Our Human Boundaries – Part I

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“Humans are taking colossal risks with the future of civilization and everything that lives on Earth.” So begins a report in Nature attempting to quantify Safe and Just Earth boundaries. The findings “are meant as a transparent proposal for further debate and refinement by scholars and wider society.” I took them at their word; let’s consider and then debate their proposals.

The Problem – “Past performance is not a guarantee of future results.”

The term earth boundaries may seem a bit foreign, but it refers to our ability to continue to live on the only planet that has been our home – an older term is carrying capacity and reflects how many of us, in how many ways, can be sustainably supported. There have been other dominant species before us; historically, our dominance is not guaranteed. Their reference “for a ‘safe’ Earth climate system state" is the Holocene, the period of Earth’s life that we live in – 11,700 years of exceptionally stable global climate. The Earth’s systems, as the Holocene has arranged them, are ... 

the fundamental prerequisites for human development as we know it … There is no evidence that billions of humans and complex societies can thrive in other known climates, such as a glacial ice age or ‘Hothouse Earth’ ”

Humans as a species have been around for perhaps 300,000 years in the Pleistocene epoch, so we might well live within the confines of that climate, but “the Holocene has witnessed all of humanity's recorded history and the rise and fall of all its civilizations.” So I’m with the experts on this; a Holocene climate seems to be our niche.

The other term often used to describe the Earth’s current age is the Anthropocene – the age of man. This is a bit of species-specific naval gazing. The Anthropocene has seen rapid, at least based on the Earth’s life, changes in how the Earth’s climate has been reconfigured. [1] You may attribute climatic change to a statistical anomaly, the way it is, or as the authors do.

“These changes are mostly driven by social and economic systems run on unsustainable resource extraction and consumption.”

In short, we are exceeding our carrying capacity and without another home or niche that is problematic for us. I emphasized those words because the authors, while speaking on behalf of “the Earth,” writ large, are primarily concerned with one species – us. There is a species-centrism throughout the report, and it simultaneously makes sense and brings to mind the words of stand-up comedian George Carlin ...

“The planet has been through a lot worse than us. Been through all kinds of things worse than us: been through earthquakes, volcanoes, plate tectonics, continental drifts, solar flares, sunspots, magnetic storms, the magnetic reversal of the poles, hundreds of thousands of years of bombardment by comets and asteroids, and meteors, worldwide floods, tidal waves, worldwide fires, erosion, cosmic rays, recurring ice ages, and we think some plastic bags and aluminum cans are going to make a difference? The planet isn’t going anywhere… we are!”

The authors discuss the carrying capacity of the Earth’s systems as we currently know them. They include fresh water, climate, aerosols (a more general term for components of our atmosphere, including water and pollutants), and nutrients. A quick note here, the nutrients, nitrogen and phosphorus, are those that feed the plants, which in turn feed the animals, both of which feed us – still species-centric. The final system, the biosphere, is really the relationship between all the other innumerated systems.

These systems were chosen because they are major interlinked components underpinning “the planet’s life-support systems” and can be impacted for good or bad by human activities in “policy-relevant timescales.” They are subject to what Donald Rumsfeld described as the

“… known unknowns; that is to say, we know there are some things we do not know. But there are also unknown unknowns—the ones we don't know, we don't know. …it is the latter category that tends to be the difficult ones.”


To understand the knowns, unknowns, and unknown unknowns, it would be best to begin by considering how the expert panel arrived at their conclusions. In considering the underlying assumptions, it is best to start with those quantifying the “safe boundaries for maintaining Earth system resilience.”

“These boundaries are aimed at protecting Earth system stability and life-support systems for as many species as possible, but they may not protect all species or all humans today….”

The term introduced as the guiding principle is again species-centric, nature’s contribution to people (NCP), as we see in this sentence,

“… we determined boundaries that avoid triggering climate tipping elements or maintain multiple local or Earth system NCPs at different levels of likelihood.”

Tipping elements are abrupt changes, “large and difficult to reverse….” These tipping moments can also be described as phase shifts, as at the moment, supercooled water can turn to ice. It is a process on the planetary scale that we cannot foresee clearly; it is a known unknown. Tipping elements or phase shifts are an important and nuanced concept that we would be wise to take into account when attempting to quantify how our systems are changing – we may not know if and when they will happen, but are hard established science gives us many examples of their occurrence in the Holocene.

The primary variable considered is the rising temperature and its impact on sea level. Both are reasonable variables to consider. There are numerous ways to measure temperature, and the experts chose a wet-bulb temperature of 35°C – the point at which the human body cannot cool itself. [2] This is well past our historical experience; wet-bulb temperatures of 28°C were responsible for 72,000 deaths in Europe in 2003 and 55,000 in Russia in 2010. The value used in their calculation involved 29°C.

A rising temperature melts ice; roughly 10% of the current Earth’s surface is covered by ice, and inevitably, based on the science we already acknowledge, that will result in rising sea levels. The study assumes that 6.8 billion people inhabit land “subject to inundation by the end of this century or on a multi-centennial timescale.” That number is subject to change because they did not account for how we might adapt to those rising levels through migration or coastal “defenses.” In any instance, unless there is some technological magic, it will be a big number.

Their boundary conditions included:

  • "surface water flows that protect freshwater ecosystems and fisheries"
  • "maximum safe average annual drawdown [of groundwater]equal to the average annual recharge [refilling]"
  • "for nitrogen, [avoiding] significant disruption to freshwater ecosystems (from total N runoff), groundwater potability (from nitrate leaching), and terrestrial ecosystems (from atmospheric N deposition due to ammonia and nitrogen oxide emissions)"
  • "for phosphorus, avoiding eutrophication from runoff.”

For aerosols, the experts were most concerned about the impact on regional monsoon systems. The most recent research suggests “interhemispheric differences in aerosol concentrations” alters the pattern and intensity of rain. [3]

How confident can we be in these boundary conditions?

“We identified boundaries at multiple levels of likelihoods to reflect underlying scientific uncertainties and variabilities. These uncertainties included epistemic uncertainty in the boundary value for a specific Earth system process or component.”

Quick translation: epistemic uncertainty refers to the unknown unknowns. The experts made judgments based on how robust the evidence base and the “degree of scientific agreement across the peer-reviewed literature and among the members of each Earth Commission Working Group.”

Of the systems considered, only climatic changes were felt to be highly confident, and by this, they meant that a 1 to 2 °C change would occur and would “fundamentally” alter the climate. They did not consider the more contentious issue of whether that temperature was solely or partially attributable to human behavior. The other systems considered had medium confidence; the confidence in the impact of aerosols was low.

Too Long Didn’t Read TL;DR

Next up in this continuing series is a look at the methodology behind what will be a more controversial part of the report – finding justice for those safe earth boundaries.


[1] If you count the 3.7 billion years since microbes were thought to appear, our current “glory years” of the Anthropocene take up 0.00003 % of the Earth’s “life.” Measuring from when we arrived on the planet means the Anthropocene reflects 0.04% of our existence.

[2] A wet bulb temperature Tw is determined based on the ambient temperature at 100% humidity using a device called a sling psychrometer. 

[3] “observations based on past volcanic eruptions and climate modeling studies show that an increased concentration of reflecting aerosols in one hemisphere leads to precipitation decreasing in the same hemisphere’s tropical monsoon regions while increasing in the opposite hemisphere.”

Here are the other parts of the article:

Part II – Earth System Boundaries, the methodologies of “Justice”

Part III – Earth System Boundaries, Climate, water, nutrients

Part IV – Earth System Boundaries, Aerosols, Biosphere and some final thoughts

Source: Safe and Just Earth System Boundaries Nature DOI: 10.1038/s41586-023-06083-8