It's no secret that Earth has undergone numerous convulsive changes in its history, from mass extinctions and climatic changes to cataclysmic collisions and long-term periods of immense volcanic eruptions - or multiple combinations of the above. But it is also undeniably clear that human activities have caused massively accelerated change - to the extent that some scientists argue that the Holocene Epoch, which began 11,700 years ago following the end of the last great Ice Age, has now given way to a period that should be called the Anthropocene.
But how to measure definitively the degree to which this latter change is outpacing more natural ones? That's a question that occurred to Owen Gaffney, anthropocene analyst and communicator at the Stockholm Resilience Centre and Future Earth, and Will Steffen, an emeritus professor of the Australian National University and one of the earliest and strongest proponents of the Anthropocene designation.
"Over the last 10 years or so, various organizations have been measuring the scale of human impact on Earth and have come to the conclusion we've entered a period of great acceleration since the 1950s, when we can see a huge acceleration in socio-economic drivers: GDP, land use change, energy use, and so on," Gaffney told Seeker.
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"As we discussed this in presentations or in the media, we would be saying things like, 'Humans now rival the great forces of Nature,' or more colloquially, 'We have the impact of an Ice Age' or 'We have the force of an asteroid hit,'" he continued. "I was thinking about this phrasing and thinking that, while that's all true, it's ambiguous and could be misinterpreted. So I wondered if it would be at all possible to formalize that in some way? I thought maybe one could formulate it in terms of an equation, and I thought the rate of change of the Earth system would be a good focus point."
Equations are generally a guaranteed way to make a lay audience's collective eyes cross or close. As Stephen Hawking wrote in the prologue to his seminal book "A Brief History of Time," "Someone told me that each equation I included in the book would halve the sales. I therefore resolved not to have any equations at all." But some equations do break through somewhat into public consciousness, even if many do not know what they main or can not recite their components - such as, for example, the one equation that Hawking ultimately did include: E = mc2.
There is probably a decent percentage of the population that is at least mildly familiar with the Drake equation, too. Presumably very few could write it out, but its name might elicit some recognition as might its function: essentially, to estimate the odds of finding intelligent life in the universe. It remains to be seen whether what Gaffney and Steffen have dubbed the Anthropocene Equation will achieve similar familiarity. But, writing in New Scientist, Gaffney argues that it "creates an unequivocal statement of the risks industrialized societies are taking at a time when action is vital."
This "unequivocal statement" is the rate at which humans are changing Earth's climate: 170 times greater than that of astronomical, geophysical and internal dynamics combined. Over the past 7,000 years, they conclude, global temperature until very recently declined at the rate of 0.01 degrees Celsius per century. Over the past 45 years, however, the rate of change has vaulted to an increase of 1.7 degrees Celsius - 170 times the baseline and in the opposite direction.
In other words, the rate of change of the Earth system over the last 40 to 50 years is "purely a function of industrialized societies."
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The equation can be applied to other elements of Earth system change, such as, Gaffney told Seeker, "changes in ocean biogeochemistry, ocean acidification, biodiversity loss, changes to the nitrogen, phosphorous and water cycles as well as the carbon cycle. So we're looking at a much broader picture of what's happening in the Anthropocene."
Their selection of a 7,000-year baseline was informed by the fact that it was a period of notable climate stability and also because seven millennia ago marks the approximate emergence of the first human civilizations.
"We've had a remarkably stable period where temperatures globally haven't gone up or down by more than 1 degree C," Gaffney noted. "Without doubt, that stability enabled agriculture to become established." He has had some pushback from those who have noted the existence of climate perturbations during that period, in the form of, for example, "little ice ages where temperatures have dropped and the rate of change has gone up a bit. But temperatures didn't go outside this Holocene envelope, and now we're pushing beyond that envelope - and in other areas of the Earth system, such as carbon dioxide emissions and biodiversity loss, we're way, way beyond the Holocene envelope."
Some critics, Gaffney acknowledges, have described him and Steffen as alarmists.
"We're not alarmists, but what this shows is alarming," he counters. "The rate of change of Earth's life-support system is accelerating. That is not sustainable in the long term. And either that rate of change will drop to zero because societies in some way collapse because they can't cope with the rates of change in Earth systems, causing chaos in social systems, or we adapt and change behavior and learn to live within planetary boundaries."
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