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David Roberts

Push the Earth over the brink?

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There’s a new scientific paper out in the journal Nature called “Approaching a state shift in Earth’s biosphere.” In a sane world, it would be front page news. This is from the abstract:

Localized ecological systems are known to shift abruptly and irreversibly from one state to another when they are forced across critical thresholds. Here we review evidence that the global ecosystem as a whole can react in the same way and is approaching a planetary-scale critical transition as a result of human influence. [my emphasis]

As examples of past global state shifts, the authors cite the Cambrian explosion (“a conversion of the global ecosystem from one based almost solely on microbes to one based on complex, multicellular life,” which took a comparatively brief 30 million years), the Big Five mass extinctions, and the last glacial-interglacial transition, which started about 14 thousand years ago.

The difference today is that human beings are generating “forcings” (influences on biophysical systems) of unprecedented power at an unprecedented rate. These forcings include “human population growth with attendant resource consumption, habitat transformation and fragmentation, energy production and consumption, and climate change.” The authors emphasize that all these forcings “far exceed, in both rate and magnitude, the forcings evident at the most recent global-scale state shift, the last glacial-interglacial transition.”

This write-up from Wired adds some context:

Human activity now dominates 43 percent of Earth’s land surface and affects twice that area. One-third of all available fresh water is diverted to human use. A full 20 percent of Earth’s net terrestrial primary production, the sheer volume of life produced on land every year, is harvested for human purposes. Extinction rates compare to those recorded during the demise of dinosaurs and average temperatures will likely be higher in 2070 than at any point in human evolution.

Humanity’s footprint now covers Earth. We are shaping the biosphere in our image.

So, what would such a global state shift entail? There’s no way to know for sure, given the number of systems interacting in unpredictable ways, but we can learn from history:

On the timescale most relevant to biological forecasting today, biotic effects observed in the shift from the last glacial to the present interglacial included many extinctions; drastic changes in species distributions, abundances and diversity; and the emergence of novel communities. New patterns of gene flow triggered new evolutionary trajectories, but the time since then has not been long enough for evolution to compensate for extinctions.

At a minimum, these kinds of effects would be expected from a global-scale state shift forced by present drivers, not only in human-dominated regions but also in remote regions not now heavily occupied by humans; indeed, such changes are already under way.

Given that it takes hundreds of thousands to millions of years for evolution to build diversity back up to pre-crash levels after major extinction episodes, increased rates of extinction are of particular concern, especially because global and regional diversity today is generally lower than it was 20,000 years ago as a result of the last planetary state shift. … Possible too are substantial losses of ecosystem services required to sustain the human population. … Although the ultimate effects of changing biodiversity and species compositions are still unknown, if critical thresholds of diminishing returns in ecosystem services were reached over large areas and at the same time global demands increased … widespread social unrest, economic instability and loss of human life could result.

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Full Story: We’re about to push the Earth over the brink, new study finds
Source: Grist, June 7, 2012

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