If the mission is emissions, then solar is the answer

A paper published in Science Advances, Quantifying effects of solar power adoption on CO₂ emissions reduction, confirms what many in the industry have long suspected: if the fundamental environmental mission is to reduce emissions, expanding solar power is a well-documented path forward. The researchers found that a 15 percent increase in solar generation (an annual growth rate the industry has maintained for the past decade) in the analyzed time windows was responsible, by itself, for achieving 12 percent of the United States’ annual emissions reduction target set by the EPA.

Unlike older models that assume solar generation directly displaces fossil fuels, this study used a more nuanced statistical approach. The analysis considered both immediate and time-delayed effects of solar generation and emphasized regional differences in how solar adoption reduces CO₂ emissions. Researchers examined five years of hourly grid data across 12 U.S. regions.

The study explored how annual CO₂ emissions change when solar generation increases by 5, 10, 15, and 20 percent. Due to regional variation, the authors found that although a 15 percent increase could theoretically reduce emissions by 21.07 MMT per year, the real-world estimate was closer to 8.54 MMT.

Estimated change in CO₂ emissions in California at noon (red) and 8 p.m. (blue), associated with a 15 percent increase in solar generation during each hour of the preceding 12-hour window. For example, the blue value at the 12:00 x-axis mark shows that a 15 percent increase in solar generation at noon is associated with a 16.08-metric ton reduction in emissions at 8 p.m. Shaded areas represent 95 percent credible intervals.
*Image: Science*

Still, that 15 percent increase, which has occurred consistently over the past decade, accounts for more than 12 percent of the U.S. Environmental Protection Agency’s (EPA) annual emissions reduction target of 69 MMT. That target is part of the broader EPA goal to cut 1,380 MMT of CO₂ over 20 years.

Notably, solar power has grown by more than 15 percent annually since 2014.

However, not all regions benefit equally. California, Texas, the Midwest, the Southwest, Florida, and the Mid-Atlantic showed the strongest reductions in CO₂ emissions as solar generation increased. By contrast, regions such as New England, Tennessee, and the Central U.S. showed little or no measurable effect, even with a 20 percent rise in solar generation.

These cases, however, reveal more about the dataset’s noted limitations than about solar’s effectiveness.

In New England, most solar capacity is distributed across rooftops and smaller systems. As a result, that generation is largely absent from the utility-scale data the researchers relied on. California also has substantial distributed solar, though its utility-scale contribution was large enough to dominate the dataset. The lack of measurable impact in New England is therefore a data artifact, not a true reflection of solar’s value. And at the national level, data complexities like this are to be expected.

Regions like California and Texas show clear emissions reductions. Others, including New England, Tennessee, and the Central U.S., appear flat—but for very different reasons. In New England, most solar is distributed and was excluded from the utility-scale dataset. In Tennessee and the Central U.S., hydro and nuclear dominate the grid, leaving less fossil fuel to displace. The model’s reliance on percentage-based scaling further downplays solar’s value in low-baseline regions.

In Central Tennessee, which is managed by the Tennessee Valley Authority, the story is different. This area is already powered primarily by hydroelectric and nuclear power. With limited fossil generation in the mix, there is simply less CO₂ to displace. Solar may still offer benefits in this region, but its emissions-reduction potential is understandably smaller.

Despite these regional quirks, the broad takeaway is clear: solar lowers emissions.

The research team, made up of experts from Rutgers, Stony Brook, Harvard, and Dana-Farber Cancer Institute, found an additional system-wide benefit tied to solar. They found that solar lowers emissions both where it is installed and in neighboring regions through electricity exports. They wrote:

We estimate that a 15% increase in solar capacity in California is associated with a total reduction of 913 and 1,942 metric tons of CO₂ emissions per day in the Northwest and Southwest regions, respectively. Therefore, increasing solar power generation in a region not only helps in the reduction of CO₂ emissions from the electricity sector within the region but also has an impact on the neighboring regions.

The study also supports a growing realization that solar exports reduce emissions beyond the point of generation. While headlines often focus on curtailment or overproduction in places like California or Germany, this work suggests that more attention should be paid to the emissions being avoided across state or national lines due to that surplus. Rather than viewing curtailment purely as wasted energy, the study reinforces that solar continues to deliver emissions reductions, even when partially curtailed or exported to neighboring grids.

The paper also examined delayed emissions reductions made possible by battery storage. In California, where large-scale storage systems have been widely deployed, solar produced in the middle of the day can now displace fossil fuel generation during the evening demand peak. The researchers modeled these time-lagged effects, showing how solar can reduce emissions four to twelve hours after it is generated.

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