The striking carbon-cutting effect of last year's lockdown has been captured in tiny blades of grass - revealing some sites saw CO2 levels plummet by 80 per cent as roads lay empty.
That was in using grass clippings to gauge the change - and recruiting families around the country to act as "citizen scientists" and send in their own backyard samples for testing.
Why could grass tell us about shifts in carbon dioxide?
"As grass grows, it absorbs carbon dioxide from the air and gains energy from it using chlorophyll and sunlight," explained Dr Jocelyn Turnbull, who leads GNS Science's radiocarbon and greenhouse gas research team.
"The carbon stored in the plant provides a useful recording of the radiocarbon content of the CO2 that has been absorbed."
Radiocarbon, particularly, was the radioactive form of carbon - and all naturally occurring carbon from forms of biogenic exchange, like photosynthesis and respiration, had a modern radioactive carbon signal.
"By measuring the amount of radiocarbon in the air, we can figure out how much CO2 in the air came from fossil fuels," she said.
"The grass samples are 'natural samplers' of local air - and testing them gives us a weekly average of fossil fuel CO2 emissions."
But because grass clippings reflected only local conditions - they captured only the environment within a 100m sq range - results varied depending on how close the sample site was to big emission sources like roads and factories.
The same approach has been used in New Zealand several times in the past - notably with a major Taranaki-based study that delivered a new way of measuring emissions from power plants.
In this case, however, lockdown offered a fascinating chance to look at what grass could tell us if human activity was effectively brought to a standstill.
"We asked our GNS staff to collect samples for us, and then saw the opportunity to engage with kids and families across New Zealand to widen our sample pool and share the science with a national audience."
With support from the Ministry for the Environment and Niwa, the Great Greenhouse Gas Grass Off citizen science campaign was born.
Volunteers across the country were called on to take cuttings from the same patch of grass at weekly intervals, as the country moved from level 4 lockdown to level 1.
"It was important that the location was the same every time to ensure that each sample was only the past weeks' growth."
Volunteers joined a Facebook group to learn more about how to take their samples and about the science behind the project.
Eventually, about 110 citizen scientists posted in samples that had been cut, dated, then stored in freezers.
The results - released today to mark Earth Day - were remarkable.
Sites across Wellington, Christchurch, Hamilton and Gisborne all showed a 75 to 80 per cent drop in emission during level 4, compared to level 1.
Central Auckland locations also showed a decrease in emissions during lockdown, but not nearly as significant as Wellington sites, where emissions fell from 40 to 60 per cent.
"What we expect that we're seeing with these central Auckland sites is a mixture of emission sources, not just traffic," Turnbull said.
"We're picking up residential and commercial emissions too, which we don't expect to have dropped as significantly during lockdown."
Other interesting results came from two sites in Wellington, both on the median strip on Cambridge Tce.
"The two sites showed similar 75 to 80 per cent decreases during level 4, but as we returned to normal life and the alert levels decreased, there were subtle differences in emissions level."
One site saw a steady increase in CO2 emissions, but the other saw a more abrupt return to pre-lockdown levels.
"The different patterns likely reflect the first monitoring site is near free-flowing traffic and the other close to traffic waiting at a set of lights."
Interestingly, results from rural Canterbury showed only a modest change.
"We expect this might be because driving dropped less significantly in these areas during level 4 as people continued with essential work."
Generally, Turnbull didn't think the nationwide results too surprising, given the lockdown's obvious impact on traffic and its emissions.
"What's interesting is the level of detail that we obtained from this type of sampling, for example differences in emissions depending on where traffic builds up at intersections."
Separately, GNS scientists collected a small dataset of direct flask measurements from Auckland during the lockdown, which showed similar results to the grass samples.
This type of sampling has been carried out at 26 sites around the city since 2017.
"Next month, we will complete install of three permanent observing sites in Auckland, to allow monitoring of emissions and changes through time," she said.
"We hope to expand to other New Zealand cities in the coming years, supporting councils and citizens to monitor and mitigate their greenhouse gas emissions.
"And we're doing ongoing urban greenhouse gas research through CarbonWatch NZ, a collaborative research programme involving Niwa, GNS Science, Manaaki Whenua and the University of Waikato, aiming to evaluate New Zealand's full carbon budget."