Is the epoch of habitability on Earth drawing to a close?

Desdemona Despair

The future of life on Earth? Stromatolites growing in Hamelin Pool Marine Nature Reserve, Shark Bay in Western Australia.

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.

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.