for carbon dioxide is the ocean, which contains 50 times more dissolved CO2 than the atmosphere. But if CO2 is going into the ocean it cannot also be coming out the ocean. The CO2 concentration in the ocean has been measured over time, and has been found to be increasing. The atmosphere contains carbon-14 caused by cosmic ray bombardment of nitrogen. Fossil fuels contain no carbon-14 because it has all decayed. Carbon-14 levels in the atmosphere have been decreasing because they have been diluted with carbon from fossil fuels. See [How Do We Know that the Atmospheric Build-up of Greenhouse Gases Is Due to Human Activity?]

How long does added carbon dioxide stay in the air?

Most of the carbon dioxide that we add to the atmosphere will be absorbed by the ocean.

The lifetime of carbon dioxide in the atmosphere is often mistakenly quoted as being on the order of a hundred years; this figure is actually the result of a fallacious and largely meaningless method of aggregating the many physical processes that operate on widely differing time scales into a single number which is supposed to represent the amount of time some extra added carbon dioxide will stay in the atmosphere. The fact is that for each kilogram of carbon dioxide put into the atmosphere today, only a small portion will be rapidly absorbed into the ocean. After five hundred to one thousand years of slow uptake by the ocean, fully a quarter of that kilogram will remain in the atmosphere. A portion of that will be taken up by the ocean over the next ten thousand years by slow processes related to ocean sediments, but fully 7 percent of our initial kilogram will stick around for hundreds of thousands of years. [ref]

Global Warming Potential and the Relative Effect of Carbon Dioxide and Methane on the Greenhouse Effect

Global Warming Potential (GWP) is a term used to compare the relative effect of greenhouse gases. For example, methane is said to be 22 times more potent than carbon dioxide. However, this value depends on the concentration of that gas in the atmosphere, and its atmospheric lifetime.

The common statement that methane is, molecule for molecule, a better greenhouse gas than CO2 is true only for situations like the present where methane is present in far lower concentrations than CO2. In this situation, the greater power of a molecule of CH4 to reduce the OLR (outgoing longwave radiation) results simply from the fact that the greenhouse effect of both CH4 and CO2 are approximately logarithmic in concentration. For methane concentrations of around 1 ppmv, each doubling of methane reduces OLR by about 2 W/m2. On the other hand, for CO2 concentrations near 300 ppmv, each doubling of CO2 reduces the OLR by about 6 W/m2. Hence, to achieve the same OLR reduction as a doubling of CO2 one needs three doublings of methane, but since methane starts from a concentration of only 1 ppmv, this only takes the concentration to 8 ppmv, and requires only 7/300 as many molecules to bring about as was needed to achieve the same reduction using a doubling of CO2. Equivalently, we can say that adding 1 ppmv of methane yields as much reduction of OLR as adding 75 ppmv of CO2. If methane were the most abundant long-lived greenhouse gas in our atmosphere, and CO2 were present only in very small concentrations, we would say instead that CO2 is, molecule for molecule, the better greenhouse gas. [ref]

The table below shows the predicted rate at which carbon dioxide released to the atmosphere is absorbed. In 20 years almost half of it is gone, in 100 years 25% of it still remains, and in 100,000 years 7% is still there.[ref]  The ocean absorbs CO2 until the partial pressure in the ocean reaches the new level that is in the air. After that, CO2 is removed by the silicate weathering cycle over a timescale of 400,000 years. This is true for present levels of carbon dioxide; at higher levels the ability of the environment to absorb CO2 is reduced. However, that has not changed in the last fifty years, as seen here.

Carbon_Dioxide_Decay.gif CO2_Decay_Hansen.png

The atmospheric concentration of carbon dioxide appears to be increasing exponentially since 1750 [ref] Accurate measurements at Mauna Loa (in Hawaii) over the last fifty years show what looks to be a linear increase..
Carbon Dioxide Levels Mauna Loa Carbon Dioxide Record

...but the rate of increase is gradually rising, reaching 2 ppm per year.


The Direct Health Effects of Carbon Dioxide

Health effect on humans due to CO2 levels start at around 1% (10,000 ppm). For example, this source states: "The Occupational Safety and Health Administration (OSHA) and the American Conference of Governmental Industrial Hygienists (ACGIH) have set workplace safety standards of 5,000 ppm as an 8-hour time weighted average (TLV-TWA) exposure, and 30,000 ppm as the short term exposure level (STEL)."

Trends in Carbon Dioxide and Methane Concentration

Carbon dioxide is continuing to increase linearly, in line with IPCC 2001 projections (in yellow). Anthropogenic methane sources, such as rice paddies, the fossil fuel industry, and livestock, have already more than doubled the methane concentration in the atmosphere from pre-industrial levels. The atmosphere currently contains about 3.5 Gton C as methane. However, methane levels have started to level off, falling far below the projections. Since 1999 there has been essentially no growth in the mean annual atmospheric methane concentration, compared to a 15% rise over the preceding 20 years. [ref]  The cause of this change is unknown, so it may only be temporary.

CO2_and_CH4_Levels_Since_1990 Methane_Growth_Since_1985.png Recent CH4 Concentrations and Trends

(a) Time series of global CH4 abundance mole fraction (in ppb) derived from surface sites operated by NOAA/GMD (blue lines) and AGAGE (red lines). The thinner lines show the 
CH4 global averages and the thicker lines are the de-seasonalized global average trends from both networks.

(b) Annual growth rate (ppb/yr) in global atmospheric 
CH4 abundance from 1984 through the end of 2005 (NOAA/GMD, blue), and from 1988 to the end of 2005 (AGAGE, red).

[From IPCC AR4 WG1 Ch 2, p 142]

Methane Cycle Residence Time of Methane in the Atmosphere

The residence time of  methane at the present concentrations is about eight years. Methane is removed from the air by hydroxyl radicals (OH-), which are produced by ultraviolet light being absorbed by ozone, and the decomposition of nitrogen dioxide (NO2). If you increase methane levels, the hydroxyl gets used up and the residence time increases, leading to more greenhouse warming. However, it would take a tenfold increase in methane levels to double the residence time from 8 to 16 years.

Methane Hydrates

For most parts of the ocean, melting of hydrates is a slow process. It takes decades to centuries to warm up the water 1000 meters down in the ocean, and centuries more to diffuse that heat down into the sediment where the base of the stability zone is. The Arctic Ocean may be a special case, because of the shallower stability zone due to the colder water column, and because warming is expected to be more intense in high latitudes. [ref]

Total amounts of methane hydrate in permafrost soils are very poorly known, with estimates ranging from 7.5 to 400 Gton C.

The Effect of the Ocean on Carbon Dioxide Uptake and Thermal Lag

The oceans have absorbed about 48% of all carbon dioxide produced by fossil fuels in the 20th century, about 120 Gt in all. Without this oceanic uptake, atmospheric CO2 would be about 55 ppm higher today than what is currently observed  The figure below shows that this anthropogenic CO2 is not evenly distributed throughout the oceans. The highest vertically integrated concentrations are found in the North Atlantic, due to the overturning effect of the Gulf Stream. As a result, this ocean basin stores 23% of the global oceanic anthropogenic CO2, despite covering only 15% of the global ocean area. By contrast, the Southern Ocean south of 50S has very low vertically integrated anthropogenic CO2 concentrations, containing only 9% of the global inventory [Science July 2004].

Column inventory of anthropogenic CO2 in the ocean (mol/m2). High inventories are
associated with deep water formation in the North Atlantic and intermediate and mode water
formation between 30 and 50S. Total inventory of shaded regions is 106  17 Pg C.

While colder water can hold more CO2 than warmer water, the critical factor for the amount absorbed from the atmosphere depends on how saturated with CO2 the water already is.

Map of the 1994 distribution of Revelle factor, averaged for the upper 50 m of the water column. A high Revelle factor indicates that, for a given atmospheric CO2 perturbation, the oceanic equilibrium concentration of anthropogenic CO2 will be lower than that for low–Revelle factor waters. The current Revelle factors are about one unit higher than they were in the preindustrial ocean.

There are several reasons why the ocean may not always continue to abosrb carbon dioxide at the same rate as the recent past:

1)  The solubility of CO2 decreases with temperature. As the ocean gets warmer, it will be able to absorb less carbon dioxide, so more will remain in the atmosphere.
For a temperature increase of 1C, the partial pressure of CO2 in the ocean surface layer increases by 4.2% or 15 μ atm., which corresponds to an average net flux of around 4 Gt C to the atmosphere. This means that a sea surface temperature increase of 1C leads to a CO2 flux to the atmosphere that is twice as big as its current yearly take up of anthropogenic CO2. [ref]
2)  The ability of the ocean to absorb CO2 at the same temperature decreases as carbon dioxide levels increase.
If atmospheric CO2 increases 10%, the ocean concentration will rise 1%. This will occur in the top 50 to 100 meters.

Because an increased atmospheric CO2 content increases the oceanic carbon dioxide uptake, more carbonate is also used during chemical solution processes, and there is increasingly less carbonate available for the chemical reaction with carbon dioxide. Thus an increasing share of the carbon dioxide taken up remains in its original form in the water, and the possibility that the surface water takes up further carbon dioxide from the atmosphere is decreased. This effect is relatively large: for a further increase of atmospheric CO2 concentrations of 100 ppm (i.e. from 370 to 470 ppm) the CO2 uptake by the ocean is reduced by 40% compared to during the first 100 ppm increase from 280 to 380 ppm since the start of industrialisation. [ref]
3)  Rising temperatures cause increased stratification of the ocean surface layer. That reduces the rate of transfer of carbon dioxide to the deep ocean, leaving the upper layer more saturated, thus less able to absorb CO2.
Mixing by turbulence in the ocean is essential for moving CO2 down into the deep ocean, away from the top 100 meters of the ocean, where carbon absorption from the atmosphere takes place. With increased temperatures, the ocean stratifies more, mixing becomes harder, and CO2 accumulates in the surface ocean instead of in the deep ocean. This accumulation creates a back pressure, lowering CO2 absorption. [ref]
4)  The ocean is absorbing much of the energy from increased greenhouse gases. This change in equilibrium cannot continue, and the heat will eventually be returned to the atmosphere, further increasing warming.
The heat-carrying capacity of the global ocean is over 1000 times greater than that of the global atmosphere. The current unrealised warming "in the ocean pipeline" is related to the net imbalance, 0.85  0.15 W/m2 implies an further warming of around 0.5-0.7 C, regardless of future emission increases. [So why did 2.5 W/m2  of greenhouse gas forcing cause 0.8 C in the 20th century, but 0.85 W/m2 cause 0.7 C?]
5)  Rising ocean temperatures may reduce the ability of phytoplankton to remove carbon dioxide.
Diatoms are the most important group of phytoplankton for removing carbon from the atmosphere and it's the silicon in diatoms that makes carbon removal possible. Temperature dramatically influences how well diatoms can do this. A coating of carbon surrounds and protects silicon in diatoms. In cold water, like that found near Antarctica, slower bacterial action allows more carbon to remain attached to the silicon. As the diatoms sink in the cold water, they take the carbon with them to the ocean bottom. The carbon can remain there for thousands of years away from the atmosphere. Even small increases in temperature cause bacteria to quickly eat the coating. The diatoms dissolve more readily and carbon recycles back to the surface ocean instead of being sequestered in deep waters. Thus,  the warmer the ocean, the less able it is to pull carbon out of the atmosphere. [ref]
6)  Global warming will reduce the temperature difference between the polar and equatorial regions. This temperature difference is the driving force for most of the world's weather. Reducing the driving force may reduce the intensity of winds, which are responsible for causing mixing of the ocean's layers. This may lead to increased startification, thus reduced carbon dioxide uptake, and reduced oxygenation of the deep ocean.

On the other hand, this summary of a Lawrence Livermore National Laboratory study states "The model shows that ocean uptake of CO2 begins to decrease in the 22nd and 23rd centuries due to the warming of the ocean surface". This model is based on a rather high emissions scenaro lasting 300 years, predicting 8 C of warming. But change in ocean uptake does not kick in for a century. It also says "the most drastic changes during the 300-year period would be during the 22nd century in which precipitation change, an increase in atmospheric precipitable water and a decrease in sea ice size are the largest when emissions rates are the highest. During the model runs, sea ice cover disappears almost completely in the northern hemisphere by the year 2150 during northern hemisphere summers."  [In 2008, the extent of Arctic sea ice is decreasing rapidly, and now it appears that the Arctic will be ice free in summer before 2020.]

Warmer water contains less oxygen, as shown below:

The ratio of annual atmospheric 
CO2 increase to annual fossil fuel CO2 emissions has remained constant since 1950. For up to date information on CO2 emissions, see the Mauna Loa Observatory.



Carbon Dioxide fluxes due to Hurricanes (from this Nature paper)

The increase in heat content from 1957 to 1994, the period of best data coverage, is 19.0 ( 9.0) x 1022 J. The heat content increase yields 0.32 0.15 W/m2 (expressed per unit area of the entire world, not just the ocean surface). [ref]


Atmospheric CO2 and Acidification of the Ocean

Surface oceans have an average pH globally of about 8.2 units. However, pH can vary by 0.3 units due to local, regional and seasonal factors. [ref]

Only the near-surface waters, or surface layers, of the oceans (down to about 100 m on average) are well mixed and so in close contact with the atmosphere. Carbon dioxide in the atmosphere dissolves in the surface waters of the oceans and establishes a concentration in equilibrium with that of the atmosphere. Molecules of CO2 exchange readily with the atmosphere and on average only remain in the surface waters for about 6 years. However mixing and advection (vertical motions, sinking and upwelling) with the intermediate and deep waters of the oceans (down to about 1000 m and 4000 m respectively) is much slower, and takes place on timescales of several hundred years or more.

The fastest natural changes that we are sure about are those occurring at the ends of the recent ice ages, when CO2 rose about 80 ppm in the space of 6000 years (IPCC 2001). This rate is about one-hundredth that of the changes currently occurring. During slow natural changes, the carbon system in the oceans has time to interact with sediments and stays therefore approximately in steady state with them.

Although the biological uptake of CO2 per unit area of the surface oceans is lower than that in most terrestrial systems, the overall biological absorption is almost as large as that in terrestrial environment. This is because the surface area of the oceans is so much larger. Increasing CO2 in ocean water will increase productivity of photosynthesis by less than 10%, because even at today’s CO2 concentration photosynthesis is saturated with inorganic carbon.

Ocean pH has fallen by 0.1 units in the 20th century. If global emissions of CO2 from human activities continue to rise on current trends then the average pH of the oceans could fall by 0.5 units (equivalent to a three fold increase in the concentration of hydrogen ions) by the year 2100 [based on a very high emissions scenario - CO2 reaching 2000 ppm by the year 2200, compared with 380 ppm today]. This pH is probably lower than has been experienced for hundreds of millennia and, critically, this rate of change is probably one hundred times greater than at any time over this period. The scale of the changes may vary regionally, which will affect the magnitude of the biological effects.

As atmospheric CO2 levels increase so does the concentration of CO2 in the surface oceans. However it is unlikely that the past atmospheric concentrations would have led to a significantly lower pH in the oceans, as the rate at which atmospheric CO2 changed in the past was much slower compared with the modern day. The fastest natural changes that we are sure about are those occurring at the ends of the recent ice ages, when CO2 rose about 80 ppm in the space of 6000 years (IPCC 2001). This rate is about one-hundredth that of the changes currently occurring. During slow natural changes, the carbon system in the oceans has time to interact with sediments and stays therefore approximately in steady state with them.

Marine organisms that construct CaCO3 structures, such as shells, are dependent on the presence of bicarbonate and carbonate forms of dissolved inorganic carbon in seawater. Once formed, CaCO3 will dissolve back into the water unless the surrounding seawater contains sufficiently high concentrations of carbonate ions (CO3 2-), ie. it is saturated. Calcium carbonate also becomes more soluble with decreasing temperature and increasing pressure, and hence with ocean depth. A natural boundary develops in seawater as a result of these different variables. This is known as the ‘saturation horizon’ and it identifies a clear depth of seawater above which CaCO3 can form, but below it dissolves. Increasing CO2 levels and the resultant lower pH of seawater decreases the saturation state of CaCO3 and raises the saturation horizon closer to the surface.

Not all marine organisms respond to increased carbon dioxide levels the same way. In this Science paper:  From the mid-Mesozoic, coccolithophores have been major calcium carbonate producers in the world's oceans, today accounting for about a third of the total marine CaCO3 production. Here, we present laboratory evidence that calcification and net primary production in the coccolithophore species Emiliania huxleyi are significantly increased by high CO2 partial pressures. Field evidence from the deep ocean is consistent with these laboratory conclusions, indicating that over the past 220 years there has been a 40% increase in average coccolith mass.

The only period with a similar rapid increase of carbon dioxide was the Paleocene Eocene Thermal Maximum, which occured 55 million years ago. Ocean sediments from this time reveal a large die-off of calcium carbonate based organisms, which is described in this paper in Science.

A related issue is the removal of oxygen from the oceans caused by the runoff of organic matter from the land. This leads to regions of the ocean that are effectively dead. This map correlates the human footprint with observed dead zones, from [Science Aug 2008].

Finally, all the proof we ever need...


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