| Figure 1 Potential tipping cascades Individual tipping elements are color coded according to estimated temperature thresholds Arrows show potential interactions PNAS | MR Online

Climate change in the Anthropocene: an unstoppable drive to Hothouse Earth?

Originally published: Climate & Capitalism on August 12, 2018 (more by Climate & Capitalism)  |

Can the global climate be stabilized before runaway change creates conditions that are too hot for human civilization and deadly for most species?

Leading Earth System scientists warn: “The Earth System may be approaching a planetary threshold that could lock in a continuing rapid pathway toward much hotter conditions.… Incremental linear changes to the present socioeconomic system are not enough to stabilize the Earth System.” From Will Steffen et al., “Trajectories of the Earth System in the Anthropocene,” Proceedings of the National Academy of Sciences, August 6, 2018.(1)

| Climate change in the Anthropocene An unstoppable drive to Hothouse Earth | MR OnlineScientific 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.”(2)

(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.(3)

The authors of this paper, in contrast, argue that “feedback processes within the Earth System coupled with direct human degradation of the biosphere may play a more important role than normally assumed.” In that context they ask four questions.

  1. “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?”
  2. “Given our understanding of geophysical and biosphere feedbacks intrinsic to the Earth System, where might such a threshold be?”
  3. “If a threshold is crossed, what are the implications, especially for the wellbeing of human societies?”
  4. “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 CO2 absorption, 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.

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.

Figure 1, below, shows potential tipping cascades identified in this paper.

| Figure 1 Potential tipping cascades Individual tipping elements are color coded according to estimated temperature thresholds Arrows show potential interactions PNAS | MR Online

Figure 1- Potential tipping cascades. Individual tipping elements are color-coded according to estimated temperature thresholds. Arrows show potential interactions. (PNAS)

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…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 usual could 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.

| Possible pathways A simplified representation of complex Earth System dynamics PNAS | MR Online

Possible pathways. A simplified representation of complex Earth System dynamics. (PNAS)

A fork in the road

For over a million years, Earth’s climate has oscillated between glacial and inter-glacial states, in the 100,000 year cycle shown schematically in the lower left quadrant of Figure 2. Underlying that pattern are long-term shifts in Earth‘s orbit and axis known as Milankovic cycles.(4) If they still had decisive influence on our climate, we would now be heading back into a glacial age, but in the past two centuries their impact has been overridden by greenhouse gas concentrations far greater than any seen during the ice ages.

The comparatively warm and stable Holocene, indicated by Ⓐ, began 11,000 years ago. Earth has moved out of that epoch, as indicated by the dark circle, and is approaching conditions that prevailed in Ⓑ the warmest part of the Eemian inter-glacial period, over 120,000 years ago. The Holocene is behind us, and there is little chance of turning back.

If business as usual continues, the Earth will be irrevocably committed to conditions like those that prevailed millions of years ago in Ⓒ the mid-Pliocene, or Ⓓ the mid-Miocene—“conditions that would be inhospitable to current human societies and to many other contemporary species.”

Contrary to some media reports, the authors don’t say that Hothouse Earth is an immediate prospect. Indeed, it will take centuries for the full impact of some large-scale Earth System processes to be fully felt. Their concern is that once Earth is committed to the hothouse trajectory, it will be irreversible, and the point of no return may be passed soon.

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 (especially for the most climate vulnerable), and ultimately, the habitability of the planet for humans.

At a minimum, the path to Hothouse conditions would involve flooding of coastal areas and “a substantial overall decrease in agricultural production, increased prices, and even more disparity between wealthy and poor countries” Heat alone would make large parts of the planet uninhabitable.

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…

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.…

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!

+ + + + + +

“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.


  1. Will Steffen, Johan Rockström, Katherine Richardson, Timothy M. Lenton, Carl Folke, Diana Liverman, Colin P. Summerhayes, Anthony D. Barnosky, Sarah E. Cornell, Michel Crucifix, Jonathan F. Donges, Ingo Fetzer, Steven J. Lade, Marten Scheffer, Ricarda Winkelmann, Hans Joachim Schellnhuber. “Trajectories of the Earth System in the Anthropocene.” Proceedings of the National Academy of Sciences (PNAS). Published ahead of print, August 6, 2018. Except as noted, all direct quotations are from this article or the associated “Supporting Information” that was published at the same time.
  2. That definition should be a wake-up call for leftish academics who dismiss Anthropocene science as just a discussion about stratigraphy, and so irrelevant to their world-ecology theories.
  3. A recent exception was the chapter “Potential Surprises: Compound Extremes and Tipping Elements,” in the U.S. government’s 2017 Climate Science Special Report. Unfortunately it was buried in a 477-page document that the media ignored and the Trump administration did not publicize.
  4. Ian Angus, Facing the Anthropocene (Monthly Review Press, 2016), 61-2.
  5. Ian Angus, A Redder Shade of Green (Monthly Review Press, 2017), 84.