Greenhouse emissions and global warming

Fourth Assessment Report of the Intergovernmental Panel on Climate Change (26)

The International Panel on Climate Change (IPCC) is an organisation that brings together scientific work from more than 2000 climate change scientists from around the world and each six years releases assessment reports summarising the scientific consensus. The IPCC released Volume 1 of its Fourth Assessment Report (AR4) on 2nd February 2007 (26), confirming many of the projections of the Third Assessment Report in 2001:

• Global atmospheric concentrations of carbon dioxide, methane and nitrous oxide have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values determined from ice cores spanning many thousands of years. The global increases in carbon dioxide concentration are due primarily to fossil fuel use and land-use change, while those of methane and nitrous oxide are primarily due to agriculture.
  
• The understanding of anthropogenic warming and cooling influences on climate has improved since the Third Assessment Report (TAR), leading to very high confidence that the globally averaged net effect of human activities since 1750 has been one of warming, with a radiative forcing * of +1.6 [+0.6 to +2.4] W/m2.
  
• * Note: Radiative forcing is a measure of the influence that a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system and is an index of the importance of the factor as a potential climate change mechanism. Positive forcing tends to warm the surface while negative forcing tends to cool it. In this report radiative forcing values are for 2005 relative to pre-industrial conditions defined at 1750 and are expressed in watts per square metre (W/m2).
  
• Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level.
  
• Eleven of the last twelve years (1995 –2006) rank among the 12 warmest years in the instrumental record of global surface temperature (since 1850). The linear warming trend over the last 50 years (0.13 [0.10 to 0.16] °C per decade) is nearly twice that for the last 100 years. The total temperature increase from 1850–1899 to 2001–2005 is 0.76 [0.57 to 0.95] °C.
  
• The average atmospheric water vapour content has increased since at least the 1980s over land and ocean as well as in the upper troposphere. The increase is broadly consistent with the extra water vapour that warmer air can hold.
  
• Observations since 1961 show that the average temperature of the global ocean has increased to depths of at least 3000 m and that the ocean has been absorbing more than 80% of the heat added to the climate system. Such warming causes seawater to expand, contributing to sea level rise.
  
• Mountain glaciers and snow cover have declined on average in both hemispheres. Widespread decreases in glaciers and ice caps have contributed to sea level rise (ice caps do not include contributions from the Greenland and Antarctic ice sheets).
  
• New data since the Third Assessment Report (2001) now show that losses from the ice sheets of Greenland and Antarctica have very likely contributed to sea level rise over 1993 to 2003. Flow speed has increased for some Greenland and Antarctic outlet glaciers, which drain ice from the interior of the ice sheets.
  
• Global average sea level rose at an average rate of 1.8 [1.3 to 2.3] mm per year over 1961 to 2003. The rate was faster over 1993 to 2003, about 3.1 [2.4 to 3.8] mm per year. There is high confidence that the rate of observed sea level rise increased from the 19th to the 20th century. The total 20th century rise is estimated to be 0.17 [0.12 to 0.22] m.
  
• At continental, regional, and ocean basin scales, numerous long-term changes in climate have been observed. These include changes in Arctic temperatures and ice, widespread changes in precipitation amounts, ocean salinity, wind patterns and aspects of extreme weather including droughts, heavy precipitation, heat waves and the intensity of tropical cyclones.
  
• Average Arctic temperatures increased at almost twice the global average rate in the past 100 years.
  
• Satellite data since 1978 show that annual average Arctic sea ice extent has shrunk by 2.7 [2.1 to 3.3]% per decade, with larger decreases in summer of 7.4 [5.0 to 9.8]% per decade.
  
• Temperatures at the top of the permafrost layer have generally increased since the 1980s in the Arctic (by up to 3 °C). The maximum area covered by seasonally frozen ground has decreased by about 7% in the Northern Hemisphere since 1900, with a decrease in spring of up to 15%.
  
• Long-term trends from 1900 to 2005 have been observed inprecipitation amount over many large regions. Significantly increased precipitation has been observed in eastern parts of North and South America, northern Europe and northern and central Asia. Drying has been observed in the Sahel, the Mediterranean, southern Africa and parts of southern Asia. Precipitation is highly variable spatially and temporally, and data are limited in some regions. Long-term trends have not been observed for the other large regions assessed.
  
• Mid-latitude westerly winds have strengthened in both hemispheres since the 1960s.
  
• More intense and longer droughts have been observed over wider areas since the 1970s, particularly in the tropics and subtropics. Increased drying linked with higher temperatures and decreased precipitation have contributed to changes in drought. Changes in sea surface temperatures (SST), wind patterns, and decreased snowpack and snow cover have also been linked to droughts.
  
• The frequency of heavy precipitation events has increased over most land areas, consistent with warming and observed increases of atmospheric water vapour.
  
• Widespread changes in extreme temperatures have been observed over the last 50 years. Cold days, cold nights and frost have become less frequent, while hot days, hot nights, and heat waves have become more frequent.
  
• There is observational evidence for an increase of intense tropical cyclone activity in the North Atlantic since about 1970, correlated with increases of tropical sea surface temperatures.
  
• Paleoclimate information supports the interpretation that the warmth of the last half century is unusual in at least the previous 1300 years. The last time the polar regions were significantly warmer than present for an extended period (about 125 000 years ago), reductions in polar ice volume led to 4 to 6 metres of sea level rise.
  
• Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations. Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns.
  
• For the next two decades a warming of about 0.2 °C per decade is projected for a range of SRES emission scenarios (see page 16). Even if the concentrations of all greenhouse gases and aerosols had been kept constant at year 2000 levels, a further warming of about 0.1 °C per decade would be expected.
  
• Continued greenhouse gas emissions at or above current rates would cause further warming and induce many changes in the global climate system during the 21st century that would very likely be larger than those observed during the 20th century.
  
• There is now higher confidence in projected patterns of warming and other regional-scale features, including changes in wind patterns, precipitation, and some aspects of extremes and of ice.
  
• Anthropogenic warming and sea level rise would continue for centuries due to the timescales associated with climate processes and feedbacks, even if greenhouse gas concentrations were to be stabilized.

Reasons for uncertainty in climate change projections

In 2006, the Climate Impacts and Risk Group of the CSIRO prepared a report for the South Australian Government on climate change in SA and the Natural Resources Management (NRM) Regions (6). In order to project climate change in South Australia, 23 global climate model (GCM) experiments, with the addition of two regional climate models, were assessed for their ability to simulate observed average (1961–1990) patterns of mean sea level pressure, temperature and rainfall in the South Australian region. Of these 13 performed satisfactorily. Temperature and rainfall projections were made using the results from those 13 models for South Australia.

Projections from the 13 models cannot be exact due to:

• The uncertainty about the volume of greenhouse gases which will continue to be emitted into the atmosphere; this depends on the effectiveness of measures to reduce emissions and the trend in global population.
  
• Uncertainty about the impact of increased greenhouse gas emissions at a regional scale.
  
• Uncertainty in the climate science, in particular sensitivity of the climate system to increases in greenhouse gas levels.

What is clear to date are the measured trends in greenhouse emissions illustrated in many scientific papers and illustrated to the right.

What is the greenhouse effect?

Shortwave energy radiated by the sun enters the atmosphere surrounding the Earth and heats the Earth’s surface. The energy is re-radiated from the Earth’s surface back towards space in the form of long wave radiation. As the energy passes back through the atmosphere, greenhouse gases (chiefly water vapour, carbon dioxide, methane, nitrous oxide and halocarbons in the troposphere) absorb part of the heat. The absorbed energy heats the lower layers of the atmosphere, and is re-radiated by the gases heating the land surface and upper layers of the ocean. This effect has been labelled ‘greenhouse’ because of the warming effect, but is in fact a different mechanism from greenhouses used in horticulture.

The Greenhouse effect in the atmosphere occurs naturally in response to levels of greenhouse gases which have remained relatively stable in the atmosphere until the start of the Industrial Era (~1750), when humans began burning large amounts of fossil fuels and clearing forest on a large scale. Burning fossil fuels ( coal, oil etc) releases carbon dioxide into the atmosphere.

Trees are natural stores of carbon. Land clearing results in the decomposition of trees converting the carbon into carbon dioxide which is then released into the atmosphere.

The natural greenhouse effect is enhanced in the Earth’s atmosphere with increases in the concentration of greenhouse gases, chiefly from the carbon dioxide released from the processes mentioned above. Prior to 1750 carbon dioxide levels were ~280 parts per million. An increase of 30% has occurred since then to 380 parts per million at present. The rate of increase is ~1.5 parts per million per year but this rate is increasing.

An increased concentration of greenhouse gases retains more of the heat which is usually radiated into space by the Earth and causes an increase in temperature of the atmosphere, the land and upper layers of the ocean. This process is illustrated in Figure 1.

Figure 1: Details of Earth’s energy balance (source: Kiehl and Trenberth, 1997). Numbers are in watts per square meter of Earth’s surface, and some may be uncertain by as much as 20%. The greenhouse effect is associated with the absorption and reradiation of energy by atmospheric greenhouse gases and particles, resulting in a downward flux of infrared radiation from the atmosphere to the surface (back radiation) and therefore in a higher surface temperature. Note that the total rate at which energy leaves Earth (107 W/m² of reflected sunlight plus 235 W/m² of infrared [long-wave] radiation) is equal to the 342 W/m² of incident sunlight. Thus Earth is in approximate energy balance in this analysis (25).

Greenhouse gases

The major greenhouse gases are carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and halocarbons. Gases other than carbon dioxide are often measured in what are known as carbon dioxide equivalents, reflecting the the combined global warming potential of carbon dioxide, the major greenhouse gas, and the other greenhouse gases. Methane for example has a warming potential twenty times that of CO₂ but is present at much lower levels in the atmosphere. Reductions in methane consequently can have a large impact on greenhouse gas warming.

Figure 2 shows trends in carbon dioxide concentrations in the atmosphere over the last thousand years and corresponding global temperature trends and is based on data from the Third Assessment Report (TAR) of the IPCC (12). Figure 2 shows that from 1000 years ago until the present carbon dioxide and other greenhouse gas concentrations have been steadily and significantly increasing. Of major concern is the fact that in the last part of the 20th Century and today, carbon dioxide gas concentrations have increased dramatically as have methane and nitrous oxide which are not shown in figure 2.

Appendix 1 shows atmospheric carbon dioxide trends over a much longer time, measured using ice cores. Because ice traps atmospheric gases as it forms, the concentrations of carbon dioxide measured in the different layers correspond to concentrations of carbon dioxide in the atmosphere during different periods in the Earth’s history. It shows some large variations over the last 400 000 years, how carbon dioxide levels in the atmosphere are greater today according to this ‘proxy’ data than for at least the last 400 000 years. An important point is that the climate system is ‘locked in’ due to the lengthy time that carbon dioxide stays in the atmosphere, to a certain amount of global warming on top of what has already been observed.

Adaptation to these ’locked in’ changes will be necessary whether desired or not. Also important are efforts to reduce human greenhouse gas emissions to minimise further changes from global warming. It is increasingly recognised from recent research and by many scientific bodies that keeping global warming below two degrees of pre-industrial levels reduces the risk of dangerous climate change.

Figure 2: Global CO₂ concentrations (direct observations and ice core data) and Northern Hemisphere temperature anomaly (direct observations and proxy, mainly tree-ring, data) (9)

Agricultural greenhouse gas emissions

The National Greenhouse Gas Inventory reports that agriculture in Australia produced an estimated 93.1 million tonnes of carbon dioxide equivalent emissions (Mt CO₂-e¹) in terms of global warming potential or 16.5% of net national emissions in 2002 (Figure 3). The agriculture sector is the dominant national source of both methane and nitrous oxide accounting for 71.9 Mt CO₂-e (60.1%) of the net national emissions of methane and 21.3 Mt CO₂-e (86.1%) of the net national emissions of nitrous oxide.

In 2002, agriculture contributed at least 20% of South Australia’s greenhouse gas emissions, or more than 6.2 million tonnes of carbon dioxide equivalents (CO₂-e) per year. However agriculture has potential to play a significant role in managing emissions to reduce the rate of climate change. This is because the way soils, crops and pastures are managed can determine whether they are a source or a sink for greenhouse emissions .

The figures above include agricultural emissions from:

• Livestock enteric fermentation (60% of total South Australian agricultural CO₂-e emissions) — methane emissions resulting from the digestive processes of livestock (98% is emitted by cattle and sheep).
  
• Agricultural soils management (34% of total South Australian agricultural CO₂-e emissions) — nitrous oxide emissions associated with soil disturbance, fertiliser losses and manure applications.
  
• Manure management (5% of total South Australian agricultural CO₂-e emissions) — both methane and nitrous oxide emissions generated by anaerobic decomposition of animal wastes.
  
• However, the figures for agricultural emissions do not include emissions associated with agricultural transport (e.g. use of tractors and other vehicles on farms, transport of produce) or stationary energy use (e.g. electricity use by farms, for pumps, heating, and refrigeration).

Figure 3: The bar graph shows estimated greenhouse gas emissions for all Australian sectors. Pie-chart shows the components of the 18% of emissions for which Australian agriculture was responsible in 2003 (from ‘Agriculture Industry Partnerships – Climate Change Action for Multiple Benefits’, Australian Greenhouse Office, 2006).

Carbon sequestration

Plants take up and store carbon by incorporating that carbon into their structure. These plant processes are collectively known as carbon
bio-sequestration, which removes about 3.2 million tonnes of CO₂-e per year
Australia-wide.

Greenhouse gas emissions from agriculture increased by 2.2% (2.0 Mt) between 1990 and 2004, but decreased by 1.7% (1.6 Mt) from 2003 to 2004. Decreases in agricultural emissions can occur as a result of change in land use (e.g. less land being used for agricultural activities, particularly those causing emissions) or reduced stock numbers, for example.

Trends in agricultural emissions and sinks

• Enteric fermentation emissions — There was only a limited change in enteric emissions over the five years to 2005. During this time there was an increase in cattle numbers and a decrease in sheep. Reduced stock numbers because of the 2006–2007 drought will probably result in reduced methane emissions, at least in the short term.
• Soil emissions — Emissions resulting from soil management (primarily nitrous oxide) have increased by approximately 20% from 1990 base emissions levels. This may be attributed to an increase in cropping or use of fertilisers. Of concern is recent research from the UK suggesting that as soil warms due to global warming, soils that were carbon dioxide sinks can become sources of carbon dioxide emissions.
• Manure management — Emissions have increased by 40–43% from 1990 base levels.

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