Earthward: Planetary Boundaries

Last week, an interdisciplinary team of twenty-nine scientists from labs spanning four continents published a comprehensive update of a “planetary boundaries framework,” an analysis of Earth’s ability to withstand human impacts. From the temperature record of the past 10,000 years, deduced from glacier ice cores, we know that Earth’s environment was exceptionally stable while civilization developed. For almost all this time, the planet’s resilience in responding to natural environmental perturbations meant that crucial aspects of the climate system, like temperature, freshwater flows and ocean chemistry, varied little and were conducive to human flourishing. But that changed when humanity learned how to harness fossil fuels, triggering massive population growth. The consequence of the population explosion is that six of nine critical Earth system processes defined by the planetary boundaries framework surpass estimated safe limits. Strikingly, the team concludes that the overall Earth system is now “…well outside the safe operating space for humanity.” Indeed, the recent human impact is so large that it shows up as a distinct imprint on the geological record. This has led some Earth scientists to propose that the recent stable epoch (known as the Holocene) is effectively over, and that a new epoch, the Anthropocene, has begun.

The planetary boundaries framework is a heroic attempt to define a “safe operating space” for humanity within the limits of the Earth system. To develop the framework, Earth scientists use both observational data and computational models to look at the natural state of the system and how it is influenced by human activities. Through several iterations since 2009, the teams working on this project have consistently found nine Earth processes for which they believe planetary boundaries must be set. Two of these processes are considered primary: climate change (measured by energy imbalance and greenhouse gas levels) and biosphere integrity (measured by species extinction rates and human interference with carbon fixation by plants). The other seven processes are: land use change (measured by deforestation), consumption of freshwater, disruption of natural phosphorus (P) and nitrogen (N) cycles, introduction of novel entities (plastics, genetically modified organisms, synthetic chemicals, etc.), ocean acidification, ozone depletion and atmospheric aerosol buildup.

The biggest challenge the teams face is where to set the individual process boundaries. For many of the nine processes, we have good estimates for the values of key parameters before the fossil fuel era. For example, data on the atmospheric CO2 level, thickness of the ozone layer and ocean acidity is reasonably good for pre-industrial times, which helps define where to set the boundaries today. For other processes, such as forest cover and freshwater flows, the natural Earth state is more difficult to estimate. In all cases, since the precise boundaries are ultimately judgment calls, the scientific data had to be evaluated in accordance with a particular set of values, a conservative approach based on the precautionary principle. In its simplest form, this widely used guideline argues for caution in the face of uncertain threats. Applied to climate change, the precautionary principle yields the often-cited value of 350 ppm as the safe limit for atmospheric CO2 concentration. This boundary was already transgressed in the 1970s. The team found that the safe limits for biosphere integrity, land use, freshwater consumption, novel entities and P/N cycle disruption have similarly been breached. On the brighter side, planetary boundaries for ocean acidification, atmospheric aerosols and ozone depletion have not yet been exceeded.

Planetary boundaries are not the same as tipping points. The boundaries define safe spaces: staying within them offers assurance that human impacts do not risk catastrophic environmental change. In contrast, a tipping point is a threshold of imbalance that, when surpassed, causes dramatic and possibly irreversible shifts in part or all of the Earth system. By transgressing six of the nine planetary boundaries, humanity has entered into an unsafe zone, closer to but not yet exceeding tipping points. There is space between boundaries and tipping points – though we have little understanding of how much. Uncertainties in setting the boundary for each process are compounded by interactions among the processes and by the fact that many of the processes cause regional imbalances with effects on the overall Earth system that are difficult to assess.

If the Earth system is indeed well outside the safe operating space for humanity, what options do we have to do something about it? In a remarkable coincidence, the updated planetary boundary analysis has landed at almost exactly the same moment as the yearly meeting of the United Nations General Assembly in New York. The General Assembly is the main deliberative, policymaking and representative body of the UN. Unlike the UN Security Council, which can issue sanctions and even authorize the use of force, the General Assembly is only able to make recommendations. However, these can be influential because the body is universal, with all sovereign countries having a voice in its decisions. In 2015, the same year as the Paris Agreement on Climate Change, all UN member states adopted the 2030 Agenda for Sustainable Development, which offers a blueprint for a future in which Earth’s limits are not exceeded. The heart of the blueprint is a set of 17 Sustainable Development Goals (SDGs) addressing not just climate, energy and the health of the biosphere, but also human values including health, education, poverty and gender equity. Through its Department of Economic and Social Affairs, the UN provides support for achieving the 17 goals, including policy analysis and capacity building through outreach to individual countries.

This year’s General Assembly meeting also coincides with the appearance of a midpoint Global Sustainable Development Report (GSDR). Recognizing the power of science to offer clarity with respect to setting and achieving the SDGs, the UN Secretary General appointed an independent scientific team to prepare the report, made up of 15 experts with a wide range of expertise and embodying geographic and gender balance. Six of the seventeen SDGs connect well to the planetary boundaries framework, encompassing climate change, the green energy transition, clean water, pollution, and sustainable use of land and water resources. Each SDG lays out some specific targets for action, which are framed as policy goals. For example, one target in the climate change area is integration of climate change measures into national policies, strategies and planning, and one way of assessing whether the target is met is by measuring greenhouse gas emissions. Similarly, the targets addressing sustainable use of terrestrial ecosystems relate to issues such as increasing forest cover and combating desertification, and are assessed by evaluating outcomes in those areas.

The UN’s sustainable development agenda offers a policy blueprint to return the Earth system to a safe operating space, and the midpoint GSDR tells us how well we are doing so far. Although there are some positive comments about progress in the green energy transition, the report mainly notes little improvement or deterioration in preserving land or water ecosystems and maintaining biodiversity. The environmental degradation adversely affects human development, especially in the global South where access to these resources is crucial to sustainable economic development. A large proportion of the report is then dedicated to reframing pathways to better reach the development goals over the remainder of the decade.

The UN meetings are still going on as I write, but it does not take much prescience to anticipate the bottom-line message that will emerge: the situation is dire, but we can still save the day if we just act with the necessary urgency. Increasingly, this message is unsatisfying to many people, and is indeed driving the popular protests that now regularly accompany any high-level meeting about climate. As Greta Thunberg famously said, enough with the bla bla bla. Many of the protesters in New York this week are young people following her lead, who don’t hesitate to describe the anxiety that they feel from the worsening environmental situation. But unlike climate doomists who want to retreat into bunkers, these youth are inspiring because they hold out hope that solutions are possible. And they are right about that.

The protesters did not converge on New York because of the planetary boundaries framework or the sustainable development report. They are there because the UN Secretary-General, Antonio Guterres, decided to use the General Assembly meeting as an opportunity to hold a special Climate Ambition Summit before the dedicated global climate meeting, COP28, this November in the United Arab Emirates. Interestingly, and probably contrary to the protester’s expectations, this week’s summit might actually not be more of the same. The New York Times reported yesterday that Secretary-General Guterres only extended speaking invitations to countries that have taken aggressive climate action, including the European Union, Brazil and South Africa. China and the United States were pointedly excluded from the dais. It appears that the Secretary-General’s intention is to lay the groundwork for COP28 by emphasizing that “more of the same” is no longer acceptable.

If this is so, it might mean that the protester’s past messages were actually heard. Unfortunately, though, November’s COP28 meeting already promises to be anything but reconciliatory. The meeting will be led by Sultan al-Jaber, who directs the UAE’s national oil company, Adnoc. Mr. al-Jaber believes that past summits have failed because the fossil fuel industry was excluded. It appears unlikely that the young protesters will agree.


Welcome to the Earthward Newsletter. Earthward is a weekly nonpartisan newsletter that covers recent events in the climate and renewable energy space, including science, technology, policy, politics and citizen advocacy.

Subscription is free and will generate one (1) weekly email that will arrive on Thursday mornings. The email will include an “unsubscribe” link. 

Earthward is written by Dr. John Perona and is an outgrowth of the climate education work begun with From Knowledge to Power: The Comprehensive Handbook for Climate Science and Advocacy (K2P).