Trajectories of the Earth System in the Anthropocene. Will Steffen, Johan Rockström, Timoth Lenton, Hans Joachim Schellnhuber, et al., PNAS. Aug. 6, 2018.
Abstract
We explore the risk that self-reinforcing feedbacks could push the Earth System toward a planetary threshold that, if crossed, could prevent stabilization of the climate at intermediate temperature rises and cause continued warming on a “Hothouse Earth” pathway even as human emissions are reduced. Crossing the threshold would lead to a much higher global average temperature than any interglacial in the past 1.2 million years and to sea levels significantly higher than at any time in the Holocene. We examine the evidence that such a threshold might exist and where it might be. If the threshold is crossed, the resulting trajectory would likely cause serious disruptions to ecosystems, society, and economies. Collective human action is required to steer the Earth System away from a potential threshold and stabilize it in a habitable interglacial-like state. Such action entails stewardship of the entire Earth System—biosphere, climate, and societies—and could include decarbonization of the global economy, enhancement of biosphere carbon sinks, behavioral changes, technological innovations, new governance arrangements, and transformed social values.
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. The proposed planetary threshold at ∼2 °C above the preindustrial level is also shown. The letters along the Stabilized Earth/Hothouse Earth pathways represent four time periods in Earth’s recent past that may give insights into positions along these pathways (SI Appendix): A, Mid-Holocene; B, Eemian; C, Mid-Pliocene; and D, Mid-Miocene. Their positions on the pathway are approximate only. Their temperature ranges relative to preindustrial are given in SI Appendix, Table S1.
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. 1 is shown here as the two divergent pathways of the Earth System in the future (broken arrows). Currently, the Earth System is on a Hothouse Earth pathway driven by human emissions of greenhouse gases and biosphere degradation toward a planetary threshold at ∼2 °C (horizontal broken line at 2 °C in Fig. 1), beyond which the system follows an essentially irreversible pathway driven by intrinsic biogeophysical feedbacks. 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. “Stability” (vertical axis) is defined here as the inverse of the potential energy of the system. 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.
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, 34). Arrows show the potential interactions among the tipping elements based on expert elicitation that could generate cascades. Note that, although the risk for tipping (loss of) the East Antarctic Ice Sheet is proposed at >5 °C, some marine-based sectors in East Antarctica may be vulnerable at lower temperatures (35⇓⇓–38).
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Conclusion
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.
Where such a threshold might be is uncertain, but it could be only decades ahead at a temperature rise of ∼2.0 °C above preindustrial, and thus, it could be within the range of the Paris Accord temperature targets.
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. Humanity is now facing the need for critical decisions and actions that could influence our future for centuries, if not millennia.
reviewed by Ian Angus. Aug. 12, 2018.
Scientific papers don’t often make front page news, but this one certainly did.
On August 6, the UK Guardian declared that a “Domino-effect of climate events could move Earth into a ‘hothouse’ state.” The New York Times warned of a “World at risk of heading towards irreversible ‘hothouse’ state.” Sky News said that “Earth is 1°C away from hothouse state that threatens the future of humanity.”
The basis for those excited headlines was an article with the distinctly unexciting title “Trajectories of the Earth System in the Anthropocene,” published in the Proceedings of the National Academy of Sciences. Normally, PNAS articles can only be read by those who pay high subscription fees, but interest in this one ran so high that after one day the publisher removed the paywall, making it accessible to all.
For once — rarely for a climate change story — the mainstream media was right to focus attention on this paper. The authors, a virtual who’s who of the world’s most respected experts on the Anthropocene and Earth System Science, make a major contribution to our understanding of the planetary emergency. They extend the discussion of global warming beyond the usual narrow focus on greenhouse emissions, incorporating the complex cycles and feedbacks that shape the entire Earth System.
This is global warming in the context of the Anthropocene, the epoch they define as “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.”
(It’s important to note that, contrary to charges that Anthropocene science blames humanity in general for environmental problems, this paper explicitly recognizes “that different societies around the world have contributed differently and unequally to pressures on the Earth System and will have varied capabilities to alter future trajectories,” and that “the wealthiest one billion people produce 60% of GHGs [Greenhouse Gases] whereas the poorest three billion produce only 5%.”)
Tipping points and planetary thresholds
With a few exceptions, scientific discussion of climate change has tended to focus on how warm the world will get at various CO2 concentration levels, or on what conditions may be like if a given temperature is reached, or on how to slow or stop greenhouse gas emissions. Other Earth System processes are played down or omitted entirely.
The authors of this paper, in contrast, argue that “feedback processes within the Earth System coupled with direct human degradation of the biospheremayplay a more important role than normally assumed.” In that context they ask four questions.
- “Is there a planetary threshold in the trajectory of the Earth System that, if crossed, could prevent stabilization in a range of intermediate temperature rises?”
- “Given our understanding of geophysical and biosphere feedbacks intrinsic to the Earth System, where might such a threshold be?”
- “If a threshold is crossed, what are the implications, especially for the wellbeing of human societies?”
- “What human actions could create a pathway that would steer the Earth System away from the potential threshold and toward the maintenance of interglacial-like conditions?”
The long-term evolution of the Earth System is influenced by a multitude of cycles and feedbacks that weaken or amplify climate changes, by controlling the movement of matter and energy in the oceans, soil, and atmosphere. As the Earth warms, positive (amplifying) feedbacks are becoming stronger: the authors identify ten that have global impacts and that could be radically accelerated by relatively small temperature increases, including thawing permafrost, release of ocean floor methane hydrates, weakened land and oceanic CO2absorption, increasing bacterial respiration in the oceans, dieback of Amazon and/or boreal forests, reduced northern snow cover, loss of Arctic and/or Antarctic sea ice, and melting of polar ice sheets.
Any one of these could substantially accelerate global warming, and if one passes a tipping point, it may trigger a “tipping cascade,” permanently accelerating others.
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The authors do not say that Hothouse Earth is inevitable, or that any of these tipping points and cascades are certain to happen at any particular time or speed: the extreme complexity of the Earth System makes such predictions impossible. However, the evidence of past climate shifts indicates any of them could occur at temperatures and CO2 concentrations that are likely to be reached in this century if business as usual continues — and some could occur before 2040.
“Current rates of human-driven changes far exceed the rates of change driven by geophysical or biosphere forces that have altered the Earth System trajectory in the past; even abrupt geophysical events do not approach current rates of human-driven change.
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In terms of their influence on the carbon cycle and climate, the human-driven changes of the Anthropocene are beginning to match or exceed the rates of change that drove past, relatively sudden mass extinction events, and are essentially irreversible.”
Continuing business as usualc[w]ould lock us into a trajectory for Hothouse Earth, and the point of no return, beyond which stabilization will be impossible, may be reached when the average global temperature rises to 2.0°C above the pre-industrial level. In fact, “even if the Paris Accord target of a 1.5°C to 2.0°C rise in temperature is met, we cannot exclude the risk that a cascade of feedbacks could push the Earth System irreversibly onto a ‘Hothouse Earth’ pathway.”
Maybe we’ll get lucky, but don’t bet on it.
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However, the authors argue, there is still time to shift to an “Alternative Stabilized Earth Pathway,” but only if radical changes are made in society’s relationship with the rest of the Earth System.
“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. … 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.”
They stress that getting to Stabilized Earth will involve taking “a turbulent road of rapid and profound changes and uncertainties … that challenge the resilience of human societies,” and that even then, it will not return to Holocene conditions. “Stabilized Earth will likely be warmer than any other time over the last 800,000 years at least (that is, warmer than at any other time in which fully modern humans have existed).”
We are, in short, at a fork in the road. “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.”
What is to be done?
In addressing the fourth question — what can be done to stabilize the Earth System? — the authors challenge the incremental reformism of liberal greens and most environmental NGOs. They point out that “the present dominant socioeconomic system … is based on high-carbon economic growth and exploitative resource use,” and that attempts to reform it have been unsuccessful.
“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.
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“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.
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“To avoid crossing a planetary threshold … a deep transformation based on a fundamental reorientation of human values, equity, behavior, institutions, economies, and technologies is required.”
Although stated in very general terms, these points have deeply radical implications. These Earth System scientists have clearly concluded only system change can stop climate change. This reconfirms the argument I have made before, that “the possibility of a powerful science-based challenge to the present social order is opening before us.”[5]
However, the real test of any policy framework for climate change is in the concrete actions it advocates, and that is the weakest part of this paper.
The authors say that achieving a Stabilized Earth would require “deep cuts in greenhouse gas emissions, protection and enhancement of biosphere carbon sinks, efforts to remove CO2 from the atmosphere, possibly solar radiation management, and adaptation to unavoidable impacts of the warming already occurring,” but they offer no plan for implementing such measures. A table in the “Supporting Information” identifies seventeen “Human actions that could steer the Earth System onto a ‘Stabilized Earth’,” but it is not a program for action. For example, it includes “Replacement of fossil fuels with low or zero emission energy sources” as a goal, but is silent on actually reducing emissions by concrete measures such as shutting down coal-fired generators, banning fracking and tar sands mining, and blocking pipeline construction.
A “deep transformation” will not be achieved without breaking the power of the fossil fuel industry, so it is disappointing that there is not even a nod in that direction in this important paper.
On the other hand, it is encouraging that although their list includes some forms of geoengineering, they point out that ocean fertilization has uncertain effects and may cause dead zones, that bio-energy with carbon capture and storage (BECSS) is not economically feasible and would compete with food production, and that blocking solar radiation “entails very large risks of destabilization or degradation of several key processes in the Earth System.” There is no support here for blind ecomodernist faith in technological silver bullets.
Early in the paper, the authors say that answering their four questions “requires a deep integration of knowledge from biogeophysical Earth System science with that from the social sciences and humanities on the development and functioning of human societies.”
The absence of a concrete program for change suggests the integration with social sciences and humanities is still more wish than reality, and their list of references indicates that integration hasn’t gone beyond consulting a few academic papers.
In fairness, the authors describe their paper as an “initial analysis” that needs to be strengthened by “more in-depth, quantitative Earth System analysis and modeling studies.” As they move forward, I hope their research will extend beyond liberal sociology and economics, and consider the concrete programs advanced in such books as This Changes Everything by Naomi Klein, Red-Green Revolution by Victor Wallis, and Creating an Ecological Society by Fred Magdoff and Chris Williams. Earth System Science and an ecosocialist program would be a powerful combination!
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“Trajectories of the Earth System in the Anthropocene” is a powerful and convincing argument for rapid and radical action. Its weaknesses should not distract us from recognizing it as an important contribution that should inform all serious efforts to understand and respond to the global crisis. By firmly placing climate change in the context of the Anthropocene and Earth System Science, it breaks from the dominant view that global warming is a linear process that can be solved by market reforms. Incremental measures like carbon pricing cannot address the systemic problems that are relentlessly driving Earth’s temperature upward.
If an irrevocable trajectory to Hothouse Earth is even possible — and this paper shows that it is — then decisive counter-measures must be at the top of the agenda for everyone who is concerned about humanity’s future.
"Hothouse Earth" Co-Author: the problem is neoliberal economics. Kate Aronoff, The Intercept. Aug. 14, 2018.
By shifting to a “wartime footing” to drive a rapid shift toward renewable energy and electrification, humanity can still avoid the apocalyptic future laid out in the much-discussed “hothouse earth” paper, a lead author of the paper told The Intercept. One of the biggest barriers to averting catastrophe, he said, has more to do with economics than science.
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Asked what could be done to prevent a hothouse earth scenario, co-author Will Steffen told The Intercept that the “obvious thing we have to do is to get greenhouse gas emissions down as fast as we can. That means that has to be the primary target of policy and economics. You have got to get away from the so-called neoliberal economics.” Instead, he suggests something “more like wartime footing” to roll out renewable energy and dramatically reimagine sectors like transportation and agriculture “at very fast rates.”
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“We need to immediately stop deforestation of the Amazon rainforest and other tropical forests, and start reforesting them. That means a U-turn in terms of how we operate the world’s economic systems,” Steffen told me via Skype. “The only way you’re going to change that is if you actually change value systems, perhaps even changing the way political systems operate and so on. The social scientists in our group have said this really is a fundamental change in human societies we need to have if we’re going to solve this problem.”
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For high-emitting countries like the U.S., Steffen says the first step to avoiding planetary apocalypse is basically self-evident: “absolutely no new fossil fuel developments. None. That means no new coal mines, no new oil wells, no new gas fields, no new unconventional gas fracking. Nothing new. Second, you need to have a rapid phase-out plan for existing fossil fuels,” starting with coal, he says.
Many of the solutions to climate change, Steffen and his co-authors argue, already exist and are starting to work; the appendix to their paper lists out several such measures. “It’s not that the solutions aren’t there. It’s that we don’t have the economic and policy setting right to really ramp those up,” he said. The main constraints on action are “our value systems, politics, and legal systems,” Steffen told me, adding that taking climate change seriously also means taking “a completely different view of economics, going away from viewing the natural world as resources to viewing it as an essential piece of our life support system that needs to be maintained and enhanced.
“I think you simply have to go right back to the fundamental science of who we are, the planet we evolved into, how that planet operates and what’s happening to it,” Steffen maintains, “and that will tell you immediately that so-called neoliberal economics is radically wrong in terms of how it views the rest of the world.”
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