These temperatures represent Earth systems evolutionary boundaries that once exceeded, result in collapse of those systems so that new species and mechanisms can evolve that are tolerant of the new conditions.  Once a system enters into collapse it does not self-restore unless the perturbation to the system that caused the collapse is removed.

Self-fulfilling Tipping and Cascading Collapses

Because we have now warmed beyond the evolution of Earth systems, many of them are in degradation collapse. The collapses to biologic systems can be caused by chronic stressors such as water stress, or acute events such as fire or insect attack. Collapse other than acute events can be as short as a few decades or years once triggered. Most species in a system succumb simultaneously as they all evolved together. Tipping can also initiate system evolutionary feedbacks that then causes collapse. Physical systems like sea level rise are not immune where beaches erode, barrier islands disintegrate and wetlands are submerged faster than they can self-regenerate or colonize new land.

Ecosystem Services and Evolutionary Boundary Exceedance

When a system collapses, its environmental services are at first degraded, then eliminated and often reversed. These services are things like forest and agricultural production, a stable hydrologic cycle for potable and irrigation water, a stable sea level where much of our industry, commerce and occupied lands are located, stable oceans and forests for sequestration, oxygen generation, and biomass production, and stable ocean circulation to provide consistent temperatures over land and critical heat and carbon burial in the abyss. For 32 years (1981 to 2013) James Hansen directed the NASA Goddard Institute for Space Studies, the de facto US climate modeling agency. His work on a safe 350 ppm CO2 limit focused on restoring our climate back to within the evolutionary boundaries of our former climate, so as to prevent loss and reversal of environmental services where our advanced civilization evolved, and preserve life as we know it.

Over half of global GDP is dependent upon intact ecologies. These ecologies have been and continue to be impacted because climate change is faster than natural and social adaptive processes; our advanced civilization has not encountered climate change during its evolution; and the approach toward limits in Earth systems is not well developed in our climate culture.

The Point of No Return

The term “point of no return” (PONR) was first used by James Hansen in 2008 to describe that point in a system collapse where even if the thing that caused the system to collapse is removed, the system can no longer self-restore or even be restored. This is his rationale for the 350 ppm CO2 atmospheric limit, which he defines as the upper boundaries of the Holocene. i.e. the climate where our Earth systems and advanced human civilization evolved over about the last 10,000 years. This is simple tenth grade science where, once a collapse begins it does not spontaneously stabilize and complete collapse fulfillment is guaranteed unless the evolutionary boundary conditions of the system are restored. Different systems have different collapse or degradation activation thresholds (tipping activation thresholds) but generally they all lie within the boundaries of our former climate where warming above 1 C causes collapses to initiate.

Systems collapses and climate tipping responses create feedbacks that increase the speed and extremeness of climate change. When a forest is burned, a dramatic ecological change takes place from a cool moisture laden ecology to a dry barren one. In our old climate, forest species were adapted to regenerating in the resulting dry heat of a burned forest. With climate change, the dryness and heat are compounded from warming that creates nonlinearly more evaporation. These conditions are now limiting forest regeneration after fire, and likely after beetle and insect mortality events as well. Across the American West, a third of forests  are not regenerating after fires that have burned since 2000, and half of the remaining burns are regenerating at half the 20th century rate.

Supporting Literature

Below is a sampling of critical literature about Earth systems tipping processes, systems at risk, effects of tipping, and responses that have been activated. 

November 27, 2024 – Polar ice is at or has exceeded the tipping point… “At a global warming level of 1.5  °C, neglecting the polar ice sheets can alter the expected number of tipped elements by more than a factor of 2. This is concerning as overshooting 1.5°C of global warming is becoming inevitable, while current state-of-the-art IPCC-type models do not (yet) include dynamic ice sheets.”
Rosser et al., Polar ice sheets are decisive contributors to uncertainty in climate tipping projections, Nature Communications, Earth and Environment, November 27, 2024.
https://esd.copernicus.org/articles/15/41/2024/esd-15-41-2024.pdf

January 26, 2024 – Review of tipping cascades and feedback literature, 31 authors… “We conclude that tipping elements should not only be studied in isolation, but also more emphasis has to be put on potential interactions. This means that tipping cascades cannot be ruled out on centennial to millennial timescales at global warming levels between 1.5 and 2.0  C or on shorter timescales if global warming surpassed 2.0 C.”TIPPING DEFINITION  – “In the context of this paper, we refer to a tipping element as any climate subsystem that shows threshold behavior (at its so-called tipping point) beyond which self amplifying feedbacks to forcing reorganize the system qualitatively (e.g., from an ice-covered to an ice-free state in Greenland). This means that, at the tipping point, a small shift in the background climate can trigger a large-scale qualitative system change. Once the tipping element is triggered, its self-amplifying feedbacks dominate the dynamics of the tipping element during the tipping process. This definition is taken from Levermann et al. (2012) and includes large-scale climate tipping elements such as the AMOC (Weijer et al., 2019) and polar ice sheets (Rosier et al., 2021), where the associated feedbacks (e.g., salt–advection, melt–elevation, or ice–albedo) are well known. Our definition also includes more regional bistabilities between savanna and forest vegetation in the Amazon region. In addition, we also consider elements that can show nonlinear behavior but it is speculated whether they should be considered tipping elements (Armstrong McKay et al., 2022).”Wunderling et al., Climate tipping point interactions and cascades: a review, Earth Systems Dynamics, January 26, 2024.
https://esd.copernicus.org/articles/15/41/2024/esd-15-41-2024.pdf

January 16, 2024 – 1.5°C is a physical limit: Here’s why this target can’t be negotiated, World Economic Forum… “1.5°C is a physical limit beyond which Earth systems enter a danger zone of cascading climate tipping points that propel further warming… Scientists have long warned that 1.5°C is a physical limit, not a political target. Tipping points are critical thresholds beyond which a system reorganises, often abruptly and/or irreversibly, according to the IPCC. Breaching 1.5°C has a domino effect – triggering critical changes in Earth systems that reinforce rather than reduce warming – with cascading consequences for economies and societies.”
1.5°C is a physical limit: Here’s why this target can’t be negotiated, World Economic Forum review, January 16, 2024.
https://www.weforum.org/stories/2024/01/earth-systems-tipping-points-1-5-degrees-celsius/

October 18, 2023 – Greenland tipping 1.7 °C and 2.3 °C… Abstract – “Melting of the Greenland ice sheet (GrIS) in response to anthropogenic global warming poses a severe threat in terms of global sea-level rise (SLR)1. Modelling and palaeoclimate evidence suggest that rapidly increasing temperatures in the Arctic can trigger positive feedback mechanisms for the GrIS, leading to self-sustained melting2–4, and the GrIS has been shown to permit several stable states5. Critical transitions are expected when the global mean temperature (GMT) crosses specific thresholds, with substantial hysteresis between the stable states6. Here we use two independent ice-sheet models to investigate the impact of different overshoot scenarios with varying peak and convergence temperatures for a broad range of warming and subsequent cooling rates. Our results show that the maximum GMT and the time span of overshooting given GMT targets are critical in determining GrIS stability. We find a threshold GMT between 1.7 °C and 2.3 °C above preindustrial levels for an abrupt ice-sheet loss. GrIS loss can be substantially mitigated, even for maximum GMTs of 6 °C or more above preindustrial levels, if the GMT is subsequently reduced to less than 1.5 °C above preindustrial levels within a few centuries. However, our results also show that even temporarily overshooting the temperature threshold, without a transition to a new ice-sheet state, still leads to a peak in SLR of up to several metres.
Bochow et al., Overshooting the critical threshold for the Greenland ice sheet, Nature, October 18, 2023.
https://www.nature.com/articles/s41586-023-06503-9

Willcock 2023 – Compound and cascading tipping drivers… “A major concern for the world’s ecosystems is the possibility of collapse, where landscapes and the societies they support change abruptly. Accelerating stress levels, increasing frequencies of extreme events and strengthening intersystem connections suggest that conventional modelling approaches based on incremental changes in a single stress may provide poor estimates of the impact of climate and human activities on ecosystems. We conduct experiments on four models that simulate abrupt changes in the Chilika lagoon fishery, the Easter Island community, forest dieback and lake water quality—representing ecosystems with a range of anthropogenic interactions. Collapses occur sooner under increasing levels of primary stress but additional stresses and/or the inclusion of noise in all four models bring the collapses substantially closer to today by ~38–81%.”
Willcock et al., Earlier collapse of Anthropocene ecosystems driven by multiple faster and noisier drivers, Nature Sustainability, June 22, 2023.
https://www.nature.com/articles/s41893-023-01157-x

Ritchie 2023 – Rate dependent tipping can create a point of no return faster than steady state forcing… This work describes an accelerated forcing rate that is faster than the classic steady-state forcing, that can create irreversible tipping response before the classic tipping projection time. It works in both biologic, natural and mechanical systems. Biologic and most natural systems are dynamic and self-restoring, as long as their collapses have not passed the point of no return. If the activation phase of tipping collapse is particularly abrupt, this can literally kill a system before the steady state forcing kills it.Example: beetle attack that kills enough of the trees in a forest to break the forest-cloud machine response. If there are no longer enough trees in the forest to create the clouds and precip that made the forest possible, this fast rate collapse results in completion of irreversible tipping before the steady rate increase state is encountered and the system does not self-restore. This is compared to the slow or steady state rate of change that most modeling uses, where tipping is suggested to occur at some point after collapse initiation – Hansen’s point of no return. With mechanical systems an electrical grid is considered with the UK grid collapse in 2007 where grid operators scheduled increased generation after the conclusion of a the 2007 semi-final soccer match. The match went into overtime with penalties and its abrupt conclusion gave grid operators too little time to react and the grid went down.
Ritchie et al., Rate-induced tipping in natural and human systems, Earth Systems Dynamics, June 14, 2023.
https://esd.copernicus.org/articles/14/669/2023/esd-14-669-2023.pdf

Rockstrom 2023 – Safe and just Earth systems boundaries identified where seven of eight quantified boundaries in over half of global land area are now exceeded… “We identify safe ESBs (Earth systems boundaries) for warming based on minimizing likelihoods of triggering climate tipping elements; maintaining biosphere and cryosphere functions; and accounting for Holocene (<0.5–1.0 °C) and previous interglacial (<1.5–2 °C) climate variability… Some climate tipping points, such as circulation collapse or Amazon dieback, have high uncertainty or low confidence in their dynamics and potential warming thresholds 16, but the complementary palaeoclimate and biosphere analyses independently support the safe climate ESB assessment. Cryosphere function includes maintaining permafrost in the northern high latitudes, permanent polar  ice sheets and mountain glaciers and minimizing sea ice loss. We find that global warming beyond 1.0 °C above pre-industrial levels, which has already been exceeded 9, carries a moderate likelihood of triggering tipping elements, such as the collapse of the Greenland ice sheet or localized abrupt thawing of the boreal permafrost 16. One-degree Celsius global warming is consistent with the safe limit proposed in 1990 of 350 ppm CO2. Above 1.5 °C or 2.0 °C warming, the likelihood of triggering tipping points increases to high or very high, respectively (high confidence in Extended Data Table 1). Biosphere damage and the risk of global carbon sinks becoming carbon sources, potentially triggering further climate feedbacks, increase substantially.”Seven ESB’s beyond their safe zone: Biosphere – natural ecosystem area, Biosphere – functional integrity, Water – surface flows, Water – groundwater flows,  Green water – soil moisture, Nutrient cycles – nitrogen, Nutrient cycles – phosphorus, Atmospheric loading – aerosols.
Rockstrom et al., Safe and just Earth system boundaries Nature, May 31, 2023.
https://www.nature.com/articles/s41586-023-06083-8
Supplementary Materials – boundaries and discussion of climate tipping points begins on page 4.
https://static-content.springer.com/esm/art%3A10.1038%2Fs41586-023-06083-8/MediaObjects/41586_2023_6083_MOESM1_ESM.pdf

Ripple 2023 – Twenty physical and 21 biological tipping feedback loops threatened… “Many feedback loops significantly increase warming due to greenhouse gas emissions. However, not all of these feedbacks are fully accounted for in climate models. Thus, associated mitigation pathways could fail to sufficiently limit temperatures. A targeted expansion of research and an accelerated reduction of emissions are needed to minimize risks…  We suggest that further small increases in short-term warming are a big risk, considering the suffering that we are already experiencing from climate disasters of ‘‘unprecedented’’ wildfires, intense storms, coastal flooding, permafrost thaw, and extreme weather that have occurred with just 1.1C to 1.2C global average warming. Second, as part of a longer timeline, positive feedback loops and tipping points may pose a major threat. Given the potential for catastrophic climate change and the lack of complete scientific understanding to date, policymakers should strongly consider the potentially dangerous effects of feedback loops, tipping points, and climate cascades, even if all desired scientific data are not available at this time… if the worst-case risks posed by feedback loops and tipping points have been underestimated, the future of a hospitable planet Earth may be at stake.”
Ripple et al., Many risky feedback loops amplify the need for climate action, One Earth, February 17, 2023.
https://scientistswarning.forestry.oregonstate.edu/sites/default/files/feedbacks.pdf

Kemp 2023 – Climate Endgame, Exploring catastrophic climate change tipping scenarios… “There are feedbacks in the carbon cycle and potential tipping points that could generate high GHG concentrations that are often missing from models. Examples include Arctic permafrost thawing that releases methane and CO2, carbon loss due to intense droughts and fires in the Amazon, and the apparent slowing of dampening feedbacks such as natural carbon sink capacity. These are likely to not be proportional to warming, as is sometimes assumed. Instead, abrupt and/or irreversible changes may be triggered at a temperature threshold. Such changes are evident in Earth’s geological record, and their impacts cascaded across the coupled climate–ecological–social system. Particularly worrying is a “tipping cascade” in which multiple tipping elements interact in such a way that tipping one threshold increases the likelihood of tipping another… The potential for tipping points and higher concentrations despite lower anthropogenic emissions is evident in existing models… There are even more uncertain feedbacks, which, in a very worst case, might amplify to an irreversible transition into a “Hothouse Earth” state (although there may be negative feedbacks that help buffer the Earth system). In particular, poorly understood cloud feedbacks might trigger sudden and irreversible global warming. Such effects remain underexplored and largely speculative “unknown unknowns” that are still being discovered. For instance, recent simulations suggest that stratocumulus cloud decks might abruptly be lost at CO2 concentrations that could be approached by the end of the century, causing an additional ∼8 °C global warming . Large uncertainties about dangerous surprises are reasons to prioritize rather than neglect them.”
Kemp et al., Climate Endgame, Exploring catastrophic climate change scenarios. PNAS, August 1, 2022.
https://www.pnas.org/doi/10.1073/pnas.2108146119

McKay 2022 – Exceeding 1.5 C global warming could trigger multiple climate tipping points with self-perpetuating change… (Note: some  of these tipping points have already been identified as being crossed by others: Permafrost, the Amazon, West Antarctic ice Sheet, Greenland Ice Sheet, and the Gulf Stream.) “We show that even the Paris Agreement goal of limiting warming to well below 2°C and preferably 1.5°C is not safe as 1.5°C and above risks crossing multiple tipping points. Crossing these CTPs can generate positive feedbacks that increase the likelihood of crossing other CTPs. Currently the world is heading toward ~2 to 3°C of global warming; at best, if all net zero pledges and nationally determined contributions are implemented it could reach just below 2°C. This would lower tipping point risks somewhat but would still be dangerous as it could trigger multiple climate tipping points.” Tipping likely with a threshold of 1.5 C to less than 2 C: Greenland Ice Sheet, West Antarctic ice Sheet, Labrador Sea/Subpolar Gyre, low-latitude corals, Barents Sea ice, widespread permafrost thaw. At 2 to 4C: Amazon Rainforest dieback, East Antarctic Ice Sheet, Sahel/West African Monsoon, Mountain Glaciers. At +4 C: Atlantic Meridonal Overturning (Gulf Stream) Boreal Forest, Arctic Winter Sea Ice.” “Self-perpetuation mechanisms are critical to the existence of a tipping point in a system, beyond which they propel qualitative change such that even if forcing of the system ceases the qualitative change usually continues to unfold regardless.”
McKay, Lenton et al., Exceeding 1.5 C global warming could trigger multiple climate tipping points, Science, September 9, 2022.
(Full)  https://www.science.org/doi/10.1126/science.abn7950
Press Release  – University of Exeter
https://earthcommission.org/wp-content/uploads/2022/09/Press-Release-for-Climate-Tipping-Points-Reassessment-paper.pdf

Canadell and Jackson 2021 – In-depth description of a dozen global ecologies currently in collapse (13 papers)… Here are descriptions of regional or sub-continental scale ecological collapses that are ongoing. ” Ecosystem collapse is associated with the crossing of critical thresholds not only as a result of gradual climate changes, but more often due to abrupt climate extremes or compounded effects of multiple disturbances occurring at greater than historical frequencies… In each case of ecosystem collapse documented here, the authors believe that climate change is playing a dominant or contributing role to the ecosystem’s apparent demise, often interacting with other pressures. The book covers three latitudinal regions: (1) polar and boreal ecosystems, such as the impacts of water stress with a novel pathogen on alpine vegetation mosaics in Antarctic Macquarie Island, forest transitions to shrublands and grasslands due to interactions between temperature and fire in Southern Siberia and North America, and rapid changes in vegetation and ecosystem functions in the permafrost region as thawing is rapidly taking place; (2) temperate and semi-arid ecosystems, including the impacts of fire on a paleo-endemic alpine ecosystem in Tasmania, and the interactions between drought and heatwaves in forests in the Mediterranean, Western USA, and elsewhere; (3) tropical and temperate coastal ecosystems, including the impacts of marine heatwaves on coral reefs of the Great Barrier Reef and Mediterranean Sea, and kelp forests and seagrass meadows in Western Australia. One goal of this book is to raise awareness of the abrupt transitions already occurring today.”
Canadell and Jackson, Ecosystem Collapse and Climate Change, Springer Nature Switzerland, 364 pages, 2021.
https://link.springer.com/content/pdf/bfm:978-3-030-71330-0/1?pdf=chapter%20toc/

Brovkin 2021 – Paleo abrupt changes and tipping responses with vast similarities to collapse already initiated today…“The cause of these changes in the AMOC is widely believed to be related to cryosphere–ocean interactions. The likely candidate mechanisms, including surging ice sheets, ice-shelf breakup, a coupled ocean– ice ‘salt oscillator’, catastrophic ice stream retreat and deep ocean warming due to deglaciation, are all considered to be threshold responses to slowly varying forcing.” All of these changes are in operation today. The root cause as determined from paleo history is warming from the deep ice age to our current interglacial. Though our Earth systems were different during these cold intervals, the warming of Earth systems and their responses is likely quite similar.
“During the rapid deglacial transition into the Bølling–Allerød warm period (14.7–12.9 ka), abrupt changes cascaded through the whole Earth system. The strengthening of the AMOC, rapid sea-level rise during the Meltwater Pulse 1 event and an abrupt increase in atmospheric CO2 and CH4 concentrations (Fig. 3) led to abrupt changes in the terrestrial climate, water availability and vegetation composition in the Northern and Southern hemispheres.” Again, these changes occurred in a different Earth than today with the average temperature 5 or 6 C colder than today, and Earth systems greatly different than today. The fundamental of abrupt and cascading changes remains however, as today’s systems evolutionary requirements are little different than any systems. Once their evolutionary boundaries are crossed, the systems collapse so new species and mechanisms can evolve that are tolerant of the new conditions. “The Holocene is often considered a period of relatively stable climate and a ‘safe operating space’ for humankind. While this is true globally, geological records show a number of abrupt changes originating and cascading through coupled climate, ecological and social systems on regional scales. For example, an abrupt climate event at about 8.2 ka, caused by ice-sheet meltwater discharge into the North Atlantic, led to cold and dry conditions in the Northern Hemisphere visible in rapid changes in vegetation composition in Europe and North America (Table 1 and Supplementary Information). Key characteristics of the current interglacial include a warm and hydrologically variable atmosphere, a growing anthropogenic footprint and multiple instances of abrupt change in hydroclimate, vegetation and societies.” This reporting shows abrupt climate change are possible in our current climate with forcings that are far less than what humanity is doing today with their greenhouse gas experiment.
Brovkin et al., Past abrupt changes, tipping points and cascading impacts in the Earth system, Nature Geoscience, July 29, 2021.
(paywall) https://www.nature.com/articles/s41561-021-00790-5
(Researchgate – free account required) https://www.researchgate.net/publication/353559458_Past_abrupt_changes_tipping_points_and_cascading_impacts_in_the_Earth_system

November 27, 2019 – Lenton et al., Climate tipping points-too risky to bet against… Identifies nine tipping systems activated within the last ten years with warming above 0.5 C… Nine Earth systems collapses have been identified by scientists as active: Arctic sea ice, Greenland ice sheet, boreal forests, permafrost, the Gulf Stream, the Amazon, coral, the West Antarctic Ice Sheet and parts of the East Antarctic Ice Sheet. Until 2018, the Intergovernmental Panel on Climate Change (IPCC) has assumed that tipping would not occur before 5 C of warming above preindustrial times, something that the worst-case scenario put well into the 22^nd century. In 2018 however, IPCC lowered this limit to between 1 and  2 C above preindustrial times in both the 1.5 C Report and the Cryosphere Report.  Lenton tells us, “The Intergovernmental Panel on Climate Change (IPCC) introduced the idea of tipping points two decades ago. At that time, these ‘large-scale discontinuities’ in the climate system were considered likely only if global warming exceeded 5 °C above pre-industrial levels. Information summarized in the two most recent IPCC Special Reports (published in 2018 and in September this year) suggests that tipping points could be exceeded even between 1 and 2 °C of warming.”  Climate tipping is now active greater than 100 years ahead of projections. Press release selected quotes from the Exeter press release: “More than half of the climate tipping points identified a decade ago are now “active”, a group of leading scientists have warned. This “cascade” of changes sparked by global warming could threaten the existence of human civilization, Evidence is mounting that these events are more likely and more interconnected than was previously thought, leading to a possible domino effect. It is not only human pressures on Earth that continue rising to unprecedented levels. It is also that as science advances, we must admit that we have underestimated the risks of unleashing irreversible changes, where the planet self-amplifies global warming. This is what we now start seeing, already at 1°C global warming. No amount of economic cost–benefit analysis is going to help us. We need to change our approach to the climate problem.” Selected quotes from the paper, “The Intergovernmental Panel on Climate Change (IPCC) introduced the idea of tipping points two decades ago. At that time, these ‘large-scale discontinuities’ in the climate system were considered likely only if global warming exceeded 5 °C above pre-industrial levels. Information summarized in the two most recent IPCC Special Reports (published in 2018 and in September this year) suggests that tipping points could be exceeded even between 1 and 2 °C of warming.” Perhaps the most salient part of this work is that tipping points seem to be interrelated. The authors make strong links that 45 percent of tipping points create positive feedbacks that increase the reaction of other tipping points through dynamic global effects.
Lenton et al., Climate tipping points-too risky to bet against, Nature, November 27, 2019.
https://www.nature.com/articles/d41586-019-03595-0
University of Exeter Press Release must read… More than half of identified tipping points are active.
http://www.exeter.ac.uk/news/featurednews/title_767753_en.html

November 5, 2019 – Climate Emergency Declared in Bioscience… Based on catastrophic threats from climate tipping that represent unrecoverable scenarios that could lead to “hothouse Earth well beyond the control of humans,” and “potentially making large areas of Earth uninhabitable.”… Over 13,000 scientists have signed a letter declaring a climate emergency is underway in the journal Biosciences…”Scientists have a moral obligation to clearly warn humanity of any catastrophic threat and to “tell it like it is.” On the basis of this obligation and the graphical indicators presented below, we declare, with more than 11,000 scientist signatories from around the world, clearly and unequivocally that planet Earth is facing a climate emergency.” Selected Quotes from Ripple 2019: “The climate crisis has arrived and is accelerating faster than most scientists expected…. It is more severe than anticipated, threatening natural ecosystems and the fate of humanity…. Especially worrisome are potential irreversible climate tipping points and nature’s reinforcing feedbacks (atmospheric, marine, and terrestrial) that could lead to a catastrophic ‘hothouse Earth,’ well beyond the control of humans…. These climate chain reactions could cause significant disruptions to ecosystems, society, and economies, potentially making large areas of Earth uninhabitable. An immense increase of scale in endeavors to conserve our biosphere is needed to avoid untold suffering due to the climate crisis.”
Ripple et al., World Scientists’ Warning of a Climate Emergency, Bioscience, November 5, 2019.
https://academic.oup.com/bioscience/article/70/1/8/5610806

Hansen 2017 “Young people’s burden…” Hansen models 350 ppm CO2 to define the maximum warming or our old climate, or the maximum range of natural variability of our old climate as 0.25 to 0.75 C warming, with a midpoint of 0.5 C. Hansen, Young People’s Burden: Requirement of Negative CO2 Emissions, Earth Systems Dynamics, July 18, 2017, Figure 12.
https://www.earth-syst-dynam.net/8/577/2017/esd-8-577-2017.pdf 

January 15, 2015 – Planetary boundary for our climate is 350 ppm CO2… “There is increasing evidence that human activities are affecting Earth systems (ES) functioning to a degree that threatens the resilience of the ES—its ability to persist in a Holocene-like state in the face of increasing human pressures and shocks. The planetary boundary (PB) framework is based on critical processes that regulate ES functioning. By combining improved scientific understanding of ES functioning with the precautionary principle, the PB framework identifies levels of anthropogenic perturbations below which the risk of destabilization of the ES is likely to remain low—a “safe operating space” for global societal development.”
Steffen et al., Planetary boundaries – Guiding human development on a changing planet, Science, January 15, 2015.
https://www.science.org/doi/epdf/10.1126/science.1259855

January 2009 – Safe planetary boundary for climate is 350 ppm CO2… This novel presentation of planetary boundaries defines a planet’s basic operational criteria where the human civilization evolved, that define a safe operating space for humanity, where beyond such boundaries are risks catastrophe for our advanced civilization. “The exponential growth of human activities is raising concern that further pressure on the Earth System could destabilize critical biophysical systems and trigger abrupt or irreversible environmental changes that would be deleterious or even catastrophic for human well-being. This is a profound dilemma because the predominant paradigm of social and economic development remains largely oblivious to the risk of human induced environmental disasters at continental to planetary scales.
Rockstrom, et al., Planetary boundaries – exploring the safe operating space for humanity, Ecology and Society, January 2009.
https://www.stockholmresilience.org/download/18.8615c78125078c8d3380002197/ES-2009-3180.pdf

Hansen 2008 – Evolutionary boundaries of our Earth systems in the Holocene, Tipping and The Point of No Return… (First use of “point of no return.”) We define: (1) the tipping level, the global climate forcing that, if long maintained, gives rise to a specific consequence, and (2) the point of no return, a climate state beyond which the consequence is inevitable, even if climate forcings are reduced. A point of no return can be avoided, even if the tipping level is temporarily exceeded. Ocean and ice sheet inertia permit overshoot, provided the climate forcing is returned below the tipping level before initiating irreversible dynamic change… Points of no return are inherently difficult to define, because the dynamical problems are nonlinear. Existing models are more lethargic than the real world for phenomena now unfolding, including changes of sea ice [65], ice streams [66], ice shelves [36], and expansion of the subtropics [67, 68].”
Hansen et. al., Target Atmospheric CO2 Where should humanity aim, Open Atmospheric Science Journal August 2008.
https://openatmosphericsciencejournal.com/contents/volumes/V2/TOASCJ-2-217/TOASCJ-2-217.pdf