Alex Turner

The abrupt shuttering of the economy in response to the COVID-19 global pandemic led to widescale speculation in both the scientific community and broader public about how our carbon emissions would change (e.g., “The Coronavirus and Carbon Emissions” from the New York Times).  All of the early estimates were based on projections from models that predict carbon emissions using economic data.  In other words, they did not incorporate measurements of CO₂ in the atmosphere.  As such, it was unclear if the atmospheric measurements would find similar changes or not.  Our paper, published in October 2020 in Geophysical Research Letters, was the first paper to quantify the changes in carbon emissions using atmospheric measurements.

We used a dense network of sensors across the San Francisco Bay Area (yellow stars in Image 1) to quantify the impacts of COVID-19 on our carbon emissions using measurements of CO₂ in the atmosphere.  The observed changes in CO₂ concentration were quite striking (Image 2).  We observed a pronounced change to both the daily and weekly cycle of CO₂. Before the lockdown the highest concentrations were typically during the middle of the week near the morning rush hour, this signal was largely absent after the lockdowns were implemented.  Instead, the weekdays and weekends showed a nearly identical daily cycle after the lockdowns.  These observations suggested large reductions in CO₂ emissions.  However, quantifying and attributing the changes to emissions requires accounting for changes in both weather patterns and vegetation (plants take up carbon during photosynthesis).  To isolate the impacts of changes in emissions, we used a weather model and satellite observations related to photosynthesis.  After accounting for variations in weather and plant activity, we found that urban CO₂ emissions decreased by 30% in response to COVID-19.  The bulk of these changes were due to traffic (-48%) with only an 8% decrease in stationary CO₂ sources.

This unnatural experiment conducted in response to COVID-19 has highlighted the subset of carbon emissions that are elastic and those that are more entrenched.  Emissions from traffic are highly elastic and could be rapidly mitigated in response to either technological advances or regulations.  In contrast, the stationary emissions (e.g., industrial sources) showed minimal changes in response to COVID-19.  These sources will require longer time scales and focused regulation to mitigate if we hope to limit future warming.  Ultimately, our work helps to quantify what our future carbon emissions might look like if we were to electrify the vehicle fleet.