The three most important greenhouse gases in the atmosphere are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). While carbon dioxide is the greenhouse gas we hear the most about, methane and nitrous oxide have greater global warming potential (GWP). Methane has a GWP of 25 for a 100-year period and nitrous oxide has a GWP of 298.
For most developed countries, carbon dioxide plays a central role in global climate change. CO2 emissions come from energy production and industrial processes. New Zealand is different in that nearly half of the country’s greenhouse gas emissions come from agriculture – mostly from the pastoral sector.
It is important to make observations and collect data regarding greenhouse gas levels. The information helps experts understand trends and test whether actions to reduce greenhouse gases are working. So how do scientists measure greenhouse gases and monitor changes over time?
Measuring greenhouse gases in the atmosphere
The National Institute of Water and Atmospheric Research (NIWA) operates the Baring Head Clean Air Monitoring Station near Wellington. The station has been collecting monthly data about atmospheric gases for nearly 50 years. What is very special about Baring Head is its location – when a southerly wind blows in, it arrives from areas that have not been influenced by humans. The air has not touched land for 4 or 5 days, so it has not been affected by local emissions like car exhaust or industrial activity. Air samples from these southerly winds are collected in glass flasks. They are analysed by NIWA and are also sent to the Scripps Institution of Oceanography in the United States. Between 60 and 100 samples are collected each year and are used to establish baseline greenhouse gas concentrations. NIWA also measures atmospheric gases every few hours as part of its national monitoring systems.
The Baring Head monitoring station sits on top of a 30 m cliff and uses a tall mast to avoid gases coming off the soil and plants. For the scientists measuring greenhouse gases over terrestrial ecosystems, it’s the soil, plant and animal interactions happening near ground level that are of interest.
Measuring greenhouse gases from the land
In New Zealand, most methane emissions come from burps from ruminant animals – sheep, cows, deer – and from their manure. Measuring methane emissions from animals has a few challenges as nearly all of the country’s animals are kept outdoors. Respiration chambers provide the most precise measurement methods. The chambers are acrylic compartments that continuously sample the air and the amount of methane the animals produce. Scientists briefly house animals in the chambers to measure different feeds, effectiveness of treatments and animals with different genotypes. Portable chambers and monitoring yokes measure burps/emissions when the animals are out in the paddocks. Animal experiments like these are subject to strict ethics approval.
Nitrous oxide emissions come primarily from urine patches in paddocks and less so from dung and fertilisers. Soil chambers are the traditional method for measuring N2O. These small enclosures effectively measure gases given off by the soil. Each chamber covers a small area of pasture, so they are unable to measure emissions coming from an entire paddock.
Micrometeorology is a way of measuring nitrous oxide at a larger scale. Sensors measure gas concentrations multiple times per second. They help to check whether small-scale measurements from individual animals or soil chambers are representative of an entire herd or flock. Learn more about this technique in the article Measuring gases using eddy covariance.
Measuring ancient greenhouse gases
New Zealand scientists, like others around the globe, also measure greenhouse gas levels from prior centuries. Although they cannot go back in time, the scientists can use air bubbles in ancient ice cores to measure atmospheric gases at the time the ice was formed. This information helps scientists link ancient climate conditions with their causes and effects.
Palaeoclimatologists use clues from the past as proxies to build a picture of what the world once looked like. Air bubbles in the ice cores hold information about the concentration of greenhouse gases in the atmosphere at the time the ice was formed.