Hear from alumni who participated in the UCDC Law Program during law school: Robina Henson ’25 worked with the U.S. Trade Representative’s Office (USTR) and Dan Wetterhahn ’24 externed with the U.S. Department of Justice.

Notes to understand and interpret this data:

How important are these sources of emissions in global context? To illustrate how important these sources are, and what big opportunities they present for near-term reductions, consider that over a year, a landfill emitting 4 tons of methane per hour (in the middle of the pack of our “top 25” list) would contribute about as much to global warming as one million SUVs or one large (500 megawatt) coal-fired power plant.

How did we calculate these emissions rates? We didn’t do the calculations. We are reporting calculations done by Carbon Mapper and included on their public data portal, making some judgment calls about what to include or exclude from our list as we explain below. Carbon Mapper’s reported emissions rate for a landfill is the average of the emissions rates determined each time an instrument flew over that landfill.

Why do our emissions rates look a little different than those reported on the Carbon Mapper site, e.g., the largest source is 7.4K on Carbon Mapper and 7.4 tons per hour on our ranker? We are reporting the same emission rates but are translating them into a different unit to make it simpler. One metric ton equals 1,000 kilograms. Because the Carbon Mapper portal has to provide consistent formatting for hundreds of sources whose emissions rates vary from very small to very large, it uses the smaller unit (kg/hr). Since the top 25 sources all have emissions in the tons range, we stated them as tons, to make it simpler and to avoid having to say “7.4 K Kg.”

Is your list equivalent to a list of the biggest landfill methane sources in 2025? Not entirely, for a few reasons, but it’s pretty close. These are the landfills with the largest hourly emission rates that two instruments saw thus far in 2025, but 2025 isn’t over yet and the instruments can’t see everywhere. In particular, they can’t see in the dark or see through heavy clouds. So while the observations of these sources are solid, it’s possible that there were even more extreme emissions events in 2025 that the instruments couldn’t see (or that we excluded from this list because of too-few observations, as noted below). Moreover, landfill emission rates do vary somewhat over time, so capturing high hourly emission rates on a set of particular dates is not the same as knowing how much a landfill emitted, in total, across an entire year.  Nevertheless, in our judgment this is about as close as one can get, given current data, to a list of the largest landfill emitters in 2025 so far.

Does the list include every big emissions event the satellites saw? No. Because of differences in the satellites’ orbits, some of these landfills were seen dozens of times in 2025, others just a few times. To be careful, we only included landfills that were observed at least twice. So there are a few landfills with extremely high emissions rates that appear on Carbon Mapper’s portal, but that we excluded from this list because they were only observed once.

Where can I learn more about methane observation from satellites? To learn more about remote observation of methane and the opportunities and limitations presented by the new satellite data, see the Emmett Institute’s “Remote Sensing of Atmospheric Methane: A Primer for Policymakers on the Science of Methane Satellites.” 

Something on the list doesn’t look right. Think we’ve got something wrong?  Do you know something about one of these landfills that suggests there’s an error in Carbon Mapper’s observation and calculation or our compilation and interpretation?  If so, please explain it to us here.