How Much Human Activity Can Earth Handle?

The scientific name is the Holocene Age, but climatologists like to call our current climatic phase the Long Summer. The history of Earth's climate has rarely been smooth. From the moment life began on the planet billions of years ago, the climate has swung drastically and often abruptly from one state to another — from tropical swamp to frozen ice age. Over the past 10,000 years, however, the climate has remained remarkably stable by historical standards: not too warm and not too cold, or Goldilocks weather. That stability has allowed Homo sapiens, numbering perhaps just a few million at the dawn of the Holocene, to thrive; farming has taken hold and civilizations have arisen. Without the Long Summer, that never would have been possible.

But as human population has exploded over the past few thousand years, the delicate ecological balance that kept the Long Summer going has become threatened. The rise of industrialized agriculture has thrown off Earth's natural nitrogen and phosphorus cycles, leading to pollution on land and water, while our fossil-fuel addiction has moved billions of tons of carbon from the land into the atmosphere, heating the climate ever more.

Now a new article in the Sept. 24 issue of Nature says the safe climatic limits in which humanity has blossomed are more vulnerable than ever and that unless we recognize our planetary boundaries and stay within them, we risk total catastrophe. "Human activities have reached a level that could damage the systems that keep Earth in the desirable Holocene state," writes Johan Rockstrom, executive director of the Stockholm Environmental Institute and the author of the article. "The result could be irreversible and, in some cases, abrupt environmental change, leading to a state less conducive to human development."

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