Possible Causes of Global Warming

Natural and Human Activity

The possible causes of global warming are sometimes classified as:-
i) external forcing
ii) internal variability
iii) human (anthropogenic) activity

External Forcings
Effects arising from outside the earth and its atmosphere eg: variations in solar radiation

The sun of course exerts the strongest influence on variations in global temperature. About half of all solar radiation is absorbed by the surface & returned to the atmosphere (evapotranspiration, thermals; infrared radiation).The atmosphere then emits radiation both up and down. Radiation lost to space comes from cloud tops and atmospheric regions much colder than the surface.

“The amount of energy released by the Sun changes slightly over the years, with some small long-term trends that span centuries, and a well known variation throughout the 11-year sunspot cycle. Changes in solar radiation during the early 1900s explain much of the global warming that occurred at that time. However, solar changes cannot explain the rapid warming the Earth has experienced since the 1970s. The solar changes account for just a fraction of this recent warming.” [1]

Internal Forcings
Non climatic effects associated with the earths activity and not caused by mankind eg: aerosols ejected by volcanic eruption, CO2 emitted by chemical reaction’s in rocks

Internal Variability
Climatic, atmospheric or biosphere variations, eg: southern oscillation (El Nino, El Nina) or changes in the geo-sphere and bio-sphere

Human (Anthropogenic) Activity
– the principal activity of concern here is anthropogenic greenhouse gas

The Total Effect
In their paper "Earth’s Annual Global Mean Energy Budget” Kiehl and Trenberth [2] have produced estimates of the energy fluxes that follow different paths and transformations before being absorbed or reflected from the earth. These are summarised and presented in units of W/m2 in the figure below from their paper.

The Earth’s Annual Global Mean Energy Budget - units of W/m2
Kiehl and Trenberth [2]

References & Links

“Is global warming predominantly due to solar variability” Australian Greenhouse Office.
Available: » www.greenhouse.gov.au/science/hottopics/pubs/topic6.pdf [accessed 2007, Feb. 24]
“Earth’s Annual Global Mean Energy Budget” Kiehl and Trenberth, 1997: Bull. Am. Met. Soc. 78, 197-208
“Climate Change 2001: Working Group I: The Scientific Basis”.
Available: » http://www.grida.no/climate/ipcc_tar/wg1/042.htm [accessed 2007, Feb. 24]

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Solar variations

One of the proposed explanations for global warming is the effect of changes in the earths orbit around the sun. Well understood examples are the orbital variations which lead to seasonal change and change in daylight hours.

Obliquity of the ecliptic NASA [1]
Earths rotation axis is not perpendicular to its orbital plane. Half the year the northern hemisphere tilts tp the sun, the other half year the southern hemisphere tilts to the sun (the main contributor to seasonal climate change)

Earths elliptical orbit [1]
During earths annual orbit of the sun the distance between them varies causing a change in daylight hours

Milankovitch cycles [1]
Miamkovitch (1879 1958) proposed that three well known orbital variations: eccentricity, obliquity and precession, could be combined and then correlated with variations in ice cap size and the earth's temperature. This theory was validated in 1976 by using deep sea sediment cores to derive global temperatures going back 450,000 years Hays et al [2]. However, the variations are too slow to account for the climate change of the last 50 to 100 years.

Eccentricity – nearest and furthest distance of earth from sun thought to vary over 100,000 years

Obliquity – tilt of earth's axis from its orbital plane thought to vary over 40,000 years

Precession - orientation of the Earth's rotational axis thought to vary with a period of about 20,000 years, seasonal contrast increases in one hemisphere and decreases in the other

Milankowitch Cycles - Past & Predicted
NASA [1]

Solar flux variability AGO [3]
Energy from the Sun changes slightly over the years, with some long-term trends that span centuries. A well known variation is the 11-year sunspot cycle. Changes in solar radiation during the early 1900s explain much of the global warming for that time. However, solar changes cannot explain earth’s rapid warming since the 1970s.

Solar Flares Damon et al [3]; Solanki et al [4]
In the last 5 years there has been a debate about the correlation between solar flares and global warming. The early papers suggested that the effect was so dominant that anthropogenic warming could not be observed. More recently errors have been found in this work. Damon et al [3] and the relationship between solar flares and temperature has been investigated over a longer span of time. The evidence so far suggests that “Although the rarity of the current episode of high average sunspot numbers may indicate that the Sun has contributed to the unusual climate change during the twentieth century, we point out that solar variability is unlikely to have been the dominant cause of the strong warming during the past three decades” Solanki et al [4]

References & Links

"Milutin Milankovitch - On the Shoulders of Giants" Earth Observatory. NASA » [home page]
Available: » http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/milankovitch_3.html [accessed 2007, April.30]
J.D Hays, John Imbrie, and N.J. Shackleton, "Variations in the Earth's Orbit: Pacemaker of the Ice Ages," Science, 194, no. 4270 (1976), 1121-1132.
“Is global warming predominantly due to solar variability” Australian Greenhouse Office. [home page]
Available: » http://www.greenhouse.gov.au/science/hottopics/pubs/topic6.pdf [accessed 2007, Feb. 24]
Damon, P.E., and P.Laut (2004), "Pattern of Strange Errors Plagues Solar Activity and Terrestrial Climate Data", Eos Trans. AGU, 85(39), 370.
Available: » http://www.agu.org/pubs/crossref/2004/2004EO390005.shtml [accessed 2007, April 5] » [home page]
S. K. Solanki, I. G. Usoskin, B. Kromer, M. Schussler & J. Beer
“Unusual activity of the Sun during recent decades compared to the previous 11,000 years” Nature Vol. 431. no. 28, (Oct, 2004)
Available: » http://www.nature.com/nature/journal/v431/n7012/abs/nature02995.html [accessed 2007, April 5]

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Volcanoes

Greenhouses gases have occasionally been cited as producing more CO2 than mankind. This might sometimes be true while they are erupting but on average they produce about 0.5% the volume of CO2 that humans do. Volcano's tend to contribute to cooling of the earth rather than its warming because of the sulfur dioxide they emit. This turns into sulfuric acid aerosols that reflect solar radiation away from the earth.

Green house gases [1]

Volcanoes emit water vapour and CO2
Average volcanic CO2 is 150Mt/yr compared with anthropogenic emissions of 30bt /yr (Ref USGS & WEC)

Cooling [2]

Eruptions emit sulfur dioxide into the upper atmosphere (the stratosphere).
The sulfur dioxide is converted into tiny particles of sulfate aerosol.
These particles reflect solar energy back into space cooling the Earth
Maximum cooling occurs up to a year after the eruption.
Sulfur dioxide -> sulfuric acid aerosol -> clears slowly (up to 7 years) from stratosphere

These effects are considered by the IPCC analysis

They do not account for recent global warming

References & Links

“Annual volcanic carbon dioxide emission” Leavitt, S, Environmental Geology, Vol 4, March, 1982.
» Available: http://www.springerlink.com/content/631t022372116213/ [accessed 2007, Feb. 24]

“Climate Change Science” Australian Greenhouse Office.
» Available: http://www.greenhouse.gov.au/science/qa/pubs/science-qa.pdf [accessed 2007, Feb. 24]

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Internal Forcing and Natural Cycles

Some of the natural cycles that can increase global CO2 are: -

Chemical Reactions between water & exposed rock increasing global CO2 e.g 40 million years ago during the formation of the Himalayas and Tibetan plateau
Rearrangement of continents > Changes in land area covered by snow > Changes in Albedo
Changes in the natural carbon cycle (see figures below)

Processes 1 and 2 are too slow to account for recent global warming. The global carbon cycle is illustrated below and suggests that human agriculture and forestry practices can play a significant role. [1]

Processes 1 & 2 are too slow for recent global warming

References & Links

“The Climate Change 2001: Working Group I: The Scientific Basis: Figure 3.1”.
» Available: http://www.grida.no/climate/ipcc_tar/wg1/104.htm [ accessed 2007, Feb. 24]

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Combining causes

This IPCC graph compares various sources of global warming or cooling and indicates that greenhouse gas increases are by far the greatest contributor

References & Links

“IPCC Summary for Policymakers”.
» Available: http://www.ipcc.ch/SPM2feb07.pdf [accessed 2007, Feb. 24]

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Combining Causes & Models

This series of model fits show the degree to which IPCC models accounting for different phenomena can fit the available data

Coupled Ocean-Atmosphere Climate Model

“Global mean surface temperature anomalies relative to the 1880 to 1920 mean from the instrumental record compared with ensembles of four simulations with a coupled ocean-atmosphere climate model (from Stott et al., 2000b; Tett et al., 2000)” [1] (subtitle of picture)

“The thick line shows the instrumental data while the thin lines show the individual model simulations in the ensemble of four members. Note that the data are annual mean values. The model data are only sampled at the locations where there are observations.” [1]

References & Links

“Detection of Climate Change and Attribution of Causes” IPCC Climate Change 2001: The Scientific Basis, Chapter 12
» Available: http://www.grida.no/climate/ipcc_tar/wg1/pdf/TAR-12.PDF [accessed 2007, Feb. 24]

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Feedback mechanisms, sudden change and complexity

The idea of positive and negative feedback is used to describe mechanisms which tend to either i) correct or slow down an increase in global warming (negative feedback) or ii) reinforce or speed up the increase. In the latter case the reinforcing might lead to much faster change than was anticipated. This is sometimes referred to as “crossing a climatic threshold” or “a discontinuous response”. The effect can be described in basic terms by the feedback loop in the diagram below. It is analogous to a loudspeaker/microphone system starting to whistle if they are brought too close together.

Examples of Possible Positive Feedbacks

increased warming leads to increased lower troposphere water vapor
ocean warming releases sedimentary methane
thaw in peat bogs and tundra releases carbon dioxide and methane as well as changing reflectivity of the land

Examples of Negative Feedback

warmer climate conditions create improved tree growth and some absorbtion of CO2
increased water vapour leads to rainfall to reduce atmospheric water vapour
increased water vapour produces clouds that reflect more solar radiation back into space

Sudden Change May Be Associated With

crossing a climatic threshold
feedbacks in the climate system
such change, if it occus, is likely to take decades rather than days [3]

References & Links

“7. Physical Climate Processes and Feedbacks” IPCC - Climate Change 2001: The Scientific Basis
» Available: http://www.grida.no/climate/ipcc_tar/wg1/260.htm [accessed 2007, Feb. 24]

“Climate Variability and Change” US Global Change Research Program.
» Available: http://www.usgcrp.gov/usgcrp/ProgramElements/understanding.htm [accessed 2007, Feb. 24]

“The Prospect of Sudden Climate Change” NASA.
» Available: http://www.nasa.gov/vision/earth/environment/Sudden_Climate_Change.html [accessed 2007, Feb. 24]

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