Eighteen years after publishing a paper on greenhouse gas calculations in agriculture, the debate on the best method to calculate methane emissions remains a topic of discussion in New Zealand.
The Science and Value Judgments
Understanding the science behind methane emissions is crucial, but it’s not just about the science. Greenhouse gas policies also involve value judgments, which can vary and are neither right nor wrong. A key judgement in this context is the time horizon used to compare short-lived gases like methane with long-lived gases such as carbon dioxide and nitrous oxide.
New Zealand’s Agricultural Emissions
In New Zealand, agriculture is often reported to contribute half of the country’s greenhouse gases. This figure is based on a 100-year time horizon, but it’s important to note that using a 500-year horizon would yield a significantly different result. Over a century, most of methane’s warming effects are realised, whereas only a quarter of carbon dioxide’s effects occur in the same period. Extending the horizon to 500 years reduces methane’s relative impact significantly.
The Role of Time Horizons
If the time horizon is shortened to 20 years, the impact of methane increases compared to carbon dioxide. This perspective implies concern only for the immediate future, disregarding long-term effects on the planet.
Non-Fossil Methane in New Zealand
The methane produced in New Zealand’s agriculture is non-fossil in origin. Shifting from a 100-year to a 500-year horizon changes its global warming potential from the equivalent of 27 tonnes of carbon dioxide to just 7.2 tonnes. This adjustment alters the perceived impact of agriculture on global warming.
Value Judgments in Science
The choice of time horizon is not a scientific decision but a societal one, based on value judgments. It’s essential for society to be informed in making these decisions.
Methane and the Pastoral Carbon Cycle
Some argue that methane emissions should not be taxed because they are part of the pastoral carbon cycle. While this cycle is a fact, it doesn’t negate methane’s contribution to global warming.
Distinguishing Non-Fossil and Fossil Methane
The Intergovernmental Panel on Climate Change (IPCC) recognizes that non-fossil sourced methane should have a lower carbon-dioxide equivalent than fossil-sourced methane. This distinction is reflected in the latest IPCC report but doesn’t eliminate the warming effect of non-fossil methane.
The Debate on Climate Change Science
While the concept of global warming is widely accepted, the specific numbers and their implications are still debated. The IPCC report’s figures, including uncertainties, indicate that the science is
not entirely settled. This uncertainty is crucial in understanding the complexity of climate change and its impact.
GWP Metric and Its Implications*
GWP* (Global Warming Potential star) is a metric discussed in the IPCC’s latest report. It’s popular among farmers and agricultural organisations but is often misunderstood. GWP* is not new science but a different perspective on existing science. It considers both current emissions and those from the past 20 years. For countries like New Zealand, with stable methane levels over two decades, GWP* significantly reduces the warming value of current methane emissions. However, for countries increasing livestock production, like India, GWP* would result in a higher assessment per unit of current emissions.
Ethical and Practical Criticisms of GWP*
Critiques of GWP* focus on both ethical and practical grounds. One argument is that it could unfairly benefit countries with high historical emissions, contradicting the goals of the Paris Agreement, which aims to support rather than penalise developing countries.
Looking Ahead: Methane Metrics Review
The new coalition government in New Zealand has committed to reviewing methane metrics in 2024, which promises to be an insightful exercise. Understanding these metrics is crucial for both farmer organisations and the government to address the challenges of climate change effectively.
