In discussions of climate change, much of the focus has been on the combustion of fossil fuels: coal, oil and its gasoline and diesel-fuel derivatives, and natural gas.

Collectively, the trillions of tons of carbon dioxide emitted into the atmosphere by man since the start of the industrial revolution around 1750 have contributed to a massive rise in airborne concentrations of that gas.

But that's only half the story, as scientists have long known.

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A recent study now estimates a far higher impact from deforestation and other human activities than previously estimated.

Published late last year in the science journal Nature, the study was conducted by 12 researchers at the Institute of Social Ecology in Austria and several other institutions in Germany, The Netherlands, Portugal, and Sweden.

As summarized in mid-December by an article in The Washington Post, "If true, it’s a finding that could shape not only our response to climate change, but our understanding of ourselves as agents of planetary transformation."

The researchers estimated the amount of carbon trapped in the globe's total vegetation, based on satellite maps and other ecological studies.

It's a huge number: 450 billion tons, which with the addition of oxygen would produce more than 1 trillion tons of carbon dioxide emissions if released into the atmosphere.

But the study also estimated the amount of carbon that could be contained in the earth's vegetation if human activity ceased and the land returned to the state it was in, say, 10,000 years ago.

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That number was 916 billion tons of carbon.

In other words, human activities—from clearing land for everything from agriculture to 2-acre lots for suburban McMansions—have released roughly half the carbon contained in the world's forests.

But it's not just deforestation that has contributed to this huge release of carbon.

Chevrolet Silverado Heavy duty drive with John Deere, Jessica Walker, courtesy of Chevrolet

Chevrolet Silverado Heavy duty drive with John Deere, Jessica Walker, courtesy of Chevrolet

It's also two additional factors: large-scale livestock grazing on grasslands, and forest "management' that thins out forests by selective logging and removal of particular species.

Together, the researchers estimate, those two factors together have produced carbon emissions equal to those of straightforward deforestation.

The impact of deforestation has been well-studied, but the collective impacts of grazing and of forest management have not.

The study's conclusion that those two factors together have produced carbon emissions as high as those of deforestation only worsens the total carbon emissions associated with human activities.

Earth's population has grown explosively for eight centuries; it has been estimated at 370 million humans at the end of the Black Death in 1350.

It reached 1 billion just after 1800, 2 billion in the early 1920s, 4 billion around 1970, and is estimated at 7.6 billion as of December 2017.

Chevrolet Silverado Heavy Duty drive with John Deere, “Jessica Walker, courtesy of Chevrolet

Chevrolet Silverado Heavy Duty drive with John Deere, “Jessica Walker, courtesy of Chevrolet

Growth rates peaked between 1965 and 1970 and have fallen steadily since then. The United Nations estimated in 2012 that total global population would be 8.3 billion to 10.5 billion in 2050.

All those humans have had a remarkable impact on the planet we share; its true scope is only now beginning to come into focus.

And, the authors of the study suggest, restoration of the huge amounts of "degraded" land will be a necessary component in limiting climate change to the 2-degree-C maximum identified by scientists as necessary to avert the worst impacts.

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