On Planetary Boundaries

The fragility of our planet is evident in the intricate balance of its ecosystems. Over-fertilizing farms can lead to toxic runoff, endangering the oceans and marine life. Excessive carbon emissions from cars contribute to global warming, causing glaciers to melt and sea levels to rise, threatening both human and animal habitats. This interconnectedness highlights the potential for a chain reaction triggered by seemingly small changes. In 2009, a group of scientists introduced the concept of "planetary boundaries," identifying nine crucial limits that, if crossed, could lead to catastrophic consequences. This framework has since been embraced by scientists and policymakers worldwide, including at the UN.

For the past 10,000 years, during the Holocene epoch, Earth's climate has remained relatively stable, enabling the development of human civilization. However, this stability is dependent on a specific range of climatic conditions. The concept of planetary boundaries seeks to define this range by establishing measurable limits across various Earth systems. These boundaries, proposed in a 2009 paper published in Ecology and Society, cover areas such as climate change, ocean acidity, ozone levels, biogeochemical flows of nitrogen and phosphorous, biodiversity loss, freshwater use, and land system change.

There is a certain desirable state of the world, in other words, at least as far as human beings are concerned. Wind back the clock beyond the Holocene, and you find eras when it was too hot for us, or too cold, or where there was so much carbon in the atmosphere we'd choke on it. It would behoove us to do what we can to keep the earth within the safe conditions we've known.

The planetary boundaries concept is an attempt to help us identify that range. The paper that first proposed the idea was published in Ecology and Society in 2009 by a set of experts who studied various earth systems. Together, they suggested seven categories where tipping points might be likely. For each, they selected a specific, measurable "boundary"—a limit that would keep us well short of any tipping points.

For climate change, the proposed boundaries include a limit on atmospheric CO2 concentration at 350 parts per million and a cap on radiative forcing at 1 Watt per square meter higher than pre-industrial levels. Ocean acidification is addressed by maintaining aragonite concentration in seawater above 80% of pre-industrial levels. The ozone layer's protection requires keeping ozone concentrations at 95% of historic levels.

Biogeochemical flows, specifically of nitrogen and phosphorous, are of concern due to their impact on ecosystems. The proposed boundaries include limiting the flow of nitrogen out of the atmosphere to less than 35 tera-grams per year and restricting phosphorous flow into oceans to less than 10 times the natural "background rate."

Biodiversity loss is measured as the ratio of species lost per year compared to the total species in a given year, with a proposed planetary boundary of less than 10 extinctions per million species annually. Additionally, human activities should withdraw less than 4,000 cubic kilometers of freshwater per year and utilize no more than 15% of ice-free land for crops.

There were two other tipping points the scientists were worried about, but for which they could not find a good way to define quantitative boundaries. Chemical pollution lacks a specific numerical boundary due to the diverse and widespread effects of various chemicals. Similarly, the release of aerosols into the atmosphere lacks a quantifiable boundary due to its complex impact on climate change.

The concept of planetary boundaries traces back to the 1972 report, "The Limits to Growth," which predicted the consequences of unchecked population growth. While initially controversial, recent evaluations have validated its predictions. In light of the climate crisis, the 2009 paper on planetary boundaries emphasizes the importance of staying within the proposed safe zone to avoid irreversible tipping points. Unfortunately, current data indicate that we have already exceeded three boundaries, emphasizing the urgent need for global action to address these environmental challenges.

The report was enormously controversial, especially among economists. Some pointed out that breakthroughs—especially in farming—had allowed us to increase our efficiency, and thereby avoid straining finite resources. As human populations grew, so too might innovation, which means there needn't be any end to growth. We'll just find more and more breakthroughs.

However, recent scrutiny, especially in light of the climate crisis, prompted a reevaluation of the fundamental concepts presented in the book. In 2008, a researcher analyzed the report's predictions and found them to be surprisingly accurate up to that point. Concurrently, scientific discourse in the same year began exploring the notion of "tipping points," points of no return that trigger irreversible changes. The 2009 paper on "planetary boundaries" became part of this growing awareness.

The authors of the paper made a careful distinction between their concept of "boundaries" and what they termed "thresholds." Thresholds represented critical points, with the abrupt retreat of Arctic sea ice being a familiar example. While the goal is to avoid all thresholds, their exact locations remain uncertain. The proposed "planetary boundaries" were introduced as a safety zone, designed to keep us far from these tipping points. Importantly, these boundaries were entirely human-defined. Establishing a boundary required identifying specific variables as proxies for larger, more intricate systems and making subjective decisions on acceptable limits.

Regrettably, we have already exceeded three boundaries. CO2 concentrations have surpassed 415ppm, well beyond the 350ppm boundary. Radiative forcing now exceeds 2 watts per square meter compared to 1750. Consequently, concerns about climate change are warranted.

There are additional boundaries that scientists acknowledge we have crossed, even if pinpointing them numerically proves challenging. Extinction rates, for instance, are challenging to precisely quantify but are likely a thousand times higher than the current background rate. Scientists also believe we have surpassed the boundary for nitrogen flows, though obtaining precise global figures remains challenging.

Image credit: Felix Mueller/Creative Commons.

There's a silver lining to consider: a couple of decades ago, the global concern revolved around ozone depletion. Thanks to collaborative efforts on an international scale, governments successfully tackled this issue, preventing us from reaching a tipping point. Anyway, with that, I'll leave you here. I hope this helped elaborate on the current state of affairs concerning climate change.