New study reveals complex future for atmosphere's methane-cleansing molecules

A new MIT study reveals that the atmosphere's ability to cleanse itself of methane and other pollutants will face a complex future as global temperatures rise. The research, published today in the Journal of Advances in Modeling Earth Systems, shows that while warming will boost levels of hydroxyl radicals through increased water vapor, it will also reduce them through higher biogenic emissions from plants and trees.

The hydroxyl radical, often called the "atmosphere's detergent," is a highly reactive molecule made up of one oxygen atom and one hydrogen atom. These molecules act as a natural vacuum cleaner, breaking down methane and other harmful compounds in the atmosphere. About 90 percent of methane removal from the atmosphere occurs through reactions with hydroxyl radicals, making them crucial for controlling greenhouse gas concentrations.

As the planet warms, scientists have been uncertain how these air-cleaning agents would respond. The MIT team, led by former postdoc Qindan Zhu and Professor Arlene Fiore, developed a new model called AquaChem to study this question. Their findings paint a nuanced picture of the future.

When global temperatures rise, several competing processes come into play. First, warmer air holds more water vapor, and since hydroxyl radicals are primarily produced when ozone interacts with sunlight in the presence of water vapor, this leads to increased OH production. The researchers found that a 2-degree Celsius temperature rise would increase hydroxyl radical levels by about 9 percent through this water vapor effect alone.

However, rising temperatures also trigger another important process. Plants and trees naturally release gases called biogenic volatile organic compounds (BVOCs), such as isoprene, through tiny pores in their leaves. These emissions increase with warming temperatures. While these compounds are natural, they react with and break down hydroxyl radicals, effectively reducing their availability to clean the atmosphere. The study found that the same 2-degree warming would increase biogenic emissions enough to reduce hydroxyl radical levels by 6 percent.

When these competing effects are combined, the result is a net increase of about 3 percent in the atmosphere's ability to break down methane and other chemical compounds. While this might seem like good news, the researchers caution that the picture is more complicated than it appears.

"Hydroxyl radicals are important in determining the lifetime of methane and other reactive greenhouse gases, as well as gases that affect public health, including ozone and certain other air pollutants," Zhu explains. The team's model, which builds on simplified Earth system models, allows them to isolate and understand these individual processes without the computational complexity of full climate models.

The study also highlights significant uncertainties, particularly regarding biogenic emissions. These natural emissions are the most unpredictable factor in the model, and other factors not considered in this study, such as rising carbon dioxide levels, could further complicate the response.

Looking ahead, the researchers plan to update AquaChem to study how different climate scenarios and additional processes might affect hydroxyl radical concentrations. Understanding these trends is crucial for predicting future methane accumulation in the atmosphere.

"We know that changes in atmospheric OH, even of a few percent, can actually matter for interpreting how methane might accumulate in the atmosphere," Zhu notes. "Understanding future trends of OH will allow us to determine future trends of methane."

The research was supported by Spark Climate Solutions and the National Oceanic and Atmospheric Administration, reflecting the importance of understanding these atmospheric processes for both climate science and public health.