Counting carbon – NZ contributes to global CO2 stocktake
New Zealand researchers have contributed to the first ever stocktake of global greenhouse gas emissions.
The project involved over 60 researchers from more than 100 countries, including scientists from NIWA.
Measurements from NASA’s Orbiting Carbon Observatory-2 (OCO-2) mission were used to estimate atmospheric CO2 concentrations from 2015 to 2020, in addition to surface-based observations.
Dave Pollard is the Principal Investigator for the Total Column Carbon Observing Network (TCCON) station operated by NIWA at Lauder. He says that no study has been done on this scale before.
“We’ve been given a unique view of how much carbon dioxide each country emits and removes from the atmosphere. Factors that influence CO2 concentrations include natural processes like ocean absorption, and human-induced processes such as the burning of fossil fuels and deforestation. These all vary widely between nations due to their size, terrain, location, socioeconomic status, and population,” said Mr Pollard.
For the global stocktake, NIWA provided remote sensing and ground-based measurements from their Baring Head clean air station near Wellington, home to the longest running continuous CO2 measurements in the Southern Hemisphere.
Interestingly, the data from this station differed from the estimates provided by the satellites. Dr Sara Mikaloff-Fletcher, NIWA’s Principal Scientist for Carbon Chemistry and Modelling, says this is because satellite systems aren’t well designed for small countries, such as New Zealand.
“Where there is cloud or complex topography present – features across much of Aotearoa’s landscape – satellites have trouble measuring CO2. Their estimates are not in line with what we know on the ground; satellite data alone suggest that New Zealand’s land vegetation is emitting carbon dioxide to the atmosphere, whereas our own data shows New Zealand’s terrestrial biosphere absorbs carbon dioxide and offsets a substantial portion of our emissions.
“This is why it’s crucial that NZ and other countries have their own ground-based observations and that these have been included in this stocktake - because they give a fuller picture. It’s likely there will be more global emission estimates done like this in the future, which will highlight gaps between the emissions countries are reporting and what is being observed in the atmosphere,” said Dr Mikaloff-Fletcher.
NIWA’s ground-based observations come from CarbonWatch NZ – a project that uses air monitoring stations around New Zealand to combine measurements of greenhouse gases with high resolution weather models. This gives a more accurate picture of overall CO2 concentrations, as information from the land’s surface complements what satellites see from above.
“This comes at the perfect time, as the 2015 Paris Agreement’s first Global Stocktake – an assessment of the world’s progress toward slowing global warming – is taking place this year. These results will contribute to that international effort,” said Dr Mikaloff-Fletcher.
About the data
These new estimates are underpinned by global models and satellites that have coarse spatial resolution. The system is designed to track emissions from large countries and can add a great deal of new information for large countries for which greenhouse gas emissions and uptake are not well known. However, the authors acknowledge that the system is not well suited for small countries, such as New Zealand.
The estimates using satellite data alone suggest that New Zealand’s terrestrial biosphere is emitting carbon dioxide to the atmosphere, while our own data shows New Zealand’s terrestrial biosphere absorbs carbon dioxide and offsets a substantial portion of our emissions. This conclusion is supported by measurements of trees at forest study plots, terrestrial biosphere models that model physical processes, and atmospheric measurements of carbon dioxide and other gases at a national network of observing sites.
Satellites have difficulty measuring CO2 where there is cloud or complex topography, which describes a lot of the forested areas across Aotearoa. In addition, the spatial resolution of the satellite used in this study is quite coarse, so it is no surprise that the satellite estimates are not in line with what we know on the ground. It is not very good at analysing terrains with mountainous areas or seeing through high levels of cloud cover.