• Rural NZ
• Adverse Events
• People issues
• Profitability
• Research
• Rural Proofing
• SLM Hill Country Erosion Programme
• Community Irrigation Fund
• Stats & forecasts
• Sustainability
  • Best practices
  • Biodiversity
  • Climate
  • Irrigation
  • Land
  • Native forests
  • Organics
  • Resources
  • Water
• Legislation
• Media Centre
• Publications
• What's New

---

• 2.1.1 Greenhouse Gas Emission Scenarios
• Table 2.1 Summary statistics for IPCC scenario IS92a
• 2.1.2 Predictions of Climate Change
• 2.1.3 International Policy Issues
• 2.1.4 Combining Greenhouse Gases - The Global Warming Potential Issue
• Table 2.2 Direct GWPs for agriculturally important greenhouse gases

---

CLIMATE CHANGE & AGRICULTURE

2.1 REVIEW OF NEW ZEALAND AND IPCC SCENARIOS FOR GREENHOUSE GAS EMISSIONS

This section provides background information on estimates of present and anticipated greenhouse gas emissions, on anticipated climate change likely to arise from the consequent higher greenhouse gas levels in the atmosphere, and on international policy issues. In each area the global situation is reviewed and the New Zealand situation is placed in the global context.

2.1.1 Greenhouse Gas Emission Scenarios

The predominant cause of increasing CO₂ concentrations in the atmosphere is the burning of fossil fuel, with deforestation being a secondary cause. Most studies of future scenarios for climate forcinu due to an enhanced greenhouse effect, consider only CO₂ emissions and include models for future production and demand of energy, because CO₂ emissions are closely linked with fossil fuel derived energy. However recent work of the Intergovernmental Panel on Climate Change (IPCC) considers all significant greenhouse gases.

Scenarios used by the IPCC for their 1992 report on climate change are the result of continuous developments in this area in the USA and Europe which can be traced back to the 1970s. Studies for the US Environmental Protection Agency and US Department of Energy, documented by Lashof and Tirpak (1990), were updated to produce four scenarios for the IPCC 1990 report. These were in turn superseded by 6 scenarios for the IPCC 1992 report which are documented separately in some detail (Pepper et al, 1992). The latest IPCC scenarios consider a wide range of greenhouse gases, not just CO₂.

"Business as usual" type scenarios such as the IS92a scenario of IPCC 1992 involve an increase in the fraction of energy generated by coal from just below 30% at present to 50-60% by 2100. This is based on the result of partial equilibrium models for energy supply and demand in different economic sectors and different geographic regions. The increase in coal use reflects the effect of diminishing resources for oil and gas, offset to some extent by increases in nuclear energy and new technology. The models used are driven by economic growth and population assumptions based on World Bank projections (e.g. Zachariah and Vu, 1988), together with assumptions about the costs of new technology. More recently the International Energy Agency and the World Energy Council have made inputs from the standpoint of energy producer organizations.

The dominant factors determining future greenhouse gas emissions in these scenario analyses are world population, the energy intensity of global economics, and the efficiency of energy use. Ironically the impact of policy to control emissions does not appear as a major factor.

This report considers the IPCC IS92a scenario as a reference scenario for what would happen globally without the intervention of climate related policy. We also limit consideration to the period 1990 to 2025 and over this range most scenarios agree relatively closely. The main divergences in scenarios occur in the latter half of the next century as a result of different assumptions about population and energy intensity.

The key assumptions in the IS92a scenario are:

• World Bank projections of population rising from 5.3 billion (1990) to 8.4 billion (2025);
• global average economic growth rate of 2.9% per annum;
• mid range estimates of oil and gas reserves, slight increases in the cost of nuclear power, but decreases in the cost of solar;
• current air quality emission legislation but no specific limitation of greenhouse gases;
• partial compliance with the Montreal protocol for CFCs and some CFC phaseout by non-signatories.

The effect of the global agricultural sector in these models is only considered through methane and nitrous oxide emissions. That is the direct emission Of CO₂ in global agriculture is not regarded as worth modelling explicitly and it is absorbed in the energy production sector. Global agriculture's methane emissions increase by 47%, primarily due to increases in livestock numbers, and nitrous oxide emissions increase by 57%, primarily due to increased fertilizer use.

The growth of methane emissions in this scenario is largely due to increases in livestock numbers. Livestock populations grow faster than human population in OECD countries and slower than human population in other countries.

Table 2.1 Summary statistics for IPCC scenario IS92a

	1990	2025	% increase
WORLD
Population (billions)	5.252	8.414	60
CO2 emissions (Gt)	27.1	44.7	65
CO2 emissions (tonnes/capita)	5.2	5.3	3
Methane emissions from
... Agriculture (Mt)	170	259	52
(Agricultural methane Kg/capita)	32	31	-5
. . . Landfills (Mt)	38	63	66
... Other anthropogenic (Mt)	144	182	26
Total Methane emissions (Mt)	351	504	44
OECD only
Population (billions)	0.838	0.939	12
Agricultural m thane emissions (Mt)	28	36	29
(Agricultural methane Kg/capita)	33	38	15
Landfill methane (Mt)	23	23	3

The New Zealand perspective

New Zealand's total CO₂ emissions for 1988 have been estimated at 26.4 Mt or about 8 tonnes per capita per annum. This is less than the estimated OECD average C02 emission rates of about 12 tonnes per capita per annum. New Zealand CO₂ emissions were attributed principally to transport (43%), industrial use (29%), and electricity generation (14%). As an alternative attribution, about 45% of CO₂ emissions are from the consumption of liquid fuels. 11% from natural gas, and 23% from solid fuels. Because of the relatively high proportion of hydro generation in New Zealand CO₂ emissions are expected to be lower than average for our energy intensity.

The uncertainty in current CO₂ emission estimates is generally taken to be about 10%. However interpretation of energy and CO₂ statistics is often confused by the lack of consistent data. Data for New Zealand is not sufficiently complete or consistent to allow disaggregation of CO₂ emissions to the level at which the agricultural sector or farm activities is properly identified. Some energy statistics for 1991 prepared by Ministry of Commerce for the International Energy Agency suggest that the agricultural sector accounts for about 4% of fuel consumption. Direct agricultural emissions of CO₂ are therefore a minor component of the New Zealand emissions inventory.

New Zealand's anthropogenic methane emissions are estimated to be about 1.5 Mt per annum or about 440 kg per capita per annum, compared with a global average of about 100 kg per annum. The uncertainty in such estimates of methane emissions from primarily agricultural sources is around 25%.

Clearly New Zealand has one of the highest ratios of methane to CO₂ emissions in the world. The estimated proportion of New Zealand's total greenhouse forcing attributable to methane emissions will depend on the GWP used (see section 2.1.4). Typical GWPs in use imply that about half the national contribution to increasing the greenhouse effect is attributable to methane.

The major implications of issues covered in this section are:

• direct emissions of CO₂ from the agricultural sector are low and New Zealand is no exception in this regard;
• emissions of methane and nitrous oxide by agriculture are much more significant from a greenhouse effect perspective;
• New Zealand has relatively low CO₂ emissions for our energy intensity, but high methane emissions due to high livestock numbers (on a per capita basis), thus nationally we have one of the highest ratios of methane to CO₂ emissions.

2.1.2 Predictions of Climate Change

Estimates of global mean temperature increases in the near future, arising from "business as usual" gas emission scenarios, are for a trend of about 0.3% per decade. This is based on the results of General Circulation Models (GCMs), which incorporate the basic thermodynamics of the atmosphere. Such models have produced fairly consistent estimates for about 5 years indicating a reasonable maturity in such models.

There are still some concerns about proper treatment of the more complex aspects of climate in climate models. In particular the treatment of clouds, and possible changes in average cloud cover or cloud height are difficult to validate in such models. Although there are some strident critics of current climate predictions most scientists believe the current estimates of change are within a factor of two of what will occur. Recent reviews of estimated temperature trends are in IPCC 1992, and Wigley et al, 1992.

Actual variations in global mean temperatures are driven by many factors other than increasing greenhouse gas concentrations. In particular volcanic eruptions and irregular cycles in the climate system. such as the EI Nino Southern Oscillation Anomaly, cause significant changes in global temperatures. Analyses comparing the observed temperature increase of about 0.5^OC over the last 100 years conclude that this is not inconsistent with climate model predictions and greenhouse gas increases. However, until observed temperature increases rise above the "noise" level in climate records it will not be possible to prove that the models are correct.

Between 1990 and 2025 the steady temperature trend can be expected to increase global mean temperatures by about 1^OC. New Zealand mean temperatures are expected to change by an amount close to this global mean value. This is about the same as short term variations in temperature that appear in climate records, and to which agriculture is already adapted. However, as the temperature increase will be persistent it will have impacts both in agriculture and elsewhere. In particular both high and low year to year temperature extremes are expected to increase by about I'C thus there will be an increasing occurrence of high-temperature years and a decreasing occurrence of low temperature years.

While the upward trend in temperatures will lead to more frequent high temperature extremes, there is also the possibility that changes in climate will lead to a wider range of extremes and to more frequent extreme events. This applies to rainfall and storms as well as temperature. For example regional modelling studies carried out in Australia suggest that heavy rainfall events are likely to occur more frequently, at least in warm regions, even where total rainfall does not increase.

At present projects are underway in Australia and New Zealand to develop accurate regional climate models that could be used to predict changes in climate at a scale relevant to activities such as farming in a specific region. This work will probably produce useful results in 1 to 2 years time. So far scientists studying climate change have not felt sufficiently confident about detailed changes at the regional level to make predictions. For example some scenarios have been considered in which the mid-western USA becomes drier, and the consequent impact on US agriculture, but this would not be regarded as a reliable prediction by most climate scientists.

Salinger and Hicks, 1989, have produced a scenario for climate change in New Zealand until 2050. The "more likely" of these includes increased rainfall in northern and western districts, and decreased rainfall in southern and eastern ones. The impacts of this have been considered by Martin et al, 1991. These scenarios should be regarded more as a way of focusing discussion than as calculated predictions for future change, however some general conclusions are: horticulture is most susceptible to climate change; crop production is most likely to be enhanced by climate change; and as climate zones move south and to higher elevations climate sensitive agriculture will have to follow.

In addition to impacts due to changes in climate, consideration should be given to possible increased productivity due to the "CO₂ fertilization effect". This effect, by which plants grow more rapidly in atmospheres enriched in CO₂, has been known for some time. There is however considerable debate among plant physiologists, and field botanists as to whether the effect will have long term impacts on plant production. Some studies in the USA suggest that most additional production goes into fine root material below ground, others suggest that plants may initially grow more rapidly but then "adapt" to the higher CO₂ levels and production returns to original levels.

In practice CO₂ fertilization is closely coupled to two other plant physiological responses. Firstly increased CO₂ levels leads to higher stomatal resistance hence to reduced evapotranspiration and higher water use efficiency by most plants. Secondly higher temperatures lead to higher rates of microbial activity and respiration of CO₂ from fine root and sub-surface labile carbon.

A recent IPCC workshop on biosphere responses to climate change, held in the USA in December 1992, was unable to reach any unanimous conclusion as to whether in general plant productivity would increase or not. On the other hand several recent attempts to explain observed increases in atmospheric CO₂ in terms of known emissions. estimated rates of deforestation, and ocean uptake, have all concluded that some CO₂ fertilization should have occurred over the last 150 years in order to match the observed rise in CO₂ concentrations to estimates of historical release and uptake of CO_2.

In our reference "business as usual" scenario, by 2025 atmospheric C02 Concentrations will be around 60% higher than in pre-industrial times. The proponents of CO₂ fertilization theories would predict about a 17% increase in global plant productivity due to this enriched CO₂ atmosphere. This would not be spread equally across all plant species. Studies of agricultural impacts in the USA suggest that such an increase in plant productivity approximately offsets a loss of productivity due to reduced rainfall (Adams et al, 1989).

It appears clear that any change in agricultural productivity before 2025 as a result of climate change would be small and of the same order as the effect of current year to year climate fluctuations. There is no good reason to believe that there will be some dramatic change in the physical climate. In addition there is considerable uncertainty in predicted climate change at the level of detail necessary to estimate agricultural impacts. Extreme events models are not reliable enough to base decisions on at this stage. It is likely that New Zealand agriculture will be influenced more by changes in our markets than by physical climate. The implementation of an emissions tax, in order to be effective. would have to be of greater impact than climate. Consequently, this report does not consider changes in agricultural productivity in determining short term impacts of policy.

The major implications arising from the issues covered in this section are:

• the impact of changes due to trends in physical climate prior to 2025 will be on average comparable to the year to year variability currently experienced
• the balance between different regions in New Zealand is more likely to change than our total production;
• other countries will experience larger changes in agricultural productivity as a result of climate change than New Zealand, thus the strongest impact on New Zealand is likely to be through shifts in actual and potential markets:

2.1.3 International Policy Issues

The development of climate change policy in New Zealand is occurring largely as response to international developments, particularly the development of international policies to manage greenhouse gas loadings in the atmosphere being promoted by the Nations. In order to understand how New Zealand policy is likely to develop an the background to Ministry for the Environment initiatives, it will be helpful to review the current status of international policies. This is done most easily by considering the Framework Convention on Climate Change (FCCC).

New Zealand along with 176 other countries has endorsed the FCCC which was finalise at the United Nations Conference on Environment and Development in Rio de Janeir in June 1992. This convention will go for-ward for approval by the United Nations General Assembly and become legally binding on its signatories once there are 50 c more of these.

The primary objective of the FCCC (INC, 1992) is to

"achieve . . . stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system."

The timing for achieving this objective is specified only as

". . within a time frame sufficient to allow ecosystems to adapt naturally to climate change, to ensure that food production is not threatened and to enable economic development to proceed in a sustainable manner".

The FCCC reaffirms a principle of international law that, while states have authority within their own boundaries they have a responsibility to avoid damaging the interests of other states. This pre-empts a -winners and losers" approach to global climate change For example several studies indicate that highly developed countries, such as the USA might be able to adapt to anticipated climate change under "business as usual" scenario: However, their endorsement of the FCCC recognizes a responsibility to potential ''Losers under climate change, and accepts at least an implicit commitment to reduce an ultimately prevent further climate change.

The general form of commitment accepted by all signatories to the FCCC is spelled out in article 4 of the convention through which they undertake, among other things,

to make available national inventories of anthropogenic emissions by sources and removals by sinks of all greenhouse gases not covered by the Montreal Protocol;

and

to cooperate in the development and transfer of technology for control of greenhouse gases in all sectors including energy, transport, industry, agriculture, forestry and waste management sectors;

and

to cooperate in adaptation to climate change and develop integrated plans for coastal zone management, water resources and agriculture;

Under article 4 of the FCCC developed countries (including New Zealand) undertake rather more specific commitments to adopt national policies for

"modifying longer term trends in anthropogenic emissions .... recognizing that the return by the end of the present decade to earlier levels of anthropogenic emissions of carbon dioxide and other greenhouse gases ... would contribute to such modification".

and

"each of these parties shall communicate . . . detailed information on its policies .... as well as on its resulting projected anthropogenic emissions by sources and removals by sinks of greenhouse gases . . . . with the aim Of returning individually or jointly to their 1990 levels of these anthropogenic emissions".

This last commitment is the one that will cause the most pressure in the near term for nations to limit greenhouse gas emissions. In particular compilation and publication of national greenhouse gas emission inventories, in an international environment that is placing higher values on conservation and sustainability ethics, will provide the strongest political pressure to act on climate change issues in the next 5 years.

Article 12 of the MCC gives more details of procedures for submission of emission inventories and plans for reducing emissions. A methodology for preparing national emission inventories has already been well developed by study Groups operating under the auspices of the OECD and the Intergovernmental Panel on Climate Change (IPCC), e.g. see OECD 1991, and it seems clear that this will be used by the United Nations as a basis for obligations under the FCCC.

The present methodology for preparing and submitting greenhouse gas emission inventories treats each greenhouse gas, e.g. CO, methane, or nitrous oxide, separately. Emissions for each of the major Greenhouse gases are categorized by the type of activity that produces the emission. This includes a category for agriculture.

A significant amount of work has been carried out to develop a measure of the total Greenhouse gas climate forcing from a single nation or technology. This means that emissions from different greenhouse gases have to be combined. The IPCC 1990 report endorsed the concept of a "Global Warming Potential" (GWP) for this purpose. The GWP is different for each greenhouse gas and measures the greenhouse effect forcing of that gas relative to the same mass Of CO₂ -i.e., it enables emissions of each gas to be expressed in C02 equivalents. The GWP concept is discussed in more detail in the following subsection, 2.1.4., but for the moment it is important to recognise that the GWPs for different greenhouse gases are subject to considerable uncertainty, both conceptual and numerical. Despite these difficulties there is clearly a desire within the international science and environmental communities to use GWPs or some similar weighting factor to combine emissions and thus establish protocols for monitoring a the total greenhouse gas forcing from each nation. Even before there is agreement on this use, we expect GWPs to be used in assessing the environmental value of trading emissions in one greenhouse gas for emissions in another. Thus it is widely recognized that burning methane at an emission source such as a landfill reduces greenhouse effect forcing (and thence emissions of CO₂ equivalents) and is therefore a desirable goal.

The short-term obligations under the FCCC are for each developed nation separately to reduce its greenhouse gas emissions. Clearly governments have the authority to choose different ways of achieving this, ranging from "command control" philosophies to targeted "market intervention" with intermediate policies available. The implications of government policies on agriculture in this regard are the main subject of this report.

In addition to controlling greenhouse gas emissions country by country, consideration is also being given to controlling emissions internationally through partnership arrangements. In particular proponents of "tradeable emission permits" see this as a way of allowing some countries, for which the cost of greenhouse gas reduction might be relatively high, to in effect purchase greenhouse gas reductions made in countries where the costs were relatively low. This would be done in such a way that the combined emissions of all countries involved was kept to an agreed level. This system could obviously be developed into an international market in greenhouse gas emission quotas

To summarize the international policy issues from a New Zealand perspective:

• the main political pressure to act on climate change in the short term will come through compilation and publication of greenhouse gas emission inventories, broken down by economic sectors;
• New Zealand is free to choose its own approach to limiting greenhouse gas emissions, the only external pressure being that these should be seen to be effective. There is still a debate over the relative merits of a "carbon tax" or "tradeable emission permits" both nationally and internationally;
• we should anticipate international agreement on a methodology for combining emissions of different greenhouse gases to arrive at a total greenhouse forcing figure for each economic sector, including agriculture. This will weight methane emissions about 15 to 20 times more heavily than emissions of the same mass of CO₂;
• there may become an international market in greenhouse gas emission quotas which will provide export potential for greenhouse gas "sinks" particularly to heavily industrialised countries.

2.1.4 Combining Greenhouse Gases - The Global Warming Potential Issue

It is known that incremental additions to the atmosphere of many of the non- CO₂ greenhouse gases, such as methane or nitrous oxide, are much more potent greenhouse effect forcers than an increment of CO₂ itself. This is principally because each is much more effective, molecule for molecule, at trapping long-wave radiation that would otherwise escape to space. This is partially offset in some cases, notably methane, by the much shorter lifetime (e.g. methane about 10 years) than CO₂ (about 120 years). A measure of the relative potency is the 'Global Warming Potential' (GWP) for each gas.

The GWP for a greenhouse gas is a measure of the impact of that gas on the greenhouse effect relative to the same amount of CO₂. IPCC (1990) defines the GWP as: "the time integrated commitment to climate forcing from the instantaneous release of 1 kg of a trace gas expressed relative to that from 1 kg of carbon dioxide". The 1992 Supplement Report of the IPCC (IPCC 1992) essentially continues with this definition. In fact this definition is incomplete (Manning, 1991) and practical calculations implicitly use slightly different definitions.

The simplest calculations of GWP (e.g. Rodhe, 1990) are based on the direct radiative forcing, that is the additional infra-red radiation absorbed, due to 1 kg of the greenhouse gas and integrated over time, expressed relative to the corresponding value for 1 kg of CO₂. If this is taken over a very long time then in effect the GWP is the product of two terms, one being the additional infra-red radiation absorbed and the other the average residence time of an increment of the gas in the atmosphere, divided by the same two terms evaluated for CO₂.

One complication in the definition of GWPs is that it is not possible to calculate the "time integrated commitment" unless some fixed horizon is set limiting the period being considered. A GWP will depend upon the 'time horizon' over which the commitment is accumulated. The 'time horizon' refers to future time after emission during which the greenhouse effect forcing of the gas is considered and beyond which it is ignored. A GWP will depend quite strongly upon the time horizon if the atmospheric lifetime of the gas in question is comparatively short. An increment of CO₂ to the atmosphere has an extremely long residence time there, and in most mathematical models this is strictly infinite. But for methane in particular, the residence time is much shorter, at about 10 vents. The effect of this is that when GWPs are calculated over time horizons of a few decades only part of the long-term effect of CO₂ is taken into account and the GWP is correspondingly larger than for a several-century horizon.

The IPCC (1990) definition of GWP and most recent calculations include additional "indirect effects" due to chemical interactions of the gas in the atmosphere. While CO₂ does not have such indirect effects methane in particular does: methane emissions lead to additional water vapour in the lower stratosphere, increases in tropospheric ozone, and reduction of the oxidising power of the atmosphere (Manning, 1991; Lelieveld and Crutzen, 1992). The IPCC (1990) report had included an estimate of indirect effects in its methane GWP. IPCC (1992) adjudged that the calculation of these indirect effects was not yet sufficiently understood to be incorporated in GWPs. The value of 11 is quoted for the "direct" component of the methane GWP over a time period of 100 years, and the statement made that the indirect effect is positive "possibly as large as the direct effect". Thus there is no agreed number for the GWP of methane at this stage. Lelieveld and Crutzen (1992) using different atmospheric chemistry scenarios calculate full GWPs of 15 and 20. Manning (1991) has estimated similar values of 16 and 21 considering additional unpublished work.

Table 2.2 reports the GWPs for the important agriculturally sourced greenhouse gases methane (CH4) and nitrous oxide (N20), for time horizons of 20, 100 and 500 Years, taken from IPCC (1992).

Table 2.2 Direct GWPs for agriculturally important greenhouse gases

(from IPCC, 1992)

Gas	Residence Time	Direct GWP for Time Horizons of:
		20 year	100 year	500 year
CO2	approx 120 yr	1	1	1
CH4	10.5 yr	35	11	4
N2O	132 yr	260	270	170

Despite these potential ambiguities in how GWPs are defined, one should anticipate general agreement to use GWPs for about 100 year horizons. In this report we have weighted emissions of methane simply by the direct GWP 11, to produce CO₂ equivalent emission. This is done on the basis that there are many other uncertainties in the calculations involved, and in the face of the lack of any internationally accepted GWP for methane. However, we expect that if final GWPs for methane are agreed internationally, and the 100 time horizon is used, then these will be around 15 to 20 and the greenhouse impact of methane will be taken to be proportionally larger. On the other hand, there may be a trade off between the increase due to indirect effects and a decrease due to adapting a longer time horizon eg. 150 to 200 years.

Contact for Enquiries

MAF Information Services
Pastoral House
25 The Terrace
PO Box 2526
Wellington, NEW ZEALAND

Fax: +64 4 894 0721
Contact this person