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In their paper in Science of the Total Environment, researchers from IIASA and Lviv Polytechnic National University in Ukraine presented a novel approach to measure and understand human pressure on planet Earth. The researchers explored how carbon emissions can be translated into measures of "stress" and "strain" to derive new insights into how the planet is changing.

"Until now, the scientific community has mainly measured Earth's condition in gigatons of carbon per year. That's important, but it doesn't show how Earth as a physical system responds to the growing pressure we're putting on it," explains lead author Matthias Jonas, a researcher in the IIASA Advancing Systems Analysis Program. "We wanted to see how the entire Earth system stretches and strains under that burden."

One of their key findings is the quantification of "stress power," which is the rate at which humans are adding energy per volume to Earth's system. In 2021, this stress power reached between 12.8 and 15.5 pascals per year.

While this pressure may sound small (it is similar to the gentle push of a light breeze), spread over the entire atmosphere, land, and oceans, it is enough to signal that Earth's system might be pushed outside its natural balance. For comparison, both strain and stress power center around zero for a balanced Earth not exposed to human-induced global warming.

The researchers also analyzed changes over time in Earth's "delay time," which describes how quickly the planet's carbon system reacts to stress and identified a turning point between 1925 and 1945, suggesting that Earth's system began shifting its response to stress much earlier than previously believed.

"This early turning point was unexpected," says Jonas. "It suggests that Earth's land and oceans may have started changing from their usual patterns as early as the first half of the 20th century. After that, instead of working as they used to, these systems were increasingly overwhelmed by human activities and eventually stopped absorbing COâ‚‚ as effectively."

This could mean countries need to act sooner than planned to cut greenhouse gas emissions.

"Meeting future emissions targets is important, but we also need to pay attention to how quickly Earth is becoming more fragile," Jonas says. "Even if we hit our targets, the weakening of Earth's natural systems could still leave us facing major disruptions sooner than expected. Earth's shift to earlier fragility isn't captured in climate models yet, but it needs to be."

The team emphasizes the need for further research to quantify this shift and include their stress-strain approach in global climate modeling. They hope that by expanding how scientists track Earth's condition from counting carbon alone to understanding how the planet physically reacts under pressure, the world can better prepare for the challenges ahead.