Is the epoch of habitability on Earth drawing to a close?
The future of life on Earth? Stromatolites growing in Hamelin Pool Marine Nature Reserve, Shark Bay in Western Australia.
By Jim Galasyn
18 January 2016 (Desdemona Despair) – Humans are collecting more biogeophysical data than ever, leading to a potential paradox: are we finally getting the data we need to manage the world, only to find that it’s too late to manage the world? One way to answer this question is to synthesize a single metric comprising the available biogeophysical data. This metric may enable us to estimate the future trajectory of the Earth system and assess the prospects for life on this planet. At least three such metrics have been proposed recently. One study, by Abel Mendez, et al., presents a habitability measurement:
Habitability refers to the quality of an environment for life and it is usually correlated with the presence of life. The habitability of our planet is controlled by a complex interaction of biogenic gases (particularly carbon dioxide, oxygen, and nitrogen), the availability of water (both as liquid and vapor), temperature, and sunlight. For primary producers like plants on land, temperature, and water availability are the most important factors controlling their distribution. The distribution of phytoplankton in the oceans is controlled by temperature too, however it is not limited by water availability but by nutrients. Both temperature and water availability are more important factors at seasonal scales, while oxygen, carbon dioxide, nitrogen and sunlight are more relevant at much longer time scales. [Habitability of the Paleo-Earth as a Model for Earth-like Exoplanets]
The Mendez study uses two different ways to compute this metric:
We calculated the habitability of Earth during the Phanerozoic using two proposed independent methods. The Relative Vegetation Density (RVD) derived from our vegetation datasets of the Visible Paleo-Earth was used as our first proxy for habitability. The RVD is very similar to vegetation indices such as the Normalized Difference Vegetation Index (NDVI) and gives a general idea of the global area-weighted fraction of vegetation cover. Vegetation, as a primary producer, is a good indicator of global habitability, as they provide the resources for many other simple to complex life forms in the trophic scale. Our second habitability indicator was the Standard Primary Habitability (SPH) derived from mean global surface temperatures and relative humidity. The RVD is a more robust indicator of the actual habitability of a planet as it uses the vegetation cover as a proxy. The SPH is more a measure of climate habitability but it is much easier to estimate.
When they crunched the numbers using these two methods over the entire Phanerozoic eon, they got these results: 
Habitability of Earth during the Phanerozoic as measured by two methods, the Relative Vegetation Density (RVD) and the Standard Primary Habitability (SPH). The RVD is related to the actual abundance of vegetation while the SPH is related to the atmospheric quality for vegetation (productivity). In general, terrestrial habitability has been about 50 percent higher than today during most of the Phanerozoic, but it has been steadily decreasing since the last 100 million years. Graphic: Mendez, et al., 2011 / Planetary Hability Laboratory
There are a number of interesting features in these curves, but the most important for humans is the steep drop at the right side of the chart, which shows sharply declining habitability since the peak at ~100 million years ago. This result suggests that long before the human species appeared (a mere 200,000 years ago), the biosphere already was in a state of steep decline. This paper points out that “the distribution of phytoplankton in the oceans is controlled by temperature”. In 2015, Yadigar Sekerci and Sergei Petrovskii modeled the dynamics of ocean plankton and found that as ocean temperature rises, a sudden, catastrophic decline in plankton populations is possible. 
Simulated oxygen concentration and plankton densities, spatially averaged, vs. time. Oxygen concentration (blue) and the density of phyto- and zooplankton (green and black, respectively). Graphic: Sekerci and Petrovskii, 2015 / Bulletin of Mathematical Biology
A widespread phytoplankton extinction would sharply reduce atmospheric oxygen and kill most multicellular life on Earth, possibly paving the way for a resurgence of anaerobic microbial species. Another approach is to model Earth as a chemical battery that stores “charge” relative to the ground of space. This model was proposed by Schramski, et al., in 2015, and it applies to a much more recent time span, from the year 0 to the year 2000:
Earth is a chemical battery where, over evolutionary time with a trickle-charge of photosynthesis using solar energy, billions of tons of living biomass were stored in forests and other ecosystems and in vast reserves of fossil fuels. In just the last few hundred years, humans extracted exploitable energy from these living and fossilized biomass fuels to build the modern industrial-technological-informational economy, to grow our population to more than 7 billion, and to transform the biogeochemical cycles and biodiversity of the earth. This rapid discharge of the earth’s store of organic energy fuels the human domination of the biosphere, including conversion of natural habitats to agricultural fields and the resulting loss of native species, emission of carbon dioxide, and the resulting climate and sea level change, and use of supplemental nuclear, hydro, wind, and solar energy sources. [Human domination of the biosphere: Rapid discharge of the earth-space battery foretells the future of humankind]
The Schramski paper proposes a metric named Omega (Ω), which measures phytomass per capita. In what is now sometimes referred to as “Figure 5” in the doomer community, Schramski computes Ω for the last 2000 years: 
Number of years of phytomass food potentially available to feed the global human population, 0-2000 CE. Graphic: Schramski, et al., 2015 / PNAS
Here, Ω is expressed as the number of years of phytomass food potentially available to feed the global human population. It’s calculated from the total stored phytomass energy of the planet divided by the metabolic energy needs to feed the global population for one year. Eyeballing the linear trend in the inset graph, it looks like Ω will hit zero sometime between 2030 and 2040. But from 1970 to 2000, the curve deflects from a linear decline, suggesting that we may have a few more decades before Earth reaches equilibrium with space. In 2015, Desdemona computed a possibly related metric, arable land per capita, which is total land available for farming divided by the size of the human population: 
World arable land per capita, 1961-2012. Graphic: James P. Galasyn The decline isn’t linear; in fact, it’s almost perfectly exponential, with R2 = 0.996 for the default Excel curve fit. Currently, the world has 0.2 hectares (0.49 acres) of arable land per person, down from 0.4 in 1962. Extrapolating the exponential curve, we’ll be down to 0.1 by around 2050, and 0.05 by 2100. So, every 50 years, arable land per capita declines by half. By the year 2100, each person will be supported by just 0.05 hectares (0.12 acres) of agricultural land. In private correspondence, Prof. Schramski suggests that “(arable land)/person, and our Figure 5 (global biomass energy stored)/(energy needed to feed population for one year) are very similar and ultimately to some extent the same chart. I think there could be a constant multiplier between the two.” If this is the case, then the deflection from linear in Figure 5 is exponential, and Ω will remain above zero beyond the year 2100. It’s unknown whether there’s a critical threshold in Ω below which human civilization is unsustainable, but this seems likely. Siegfried Franck has suggested that biospheres have a “life span”, and that the reign of multicellular life is limited. Franck, et al., 2005, presents the case:
We find that from the Archaean to the future a prokaryotic biosphere always exists. 2 Gyr ago eukaryotic life first appears. The emergence of complex multicellular life is connected with an explosive increase in biomass and a strong decrease in Cambrian global surface temperature at about 0.54 Gyr ago. In the long-term future, the three types of biosphere will die out in reverse sequence of their appearance. [Causes and timing of future biosphere extinction]
They estimate the amount of carbon stored in the prokaryotes, eukaryotes, and multicellular organisms, over the entire span of Earth’s history, and the result is this graph: 
Evolution of the cumulative biosphere pools for procaryotes (red), eucaryotes (green), and complex multicellular life (brown). Graphic: Franck, et al., 2005 / Biogeosciences Discussions
The peak of multicellular life was around 500 million years ago, during the Cambrian Period, and the amount of carbon stored in multicellular species has decreased continuously since. Invoking the Copernican principle, these metrics may provide a template for the development of life on most planets, based on underlying thermodynamic considerations. Such a template may suggest a solution to the famous Fermi paradox: multicelluar life exists only for a brief span of a planet’s existence, and its appearance heralds the beginning of the end for habitability of all life on a planet. UPDATE: This brilliant new paper offers a very convincing solution to the Fermi paradox: The Case for a Gaian Bottleneck: The Biology of Habitability [pdf]
“If it takes several billion years to develop radio telescopes, then the Gaian bottleneck ensures that the vast majority of life in the Universe is either young and microbial, or extinct.”
"Are we finally getting the data we need to manage the world, only to find that it’s too late to manage the world?"
This is a false assumption. Humans do not and will not 'manage the world'. It's revealing that the author would even make such a statement.
Humans are a part of Nature as much as anything else. But we do not, cannot – manage Nature.
The incredibly complex systems that compromise Nature, or even a single ecosystem – or a single species is still outside of our ability to manage. Nor do we truly understand all that there is to know at any level. New discoveries are being made all the time, which reveals how little we really understand.
We do not manage Nature – not even close, and if the past is any indication of the future, we never will.
We can manage ourselves, but do a piss-poor job of even that.
We can manage our activities and consumption, but do a piss-poor job of that too.
Humans are a force of Nature, just like a plague of locusts. I think the comparison applies quite well.
We're now the primary mammal species that is impacting Nature, but we are not the only species doing so.
If you assess our impacts upon the natural environment, and how this affects future habitability, then it's pretty obvious that the "epoch of habitability on Earth is drawing to a close". The damage has been done and there is scant little we're really going to be able to do about it.
We do not control Nature. We still barely understand what we've done to Nature (there's your "data" for you) or how it will all play out (it's going to get mighty hot).
It comes as a considerable relief to know our fate has been sealed since before we existed, and puts all our fussing and fuming into perspective. I do think things will come to an end a bit faster than these studies suggest, because NPP of vegetation is being seriously curtailed by human-created pollution. I also wrote to Schramski, the following:
I wanted to thank you for your recent publication and had a little bit to add to your analysis that you might find interesting as it regards NPP.
You did mention the requirement for nitrogen however, there is another aspect of the nitrogen cycle which is that reactive nitrogen, which comes from applications for agriculture and also burning fuel, is a precuror of ozone. The background concentration of tropospheric ozone is at a level that is highly toxic to vegetation. It causes direct injury and also increases vulnerability to insects, disease, fungus, drought and windthrow.
Trees are dying rapidly and prematurely, all over the world, and other plants are diminished as well. The impacts on wildlife are enormous, and it is the primary reason wildfires are now unprecedented in frequency, duration, and intensity.
Nitrogen pollution in the water is also causing eutrophication and the death of aquatic species.
In fact, the entire web of life is unraveling as key species for habitat and food, such as trees and coral reefs, are disappearing.
It's nice to see the thermodynamic explanation, but also, important to recognize the insidious influence of pollution on the biosphere.
His reply: "Hi Gail, As my area of expertise is with energy, I wasn’t aware of this phenomena. Due to your email, I have spent some time researching and find this troubling. Thanks for the heads up. You’ve peaked my interest. Words are failing to convey the enormity of our plight. Take care, js"
For reference, here's Desdemona's collection of stories that are related to nitrogen: https://desdemonadespair.net//////search/label/nitrogen
This is certainly a hot topic now.
We should remember that life on this planet has built technically intelligent beings (oil and digital?) Even if you extend that back 10,000 years – humans – with the wheel and weapons – we have lived only a tiny fraction of the duration of complex life forms. Brains have been around for millions and millions of years, but locomotives and iPhones not so long at all.
The reason for higher intelligence may be nudged by ice ages… Check out the book "A Brief History of the Mind…" by Dr William H. Calvin. Human survival pushed by ice ages..- and we should realize that the very geography of our planet moved high tech forward. Vast oceanic navigation necessitated the invention of accurate timepieces, radios. If our planet was mostly rocky land then we would never move beyond the railroad age – we would not need to. And even airlines would be less important than high speed trains. Telegraphs would suffice, no need for wireless.
I like to think there is plenty of life out there everywhere.. but may not be much more than dinosaurs, squid and whales… all possibly very intelligent .. but not disposed to using technology to build traveling machines and digital cameras. It is our shape of the planet that had nurtured our smarts. Any other structures – more oceans or more land and we would not have the same brains and tools. there should be a nifty phrase for this — something about "geology drives ontology".
And it is possible that we created our ultimate challenge. A tragic flaw.