Earth System dynamics can be described, studied, and understood in terms of trajectories between alternate states separated by thresholds that are controlled by nonlinear processes, interactions, and feedbacks. Based on this framework, we argue that social and technological trends and decisions occurring over the next decade or two could significantly influence the trajectory of the Earth System for tens to hundreds of thousands of years and potentially lead to conditions that resemble planetary states that were last seen several millions of years ago, conditions that would be inhospitable to current human societies and to many other contemporary species.
The trajectory of the Earth System through the Late Quaternary, particularly the Holocene, provides the context for exploring the human-driven changes of the Anthropocene and the future trajectories of the system SI Appendix has more detail.
House of Earth - Woody Guthrie - Hardcover
Over the Late Quaternary past 1. Not every glacial—interglacial cycle of the past million years follows precisely the same trajectory 7 , but the cycles follow the same overall pathway a term that we use to refer to a family of broadly similar trajectories. The full glacial and interglacial states and the ca. This limit cycle is shown in a schematic fashion in blue in Fig. The Holocene is represented by the top of the limit cycle loop near the label A.
A schematic illustration of possible future pathways of the climate against the background of the typical glacial—interglacial cycles Lower Left. The interglacial state of the Earth System is at the top of the glacial—interglacial cycle, while the glacial state is at the bottom. Sea level follows temperature change relatively slowly through thermal expansion and the melting of glaciers and ice caps.
The horizontal line in the middle of the figure represents the preindustrial temperature level, and the current position of the Earth System is shown by the small sphere on the red line close to the divergence between the Stabilized Earth and Hothouse Earth pathways. Their positions on the pathway are approximate only. The current position of the Earth System in the Anthropocene is shown in Fig. In Fig. Stability landscape showing the pathway of the Earth System out of the Holocene and thus, out of the glacial—interglacial limit cycle to its present position in the hotter Anthropocene.
The fork in the road in Fig. The other pathway leads to Stabilized Earth, a pathway of Earth System stewardship guided by human-created feedbacks to a quasistable, human-maintained basin of attraction. Systems in a highly stable state deep valley have low potential energy, and considerable energy is required to move them out of this stable state.
Systems in an unstable state top of a hill have high potential energy, and they require only a little additional energy to push them off the hill and down toward a valley of lower potential energy. The Anthropocene represents the beginning of a very rapid human-driven trajectory of the Earth System away from the glacial—interglacial limit cycle toward new, hotter climatic conditions and a profoundly different biosphere 2 , 8 , 9 SI Appendix. More importantly, the rapid trajectory of the climate system over the past half-century along with technological lock in and socioeconomic inertia in human systems commit the climate system to conditions beyond the envelope of past interglacial conditions.
In the future, the Earth System could potentially follow many trajectories 12 , 13 , often represented by the large range of global temperature rises simulated by climate models In most analyses, these trajectories are largely driven by the amount of greenhouse gases that human activities have already emitted and will continue to emit into the atmosphere over the rest of this century and beyond—with a presumed quasilinear relationship between cumulative carbon dioxide emissions and global temperature rise However, here we suggest that biogeophysical feedback processes within the Earth System coupled with direct human degradation of the biosphere may play a more important role than normally assumed, limiting the range of potential future trajectories and potentially eliminating the possibility of the intermediate trajectories.
We argue that there is a significant risk that these internal dynamics, especially strong nonlinearities in feedback processes, could become an important or perhaps, even dominant factor in steering the trajectory that the Earth System actually follows over coming centuries. This risk is represented in Figs.
Precisely where a potential planetary threshold might be is uncertain 15 , Such cascades comprise, in essence, the dynamical process that leads to thresholds in complex systems section 4. This analysis implies that, even if the Paris Accord target of a 1. Creating such a pathway and basin of attraction requires a fundamental change in the role of humans on the planet. This stewardship role requires deliberate and sustained action to become an integral, adaptive part of Earth System dynamics, creating feedbacks that keep the system on a Stabilized Earth pathway Alternative Stabilized Earth Pathway.
We now explore this critical question in more detail by considering the relevant biogeophysical feedbacks Biogeophysical Feedbacks and the risk of tipping cascades Tipping Cascades. The trajectory of the Earth System is influenced by biogeophysical feedbacks within the system that can maintain it in a given state negative feedbacks and those that can amplify a perturbation and drive a transition to a different state positive feedbacks. Table 1 summarizes carbon cycle feedbacks that could accelerate warming, while SI Appendix , Table S2 describes in detail a more complete set of biogeophysical feedbacks that can be triggered by forcing levels likely to be reached within the rest of the century.
Carbon cycle feedbacks in the Earth System that could accelerate global warming. Many feedbacks will show some gradual change before the tipping point is reached. A few of the changes associated with the feedbacks are reversible on short timeframes of 50— years e. A few of the feedbacks do not have apparent thresholds e. For some of the tipping elements, crossing the tipping point could trigger an abrupt, nonlinear response e. There could also be considerable lags after the crossing of a threshold, particularly for those tipping elements that involve the melting of large masses of ice.
However, in some cases, ice loss can be very rapid when occurring as massive iceberg outbreaks e. For some feedback processes, the magnitude—and even the direction—depend on the rate of climate change. However, if the rate of climate change is too large or too fast, a tipping point can be crossed, and a rapid biome shift may occur via extensive disturbances e.
Trajectories of the Earth System in the Anthropocene
In some terrestrial cases, such as widespread wildfires, there could be a pulse of carbon to the atmosphere, which if large enough, could influence the trajectory of the Earth System Varying response rates to a changing climate could lead to complex biosphere dynamics with implications for feedback processes. For example, delays in permafrost thawing would most likely delay the projected northward migration of boreal forests 30 , while warming of the southern areas of these forests could result in their conversion to steppe grasslands of significantly lower carbon storage capacity.
The overall result would be a positive feedback to the climate system. However, increasing atmospheric CO 2 raises temperature, and hotter leaves photosynthesize less well. Other feedbacks are also involved—for instance, warming the soil increases microbial respiration, releasing CO 2 back into the atmosphere. Our analysis focuses on the strength of the feedback between now and However, several of the feedbacks that show negligible or very small magnitude by could nevertheless be triggered well before then, and they could eventually generate significant feedback strength over longer timeframes—centuries and even millennia—and thus, influence the long-term trajectory of the Earth System.
These feedback processes include permafrost thawing, decomposition of ocean methane hydrates, increased marine bacterial respiration, and loss of polar ice sheets accompanied by a rise in sea levels and potential amplification of temperature rise through changes in ocean circulation The tipping elements fall into three clusters based on their estimated threshold temperature 12 , 17 , Cascades could be formed when a rise in global temperature reaches the level of the lower-temperature cluster, activating tipping elements, such as loss of the Greenland Ice Sheet or Arctic sea ice.
These tipping elements, along with some of the nontipping element feedbacks e. For example, tipping loss of the Greenland Ice Sheet could trigger a critical transition in the Atlantic Meridional Ocean Circulation AMOC , which could together, by causing sea-level rise and Southern Ocean heat accumulation, accelerate ice loss from the East Antarctic Ice Sheet 32 , 40 on timescales of centuries Global map of potential tipping cascades.
The individual tipping elements are color- coded according to estimated thresholds in global average surface temperature tipping points 12 , Arrows show the potential interactions among the tipping elements based on expert elicitation that could generate cascades. Observations of past behavior support an important contribution of changes in ocean circulation to such feedback cascades. During previous glaciations, the climate system flickered between two states that seem to reflect changes in convective activity in the Nordic seas and changes in the activity of the AMOC. During extremely cold conditions in the north, heat accumulated in the Southern Ocean, and Antarctica warmed.
Eventually, the heat made its way north and generated subsurface warming that may have been instrumental in destabilizing the edges of the Northern Hemisphere ice sheets If Greenland and the West Antarctic Ice Sheet melt in the future, the freshening and cooling of nearby surface waters will have significant effects on the ocean circulation.
While the probability of significant circulation changes is difficult to quantify, climate model simulations suggest that freshwater inputs compatible with current rates of Greenland melting are sufficient to have measurable effects on ocean temperature and circulation 46 , Sustained warming of the northern high latitudes as a result of this process could accelerate feedbacks or activate tipping elements in that region, such as permafrost degradation, loss of Arctic sea ice, and boreal forest dieback.
While this may seem to be an extreme scenario, it illustrates that a warming into the range of even the lower-temperature cluster i. This human-created pathway is represented in Figs. Stabilized Earth would require deep cuts in greenhouse gas emissions, protection and enhancement of biosphere carbon sinks, efforts to remove CO 2 from the atmosphere, possibly solar radiation management, and adaptation to unavoidable impacts of the warming already occurring The short broken red line beyond Stabilized Earth in Fig.
In essence, the Stabilized Earth pathway could be conceptualized as a regime of the Earth System in which humanity plays an active planetary stewardship role in maintaining a state intermediate between the glacial—interglacial limit cycle of the Late Quaternary and a Hothouse Earth Fig. We emphasize that Stabilized Earth is not an intrinsic state of the Earth System but rather, one in which humanity commits to a pathway of ongoing management of its relationship with the rest of the Earth System.
A critical issue is that, if a planetary threshold is crossed toward the Hothouse Earth pathway, accessing the Stabilized Earth pathway would become very difficult no matter what actions human societies might take. In other words, after the Earth System is committed to the Hothouse Earth pathway, the alternative Stabilized Earth pathway would very likely become inaccessible as illustrated in Fig. Hothouse Earth is likely to be uncontrollable and dangerous to many, particularly if we transition into it in only a century or two, and it poses severe risks for health, economies, political stability 12 , 39 , 49 , 50 especially for the most climate vulnerable , and ultimately, the habitability of the planet for humans.
This variability was often much more pronounced than global, longer-term Holocene variability SI Appendix. SI Appendix , Table S4 summarizes biomes and regional biosphere—physical climate subsystems critical for human wellbeing and the resultant risks if the Earth System follows a Hothouse Earth pathway. While most of these biomes or regional systems may be retained in a Stabilized Earth pathway, most or all of them would likely be substantially changed or degraded in a Hothouse Earth pathway, with serious challenges for the viability of human societies.
For example, agricultural systems are particularly vulnerable, because they are spatially organized around the relatively stable Holocene patterns of terrestrial primary productivity, which depend on a well-established and predictable spatial distribution of temperature and precipitation in relation to the location of fertile soils as well as on a particular atmospheric CO 2 concentration. Current understanding suggests that, while a Stabilized Earth pathway could result in an approximate balance between increases and decreases in regional production as human systems adapt, a Hothouse Earth trajectory will likely exceed the limits of adaptation and result in a substantial overall decrease in agricultural production, increased prices, and even more disparity between wealthy and poor countries A Hothouse Earth trajectory would almost certainly flood deltaic environments, increase the risk of damage from coastal storms, and eliminate coral reefs and all of the benefits that they provide for societies by the end of this century or earlier In the dominant climate change narrative, humans are an external force driving change to the Earth System in a largely linear, deterministic way; the higher the forcing in terms of anthropogenic greenhouse gas emissions, the higher the global average temperature.
However, our analysis argues that human societies and our activities need to be recast as an integral, interacting component of a complex, adaptive Earth System. This framing puts the focus not only on human system dynamics that reduce greenhouse gas emissions but also, on those that create or enhance negative feedbacks that reduce the risk that the Earth System will cross a planetary threshold and lock into a Hothouse Earth pathway. This requires that humans take deliberate, integral, and adaptive steps to reduce dangerous impacts on the Earth System, effectively monitoring and changing behavior to form feedback loops that stabilize this intermediate state.
There is much uncertainty and debate about how this can be done—technically, ethically, equitably, and economically—and there is no doubt that the normative, policy, and institutional aspects are highly challenging. However, societies could take a wide range of actions that constitute negative feedbacks, summarized in SI Appendix , Table S5 , to steer the Earth System toward Stabilized Earth.
Some of these actions are already altering emission trajectories. The negative feedback actions fall into three broad categories: i reducing greenhouse gas emissions, ii enhancing or creating carbon sinks e. While reducing emissions is a priority, much more could be done to reduce direct human pressures on critical biomes that contribute to the regulation of the state of the Earth System through carbon sinks and moisture feedbacks, such as the Amazon and boreal forests Table 1 , and to build much more effective stewardship of the marine and terrestrial biospheres in general.
The present dominant socioeconomic system, however, is based on high-carbon economic growth and exploitative resource use 9.
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Attempts to modify this system have met with some success locally but little success globally in reducing greenhouse gas emissions or building more effective stewardship of the biosphere. Incremental linear changes to the present socioeconomic system are not enough to stabilize the Earth System. Widespread, rapid, and fundamental transformations will likely be required to reduce the risk of crossing the threshold and locking in the Hothouse Earth pathway; these include changes in behavior, technology and innovation, governance, and values 48 , 62 , Enhanced ambition will need new collectively shared values, principles, and frameworks as well as education to support such changes 67 , In essence, effective Earth System stewardship is an essential precondition for the prosperous development of human societies in a Stabilized Earth pathway 69 , In addition to institutional and social innovation at the global governance level, changes in demographics, consumption, behavior, attitudes, education, institutions, and socially embedded technologies are all important to maximize the chances of achieving a Stabilized Earth pathway Many of the needed shifts may take decades to have a globally aggregated impact SI Appendix , Table S5 , but there are indications that society may be reaching some important societal tipping points.
For example, there has been relatively rapid progress toward slowing or reversing population growth through declining fertility resulting from the empowerment of women, access to birth control technologies, expansion of educational opportunities, and rising income levels 72 , These demographic changes must be complemented by sustainable per capita consumption patterns, especially among the higher per capita consumers.
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Some changes in consumer behavior have been observed 74 , 75 , and opportunities for consequent major transitions in social norms over broad scales may arise Technological innovation is contributing to more rapid decarbonization and the possibility for removing CO 2 from the atmosphere Ultimately, the transformations necessary to achieve the Stabilized Earth pathway require a fundamental reorientation and restructuring of national and international institutions toward more effective governance at the Earth System level 77 , with a much stronger emphasis on planetary concerns in economic governance, global trade, investments and finance, and technological development Stabilized Earth will likely be warmer than any other time over the last , years at least 83 that is, warmer than at any other time in which fully modern humans have existed.
In addition, the Stabilized Earth trajectory will almost surely be characterized by the activation of some tipping elements Tipping Cascades and Fig. Current rates of change of important features of the Earth System already match or exceed those of abrupt geophysical events in the past SI Appendix. With these trends likely to continue for the next several decades at least, the contemporary way of guiding development founded on theories, tools, and beliefs of gradual or incremental change, with a focus on economy efficiency, will likely not be adequate to cope with this trajectory.
Thus, in addition to adaptation, increasing resilience will become a key strategy for navigating the future. Generic resilience-building strategies include developing insurance, buffers, redundancy, diversity, and other features of resilience that are critical for transforming human systems in the face of warming and possible surprise associated with tipping points Features of such a strategy include i maintenance of diversity, modularity, and redundancy; ii management of connectivity, openness, slow variables, and feedbacks; iii understanding social—ecological systems as complex adaptive systems, especially at the level of the Earth System as a whole 85 ; iv encouraging learning and experimentation; and v broadening of participation and building of trust to promote polycentric governance systems 86 , Our systems approach, focusing on feedbacks, tipping points, and nonlinear dynamics, has addressed the four questions posed in the Introduction.
Our analysis suggests that the Earth System may be approaching a planetary threshold that could lock in a continuing rapid pathway toward much hotter conditions—Hothouse Earth. This pathway would be propelled by strong, intrinsic, biogeophysical feedbacks difficult to influence by human actions, a pathway that could not be reversed, steered, or substantially slowed. The impacts of a Hothouse Earth pathway on human societies would likely be massive, sometimes abrupt, and undoubtedly disruptive.
Avoiding this threshold by creating a Stabilized Earth pathway can only be achieved and maintained by a coordinated, deliberate effort by human societies to manage our relationship with the rest of the Earth System, recognizing that humanity is an integral, interacting component of the system.