Earth beyond six of nine planetary boundaries

ENVIRONMENTAL STUDIES

Earth beyond six of nine planetary boundaries

This planetary boundaries framework update finds that six of the nine boundaries are transgressed, suggesting that Earth is now well outside of the safe operating space for humanity. Ocean acidification is close to being breached, while aerosol loading regionally exceeds the boundary. Stratospheric ozone levels have slightly recovered. The transgression level has increased for all boundaries earlier identified as overstepped. As primary production drives Earth system biosphere functions, human appropriation of net primary production is proposed as a control variable for functional biosphere integrity. This boundary is also transgressed. Earth system modeling of different levels of the transgression of the climate and land system change boundaries illustrates that these anthropogenic impacts on Earth system must be considered in a systemic context.

INTRODUCTION

The planetary boundaries framework draws upon Earth system science. It identifies nine processes that are critical for maintaining the stability and resilience of Earth system as a whole. All are presently heavily perturbed by human activities. The framework aims to delineate and quantify levels of anthropogenic perturbation that, if respected, would allow Earth to remain in a "Holocene-like" interglacial state. In such a state, global environmental functions and life-support systems remain similar to those experienced over the past approximately ten thousand years rather than changing into a state without analog in human history. This Holocene period, which began with the end of the last ice age and during which agriculture and modern civilizations evolved, was characterized by relatively stable and warm planetary conditions. Human activities have now brought Earth outside of the Holocene's window of environmental variability, giving rise to the proposed Anthropocene epoch.

Planetary-scale environmental forcing by humans continues and individual Earth system components are, to an increasing extent, in disequilibrium in relation to the changing conditions. As a consequence, the post-Holocene Earth is still evolving, and ultimate global environmental conditions remain uncertain. Paleoclimate research, however, documents that Earth has previously experienced largely ice-free conditions during warm periods with correspondingly different states of the biosphere. It is clearly in humanity's interest to avoid perturbing Earth system to a degree that risks changing global environmental conditions so markedly. Ice cover is only one indicator of substantial system-wide change in numerous other Earth system dimensions. The planetary boundaries framework delineates the biophysical and biochemical systems and processes known to regulate the state of the planet within ranges that are historically known and scientifically likely to maintain Earth system stability and life-support systems conducive to the human welfare and societal development experienced during the Holocene.

Currently, anthropogenic perturbations of the global environment are primarily addressed as if they were separate issues, e.g., climate change, biodiversity loss, or pollution. This approach, however, ignores these perturbations' nonlinear interactions and resulting aggregate effects on the overall state of Earth system. Planetary boundaries bring a scientific understanding of anthropogenic global environmental impacts into a framework that calls for considering the state of Earth system as a whole.

For more than three billion years, interactions between the geosphere (energy flow and nonliving materials in Earth and atmosphere) and biosphere (all living organisms/ecosystems) have controlled global environmental conditions. Earth system's state changed in response to forcings generated by external perturbations (e.g., solar energy input and bolide strikes) or internal processes in the geosphere (e.g., plate tectonics and volcanism) or biosphere (e.g., evolution of photosynthesis and rise of vascular plants). These forcings were processed through interactions and feedbacks among processes and systems within Earth system, shaping its often complex overall response. Today, human activities with planetary-scale effects act as additional forcing on Earth system. Thus, the anthroposphere has become an additional functional component of Earth system, capable of altering Earth system state. The planetary boundaries framework formulates limits to the impact of the anthroposphere on Earth system by identifying a scientifically based safe operating space for humanity that can safeguard both Earth's interglacial state and its resilience.

The Holocene state of Earth is the benchmark reference in this context, as many of the components comprising the planetary boundary framework were rather stable during this period. This is also the only Earth system state civilizations have historically known. Climate is a manifestation of external forcing, e.g., solar activity, orbital cycles, and interactions among Earth system components, and global mean surface temperature varied by only plus or minus zero point five degrees Celsius from the Neolithic [approximately nine thousand years before the present] until the Industrial Revolution. Biomes across Earth have also largely been stable over the past ten thousand years, with preindustrial global terrestrial net primary production (NPP) varying by not more than fifty-five point nine plus or minus one point one billion tonnes of carbon year negative one (see the Supplementary Materials). Bias-corrected data confirm that preindustrial global precipitation levels were also stable, particularly from the mid-Holocene onward. These data provide strong support for using the Holocene (see the Supplementary Materials) as the planetary boundaries reference state for a stable and resilient planet.

All of the framework's individual boundaries therefore adopt preindustrial Holocene conditions as a reference for assessing the magnitude of anthropogenic deviations. Available data and state of knowledge from analytics and modeling of the framework components dictate the methods for derivation and quantification of the individual boundaries and their precautionary guardrails. Despite data constraints, efforts have been made to identify suitable control variables for all boundaries, together with evidence of how much perturbation leads to generation of impacts or altered interactions/feedbacks that can potentially cause irreversible changes to Earth's life support systems. The focus is always at Earth system rather than regional scale, even when the evidence used to establish boundaries originates from regional studies. In these cases, regional evidence is combined to assess Earth system impacts of cumulative transgressions across multiple regional systems.

The planetary boundaries framework has attracted considerable scientific and societal attention, inspiring governance strategies and policies at all levels. The framework evolves through updates made in light of recent scientific understanding. Here, we bring together advances from different fields of science to update the framework and the status of its boundaries. Boundaries are, for the first time, proposed for all of the individual components of the framework. Updates of the functional biosphere integrity and aerosol loading boundaries are based on analyses presented here. Recent analyses form the basis for updates of the freshwater change and novel entities boundaries. Last, the importance of considering human impacts on components of the global environment in a system context is illustrated using a modeling exercise exploring how various scenarios of transgression of the land system (representing the biosphere) and climate change boundaries combine to affect Earth system characteristics.