The Backstory
John Lin
The backstory to this tale first begins in the Pacific Ocean. Tracking carbon emissions dates back decades to a single site in Hawai’i where, on a largely inactive volcano on the Big Island, instruments measured carbon dioxide in the atmosphere. At a high elevation, the site was very good at characterizing broad scale changes in carbon dioxide, globally, a “poster child for climate change because over time,” explains Lin who is also associate director of the Wilkes Center for Climate Science and Policy, “we know that from these Hawai’i measurements, CO2 has this distinct cycle, seasonally, but then this upward trend due to all of us burning fossil fuels.”
Human-caused carbon emissions are not only leading to CO2 buildup everywhere in the atmosphere but the issue is widespread in public discourse. Whether it’s on the micro level of mitigating one’s personal “carbon footprint” by taking the bus, or on the meta level of international initiatives like the Kyoto Accords or the United Nations-brokered Paris Agreement, the effects of carbon emissions are on everyone’s mind. A cascade of cities and whole nations have established goals for mitigating emissions, but their estimates of carbon emissions have been relying on data that are inconsistent and sometimes missing altogether in parts of the world.
That cities have singly established and even accelerated their carbon-neutral goals is a good thing, considering that over 70 percent of human-emitted CO2 into the atmosphere stems from cities around the globe.
Tracking progress toward city-scale emissions reduction targets is essential by providing “actionable information for policy makers,” the paper states. This while the authors acknowledge that earlier measurements and claims from municipal entities are based on “self-reported emissions inventories,” whose methodology and input data often differ from one another. These practices hamper “understanding of changes in both city-scale emissions and the global summation of urban emissions mitigation actions.”
Strangely beautiful animations
Kai Wilmot
In short, Lin and company’s findings, published in Environmental Research Letters, represents a new capacity based on recent developments in modeling. And the animations of the assembled and interpreted satellite CO2 data delivered by the team are startling, even strangely beautiful. In one chart the left side displays latitude vs CO2. “This narrow swath,” explains Lin, indicates “each time … [the satellite] orbits. There's this narrow slice of data that becomes available.”
Using that data, he continues, “the NASA scientists can construct this nice animation of CO2 change in each latitude band over time.” Lin points to what he calls “ridges and valleys” on the the chart that represent the seasonal cycle, and he personifies the entire Earth as if it is “breathing in the carbon dioxide through photosynthesis during the summer growing season and then releasing it in the winter. They have these very sharp ridges — high CO2, low CO2, higher CO2 [the breaths] — but overall, the rug is going up, because we're emitting carbon dioxide into the atmosphere.”
Here, researchers are only looking at a small fraction of data points, the ones that intersect the targeted cities. They then do a more detailed look at whether they’re seeing a signal or not and whether they’re getting enough data.
“Personally,” says Wilmot, “I think the particularly neat aspect of this work is the capacity for global application. Leveraging satellite data and atmospheric modeling, we are able to gain some insight into urban emissions at cities around the world. We can see interactions between these emissions and socioeconomic factors, and we can identify large changes in emissions over time.”
The possibilities of creating more rigorous models, and more revealing data about how much cities emit carbon to the atmosphere are tantalizing. And so are the findings of the research. “This kind of information can be used by cities and the UN process,” Lin says. “But I’m pretty sure what they want is something more dynamic through time, how these emissions evolve. And also, probably more frequent updates.” As it was in this study, researchers had to aggregate multiple years of data to get enough points for each city. “So the challenge, I think, is to be able to track more dynamically these emissions over time.”
More to come
NASA’s next iteration of the Orbiting Carbon Observatory — OCO3 — has already been successfully docked on the International Space Station, although it was de-installed for a period of time recently to allow another instrument to carry out measurements. (It turns out that prime real estate on the crowded station is, well, at a premium.) But new data is forthcoming.
Meantime, researchers have their work cut out for themselves in the data crunching/parsing/interpreting part of this saga. Scientists typically accrue data far faster than they are able to use and interpret them . . . and create cool animations for general consumption.
A log-log plot of the scaling relationship between direct emissions per capita and effective population density for all 77 cities.
“Naturally,” concludes Lin, “to bend the curve in terms of trying to reduce carbon emissions in cities is a primary focus. And there's a lot of excitement and social energy around reducing carbon emissions in cities, including here in Salt Lake. Many mayors have pledged carbon reduction plans, and the University of Utah has their own [pledge]. Lots of cities have very ambitious goals to reduce carbon.”
For Wilmot, this project will only add to the increased “social energy” around the issue of carbon emission mitigation. Satellite measuring will help identify a path toward monitoring urban emissions at the global scale in order to identify effective policy levers for emissions reductions. “Of course, realizing this monitoring ability is contingent on further development of the modeling, satellite observations, and a number of necessary input datasets,” he says. “So by no means am I saying that we are there already.”
Clearly, this research has shown that the co-authors’ designed, multi-component satellite framework is capable of monitoring CO2 emissions across urban systems and identifying relevant driving factors. Their analysis not only pulled out data of the emissions from individual cities, but, because it is global, they could then do pattern analyses. In fact, the researchers, using an established relationship between emission-per-capita vs population density were able to plot from the data what happened, emissions-wise, during the COVID shutdown.
But, as co-author Kai Wilmot infers about work yet to be done, the ending to this story — from the Hawaiian Islands to outer space — is one of not-quite-yet “mission accomplished.”
“It’s more like mission half-accomplished,” John Lin concedes, “which is often the case in research.”
By David Pace
Read the complete paper in Environmental Research Letters.
