Hothouse Earth


Tipping Points

By David Kattenburg

Earth’ surface is one degree warmer today, on average, than it was at the start of the industrial revolution 200 years ago. One degree doesn’t seem like much. The Paris Agreement (which Donald Trump has withdrawn from) keyed greenhouse gas emission reduction targets to a 1.5-2.0 degree rise. Sound like a conservative precautionary measure?

Perhaps it isn’t. In a  landmark report published in the prestigious Proceedings of the National Academy of Scientists, researchers warn that a two degree rise in global surface temperature may actually exceed a critical planetary threshold, pushing Earth down a cascade of tipping points into “hothouse” mode, unlike anything this third rock from the sun has experienced since the mid-Miocene epoch, fifteen million years ago.

Possible future pathways for Earth’s climate

If two degrees is a point of no return (no one knows for sure where the tipping point lies), then efforts to reduce greenhouse gas emissions at that point would be for nought. The ensuing climate cascade “could not be reversed, steered, or substantially slowed,” the research paper’s 16-author team warns — and the consequences for human life on Earth would likely be catastrophic: Heat expanded oceans, engorged by melting polar ice sheets, would flood coastal cities. Species extinction rates would skyrocket. Countless people would perish.

So, the gravity of the planetary crisis is huge. Decisions made today will determine the future of the human species for millennia, the authors of the paper say.

Stability landscape for Earth system out of the Holocene

The enormity of the challenge — turning greenhouse gas emissions around on a dime — is even greater than it would seem on the basis of complex computer models.  “The present dominant socioeconomic system … is based on high-carbon economic growth and exploitative resource use,” the authors point out, and “technological lock” and the “socioeconomic inertia in human systems” stand in humanity’s way.

“[We] argue that social and technological trends and decisions occurring over the next decade or two could significantly influence the trajectory of the Earth System for tens to hundreds of thousands of years, and potentially lead to conditions that resemble planetary states that were last seen several millions of years ago,” the authors write. And the Earth system “may already have passed a “fork in the road” of potential pathways.”

Global map of potential tipping cascades.

On the bright side (for those looking for silver linings), confronting the crisis in bold and determined fashion will necessitate the creation of diverse, resilient human systems in harmony with the planetary systems around us. These will benefit humanity in the long run.

Will Steffen and Johan Rockström are lead authors of the landmark paper entitled “Trajectories of the Earth System in the Anthropocene.” Listen to my conversation with Will Steffen:


All images reproduced with the permission of the Proceedings of the National Academy of Sciences.

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  1. Cryosphere - Green Planet Monitor

    […] Like that proverbial canary in a coal mine, Earth’s cryosphere is in trouble. As the atmosphere warms, ‘positively forced’ by an unrelenting flow of human-generated greenhouse gases, the cryosphere is slowly melting. Various positive feedbacks, in turn, threaten to push Earth into ‘hothouse mode’. […]

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