Trajectories of the Earth System in
Will Steffena,b,1, Johan RockstrÃ¶ma
, Katherine Richardsonc
, Timothy M. Lentond
, Carl Folkea,e, Diana Livermanf
Colin P. Summerhayesg
, Anthony D. Barnoskyh
, Sarah E. Cornella
, Michel Crucifixi,j, Jonathan F. Dongesa,k,
, Steven J. Ladea,b, Marten Schefferl
, Ricarda Winkelmannk,m, and Hans Joachim Schellnhubera,k,m,1
Edited by William C. Clark, Harvard University, Cambridge, MA, and approved July 6, 2018 (received for review June 19, 2018)
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 â€œHothouse Earthâ€ 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 Systemâ€”biosphere,
climate, and societiesâ€”and could include decarbonization of the global economy, enhancement of biosphere
carbon sinks, behavioral changes, technological innovations, new governance arrangements, and transformed social values.
Earth System trajectories | climate change | Anthropocene | biosphere feedbacks | tipping elements
The Anthropocene is a proposed new geological epoch (1) based on the observation that human impacts
on essential planetary processes have become so profound (2) that they have driven the Earth out of the
Holocene epoch in which agriculture, sedentary communities, and eventually, socially and technologically
complex human societies developed. The formalization of the Anthropocene as a new geological epoch is
being considered by the stratigraphic community (3),
but regardless of the outcome of that process, it is
becoming apparent that Anthropocene conditions
transgress Holocene conditions in several respects
(2). The knowledge that human activity now rivals geological forces in influencing the trajectory of the Earth
System has important implications for both Earth System science and societal decision making. While
recognizing 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 (4), the sum total of
human impacts on the system needs to be taken into
account for analyzing future trajectories of the
Here, we explore potential future trajectories of the
Earth System by addressing the following 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?
Stockholm Resilience Centre, Stockholm University, 10691 Stockholm, Sweden; b
Fenner School of Environment and Society, The Australian
National University, Canberra, ACT 2601, Australia; c
Center for Macroecology, Evolution, and Climate, University of Copenhagen, Natural History
Museum of Denmark, 2100 Copenhagen, Denmark; d
Earth System Science Group, College of Life and Environmental Sciences, University of Exeter,
EX4 4QE Exeter, United Kingdom; e
The Beijer Institute of Ecological Economics, The Royal Swedish Academy of Science, SE-10405 Stockholm, Sweden; f
School of Geography and Development, The University of Arizona, Tucson, AZ 85721; g
Scott Polar Research Institute, Cambridge University, CB2 1ER
Cambridge, United Kingdom; h
Jasper Ridge Biological Preserve, Stanford University, Stanford, CA 94305; i
Earth and Life Institute, Universit Â´e catholique de
Louvain, 1348 Louvain-la-Neuve, Belgium; j
Belgian National Fund of Scientific Research, 1000 Brussels, Belgium; k
Research Domain Earth System Analysis,
Potsdam Institute for Climate Impact Research, 14473 Potsdam, Germany; l
Department of Environmental Sciences, Wageningen University & Research,
6700AA Wageningen, The Netherlands; and mDepartment of Physics and Astronomy, University of Potsdam, 14469 Potsdam, Germany
Author contributions: W.S., J.R., K.R., T.M.L., C.F., D.L., C.P.S., A.D.B., S.E.C., M.C., J.F.D., I.F., S.J.L., M.S., R.W., and H.J.S. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Published under the PNAS license.
To whom correspondence may be addressed. Email: [email protected] or [email protected]
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1810141115/-/DCSupplemental.
www.pnas.org/cgi/doi/10.1073/pnas.1810141115 PNAS Latest Articles | 1 of 8
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?
Addressing these 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 (5). Integrating the requisite knowledge can be difficult, especially in light of the formidable range of
timescales involved. Increasingly, concepts from complex systems
analysis provide a framework that unites the diverse fields of inquiry relevant to the Anthropocene (6). 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.
Risk of a Hothouse Earth Pathway
Limit Cycles and Planetary Thresholds. 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). Fig. 1 shows a simplified
representation of complex Earth System dynamics, where the
physical climate system is subjected to the effects of slow changes
in Earthâ€™s orbit and inclination. Over the Late Quaternary (past
1.2 million years), the system has remained bounded between
glacial and interglacial extremes. Not every glacialâ€“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. 100,000-years oscillations between them in the Late Quaternary loosely constitute limit cycles
(technically, the asymptotic dynamics of ice ages are best modeled
as pullback attractors in a nonautonomous dynamical system). This
limit cycle is shown in a schematic fashion in blue in Fig. 1, Lower
Left using temperature and sea level as the axes. The Holocene is
represented by the top of the limit cycle loop near the label A.
The current position of the Earth System in the Anthropocene
is shown in Fig. 1, Upper Right by the small ball on the pathway
that leads away from the glacialâ€“interglacial limit cycle. In Fig. 2, a
stability landscape, the current position of the Earth System is
represented by the globe at the end of the solid arrow in the
deepening Anthropocene basin of attraction.
The Anthropocene represents the beginning of a very rapid
human-driven trajectory of the Earth System away from the glacialâ€“interglacial limit cycle toward new, hotter climatic conditions
and a profoundly different biosphere (2, 8, 9) (SI Appendix). The
current position, at over 1 Â°C above a preindustrial baseline (10), is
nearing the upper envelope of interglacial conditions over the
past 1.2 million years (SI Appendix, Table S1). More importantly,
the rapid trajectory of the climate system over the past halfcentury along with technological lock in and socioeconomic
inertia in human systems commit the climate system to conditions
beyond the envelope of past interglacial conditions. We, therefore, suggest that the Earth System may already have passed one
â€œfork in the roadâ€ of potential pathways, a bifurcation (near A in
Fig. 1) taking the Earth System out of the next glaciation cycle (11).
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 (14). 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 beyondâ€”with a presumed quasilinear relationship
between cumulative carbon dioxide emissions and global temperature rise (14). 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.
Fig. 1. A schematic illustration of possible future pathways of the
climate against the background of the typical glacialâ€“interglacial
cycles (Lower Left). The interglacial state of the Earth System is at the
top of the glacialâ€“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 âˆ¼2 Â°C above the
preindustrial level is also shown. The letters along the Stabilized Earth/
Hothouse Earth pathways represent four time periods in Earthâ€™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.
2 of 8 | www.pnas.org/cgi/doi/10.1073/pnas.1810141115 Steffen et al.
This risk is represented in Figs. 1 and 2 by a planetary threshold
(horizontal broken line in Fig. 1 on the Hothouse Earth pathway
around 2 Â°C above preindustrial temperature). Beyond this
threshold, intrinsic biogeophysical feedbacks in the Earth System
(Biogeophysical Feedbacks) could become the dominant processes controlling the systemâ€™s trajectory. Precisely where a potential planetary threshold might be is uncertain (15, 16). We
suggest 2 Â°C because of the risk that a 2 Â°C warming could activate important tipping elements (12, 17), raising the temperature
further to activate other tipping elements in a domino-like cascade that could take the Earth System to even higher temperatures (Tipping Cascades). Such cascades comprise, in essence, the
dynamical process that leads to thresholds in complex systems
(section 4.2 in ref. 18).
This analysis implies that, even if the Paris Accord target of a
1.5 Â°C to 2.0 Â°C rise in temperature is met, we cannot exclude the
risk that a cascade of feedbacks could push the Earth System
irreversibly onto a â€œHothouse Earthâ€ pathway. The challenge that
humanity faces is to create a â€œStabilized Earthâ€ pathway that steers
the Earth System away from its current trajectory toward the
threshold beyond which is Hothouse Earth (Fig. 2). The humancreated Stabilized Earth pathway leads to a basin of attraction
that is not likely to exist in the Earth Systemâ€™s stability landscape
without human stewardship to create and maintain it. 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
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).
Biogeophysical Feedbacks. 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). Some of the key negative feedbacks that
could maintain the Earth System in Holocene-like conditionsâ€”
notably, carbon uptake by land and ocean systemsâ€”are weakening
relative to human forcing (19), increasing the risk that positive
feedbacks could play an important role in determining the Earth
Systemâ€™s trajectory. 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.
Most of the feedbacks can show both continuous responses
and tipping point behavior in which the feedback process
becomes self-perpetuating after a critical threshold is crossed;
subsystems exhibiting this behavior are often called â€œtipping elementsâ€ (17). The type of behaviorâ€”continuous response or
tipping point/abrupt changeâ€”can depend on the magnitude or
the rate of forcing, or both. 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 50â€“100 years (e.g., change in
Arctic sea ice extent with a warming or cooling of the climate;
Antarctic sea ice may be less reversible because of heat accumulation in the Southern Ocean), but most changes are largely
irreversible on timeframes that matter to contemporary societies
(e.g., loss of permafrost carbon). A few of the feedbacks do not
have apparent thresholds (e.g., change in the land and ocean
physiological carbon sinks, such as increasing carbon uptake due
Table 1. Carbon cycle feedbacks in the Earth System that could accelerate global warming
Strength of feedback
by 2100,* Â°C
Refs. (SI Appendix, Table
S2 has more details)
Permafrost thawing 0.09 (0.04â€“0.16) 20â€“23
Relative weakening of land and ocean physiological C sinks 0.25 (0.13â€“0.37) 24
Increased bacterial respiration in the ocean 0.02 25, 26
Amazon forest dieback 0.05 (0.03â€“0.11) 27
Boreal forest dieback 0.06 (0.02â€“0.10) 28
Total 0.47 (0.24â€“0.66)
The strength of the feedback is estimated at 2100 for an âˆ¼2 Â°C warming.
*The additional temperature rise (degrees Celsius) by 2100 arising from the feedback.
Fig. 2. Stability landscape showing the pathway of the Earth System
out of the Holocene and thus, out of the glacialâ€“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 âˆ¼2 Â°C (horizontal broken line at 2 Â°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.
â€œStabilityâ€ (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.
Steffen et al. PNAS Latest Articles | 3 of 8
to the CO2 fertilization effect or decreasing uptake due to a decrease in rainfall). For some of the tipping elements, crossing the
tipping point could trigger an abrupt, nonlinear response (e.g.,
conversion of large areas of the Amazon rainforest to a savanna or
seasonally dry forest), while for others, crossing the tipping point
would lead to a more gradual but self-perpetuating response
(large-scale loss of permafrost). 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.g., Heinrich Events).
For some feedback processes, the magnitudeâ€”and even the
directionâ€”depend on the rate of climate change. If the rate of
climate change is small, the shift in biomes can track the change in
temperature/moisture, and the biomes may shift gradually, potentially taking up carbon from the atmosphere as the climate warms
and atmospheric CO2 concentration increases. 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.g.,
wildfires, insect attacks, droughts) that can abruptly remove an
existing biome. 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 (29).
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.
The so-called â€œgreeningâ€ of the planet, caused by enhanced
plant growth due to increasing atmospheric CO2 concentration
(31), has increased the land carbon sink in recent decades (32).
However, increasing atmospheric CO2 raises temperature, and
hotter leaves photosynthesize less well. Other feedbacks are also
involvedâ€”for instance, warming the soil increases microbial respiration, releasing CO2 back into the atmosphere.
Our analysis focuses on the strength of the feedback between
now and 2100. However, several of the feedbacks that show
negligible or very small magnitude by 2100 could nevertheless be
triggered well before then, and they could eventually generate
significant feedback strength over longer timeframesâ€”centuries
and even millenniaâ€”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 (33).
Tipping Cascades. Fig. 3 shows a global map of some potential
tipping cascades. The tipping elements fall into three clusters
based on their estimated threshold temperature (12, 17, 39).
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.g., gradual weakening of land and
ocean physiological carbon sinks), could push the global average
temperature even higher, inducing tipping in mid- and highertemperature clusters. 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 (41).
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. These
variations caused typical temperature response patterns called the
â€œbipolar seesawâ€ (42â€“44). 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 (45).
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, 47).
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.e., the Paris targets) could lead to tipping in the mid- and
higher-temperature clusters via cascade effects. Based on this
analysis of tipping cascades and taking a risk-averse approach, we
suggest that a potential planetary threshold could occur at a
temperature rise as low as âˆ¼2.0 Â°C above preindustrial (Fig. 1).
Alternative Stabilized Earth Pathway
If the worldâ€™s societies want to avoid crossing a potential threshold
that locks the Earth System into the Hothouse Earth pathway, then
it is critical that they make deliberate decisions to avoid this risk
Fig. 3. 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 Â°C, some marine-based sectors in East Antarctica may
be vulnerable at lower temperatures (35â€“38).
4 of 8 | www.pnas.org/cgi/doi/10.1073/pnas.1810141115 Steffen et al.
and maintain the Earth System in Holocene-like conditions. This
human-created pathway is represented in Figs. 1 and 2 by what
we call Stabilized Earth (small loop at the bottom of Fig. 1, Upper
Right), in which the Earth System is maintained in a state with a
temperature rise no greater than 2 Â°C above preindustrial (a
â€œsuper-Holoceneâ€ state) (11). 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 (48). The short broken red
line beyond Stabilized Earth in Fig. 1, Upper Right represents a potential return to interglacial-like conditions in the longer term.
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 glacialâ€“interglacial limit cycle of the Late
Quaternary and a Hothouse Earth (Fig. 2). 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. Beyond the threshold, positive (reinforcing)
feedbacks within the Earth Systemâ€”outside of human influence or
controlâ€”could become the dominant driver of the systemâ€™s pathway,
as individual tipping elements create linked cascades through time
and with rising temperature (Fig. 3). 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. 2.
What Is at Stake? 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.
Insights into the risks posed by the rapid climatic changes
emerging in the Anthropocene can be obtained not only from
contemporary observations (51â€“55) but also, from interactions in
the past between human societies and regional and seasonal
hydroclimate variability. This variability was often much more
pronounced than global, longer-term Holocene variability (SI
Appendix). Agricultural production and water supplies are especially vulnerable to changes in the hydroclimate, leading to hot/
dry or cool/wet extremes. Societal declines, collapses, migrations/
resettlements, reorganizations, and cultural changes were often
associated with severe regional droughts and with the global
megadrought at 4.2â€“3.9 thousand years before present, all occurring within the relative stability of the narrow global Holocene
temperature range of approximately Â±1 Â°C (56).
SI Appendix, Table S4 summarizes biomes and regional biosphereâ€“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 CO2 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 (57).
The worldâ€™s coastal zones, especially low-lying deltas and the
adjacent coastal seas and ecosystems, are particularly important
for human wellbeing. These areas are home to much of the worldâ€™s
population, most of the emerging megacities, and a significant
amount of infrastructure vital for both national economies and international trade. 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 (58).
Human Feedbacks in the Earth System. 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.
Humanityâ€™s challenge then is to influence the dynamical
properties of the Earth System in such a way that the emerging
unstable conditions in the zone between the Holocene and a very
hot state become a de facto stable intermediate state (Stabilized
Earth) (Fig. 2). 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
doneâ€”technically, ethically, equitably, and economicallyâ€”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.g., protecting and enhancing biosphere carbon sinks and creating new types of sinks) (59), and (iii)
modifying Earthâ€™s energy balance (for example, via solar radiation
management, although that particular feedback entails very large risks
of destabilization or degradation of several key processes in the Earth
System) (60, 61). 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). Attempts to modify this system have met with some
Steffen et al. PNAS Latest Articles | 5 of 8
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, 63).
International efforts to reduce human impacts on the Earth
System while improving wellbeing include the United Nations
Sustainable Development Goals and the commitment in the Paris
agreement to keep warming below 2 Â°C. These international
governance initiatives are matched by carbon reduction commitments by countries, cities, businesses, and individuals (64â€“66) ,
but as yet, these are not enough to meet the Paris target. Enhanced ambition will need new collectively shared values, principles, and frameworks as well as education to support such
changes (67, 68). In essence, effective Earth System stewardship is
an essential precondition for the prosperous development of
human societies in a Stabilized Earth pathway (69, 70).
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 (71). 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, 73). These demographic changes must be
complemented by sustainable per capita consumption patterns,
especially among the higher per capita consumers. Some changes
in consumer behavior have been observed (74, 75), and opportunities for consequent major transitions in social norms over
broad scales may arise (76). Technological innovation is contributing to more rapid decarbonization and the possibility for removing CO2 from the atmosphere (48).
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 (78).
Building Resilience in a Rapidly Changing Earth System. Even if
a Stabilized Earth pathway is achieved, humanity will face a turbulent road of rapid and profound changes and uncertainties on
route to itâ€”politically, socially, and environmentallyâ€”that challenge the resilience of human societies (79â€“82). Stabilized Earth
will likely be warmer than any other time over the last 800,000 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. 3) and by nonlinear dynamics and abrupt
shifts at the level of critical biomes that support humanity (SI
Appendix, Table S4). 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 (84). Features of such a strategy include (i) maintenance of
diversity, modularity, and redundancy; (ii) management of connectivity, openness, slow variables, and feedbacks; (iii) understanding socialâ€“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, 87).
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 conditionsâ€”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 âˆ¼2.0 Â°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 (88).
How credible is this analysis? There is significant evidence from
a number of sources that the risk of a planetary threshold and thus,
the need to create a divergent pathway should be taken seriously:
First, the complex system behavior of the Earth System in the
Late Quaternary is well-documented and understood. The two
bounding states of the systemâ€”glacial and interglacialâ€”are
reasonably well-defined, the ca. 100,000-years periodicity of the
limit cycle is established, and internal (carbon cycle and ice albedo
feedbacks) and external (changes in insolation caused by changes
in Earthâ€™s orbital parameters) driving processes are generally wellknown. Furthermore, we know with high confidence that the
progressive disintegration of ice sheets and the transgression of
other tipping elements are difficult to reverse after critical levels of
warming are reached.
Second, insights from Earthâ€™s recent geological past (SI Appendix) suggest that conditions consistent with the Hothouse
Earth pathway are accessible with levels of atmospheric CO2
concentration and temperature rise either already realized or
projected for this century (SI Appendix, Table S1).
6 of 8 | www.pnas.org/cgi/doi/10.1073/pnas.1810141115 Steffen et al.
Third, the tipping elements and feedback processes that
operated over Quaternary glacialâ€“interglacial cycles are
the same as several of those proposed as critical for the future trajectory of the Earth System (Biogeophysical Feedbacks, Tipping Cascades, Fig. 3, Table 1, and SI Appendix,
Fourth, contemporary observations (29, 38) (SI Appendix) of
tipping element behavior at an observed temperature anomaly of
about 1 Â°C above preindustrial suggest that some of these elements are vulnerable to tipping within just a 1 Â°C to 3 Â°C increase
in global temperature, with many more of them vulnerable at
higher temperatures (Biogeophysical Feedbacks and Tipping
Cascades) (12, 17, 39). This suggests that the risk of tipping cascades could be significant at a 2 Â°C temperature rise and could
increase sharply beyond that point. We argue that a planetary
threshold in the Earth System could exist at a temperature rise as
low as 2 Â°C above preindustrial.
The Stabilized Earth trajectory requires deliberate management of humanityâ€™s relationship with the rest of the Earth System if
the world is to avoid crossing a planetary threshold. We suggest
that a deep transformation based on a fundamental reorientation
of human values, equity, behavior, institutions, economies, and
technologies is required. Even so, the pathway toward Stabilized
Earth will involve considerable changes to the structure and functioning of the Earth System, suggesting that resilience-building
strategies be given much higher priority than at present in decision
making. Some signs are emerging that societies are initiating some of
the necessary transformations. However, these transformations are
still in initial stages, and the social/political tipping points that
definitively move the current trajectory away from Hothouse Earth
have not yet been crossed, while the door to the Stabilized Earth
pathway may be rapidly closing.
Our initial analysis here needs to be underpinned by more indepth, quantitative Earth System analysis and modeling studies to
address three critical questions. (i) Is humanity at risk for pushing
the system across a planetary threshold and irreversibly down a
Hothouse Earth pathway? (ii) What other pathways might be possible in the complex stability landscape of the Earth System, and
what risks might they entail? (iii) What planetary stewardship strategies are required to maintain the Earth System in a manageable
Stabilized Earth state?
We thank the three reviewers for their comments on the first version of the
manuscript and two of the reviewers for further comments on a revised version
of the manuscript. These comments were very helpful in the revisions. We thank
a member of the PNAS editorial board for a comprehensive and very helpful
review. W.S. and C.P.S. are members of the Anthropocene Working Group.
W.S., J.R., K.R., S.E.C., J.F.D., I.F., S.J.L., R.W. and H.J.S. are members of the
Planetary Boundaries Research Network PB.net and the Earth Leagueâ€™s EarthDoc
Programme supported by the Stordalen Foundation. T.M.L. was supported by
a Royal Society Wolfson Research Merit Award and the European Union
Framework Programme 7 Project HELIX. C.F. was supported by the Erlingâ€“
Persson Family Foundation. The participation of D.L. was supported by the
Haury Program in Environment and Social Justice and National Science
Foundation (USA) Decadal and Regional Climate Prediction using Earth
System Models Grant 1243125. S.E.C. was supported in part by Swedish Research Council Formas Grant 2012-742. J.F.D. and R.W. were supported by
Leibniz Association Project DOMINOES. S.J.L. receives funding from Formas
Grant 2014-589. This paper is a contribution to European Research Council
Advanced Grant 2016, Earth Resilience in the Anthropocene Project 743080.
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