Scientists have long considered this argument to be baseless, and now a new study hammers that point home further. Changes in how many cosmic rays hit Earth could affect temperatures in the short term, but not in the long term, researchers reported in the Proceedings of the National Academy of Sciences.
Cosmic rays are theorized to help form clouds, which can alter temperature by bouncing sunlight back to space. The more rays that hit our atmosphere, the more clouds that form. And the more clouds that form, the more that sunlight gets bounced back to space, cooling the planet. The opposite would hold true, as well: The fewer rays that hit us, the fewer clouds that form and the less that sunlight gets reflected, warming the planet a bit.
A key factor influencing how many cosmic rays hit us to begin with is the sun. Our star undergoes a roughly decade-long cycle where its power output fluctuates up and down just a tick. And when that happens there’s a corresponding change in the solar wind, a stream of particles that can deflect cosmic rays from hitting the Earth. Some climate contrarians have suggested that the solar cycle, by altering the solar wind’s strength, and thus how many cosmic rays hit the Earth, may be responsible for recent temperature trends, including 20th century warming.
Multiple studies show that this argument is flat-out baseless. Not only might cosmic rays not be very good at boosting cloud formation, but even if they are good at it, the resulting effects on temperature would be pretty small. A 2011 paper, for instance, found that our planet’s dose of cosmic rays could only account for eight percent of the 20th century’s warming. And a 2013 paper also found at most a 14 percent link with warming after 1955. Basically, cosmic rays alone can’t account for all the warming we’ve experienced.
Those two studies focused on how well global temperatures and cosmic rays were correlated. But in the new study, Anastasios A. Tsonis of the University of Wisconsin-Milwaukee and colleagues tried to tease out whether our planet’s cosmic ray dose was influencing temperatures. To do that, they assembled temperature and cosmic-ray data for each year in the 20th century, then determined the following: Given how a given year’s temperature compares with other years’ temperatures, can we predict what the cosmic ray dose was just before that year?
The first temperature trend they focused on was how the temperature in a given year changed from the previous year. On this timescale, we know temperatures can be noisy and that the sun’s power output (and thus cosmic ray dosages) can fluctuate. From each year’s temperature change, the researchers found that they actually had “modest” success teasing out Earth’s cosmic ray dose in prior years. That finding suggests cosmic rays could potentially alter temperatures on very short timescales (just as natural cycles and aerosol pollution can), but not by very much.
But what about longer timescales? The long term is what we’re really concerned about when it comes to global warming. To answer that question, the researchers instead looked at how much each year’s temperature differed from the long-term average, instead of the one-year change. When they looked at it from that perspective, the temperature data had no link at all with the previous years’ cosmic ray data. And that seems to make sense: The long-term temperature trend is upward, while the long-term cosmic-ray trend is, overall, relatively flat.
These findings further shoot down the idea that cosmic rays caused the warming we experienced in the 20th century, the researchers conclude. Global warming is a long-term matter, and on long timescales any causal link between cosmic rays and temperature simply breaks down. The strongest explanation for the planet’s warming remains the same: greenhouse-gas emissions from the burning of fossil fuels.
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