Sedimentary rocks at Meishan, China. These rocks contain signatures of a disturbance in the carbon cycle immediately preceding Earth's greatest mass extinction. Credit: Shuzhong Shen

In the past 540 million years, the Earth has endured five mass extinction events, each involving processes that upended the normal cycling of carbon through the atmosphere and oceans. These globally fatal perturbations in carbon each unfolded over thousands to millions of years, and are coincident with the widespread extermination of marine species around the world.

The question for many scientists is whether the carbon cycle is now experiencing a significant jolt that could tip the planet toward a sixth mass extinction. In the modern era, carbon dioxide emissions have risen steadily since the 19th century, but deciphering whether this recent spike in carbon could lead to mass extinction has been challenging. That's mainly because it's difficult to relate ancient carbon anomalies, occurring over thousands to millions of years, to today's disruptions, which have taken place over just a little more than a century.

Now Daniel Rothman, professor of geophysics in the MIT Department of Earth, Atmospheric and Planetary Sciences and co-director of MIT's Lorenz Center, has analyzed significant changes in the carbon cycle over the last 540 million years, including the five mass extinction events. He has identified "thresholds of catastrophe" in the carbon cycle that, if exceeded, would lead to an unstable environment, and ultimately, mass extinction.

In a paper published in Science Advances, he proposes that mass extinction occurs if one of two thresholds are crossed: For changes in the carbon cycle that occur over long timescales, extinctions will follow if those changes occur at rates faster than global ecosystems can adapt. For carbon perturbations that take place over shorter timescales, the pace of carbon-cycle changes will not matter; instead, the size or magnitude of the change will determine the likelihood of an extinction event.

Taking this reasoning forward in time, Rothman predicts that, given the recent rise in carbon dioxide emissions over a relatively short timescale, a sixth extinction will depend on whether a critical amount of carbon is added to the oceans. That amount, he calculates, is about 310 gigatons, which he estimates to be roughly equivalent to the amount of carbon that human activities will have added to the world's oceans by the year 2100.

Does this mean that mass extinction will soon follow at the turn of the century? Rothman says it would take some time—about 10,000 years—for such ecological disasters to play out. However, he says that by 2100 the world may have tipped into "unknown territory."

"This is not saying that disaster occurs the next day," Rothman says. "It's saying that, if left unchecked, the carbon cycle would move into a realm which would be no longer stable, and would behave in a way that would be difficult to predict. In the geologic past, this type of behavior is associated with mass extinction."

History follows theory

Rothman had previously done work on the end-Permian extinction, the most severe extinction in Earth's history, in which a massive pulse of carbon through the Earth's system was involved in wiping out more than 95 percent of marine species worldwide. Since then, conversations with colleagues spurred him to consider the likelihood of a sixth extinction, raising an essential question:

"How can you really compare these great events in the geologic past, which occur over such vast timescales, to what's going on today, which is centuries at the longest?" Rothman says. "So I sat down one summer day and tried to think about how one might go about this systematically."

He eventually derived a simple mathematical formula based on basic physical principles that relates the critical rate and magnitude of change in the carbon cycle to the timescale that separates fast from slow change. He hypothesized that this formula should predict whether mass extinction, or some other sort of global catastrophe, should occur.

Rothman then asked whether history followed his hypothesis. By searching through hundreds of published geochemistry papers, he identified 31 events in the last 542 million years in which a significant change occurred in Earth's carbon cycle. For each event, including the five mass extinctions, Rothman noted the change in carbon, expressed in the geochemical record as a change in the relative abundance of two isotopes, carbon-12 and carbon-13. He also noted the duration of time over which the changes occurred.

He then devised a mathematical transformation to convert these quantities into the total mass of carbon that was added to the oceans during each event. Finally, he plotted both the mass and timescale of each event.

"It became evident that there was a characteristic rate of change that the system basically didn't like to go past," Rothman says.

In other words, he observed a common threshold that most of the 31 events appeared to stay under. While these events involved significant changes in carbon, they were relatively benign—not enough to destabilize the system toward catastrophe. In contrast, four of the five mass extinction events lay over the threshold, with the most severe end-Permian extinction being the farthest over the line.

"Then it became a question of figuring out what it meant," Rothman says.

A hidden leak

Upon further analysis, Rothman found that the critical rate for catastrophe is related to a hidden process within the Earth's natural carbon cycle. The cycle is essentially a loop between photosynthesis and respiration. Normally, there is a "leak" in the cycle, in which a small amount of organic carbon sinks to the ocean bottom and, over time, is buried as sediment and sequestered from the rest of the carbon cycle.

Rothman found that the critical rate was equivalent to the rate of excess production of carbon dioxide that would result from plugging the leak. Any additional carbon dioxide injected into the cycle could not be described by the loop itself. One or more other processes would instead have taken the carbon cycle into unstable territory.

He then determined that the critical rate applies only beyond the timescale at which the marine carbon cycle can re-establish its equilibrium after it is disturbed. Today, this timescale is about 10,000 years. For much shorter events, the critical threshold is no longer tied to the rate at which carbon is added to the oceans but instead to the carbon's total mass. Both scenarios would leave an excess of carbon circulating through the oceans and atmosphere, likely resulting in global warming and ocean acidification.

The century's the limit

From the critical rate and the equilibrium timescale, Rothman calculated the critical mass of carbon for the modern day to be about 310 gigatons.

He then compared his prediction to the total amount of carbon added to the Earth's oceans by the year 2100, as projected in the most recent report of the Intergovernmental Panel on Climate Change. The IPCC projections consider four possible pathways for carbon dioxide emissions, ranging from one associated with stringent policies to limit carbon dioxide emissions, to another related to the high range of scenarios with no limitations.

The best-case scenario projects that humans will add 300 gigatons of carbon to the oceans by 2100, while more than 500 gigatons will be added under the worst-case scenario, far exceeding the critical threshold. In all scenarios, Rothman shows that by 2100, the carbon cycle will either be close to or well beyond the threshold for catastrophe.

"There should be ways of pulling back [emissions of carbon dioxide]," Rothman says. "But this work points out reasons why we need to be careful, and it gives more reasons for studying the past to inform the present."

More information: "Thresholds of catastrophe in the Earth system," Science Advances (2017). Journal information: Science Advances "Thresholds of catastrophe in the Earth system,"(2017). advances.sciencemag.org/content/3/9/e1700906</s>The event database

After a brief discussion of possible sources of error, this section summarizes how each event has been identified and quantified. Geological, paleoenvironmental, and paleobiological context is also given, as are relevant references. All data, including the identification of abbreviations, are provided in Table 1.

When multiple observations are used to compute a mean, the sample SD, the unaveraged observations, or both are noted in the event’s description. However, these quantities only indicate the variability of a particular set of observations rather than an estimate of the observational error. As with any compilation of isotopic data [for example, Hayes et al. (43)], inadequate sampling may be a more significant problem. Here, it takes two forms: the sheer paucity of observations, and biases deriving from not only the original investigators—who may favor larger isotopic excursions—but also the needs of the present compilation, which favors carbon isotopic studies associated with strong geochronological constraints. An additional source of error derives from incompleteness of the sedimentary record. For example, a hiatus in sediment deposition, or erosion after deposition, may make the true carbon isotopic minimum effectively unavailable to the modern observer, thereby leading to an underestimate of |Δ r |.

The ideal geochronological control would provide absolute dates at the initiation and termination of isotopic excursions. Only one event in the database—at the Ediacaran-Cambrian boundary—meets this standard. Among the others, all but 1 of the 18 events extending from the Toarcian (182 Ma) to the Miocene (16 Ma) are timed by astrochronology, including, in some cases, information obtained from isotope geochronology. The duration of five of the older, pre-Jurassic events is determined from at least two dates provided by U-Pb geochronology that bracket less than twice the accumulation of sediment deriving from the event itself; a sixth event (end-Permian) comes close to this standard.

The remaining pre-Quaternary events are timed by biostratigraphy with reference to the Geologic Time Scale (44); their time scales are therefore the most poorly constrained. Two of them, the end-Ordovician and Frasnian-Famennian events, are among the Big Five mass extinctions. A third mass extinction—the end-Triassic—is constrained by both isotope geochronology and astrochronology, but these constraints are not directly associated with the carbonate–carbon isotope excursion. If these three mass extinction events and the other four events with poor temporal constraints (Cambrian Spice, Tournaisian, mid-Capitanian, and Albian-Cenomanian) are removed from the database, the trends in Figs. 2 and 3 remain intact and no new trends appear.

Nevertheless, the unknown errors in the observations render quantitative error analysis infeasible. The representative error bars for Δ r , M, and τ env in Figs. 2 and 3 are instead guides for interpretation.

The following list proceeds from the oldest to the youngest event.

Ediacaran-Cambrian boundary, ~541 Ma. The magnitude and duration of this negative isotopic excursion derive from the Oman carbon isotope data of Amthor et al. (45) and the U-Pb geochronology of Bowring et al. (46). Other data from Morocco, Siberia, Mongolia, and China (47, 48) suggest global consistency. The absence of the Ediacaran biota in the Cambrian has led to the suggestion that the Ediacaran biota vanished by mass extinction at the end of the Ediacaran, possibly related to the environmental event signaled by this excursion (45, 49–52). However, gradual extinction coupled with permanent environmental changes unfavorable to preservation of the Ediacaran biota may also be possible (53).

Nemakit-Daldynian-Tommotian boundary, ~525 Ma. Both the carbon isotopic excursion and the U-Pb dates establishing the duration of this Early Cambrian event are from the Moroccan data of Maloof et al. (47, 54); data from Siberia, Mongolia, and China confirm its global nature (47, 48). No mass extinction has been associated with this interval. However, diversity (number of genera) and disparity (number of classes) sharply increase in the Tommotian, primarily because of the rise of small shelly fossils (55). The Nemakit-Daldynian-Tommotian boundary is also associated with a transition from seawater chemistry that favored aragonite precipitation to seawater favoring calcite precipitation (47, 56).

Cambrian Spice, ~497 Ma. The duration and magnitude of this late Cambrian event, also known as the Steptoean positive carbon isotope excursion, are taken from the Australian data of Saltzman et al. (57). The event has also been found in North America, China, Kazakhstan, and Sweden (58, 59). The initiation of this positive isotopic excursion coincides with an extinction of trilobites (58).

End-Ordovician event, ~446 Ma. The latest stage of the Ordovician—the Hirnantian—simultaneously exhibits a positive isotopic excursion and one of the Big Five mass extinctions. I estimated the magnitude of the negative limb of the excursion from data obtained at Vinni Creek and Monitor Range, Nevada (60), Copenhagen Canyon, Nevada (61), Blackstone Range, Yukon (60), Wangjiawan, south China (62), the Kardla drill core, Estonia (63), and Anticosti Island, Quebec (64). The seven data sets yield a mean excursion of 5.6 ± 0.9‰. Each data set except for Copenhagen Canyon has been correlated to graptolite zones by Gorjan et al. (62). By estimating the fraction of the persculptus and extraordinarius zones in which the excursion occurs at each site and estimating those time intervals from Gradstein et al. (44), I found that the negative limb of the excursion lasted approximately 230 ky.

Silurian Mulde event, ~428 Ma. A sequence of two positive excursions, known as the “Mulde event,” occurs in the Homerian stage of the Silurian, during a period of graptolite extinctions known as the “Big Crisis” (65). The event is apparently global (65). Cramer et al. (66) have recently bracketed the earlier of the two excursions between two U-Pb dates obtained in Gotland, Sweden. I focused on the negative downswing, which occupies somewhat more than half of the dated interval, and estimated its duration to be about 260 ky. As shown by Cramer et al. (66), the Gotland excursion correlates well with observations in the West Midlands, England, with the West Midlands excursion having a magnitude of about 4‰ and the Gotland excursion about 2‰, a range that is consistent with other observations (65, 66). I therefore estimated the magnitude to be 3 ± 1‰.

Silurian Lau event, ~423 Ma. A significant global positive carbon isotopic excursion known as the “Lau Event” occurs in the Ludlow Epoch of the Silurian (65, 67–72). The excursion typically peaks at 6 to 8‰ [for example, Lehnert et al. (71)], though values as small as 3 to 4‰ and as high as 10 to 11‰ have been observed in North America (68) and Sweden (70), respectively. Given that uncertainty, I adopted a conservative mean peak value of 7‰. The negative downswing after the peak typically bottoms out at about 1‰, resulting in a typical total negative shift of about 6‰. Cramer et al. (72) have recently bracketed the duration of the excursion by U-Pb geochronology, finding that it must be less than about 1.17 My. Following their analysis, I took the upswing and downswing to be roughly symmetric, each occupying about one-half of an approximately million-year interval, implying that the duration of the downswing is about 500 ky.

Frasnian-Famennian boundary, ~372 Ma. The boundary between the Frasnian and Famennian stages in the Late Devonian is associated with one of the Big Five mass extinctions (5, 6). But the drop in diversity at the boundary is thought to be a consequence of a lack of originations rather than an elevated extinction rate (19, 20). The boundary is associated with a global positive carbon isotopic excursion, known as the Upper Kellwasser Event (73). The magnitude and duration of the excursion are taken from the compilation of European Devonian data of Buggisch and Joachimski (74).

Tournaisian event, ~352 Ma. Buggisch et al. (75) presented carbon isotopic data from Belgium, Ireland, France, western Canada, and the western United States that collectively exhibit a pronounced positive isotopic excursion in the Tournaisian stage of the Carboniferous. Using their biostratigraphic time scale, I estimated the duration of the negative limb of the excursion to be 1.51 My; the magnitude of the excursion in their compilation is about 3.5‰.

Mid-Capitanian event, ~262 Ma. A mass extinction in the Capitanian Stage of the Middle Permian is associated with a negative isotopic excursion of carbonate carbon of about 5‰ over about 500 ky (76, 77). Observations of the event in south China suggest that the extinction event coincides with the initiation of Emeishan volcanism (78) and the onset of the excursion (76).

End-Permian extinction, ~252 Ma. The end-Permian extinction is considered the most severe of the Big Five (8). A significant negative isotopic excursion occurs at the time of the extinction, just below the Permian-Triassic boundary. Korte and Kozur (16) reviewed observations of the excursion in 40 localities worldwide and concluded that the excursion ranged from 4 to 7‰; I therefore took the magnitude of the excursion to be at the center of that distribution (5.5‰). The 60-ky duration of the excursion derives from the well-studied section at Meishan, south China, where Cao et al. (79) have provided high-resolution carbon isotopic data and U-Pb geochronology provides strong constraints on the time scale (9).

Early Triassic, ~251 Ma. Several significant isotopic excursions of unknown origin follow the end-Permian extinction (80–83). Galfetti et al. (83) provided carbon isotopic data tied to U-Pb geochronology from the Loulou Formation, northwest Guanxi, south China. The only temporally constrained negative isotopic shift occurs in a negative excursion that begins approximately 251.22 Ma (83) in the mid-Smithian and declines approximately 3.5‰ over approximately 250 ky.

Triassic-Jurassic boundary, ~201 Ma. One of the Big Five extinctions, the end-Triassic event is temporally associated with the emplacement of the Central Atlantic magmatic province (40). Geochemically, the most well-studied section is at St. Audries Bay in southwest England, where the extinction interval coincides with a rapid fall and subsequent rise of the isotopic composition of organic matter (84) that lasts 20 to 40 ky according to astrochronology (85). This so-called initial excursion in organic carbon is widely observed (86). However, its significance for understanding the evolution of marine dissolved inorganic carbon is unclear (86, 87), partly because it may be associated with observed changes in flora (87)—and therefore variations in the organic matter itself—and partly because inorganic isotopic compositions need not track organic isotopic compositions (43). There are unfortunately few well-resolved isotopic studies of carbonate carbon associated with the end-Triassic event (88–93). These carbonate studies also reveal a negative excursion associated with the extinction, but it is often unclear if they represent the “initial excursion” or the later 120-ky-long “main excursion” seen in organic carbon (84, 85). A recent review (94) cautions that diagenetic alteration may have corrupted carbonate data, yet a careful study of lithological effects at an Italian section suggests that the carbonate excursion may indeed be primary (87). An important additional problem is the need to estimate the time scale of the carbonate excursion. The data of Clémence et al. (92) provide a solution to these problems. In their analysis of inorganic and organic carbonate at the Tiefengraben section in the Austrian Alps, they found an initial 2‰ negative excursion in carbonate that extended to the second significant minimum in their organic data (92), which in turn correlates reasonably well with the astrochronologically calibrated organic data at St. Audries Bay (85). Their initial carbonate excursion was then found to last approximately 50 ky [which included one precessional cycle of the “pre-recovery” interval identified by Ruhl et al. (85)]. This 2.0‰ shift is also consistent with isotopic analyses of carbonate in pristine oysters located near St. Audries Bay (91).

Toarcian oceanic anoxic event, ~183 Ma. The early Toarcian oceanic anoxic event (95) is widely observed in Europe as a negative excursion in the isotopic composition of carbonate (96–99). To estimate its magnitude and duration, I used the high-resolution carbonate record of Hesselbo et al. (96) obtained at Peniche (Portugal). The astrochronology of Suan et al. (97) found a duration of 150 ky (their interval C1) for the negative isotopic excursion, which is consistent with the subsequent analysis of Huang and Hesselbo (100). Using this chronology, recent U-Pb dating by Burgess et al. (101) found that the intrusive magmatism associated with the Ferrar large igneous province is synchronous with the negative excursion.

Aptian oceanic anoxic event, ~120 Ma. The negative isotopic excursion associated with the Aptian oceanic anoxic event has been observed worldwide in open ocean records of carbonate carbon (102–105). My estimate of the magnitude and duration of the excursion derived from the astrochronology of Malinverno et al. (106) performed on the carbonate record of Erba et al. (103). The negative excursion spans the interval C3 [originally identified by Menegatti et al. (102)] and lasts approximately 47 ky.

Albian-Cenomanian boundary, ~100 Ma. This complex carbon isotope event occurred near the Albian-Cenomanian boundary (mid-Cretaceous) in several European sections (107). Here, I focused on the carbon isotopic data obtained at Ocean Drilling Program (ODP) Site 1050 at Blake Nose, western North Atlantic, as presented by Ando et al. (108). When plotted using the time scale of Huber et al. (109), the ODP data show an unambiguous 0.7‰ negative excursion across the boundary over approximately 110 ky.

Mid-Cenomanian event, ~95.9 Ma. The mid-Cenomanian positive carbon isotopic excursion has been clearly observed in European sections (107), the North Atlantic (110), western North America (111), and elsewhere. My analysis follows from the correlation of the “English Chalk” reference carbonate curve (107) to carbon isotopic data from the Cretaceous Western Interior Seaway of North America (111, 112). I used the time scale of Eldrett et al. (112), which is based on a combination of astrochronology and U-Pb geochronology. As indicated in figure 11 of Eldrett et al. (112), the negative swing of the excursion begins at about 96.5 Ma, followed by a decline of about 0.85‰ over roughly 138 ky.

Cenomanian-Turonian boundary event, ~94.2 Ma. Otherwise known as oceanic anoxic event 2, the positive isotopic excursion at the Cenomanian-Turonian boundary is probably the most widely observed of the Cretaceous isotopic events (107, 111, 112). As for the mid-Cenomanian event, my estimate of the time scale and magnitude of the Cenomanian-Turonian event follows from the correlation of the English Chalk reference carbonate curve (107) to carbon isotopic data from the Cretaceous Western Interior Seaway of North America (111, 112). I took the initiation of the event to be at the positive peak labeled B in figure 11 of Eldrett et al. (112) and found that the ensuing negative excursion declines by 1.60‰ in roughly 553 ky.

Cretaceous-Paleogene extinction, ~65.5 Ma. The end-Cretaceous negative isotopic excursion is associated with one of the Big Five extinctions (5), widely known for the extinction of dinosaurs and the Alvarez impact hypothesis (39). Its temporal association with the eruption of the massive Deccan volcanic province in India is less widely known (38). I obtained the magnitude and time scale of the carbon isotopic event from the deep-sea bulk carbon isotopic data obtained at ODP Sites 1210 (Northwest Pacific) and 1262 (South Atlantic), as presented by Alegret et al. (113) using an orbitally tuned time scale (114). I took the initiation and termination of the approximately 1.15 ± 0.03‰ (the mean of 1.16 and 1.13‰) negative isotopic excursion at each site to be where values of δ 1 begin to change by at least 0.1‰, resulting in a mean event duration of roughly 26 ky (the average of 8.5 and 44 ky).

Early late Paleocene event, ~58.9 Ma. Also known as the mid-Paleocene biotic event, this negative excursion is synchronous with dissolution of carbonate and changes in the organization of benthic and planktonic ecosystems (115, 116). The magnitude and time scale of this negative excursion are taken from the high-resolution bulk isotopic record of Littler et al. (115).

Paleocene-Eocene Thermal Maximum, ~55.5 Ma. The negative isotopic excursion of the Paleocene-Eocene Thermal Maximum is perhaps the most studied carbon isotopic event in Earth history (11), in large part because of its association with significant climate warming and clear evidence of ocean acidification (10). The event is also associated with a significant extinction of benthic foraminifera (12); however, other groups of benthic and planktic microfossils show little or no extinction (11). The time scales of the isotopic excursion of bulk carbonate in two deep-sea cores, from ODP Site 1266 on the Walvis Ridge in the South Atlantic and ODP Site 690 in the Weddell Sea, Southern Ocean, have each been estimated independently by identification of orbital cycles (117) and the estimation of sedimentation rates from the concentration of 3He (118, 119). The mean of the resulting four estimates [corresponding to the cumulative time between the onset of the excursion and the termination of its “core” as summarized in table 1 of Murphy et al. (119)] is 83 ± 23 ky. I obtained the magnitude of the isotopic excursion, 2.7 ± 1.1‰, from the mean of 33 published analyses of bulk Paleocene-Eocene Thermal Maximum carbonates reviewed by McInerney and Wing (11). The initial value of the excursion was obtained similarly (11). Although these averages lack prejudice, they may nevertheless underestimate the excursion’s size (120) and overestimate its time scale (121).

Eocene Thermal Maximum 2, ~53.7 Ma. The Eocene is punctuated by several “hyperthermal events,” each represented by a negative excursion in the isotopic composition of carbonate carbon and dissolution of deep-sea marine carbonates. Eocene Thermal Maximum 2, one such event, follows the Paleocene-Eocene Thermal Maximum (also known as Eocene Thermal Maximum 1) by roughly 2 My. My estimate of the magnitude of Eocene Thermal Maximum 2 derived from averaging estimates from four high-resolution bulk isotopic records presented by Stap et al. (122). The records derive from ODP Sites 1262, 1263, 1265, and 1267, corresponding to paleowater depths ranging from about 1500 to 3600 m (122); the shallowest site yields an excursion of about 1.5‰, whereas the others are each about 1.0‰ (122), yielding a mean of 1.13 ± 0.25‰. The time scale is about 45 ky.

Eocene Hyperthermal H2, ~53.6 Ma. The magnitude and time scale of this Eocene hyperthermal event come from ODP Sites 1263, 1265, and 1267. Following Stap et al. (122), I took the excursion of bulk carbon isotopes to be about 0.6‰ and the time scale to be about 33 ky.

Eocene Hyperthermal I1, ~53.2 Ma. I obtained the magnitude (0.72‰) and time scale (41 ky) of this Eocene hyperthermal event from the high-resolution bulk carbon isotopic record of Littler et al. (115), which derives from ODP Site 1262.

Eocene Hyperthermal I2, ~53.1 Ma. This Eocene hyperthermal follows Eocene Hyperthermal I1 by about 100 ky. Using the same source (115) as for Eocene Hyperthermal I1, I found an excursion of about 0.61‰ and a time scale of 40 ky.

Eocene Thermal Maximum 3, ~52.5 Ma. I estimated the magnitude and time scale of this hyperthermal event from the high-resolution benthic carbon isotopic record of Lauretano et al. (123), obtained at ODP Sites 1262 and 1263. The two records are similar, yielding an excursion of about 0.8‰ and a time scale of about 37 ky.

Early Oligocene Event, ~33.5 Ma. The positive carbon isotopic excursion just above the Eocene-Oligocene boundary is associated with the initiation of permanent Cenozoic ice sheets on Antarctica (124–126). I focused on the negative downswing to lighter isotopic compositions that followed. The time scale and isotopic change are derived from the high-resolution data collected at ODP Site 1218, using the astrochronological time scale presented by Coxall et al. (125). The negative shift extends over roughly 0.6 ± 0.1‰ during a period of about 430 ± 100 ky. The three ODP records presented by Zachos and Kump (126) are consistent with these estimates.

Miocene Climatic Optimum 1, ~16.9 Ma. Holbourn et al. (127) provided high-resolution carbon isotopic records spanning most of the Miocene Climatic Optimum, a period of warm climates ranging from 17.0 to 14.7 Ma that interrupted the longer-term trend of Cenozoic cooling. The records are obtained from Integrated ODP Site U1337 in the eastern equatorial Pacific Ocean, at a paleowater depth of 3500 to 4000 m. The Miocene Climatic Optimum began with a negative isotopic excursion, which I designated Miocene Climatic Optimum 1, that has also been identified in lower-resolution records from three other sites, elsewhere in the Pacific and in the Southern Ocean (127). I obtained the magnitude (0.5‰) and time scale (28 ky) from the bulk carbonate record. The event is synchronous with shoaling of the carbonate compensation depth (CCD).

Miocene Climatic Optimum 2, ~16.4 Ma. Holbourn et al. (127) identified three other Miocene Climatic Optimum negative excursions, which I designated Miocene Climatic Optimum 2 to Miocene Climatic Optimum 4, during which δ 1 decreases sharply and the CCD appears to shoal. Using the same high-resolution record as for Miocene Climatic Optimum 1, I found that Miocene Climatic Optimum 2 has a time scale of 22 ky and a magnitude of 0.76‰.

Miocene Climatic Optimum 3, ~16.0 Ma. See Miocene Climatic Optimum 2. For Miocene Climatic Optimum 3, I found a time scale of 32 ky and a magnitude of 0.66‰. This event also appears in the records of ODP Site 1146 in the western Pacific Ocean (128) and ODP Site U1338 in the eastern equatorial Pacific (129).

Miocene Climatic Optimum 4, ~15.6 Ma. See Miocene Climatic Optimum 2. For Miocene Climatic Optimum 4, the excursion has a magnitude of 0.6‰ and a time scale of 29 ky.</s>Climate change will have sent Earth on the way to its sixth mass extinction in less than a lifetime, new research warns.

By the year 2100, about 310 gigatons of carbon will have been added to the oceans - a potential 'tipping point' for ecological disaster, according to the study.

When CO2 dissolves into the ocean - as in the case of the End-Permian mass extinction - life on Earth is at risk.

Professor Daniel Rothman from the Massachusetts Institute of Technology came up with a simple mathematical formula to predict when the next mass extinction would take place.

The formula predicts that by the end of the century oceans will hold enough carbon to launch a mass extermination of species in the future.

The model showed the critical extra amount required is about 310 gigatons, which is around the best case scenario projected by the Intergovernmental Panel on Climate Change (IPCC).

And it's well below the worst of more than 500 gigatons that would far exceed the line.

In all scenarios the study found by the end of the century the carbon cycle will either be close to, or well beyond, the threshold for catastrophe.

It also found although mass extinction won't soon follow at the turn of the century the world may have tipped into 'unknown territory.'

Professor Rothman says it would take some time, about 10,000 years, for such ecological disasters to play out.

He said: 'This is not saying disaster occurs the next day.

'It's saying - if left unchecked - the carbon cycle would move into a realm which would be no longer stable and would behave in a way that would be difficult to predict.

'In the geologic past this type of behaviour is associated with mass extinction.'

In the modern era CO2 emissions have risen steadily since the 19th century, but deciphering whether this could lead to mass extinction has been challenging.

That's mainly because it's difficult to relate ancient carbon anomalies, occurring over thousands to millions of years, to today's disruptions which have taken place over little more than a century.

Professor Rothman's analysis identified 'thresholds of catastrophe' in the carbon cycle that, if exceeded, would lead to an unstable environment, and ultimately, mass extinction.

He had previously done work on the end-Permian extinction, or Great Dying.

Since then conversations with colleagues spurred him to consider the likelihood of a sixth, raising an essential question.

Professor Rothman said: 'How can you really compare these great events in the geologic past, which occur over such vast timescales, to what's going on today, which is centuries at the longest?.

'So I sat down one summer day and tried to think about how one might go about this systematically.'

His unique equation factored in the critical rate and magnitude of change in the carbon cycle to the timescale that separates fast from slow change from 31 events in the last 542 million years in which a significant change occurred in the carbon cycle.

For each, including the five mass extinctions, he noted the change in carbon, expressed in the relative abundance of two telltale chemicals in the geologic records.

He then devised a mathematical transformation to convert these quantities into the total mass of carbon that was added to the oceans during each event.

Finally, he plotted both the mass and timescale of each event.

'It became evident that there was a characteristic rate of change that the system basically didn't like to go past,' said Professor Rothman.

There was a common threshold that most of the 31 events appeared to stay under. They were relatively benign - not enough to destabilise the system toward catastrophe.

In contrast four of the five mass extinctions lay over the threshold - with the 'Great Dying' being the farthest over the line.

'Then it became a question of figuring out what it meant,' said Professor Rothman.

He added: 'There should be ways of pulling back emissions of carbon dioxide.

'But this work points out reasons why we need to be careful, and it gives more reasons for studying the past to inform the present.'

Humans have emitted 1,540 billion tonnes of CO2 since the industrial revolution.

That's equivalent to burning enough coal to form a square tower 72 feet wide stretching 240,000 miles from Earth to the Moon.

Half of these have remained in the atmosphere causing a rise in levels at least 10 times faster than any known natural increase during Earth's long history.

Most of the other half has dissolved into the ocean, causing acidification.

The full results of the study were published today in the journal Science Advances.</s>CBS Local — Will the human race live to see the next century on Earth? According to a mathematician at MIT, the next extinction-level event for the planet may begin as early as the year 2100.

Professor Daniel Rothman has published a formula in the journal Science Advances that looks at the planet’s carbon cycle. According to Rothman, the massive amounts of CO2 emissions produced around the world are in line with previous mass extinction events in the planet’s history.

“The modern critical size for the marine carbon cycle is roughly similar to the mass of carbon that human activities will likely have added to the oceans by the year 2100,” Rothman said. The MIT professor adds that in four of the last five extinction events the critical level of carbon in his model was surpassed.

The study doesn’t draw a direct conclusion as to how life will begin to die out in the 22nd century.

“This is not saying that disaster occurs the next day,” Rothman admits. The mathematician adds that his model only points to a dangerous trend in the history of the Earth over the last 540 million years. “If left unchecked, the carbon cycle would move into a realm which would be no longer stable, and would behave in a way that would be difficult to predict. In the geologic past, this type of behavior is associated with mass extinction.”</s>Our oceans may hold enough carbon in 80 years’ time to trigger a sixth mass extinction, a scientist has warned based on his mathematical prediction.

Daniel Rothman, geophysics professor at MIT, said he made the calculation based on the significant changes in the carbon cycle over the last 540 million years and everything we know so far about the previous five mass extinctions that occurred during this time.

In his paper, Rothman proposes that mass extinction occurs when one of these two thresholds are crossed – one, where changes occur at rates faster than our ecosystems can adapt in a long-timescale carbon cycle and, two, where changes are of a significantly large magnitude for carbon perturbations that take place over shorter timescales.

Given the recent rise in carbon dioxide emissions due to human activity, he said we would reach this threshold by the year 2100 and once that happens, it could lead to an unstable environment, and ultimately, mass extinction.

Rothman calculated the critical amount of carbon to be about 310 gigatons, which would be enough to trigger the events that could lead to a mass wipe-out of all the major species on Earth.

But it doesn’t mean mass extinction will follow immediately. Rothman believes it will take another 10,000 years for the ecological disasters to play out but warns that by 2100, the world would be tipped into “unknown territory”.

“This is not saying that disaster occurs the next day,” Rothman said. “It’s saying that, if left unchecked, the carbon cycle would move into a realm which would be no longer stable, and would behave in a way that would be difficult to predict.

“In the geologic past, this type of behaviour is associated with mass extinction.”

The Earth has endured five mass extinction events, each involving processes that wreaked havoc on the the carbon cycle.

Having previously done some work on the end-Permian extinction, Rothman identified 31 events in the last 542 million years where a significant change occurred in our planet’s carbon cycle after looking through hundreds of scientific papers.

Looking at the geochemical record, Rothman noted changes in the relative abundance of two carbon isotopes – carbon-12 and carbon-13 – for each mass extinction event.

He then created a mathematical formula based on the total mass of carbon that was added to the oceans during each event and the timescale of each event.

“It became evident that there was a characteristic rate of change that the system basically didn’t like to go past,” Rothman said.

“Then it became a question of figuring out what it meant.”

The research is published in Science Advances.</s>New Delhi: Courtesy climate change as well as human behaviour towards nature and its beings, the world may need to prepare itself for its sixth mass extinction event, which researchers have said will begin by about 2100.

The reason? The amount of carbon being pumped into the atmosphere.

The threat that the ecosystem is facing due to this, has scientists worried and going by the rate at which the world is losing out on its biodiversity, they say that the possibility of a mass extinction in the future looms larger than ever.

Scientists came to this conclusion after a comprehensive mathematical study of the five previous events in the last 540 million years.

According to a report in the Independent UK, professor Daniel Rothman, co-director of the Massachusetts Institute of Technology’s Lorenz Centre, theorised that disturbances in the natural cycle of carbon through the atmosphere, oceans, plant and animal life played a role in mass die-offs of animals and plants.

After studying such changes based on when they took place 31 times, he found that four out of the five previous mass extinctions occurred when the disruption crossed a “threshold of catastrophic change”.

The 'Great Dying', known as the worst mass extinction of all – that took place 248 million years ago when 96 percent of species died out – breached one of these thresholds by the greatest margin.

Based on his analysis of these mass extinctions, Professor Rothman developed a mathematical formula to help predict how much extra carbon could be added to the oceans – which absorb vast amounts from the atmosphere – before triggering a sixth one.

The answer came as a shock.

For the figure of 310 gigatons is just 10 gigatons above the figure expected to be emitted by 2100 under the best-case scenario forecast by the Intergovernmental Panel on Climate Change. The worst-case scenario would result in more than 500 gigatons, the report said.

While many scientists argue that the sixth mass extinction has already begun, the total number of species that have disappeared from the planet comes nowhere near the most apocalyptic events of the past and the rate of species loss is comparable.

Professor Rothman emphasised that dramatic changes to the carbon cycle wasn't a necessary occurrence for a mass extinction to take place – as shown by the absence of this during the Late Devonian extinction more than 360 million years ago.

The study, published in the journal Science Advances, noted that events such as volcanic eruptions, climate change and other environmental factors could also play a role.

Does this mean that mass extinction will soon follow at the turn of the century? As per a report in Phys.org, Rothman says it would take some time—about 10,000 years—for such ecological disasters to play out. However, he says that by 2100 the world may have tipped into "unknown territory."

"This is not saying that disaster occurs the next day," Rothman says. "It's saying that, if left unchecked, the carbon cycle would move into a realm which would be no longer stable, and would behave in a way that would be difficult to predict. In the geologic past, this type of behavior is associated with mass extinction."

The Independent UK further quoted him saying, “By analyzing 31 carbon isotopic events during the past 542 million years, I identify the critical rate with a limit imposed by mass conservation.

“The modern critical size for the marine carbon cycle is roughly similar to the mass of carbon that human activities will likely have added to the oceans by the year 2100.”</s>Over the past 540 million years Earth has suffered five mass extinction events, the worst of which wiped out more than 9 per cent of marine life on the planet. A new study has suggested that the next such catastrophe might not be too far away.

Massachusetts Institute of Technology (MIT) geophysicist and mathematician, Daniel Rothman has been busy studying previous mass extinctions. He reckons the next one might be a mere 83 years away.

The previous five catastrophic events each unfolded over millions of years and involved the natural cycle of carbon through the oceans and atmosphere being disturbed, resulting, in some cases, the death of almost all life on Earth.

The award-winning mathematician identified two “thresholds of catastrophe” that, if exceeded, would upset the natural order of the cycle, leading to an unstable environment and eventually a mass extinction.

The first relates to changes in the carbon cycle over a period of thousands or millions of years. A mass extinction will occur if the rate of change in the cycle occurs faster than global ecosystems can adapt.

The second pertains to the size or magnitude of the carbon flux over a shorter period, as has been the case over the last century.

Therein lies a problem, however, as Rothman says: “How can you really compare these great events in the geologic past, which occur over such vast timescales, to what's going on today, which is centuries at the longest?”

“So I sat down one summer day and tried to think about how one might go about this systematically.”

He set about searching through hundreds of geochemistry papers trying to find an answer, classifying the events as either long or short in duration. Throughout his studies, Rothman identified 31 instances over the past 542 million years in which a major change occurred in the carbon cycle.

Following this, he devised a mathematical formula to determine the total mass of carbon added to the oceans during each event, after which “it became evident that there was a characteristic rate of change that the system basically didn't like to go past.”

Rothman estimates this to be 310 gigatons. He thinks that given the rise of carbon dioxide over the last century, a sixth mass extinction could be on the way as estimates suggest that humans will add roughly 310 gigatons to the cycle by 2100.

READ MORE: ‘What the frack! We’re destroying all life, we have a huge problem’

“This is not saying that disaster occurs the next day,” Rothman said. “It’s saying that, if left unchecked, the carbon cycle would move into a realm which would be no longer stable, and would behave in a way that would be difficult to predict. In the geologic past, this type of behavior is associated with mass extinction.”

Rothman's paper was published Wednesday in the journal Science Advances.</s>Geologists have long been fascinated with past extinction events and now it appears we're may be headed for another one by the end of the century. This finding is based on a mathematical analysis of the five mass extinction events occurring within the past 540 million years.

What do all of these mass extinction events have in common? Unusually high increases in global carbon, leading to a destabilization of practically every ecosystem, with a punctuated impact on oceans. Dr. Daniel Rothman, a professor at MIT's Earth, Atmospheric, and Planetary Sciences department took a mathematical approach to investigate why some carbon anomalies in the past have led to minor disruptions while others have led to mass extinctions.

However, before we jump into his findings, let's discuss how we understand past mass extinction events. Geologists use the rock record to understand what happened in the past. This can be done because practically every major change in our Earth & environment is preserved in the rock record.

Rocks, specifically sedimentary rocks, are formed from the gradual deposit of sediment on top of each other, that sediment, buried through time with more and more sediment, lithifies into rock. This imparts a vertical timeline recording Earth's history in sedimentary rocks, going from oldest at the bottom to youngest at the top (normally).

First, how do we know there were mass extinction events in the past? It's rather simple and only requires a sharp eye. If you understand how sediment is deposited on top of older sediment, what is called the law of superposition, you can walk through time as you look at layers and layers of rocks. What geologists do is take samples of rocks, from the bottom of an outcrop or core to the top.

They measure chemical traces and trace fossils. Much to early geologists surprise they stepped through time in the rock section, finding large numbers of plant and animal fossils, then suddenly nothing. Abruptly, in the rock record, there were no or very little fossils. Then, gradually the number of fossils increased again as they moved forward in time (higher in the rock record).

Well, that may be a coincidence and local changes that caused the large reduction in fossils. However, the more geologists measured rocks around the world, they found the exact same types of fossils, leading to an abrupt disappearance of fossils and gradual increase. This, they concluded, marked a global mass extinction event.

How We Link Mass Extinctions To Changes In Climate

Noting the above finding, geologists concluded that there is substantial evidence of mass extinction events in the past. However, the sudden disappearance of fossils does not answer the "why" that typically follows unexpected findings.

To answer why this happened at that specific time, across the globe, geologists measure the chemistry of the rock just before and just after the extinction event. What they found is typically during these mass extinction events, there's a dramatic change in the carbon within the rock.

When organisms form their carbonate shells, the shells impart specific signatures associated with the amount and type of carbon in the ocean/atmosphere. Humans do the same thing when we make our bones, we impart a carbon signature in our bones that "timestamps" the type and amount of carbon in the atmosphere while we are alive.

Now, geologists found the missing key, the amount and type of carbon leading up to these mass extinction events, as measured in the same rock that shows these extinction events, is anomalously high. Finally, we have an answer to "why" and it involves a rapid and large increase in carbon into our atmosphere.

Now that we have a good grasp of how geologists determined the existence of mass extinction events and the association with why they happened, we can be predictive! After all, that's a primary reason geologists study the past, to understand what the future holds.

To mathematically predict the required input of carbon necessary to trigger a mass extinction, Dr. Daniel Rothman analyzed 31 events in the past 542 million years. All 31 of these events are associated with changes in the amount of carbon in the atmosphere and ocean. However, only 5 of these events triggered global mass extinctions. Well, the difference may be the rate and amount of carbon increase in each event.

Dr. Rothman plotted the total mass of carbon increase by the length of time for the increase and noticed a significant finding.

The comparison found that most of the small carbon disturbances that did not trigger mass extinction events fell below the threshold. However, four of the five mass extinction events fell above the threshold. Dr. Rothman logically concluded that a mass extinction event is dependent both on the amount and speed in which carbon is added to the atmosphere and ocean.

Therefore, a mass extinction event can be triggered by a rapid and extremely large input of carbon (similar to today) or a slow but much larger total amount of carbon added to the atmosphere.

This allowed Dr. Rothman to compare the carbon rate (amount and speed) being emitted into the atmosphere today with the historical data in the figure above. One thing that makes a comparison difficult is even the fastest extinction events in the past saw carbon increasing over thousands to millions of years. Nothing is on the timescale of hundreds of years we see today.

Nonetheless, he found that given the current rate of carbon being emitted into the atmosphere, we will likely reach a mass extinction threshold by the year 2100.

This is based on a total increase of 310 gigatons of carbon, which at current rates is expected to fall right around 2100.

The upside? The effects of this mass extinction event will take a while to be fully realized. While we have increased the amount of carbon at unprecedented rates, it takes the Earth some time to equilibrate to the new conditions. This means we have thousands of years (a blip in the geologic record) for the mass extinction to fully be in effect.

The downside? The basis of how long extinction events take to fully realize is based on past extinction events. And just as we discussed above, none of those previous extinction events saw carbon increase on the order of hundreds of years. So, long story short, we can't fully know what is in store for the future.

As geologists, we look to the past to understand the future. Unfortunately, we're in a transformative time in Earth's history where things are changing faster than they ever have in the past. That makes predicting the future much harder, something that worries scientists. And should worry you.</s>It doesn't come along very often, but after some 540 million years, this world we live on has witnessed five mass extinctions – and the next curtain could fall before the century is up.

That's the grim assessment of a new mathematical analysis of Earth's revolving carbon cycle, with calculations predicting our horrendous output of CO2 emissions is inching us towards a 'threshold of catastrophe' the planet hasn't breached in millions of years.

MIT geophysicist Daniel Rothman investigated fluctuations in the carbon cycle – the pathway carbon traces through Earth's land, oceans, and atmosphere – that have occurred over the last 542 million years.

By analysing 31 established carbon isotopic events recognised by geochemists, Rothman identified the ebb and flow of carbon–12 and carbon–13 – two isotopes of carbon whose abundance has varied considerably in Earth's history.

From this, he constructed a database to assess how much carbon mass was pumped into the world's oceans in each historical event. In most of these episodes, the carbon volume stayed under a certain threshold.

But in some of them – including four of the past five mass extinction events that exterminated multitudes of life-forms on the planet – the threshold was breached.

Now, we all know that correlation doesn't equal causation, but in light of all the other evidence we have on how dangerous high levels of carbon are to life on our planet, a disturbing pattern is definitely emerging.

"It became evident that there was a characteristic rate of change that the system basically didn't like to go past," as Rothman puts it.

Now for the bad news.

Per Rothman's calculations, there are two ways carbon levels can exceed this threshold of catastrophe. One is where CO2 emissions slowly swell over thousands and millions of years, slowly triggering a global calamity.

The other case occurs on a much shorter timescale, where an immense shift in carbon volumes moving through the carbon cycle happens in the space of say, decades and years. Sound familiar?

In this context, Rothman predicts it would take about 310 gigatonnes of carbon added to the world's oceans for us to pass the threshold – which is roughly the minimum amount expected to be contributed by the year 2100 at the rate things are going, at which point the researcher says we'll enter "unknown territory".

"This is not saying that disaster occurs the next day," Rothman says.

"It's saying that, if left unchecked, the carbon cycle would move into a realm which would be no longer stable, and would behave in a way that would be difficult to predict. In the geologic past, this type of behaviour is associated with mass extinction."

In other words, unless humanity does something to drastically turn around our carbon situation – and there's cause for real optimism on that front, friends – we could lock in a dangerous extermination.

Not just for humanity necessarily either, but for many a critter that ever walked, chirped, buzzed, or purred.

It wouldn't happen overnight, mind you, but such an epic die-off could play out over something like 10,000 years or so, Rothman suggests, and the phenomenon could crystallise as soon as 2100 if things don't change.

Of course, it's only one perspective on how the world's carbon scenario could materialise, and Rothman doesn't pretend he has all the answers, but he does hope we take these numbers as another piece of evidence to galvanise our slow-moving selves into action.

"There should be ways of pulling back [emissions of carbon dioxide]," he says.

"But this work points out reasons why we need to be careful, and it gives more reasons for studying the past to inform the present."

The findings are reported in Science Advances.</s>According to the latest findings by cosmic doomsayers, Planet X is due to eclipse the sun this Saturday, September 23, marking the start of The Rapture. Although sceptics - such as NASA - have denied the very existence of Nibiru, there are still many conspiracists who are convinced that the end is nigh. But they may in fact be right - in planetary terms at least. One US geophysicist and award-winning mathematician has been studying Earth's last five mass extinction events - and believes the next one could be just 83 years away.

Daniel Rothman, of the Massachusetts Institute of Technology, has identified two "thresholds of catastrophe" that upset the world's natural order, resulting in environmental instability and, eventually, mass extinction. The first regards changes in the carbon cycle over a period of thousands or millions of years. If changes occur faster than the ecosystem can adapt, then a mass extinction will ultimately occur.

The second threshold involves the magnitude of the carbon flux. In this scenario, it is not the speed of the changes, merely the size of them which will result in extinction. Mr Rothman has now predicted that, with humans increasing the rate of carbon dioxide emissions at a faster rate over a shorter period of time, the tipping point for the sixth mass extinction could occur as early as 2100. He calculated that by then, mankind will have added some 310 gigatons of carbon to the oceans, which he believes is the critical amount needed to trigger a future apocolyptic event. However, Mr Rothman said the likelihood of ecological catastrophe occurring as soon as this point is reached is unlikely.

In his paper, publicised in Science Advances, he said: "This is not saying that disaster occurs the next day. "It's saying that, if left unchecked, the carbon cycle would move into a realm which would be no longer stable, and would behave in a way that would be difficult to predict. "In the geologic past, this type of behaviour is associated with mass extinction." Earth has suffered five mass extinctions in the past 540 million years, with the worst wiping out more than 9 per cent of the planet's marine life.
MIT physics professor Daniel Rothman's mathematical formula, published in the peer-review journal Science Advances, predicts a sixth mass extinction may be set to occur within 10,000 years if by the year 2100 current carbon dioxide emissions are not reduced, and a critical amount of carbon (310 gigatons) is added to the oceans.