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The enhanced greenhouse effect (Global warming)

What is the greenhouse effect?

The greenhouse effect is an important part of the Earth's climate without which the planet would be a far colder place. The effect is natural and not new. When sunlight hits the surface of the earth it is absorbed and the visible light (short wave radiation) is converted to heat (infrared or long wave radiation) which is radiated back into the atmosphere towards space.

Figure 1. A schematic of the electromagnetic spectrum, showing the Sun's energy output in relation to wavelength.

Some gases in the atmosphere (the so called greenhouse gases: such as carbon dioxide, water vapour, methane, etc.) absorb the infra red radiation (heat) which is converted into kinetic and potential energy. Eventually these molecules then emit heat back into the atmosphere as infrared radiation. Some of this infrared radiation is absorbed by other greenhouse gases and some is absorbed at the earth's surface and the cycles of absorption, conversion and emission are repeated. Essentially this process slows the loss of heat to space, keeping the earth's surface warmer than it would be without the greenhouse gases. Without this “greenhouse” the Earth's atmosphere would be an average of about 30-35 ^oC cooler and life as we know it would not exist.

Figure 2. An overview of the Greenhouse Effect. From IPPC Working Group 1 contribution, Science of Climate Change, Second Assessment Report 1996 [1].

The enhanced greenhouse effect, sometimes referred to as climate change or global warming, is the impact on the climate from the additional heat retained due to the increased amounts of carbon dioxide and other greenhouse gases that humans have released into the earths atmosphere since the industrial revolution.

What is causing the enhanced greenhouse effect?

Since the mid 1800's the average concentration of CO₂ in the earth's atmosphere has risen from about 280 parts per million (ppm) to just over 382ppm at the end of 2005, and methane from about 800ppb to around 1700ppb.

Figure 3. Global atmospheric concentrations of three greenhouse gases. From BOM [2].

While these changes represent only a very small change to the overall composition of the earth's atmosphere, it is a significant change to its capacity to absorb and emit heat. The main contributors are changes to the carbon cycle that have led to increased levels of carbon dioxide in the earth's atmosphere in the last 200 years. These include reduced CO₂ removal and storage through deforestation; direct CO₂ production from the burning of fossil fuels and CO₂ released from cement production.

The increased release of nitrogen oxides (NOx) from burning fossil fuels and soil denitrification (particularly with the introduction of high nitrogen fertilizers) and intensive production of livestock such as cows and pigs which produce methane have also contributed to the enhanced greenhouse effect.

The differing chemical structures of these gases produce a different absorption spectra or wavelengths of radiation which they will absorb or let through. An important aspect of this is that even if the atmosphere is saturated with water vapour there are wavelengths of infrared radiation that it will not be absorbed. However, CO₂ and other greenhouse gases can absorb the infrared radiation at the wavelengths missed by water vapour.

Figure 4. The radiation Absorption characteristics of Water Vapour and Carbon Dioxide. From BOM [2].

The capacity for a gas to absorb long wavelength (Infrared) radiation and the length of time in spends in the atmosphere both impact on its potential to act as a greenhouse gas. This potential is often expressed as its CO₂ equivalent, or the number of equivalent molecules of CO₂  it would take to absorb as much heat as one molecule of the gas in question over a given time period (usually 100 years). The CO₂ equivalents of some greenhouse gases are shown below [3].

Table 1. CO₂  equivalents of some greenhouse gases. From [3].

Note that while methane (CH₄) and N₂O both absorb more heat per molecule than CO₂, CO₂ concentrations are much higher (100 -100 times higher respectively) and therefore have more overall affect on the enhanced greenhouse affect. Residence time plays an important role as well as concentration. While water vapour is by far the greatest contributor to the natural greenhouse effect, it spends so little time in the atmosphere (days rather than centuries) that it is not well mixed and thus its affects on temperature are short lived and very localised.

Considerations for measurement and interpretation

While we can directly measure the levels of CO₂ and other greenhouse gases in the atmosphere and we know how they have changed in the past, the extent to which their concentrations will change in the future is uncertain (Figure 5). How much greenhouse gas will be emitted in the future is dependent on a number of complex factors, such population change, economic development, changes to technology along with social and political ideology. Projections of future emissions of greenhouse gases are made based on scenarios, or plausible descriptions of the future. A scenario provides a set of assumptions that describe what might happen in the future [4]. As the interactions between each of the factors within a scenario and how each of the factors will affect greenhouse emission are not completely understood, uncertainty is introduced at every step of the projection process. The possible error in projected emissions is carried into the projected levels of greenhouse gases and further compounded when a projection of temperature change is made from the greenhouse gas concentrations.

Figure 5. Flow chart illustrating that uncertainty is introduced into predictions of impacts at every step, and these uncertainties accumulate. From [5]

The uncertainty of the temperature projections are further increased due to our limited understanding of the exact sensitivity of climate to various concentrations of greenhouse gases, i.e. how much will temperature rise from a given increase in CO₂ levels (Figure 6). This is even further complicated by the issue of feedbacks in which higher temperatures will lead to increased release of greenhouse gases leading to even higher temperatures and thus releasing more greenhouse gas and so on. An example would be the release of methane from permafrost (ground currently frozen all year round) as it thaws in the Northern Hemisphere.

Figure 6. Projections of CO₂ emissions (a) and the atmospheric CO₂ concentrations (b) and temperature change (d) associated with those emissions. Note the levels of uncertainty associated with the projections of temperature change. From IPCC 2001 [4].

The other impacts of climate change such as rainfall, storm intensity or frequency and sea level rise, are even more complex as they each add even more steps to get from emissions of greenhouse gases, through a concentration of greenhouse gases and projected temperature change to a projection of another effect. Rainfall is particularly difficult as our understanding of why it rains is very limited and rainfall itself is often highly localised, occurring on a much small scale than is currently being modelled. It is even more difficult to then take this sort of modelling even further and predict how much the projected rainfall will reach streams or reservoirs. Factors such as air temperature, wind and humidity play important roles in evaporation rates and soils capacity to store water. The result is that the relationships between rainfall and runoff are not linear as can be seen in the observed changes in south-west Western Australia

Other factors such as the impact of aerosols or whether clouds serve to warm or cool the atmosphere are also difficult to model due to our lack of understanding of both the processes themselves and how human activities will change them in the future.

Figure 7. Average trend in annual mean temperature in Australia (ºc/10yrs - 1950-2001) From The Bureau of Meteorology [6].

Rainfall

Rainfall is regularly measured across Australia. The map in Figure 8 shows the average trend in total rainfall for Australia for the period 1950-2001 as millimetres per decade, eg. +20mm/10yrs for 50 years equals a 100 millimetre increase in average rainfall since 1950.

Figure 8. Average trend in total rainfall in Australia (mm/10yrs) 1950-2001. From The Bureau of Meteorology [7].

Southwest Western Australia Rainfall and Runoff

A major factor governing the form and function of coastal waterways is the availability of water. Rainfall is predicted to decrease for most of the populated areas of Australia and this will impact how much water can be collected and environmental flows. As discussed above the relationship between rainfall and runoff is not linear. Measurements taken for the reservoirs that supply Perth (Figure 9) show that for the period between 1974 and 1996 the average rainfall decreased by 14% but the inflow to the reservoirs for the same period decreased by 48%. In the past ten years (1996-2006) the rainfall has decreased by another 7% and the inflow by another 16%. Factors such as increased evaporation and decreased soil moisture are combining with the decreased rainfall to produce a much greater decrease in inflow.

Figure 9. Rainfall (top) and inflow to Reservoirs (bottom) near Perth Western Australia. From Water Corporation [8].

The other interesting point about this example is the apparent stepwise nature of the changes. These changes are not slow gradual variations that can be monitored and accounted for through planning or evolution. These are sudden rapid transformations in conditions that could devastate an ecosystem dependant on the inflow. 

Sea level Rise

Sea level rise occurs for two main reasons. Firstly the seawater expands as it gets warmer and secondly there is an increase in the amount of water in the oceans due to the melting of glaciers, ice caps and ice sheets. Figure 10 shows the increase in global mean sea level since 1870 until the end of 2001. Note that not only is the sea level rising but that the rate at which it is rising is increasing. Sea level rise leads to a variety of problems including damage to infrastructure and saline intrusion into freshwater aquifers.

Figure 10. Global mean sea level for 1870 to 2001. The bottom line shows the monthly global average, with the top line showing the same data as yearly averages (offset by 300mm). The middle line is the yearly average with a quadratic fit of the data. The insert shows the recent satellite altimeter data. The light and dark shaded areas show one and two standard deviations respectively. Reprinted with permission Church and White 2006 [9].

Ecosystems

In Australia there have been many trends observed in a variety of ecosystems that could be the result of changes to climate. Hughes [10] provides a review of these changes that include:

• Changes to woodland distribution and biomass probably due to changes in rainfall and CO₂ levels eg. Expansion of rainforest in Queensland and expansion of eucalyptus into subalpine grassland.
• Changes to migration and distribution patterns of birds and other animals eg. the southward contraction of range of the grey headed flying fox and the southward extension of the distribution of the black flying fox,
• The southward extension of the distribution of marine species such as the sea urchins and the introduced European shore crab

Ocean Acidification

CO₂ absorption by oceans has led to a decrease in the pH of about 0.1 units from pre industrial levels. This change represents about a 30% increase in the concentration of H⁺ in seawater.

Storm Frequency

Recent studies have correlated the frequency of intense cyclones (category 4 or 5 on the Saffir-Simpson scale)  with increased water temperature [13].  The Climate Projections for Australia

The Climate Projections for Australia

The CSIRO [11] has produced a series of projections for climate change in Australia. Projected changes in temperature and rainfall for Australia (2030-2070) are shown in the form of maps in Figure 11 and 12. CSIRO has also produced smaller scale, sate based projections of each of the states of Australia. These can be found through the websites of the relevant state governments.

Temperature

Figure 11. Average seasonal and annual warming ranges (in ^oC) for around 2030 and 2070 relative to 1990. The coloured bars show ranges of change for areas with corresponding colours in the maps. Reprinted with permission from CSIRO. [11].

Rainfall 

Figure 12. The ranges of average seasonal and annual rainfall change (%) for around 2030 and 2070 relative to 1990. The coloured bars show ranges of change for areas with corresponding colours in the maps. Ranges are not given for areas with seasonally low rainfall because percentage changes in rainfall can not be as reliably calculated or applied in such regions. Reprinted with permission from CSIRO. [11].

Impacts on Estuaries from Global Warming Projected for Australia

Listed below are some examples of the potential impacts of climate change could have on estuaries. All of these changes have the potential to alter the distribution of species.

Higher Air Temperatures

• Increased evaporation and lower soil moisture affecting runoff to estuaries
• Increased fire risk for surrounding vegetation
• Increased stratification of coastal lakes (with potential anoxic & hypoxic events)

Decreased Rainfall or Changes to Rainfall Patterns

• Decreased runoff and its impact on environmental flows
• Average rainfall might stay the same but how and when it falls could change (Rain falling in very large storms less often)
• Increased Fire risk for surrounding vegetation

Sea Level Rise

• Saline Intrusion with die-back of fresh water wetlands
• Larger or more frequent storm surges impact barrier ridges and or salinity of estuaries (wave dominated estuaries)

Higher Sea Surface Temperatures

• Changes to nutrient cycling
• Changes to primary productivity
• Changes to water temperature of coastal waters

Ocean Acidification

• Changes to pH
• Changes to pCO₂

Ocean Circulation Wave Patterns

• Changes to nutrient cycling
• Changes to sediment dynamics and form of estuaries

Vector-borne diseases

• Change in the occurrence and distribution of vectors which utilise coastal waterways in their life cycles.

Key questions and further research needs

• The identification of the aspects of estuaries and coastal systems most sensitive to climate change.
• The interaction between increased CO₂ and other factors such as air and, changed runoff quantities and patterns and carbon and nutrient cycles, speciation of trace metals and particle flocculation and their impact of overall estuarine health
• Possible threshold limits, or tipping points, where estuaries ecosystems will no longer be able to function in their current state.

References

1. IPCC Working Group 1 contribution, Science of Climate Change, Second Assessment Report 1996
2. Bureau of Meteorology ( Australia ) The Greenhouse Effect and Climate Change.
3. U.S. Environmental Protection Agency, Office of Atmospheric Programs (2002) Greenhouse gases and global warming potential values: Excerpt from theinventory of U.S. greenhouse emissions and sinks: 1990-2000
4. IPCC (2001). Climate Change 2001: The Scientific Basis. (Houghton, J., Ding, Y., Griggs, D., Noguer, M., vander Linden, P. and Xiaosu, D., Editors). Contribution of the Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. World Meteorological Organisation and United Nations Environment Programme. Cambridge University Press, 944pp
5. Pittock, B. 2005 Global Perspective on Climate Change. Presentation to the Climate Change Impacts and Adaptation in Gippsland stakeholder workshop, September 2005.
6. Bureau of Meteorology ( Australia ) The Greenhouse Effect and Climate Change. Average trend in annual mean temperature in Australia .
7. Bureau of Meteorology ( Australia ) The Greenhouse Effect and Climate Change. Average trend in total rainfall in Australia (mm/10yrs) 1950-2001.
8. Water Corporation, Western Australia . Rainfall and Stream flow data.
9. Church J.A. and White N.j. (2006) A 20 th Century acceleration in global sea-level rise. Geophysical Research Letter, 33, L01602 doi:10.1029/2005GLO24826
10. Hughes, L. (2003) Climate Change and Australia : Trends, Projections and Impacts. Austral Ecology 28 , 423-443.
11. CSIRO (2001) Climate Change Projections for Australia.
12. Pittock, B. (ed)(2003) Climate Change: An Australian Guide to the Science and Potential Impacts . Commonwealth of Australia, Australian Greenhouse Office Canberra Australia. 239 pages.
13. Webster, P. J., G. J. Holland, J. A. Curry and H.-R. Chang (2005): Changes in tropical cyclone number, duration and intensity in a warming environment. Science 309, 1844-1846.

Author

Chris Hepplewhite, Geoscience Australia