Greenhouse gas measurements
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The global context
Since pre-industrial times, the background atmospheric concentrations of many greenhouse gases have grown significantly. Over the last 250 years the carbon dioxide concentration (CO2) has increased by about 36%, methane concentration (CH4) has increased by 2.5-fold (ie, by 150%), and nitrous oxide concentration (N2O) by about 16% (Forster et al., 2007). Figure 1 illustrates this for all three greenhouse gases. The present global mean carbon dioxide concentration (around 379 parts per million) has not been exceeded during the past 750,000 years, and probably not for the past 20 million years. The growth in carbon dioxide, methane and nitrous oxide concentrations is due largely to human activities including fossil fuel combustion, deforestation and agriculture. The growth rates of methane and nitrous oxide also fluctuate from year to year, but their long-term trend is again upwards.
The rate at which carbon dioxide is increasing varies from year to year as shown in Figure 2. The average rate of increase since 1980 is 0.4% per year. The highest rates of increase have typically been in El Niño years.
NIWA has made trace gas measurements at Baring Head (on the South Coast of the North Island near Wellington) since 1973. When the wind blows from the South, Baring Head air has generally had a long trajectory over the ocean. Under such conditions measurements of carbon dioxide, methane and nitrous oxide represent mid-latitude Southern Hemisphere background values, since the sampled air has not been contaminated by sources on the New Zealand mainland. Baring Head is part of a global network of stations for determining trends in greenhouse gas concentrations. The data are collected during southerly winds and represent values typical of the remote ocean regions of the Southern Hemisphere.
In addition, gas samples from Baring Head can also be used to deduce information about greenhouse gas sources, through analyses of their isotopic composition.
NIWA is also measuring various other gases and aerosols which influence the earth’s radiation budget or which affect greenhouse gases through atmospheric reactions. Some of these measurements are being carried out at Baring Head, and measurements higher up in the atmosphere are made at NIWA’s Lauder research station in Central Otago. More information is available on NIWA’s atmospheric research web pages and in the NIWA booklet The Greenhouse Effect – A New Zealand perspective on climate change.
Developed countries that have ratified the Framework Convention on Climate Change, including New Zealand, have made commitments to limit their anthropogenic emissions of greenhouse gases and enhance their greenhouse gas sinks. They have also undertaken to annually report their current emissions and their estimates for the future. If the Kyoto Protocol is ratified, then New Zealand will become committed to limiting the net anthropogenic emissions of greenhouse gases for the period 2008–12 (expressed as “carbon dioxide equivalents) to 1990 emission levels. More information from a New Zealand policy perspective can be found on this New Zealand Government climate change information website.
A concept called the “Global Warming Potential” (GWP) is used in calculating the “carbon dioxide equivalent” of a mix of greenhouse gases. The “global warming” effect of a particular gas depends on how strongly it absorbs infrared radiation, and on how long it stays in the atmosphere. The GWP is a way of summarising these factors. In essence, the GWP describes the number of grams of carbon dioxide that would provide the same “warming” effect over a specified period of time (e.g. 100 years) as one gram of the gas of interest. For example methane has a 100-year GWP of 21, indicating that a given weight of methane released now would have 21 times as much effect on global warming over the next 100 years as would the same weight of carbon dioxide. (Technical definition of GWP: The cumulative radiative forcing between the present and some chosen time horizon caused by a unit mass of gas emitted now, expressed relative to that of carbon dioxide). Table 1 shows GWPs for various greenhouse gases from the 1990-2005 Ministry for the Environment greenhouse gas inventory (Ministry for the Environment, 2007), for a 100-year time horizon.
|Carbon dioxide (CO2)||Methane (CH4)||Nitrous oxide (N2O)||HFCs (HFC134a)||PFCs (CF4)||Sulphur hexafluoride (SF6)|
There is international agreement to use a 100-year time horizon for calculating “carbon dioxide-equivalent” emissions. Table 2 shows the estimated anthropogenic emissions of each major greenhouse gas from New Zealand in 1990 and in 2005 (Ministry for the Environment, 2007). Also shown in Table 2 are “CO2 removals”, also known as “carbon sinks”. These refer to changes in land use that result in uptake of CO2, for example through growth of trees or build-up of organic carbon in soils.
|Year||CO2 emissions||CO2 removals||CH4||N2O||HFCs||PFCs||SF6|
The IPCC has prepared guidelines for assessing national emissions (IPCC, 2006). However some emissions can be estimated or calculated only to limited accuracy (Penman et al., 2000). In particular, calculated emissions of methane and nitric oxide carry uncertainties because they are generated through biological processes that can be quite variable. NIWA is collaborating with AgResearch and Landcare Research to better define these agricultural emissions for New Zealand.
Figure 3 illustrates the relative “warming influence” of New Zealand’s estimated 2005 anthropogenic greenhouse gas emissions. It comes from the same numbers used to develop Table 2. About 52% of New Zealand’s anthropogenic emissions are methane and nitrous oxide, largely of agricultural origin, which makes our greenhouse gas profile unusual among developed countries.
Forster, P.; Ramaswamy, V.; Artaxo, P.; Bernsten, T.; Betts, R.; Fahey, D.W.; Haywood, J.; Lean, J.; Lowe, D.C.; Myhre, G.; Nganga, J.; Prinn, R.; Raga, G.; Schulz, M.; Van Dorland, R. (2007). Changes in atmospheric constituents and in radiative forcing. In: Solomon, S.; Qin, D.; Manning, M.; Chen, Z.; Marquis, M.; Averyt, K.B.; Tignor, M.; Miller, H.L. (eds). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, pp. 129-234. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. http://www.ipcc.ch/publications_and_data/publications_and_data_reports.s...
IPCC 2006, 2006 IPCC Guidelines for National Greenhouse Gas Inventories, Prepared by the National Greenhouse Gas Inventories Programme, Eggleston H.S., Buendia L., Miwa K., Ngara T. and Tanabe K. (eds). Published: IGES, Japan.
IPCC (2001). Climate Change 2001 – Synthesis Report. Contribution of Working Groups I, II and III to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press. 398 p.
Ministry for the Environment (2007). New Zealand’s Greenhouse Gas Inventory 1990–2005, The National Inventory Report and Common Reporting Format. Ministry for the Environment, Wellington. 158 p.
Penman, J.; Kruger, D.; Galbally, I.; Hiraishi, T.; Nyenzi, B.; Emmanul, S.; Buendia, L .; Hoppaus, R.; Martinsen, T.; Meijer, J.; Miwa, K.; Tanabe, K. (eds) (2000). IPCC Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories. IPCC NGGIP Technical Support Unit, IGES, Hayama, Kanagawa, Japan.
Prepared by David Wratt, Martin Manning, David Lowe, Ross Martin and Keith Lassey.