Timothy M. Lenton

A safe operating space for humanity

Identifying and quantifying planetary boundaries that must not be transgressed could help prevent human activities from causing unacceptable environmental change, argue Johan Rockstrom and colleagues.

Tipping elements in the Earth's climate system

The term “tipping point” commonly refers to a critical threshold at which a tiny perturbation can qualitatively alter the state or development of a system. Here we introduce the term “tipping element” to describe large-scale components of the Earth system that may pass a tipping point. We critically evaluate potential policy-relevant tipping elements in the climate system under anthropogenic forcing, drawing on the pertinent literature and a recent international workshop to compile a short list, and we assess where their tipping points lie. An expert elicitation is used to help rank their sensitivity to global warming and the uncertainty about the underlying physical mechanisms. Then we explain how, in principle, early warning systems could be established to detect the proximity of some tipping points.

Anticipating Critical Transitions

All Change Research on early warning signals for critical transitions in complex systems such as ecosystems, climate, and global finance systems recently has been gathering pace. At the same time, studies on complex networks are starting to reveal which architecture may cause systems to be vulnerable to systemic collapse. Scheffer et al. (p. 344) review how previously isolated lines of work can be connected, conclude that many critical transitions (such as escape from the poverty trap) can have positive outcomes, and highlight how the new approaches to sensing fragility can help to detect both risks and opportunities for desired change. Tipping points in complex systems may imply risks of unwanted collapse, but also opportunities for positive change. Our capacity to navigate such risks and opportunities can be boosted by combining emerging insights from two unconnected fields of research. One line of work is revealing fundamental architectural features that may cause ecological networks, financial markets, and other complex systems to have tipping points. Another field of research is uncovering generic empirical indicators of the proximity to such critical thresholds. Although sudden shifts in complex systems will inevitably continue to surprise us, work at the crossroads of these emerging fields offers new approaches for anticipating critical transitions.

The coral reef crisis: The critical importance of <350 ppm CO2

Temperature-induced mass coral bleaching causing mortality on a wide geographic scale started when atmospheric CO(2) levels exceeded approximately 320 ppm. When CO(2) levels reached approximately 340 ppm, sporadic but highly destructive mass bleaching occurred in most reefs world-wide, often associated with El Niño events. Recovery was dependent on the vulnerability of individual reef areas and on the reefs previous history and resilience. At todays level of approximately 387 ppm, allowing a lag-time of 10 years for sea temperatures to respond, most reefs world-wide are committed to an irreversible decline. Mass bleaching will in future become annual, departing from the 4 to 7 years return-time of El Niño events. Bleaching will be exacerbated by the effects of degraded water-quality and increased severe weather events. In addition, the progressive onset of ocean acidification will cause reduction of coral growth and retardation of the growth of high magnesium calcite-secreting coralline algae. If CO(2) levels are allowed to reach 450 ppm (due to occur by 2030-2040 at the current rates), reefs will be in rapid and terminal decline world-wide from multiple synergies arising from mass bleaching, ocean acidification, and other environmental impacts. Damage to shallow reef communities will become extensive with consequent reduction of biodiversity followed by extinctions. Reefs will cease to be large-scale nursery grounds for fish and will cease to have most of their current value to humanity. There will be knock-on effects to ecosystems associated with reefs, and to other pelagic and benthic ecosystems. Should CO(2) levels reach 600 ppm reefs will be eroding geological structures with populations of surviving biota restricted to refuges. Domino effects will follow, affecting many other marine ecosystems. This is likely to have been the path of great mass extinctions of the past, adding to the case that anthropogenic CO(2) emissions could trigger the Earths sixth mass extinction.

Gaia and natural selection

Evidence indicates that the Earth self-regulates at a state that is tolerated by life, but why should the organisms that leave the most descendants be the ones that contribute to regulating their planetary environment? The evolving Gaia theory focuses on the feedback mechanisms, stemming from naturally selected traits of organisms, that could generate such self-regulation.

Bistability of atmospheric oxygen and the Great Oxidation

The history of the Earth has been characterized by a series of major transitions separated by long periods of relative stability. The largest chemical transition was the ‘Great Oxidation’, approximately 2.4 billion years ago, when atmospheric oxygen concentrations rose from less than 10-5 of the present atmospheric level (PAL) to more than 0.01 PAL, and possibly to more than 0.1 PAL. This transition took place long after oxygenic photosynthesis is thought to have evolved, but the causes of this delay and of the Great Oxidation itself remain uncertain. Here we show that the origin of oxygenic photosynthesis gave rise to two simultaneously stable steady states for atmospheric oxygen. The existence of a low-oxygen (less than 10-5 PAL) steady state explains how a reducing atmosphere persisted for at least 300 million years after the onset of oxygenic photosynthesis. The Great Oxidation can be understood as a switch to the high-oxygen (more than 5 × 10-3 PAL) steady state. The bistability arises because ultraviolet shielding of the troposphere by ozone becomes effective once oxygen levels exceed 10-5 PAL, causing a nonlinear increase in the lifetime of atmospheric oxygen. Our results indicate that the existence of oxygenic photosynthesis is not a sufficient condition for either an oxygen-rich atmosphere or the presence of an ozone layer, which has implications for detecting life on other planets using atmospheric analysis and for the evolution of multicellular life.

Redfield revisited. 1. Regulation of nitrate, phosphate, and oxygen in the ocean

The ratio of phosphate and nitrate concentrations in the deep ocean matches closely the Redfield ratio required by phytoplankton growing in the surface ocean. Furthermore, the oxygen available from dissolution in ocean water is, on average, just sufficient for the respiration of the resulting organic matter. We review various feedback mechanisms that have been proposed to account for these remarkable correspondences and construct a model to test their effectiveness. The models initial steady state is cate responds to perturbation in 1000–2000 years and phosphate in 40,000-60,000 years. However, recently increased estimates oflose to the Redfield ratios and stable against instantaneous changes in the sizes of the nitrate and phosphate reservoirs. When classic flux estimates are adopted, nitr the input and output fluxes of nitrate and phosphate suggest that they respond more rapidly to perturbation, nitrate in 500–1000 years and phosphate in 10,000–15,000 years. Nitrogen fixation tends to maintain nitrate close to Redfield ratio with phosphate, while denitrification tends to keep nitrate as the proximate limiting nutrient and tie it in Redfield ratio to dissolved oxygen. Under increases in phosphorus input to the ocean, the relative responsiveness of nitrogen fixation and denitrification determine whether nitrate remains close to Redfield ratio to phosphate or to oxygen. If nitrogen fixation is strongly limited (e.g., by lack of iron), increasing phosphorus input to the ocean can cause phosphate to deviate above Redfield ratio to nitrate. Hence nitrogen dynamics can control phosphate behavior and nitrate can potentially be the ultimate limiting nutrient over geologic periods of time. When nitrate and phosphate are coupled together by responsive nitrogen fixation, negative feedbacks on organic and calcium-bound phosphorus burial stabilize their concentrations. If anoxia suppresses organic phosphorus burial, the resulting feedbacks on phosphate (positive) and oxygen (negative) improve regulation toward the Redfield ratios. Variants of the model are forced with a global record of phosphorus accumulation in biogenic sediments as a proxy for changes in phosphate input to the ocean over the past 40 Myr. Nitrate is generally regulated close to Redfield ratio to phosphate, despite large changes in phosphorus input. If nitrogen fixation is strongly limited, then there is one interval (∼15 Myr ago) when a very rapid increase in phosphate input forces phosphate above Redfield ratio to nitrate. Decreases in phosphorus input cause phosphate and nitrate to quickly deviate below Redfield ratio with oxygen, removing anoxia from the ocean, while increases in phosphorus input rapidly increase anoxia. Hence we conclude that there appears to be an element of chance in observing todays ocean “on the edge of anoxia” with nitrate, phosphate, and oxygen all close to the Redfield ratios.

No increase in global temperature variability despite changing regional patterns

Evidence from Greenland ice cores shows that year-to-year temperature variability was probably higher in some past cold periods, but there is considerable interest in determining whether global warming is increasing climate variability at present. This interest is motivated by an understanding that increased variability and resulting extreme weather conditions may be more difficult for society to adapt to than altered mean conditions. So far, however, in spite of suggestions of increased variability, there is considerable uncertainty as to whether it is occurring. Here we show that although fluctuations in annual temperature have indeed shown substantial geographical variation over the past few decades, the time-evolving standard deviation of globally averaged temperature anomalies has been stable. A feature of the changes has been a tendency for many regions of low variability to experience increases, which might contribute to the perception of increased climate volatility. The normalization of temperature anomalies creates the impression of larger relative overall increases, but our use of absolute values, which we argue is a more appropriate approach, reveals little change. Regionally, greater year-to-year changes recently occurred in much of North America and Europe. Many climate models predict that total variability will ultimately decrease under high greenhouse gas concentrations, possibly associated with reductions in sea-ice cover. Our findings contradict the view that a warming world will automatically be one of more overall climatic variation.

Land and ocean carbon cycle feedback effects on global warming in a simple Earth system model

A simple Earth system model is developed by coupling a box model of the global carbon cycle to an energy-balance approximation of global temperature. The model includes a range of feedback mechanisms between atmospheric CO2, surface temperature and land and ocean carbon cycling. It is used to assess their effect on the global change being driven by anthropogenic CO2 emissions from fossil fuel burning and land-use change. When tuned to reach the 1990 level of atmospheric CO2, the model CO2 predictions for 1832–1990 are reasonably close to ice-core and instrumental records, observed global warming of ~0.6 K from 1860–1990 is accurately predicted and the land and ocean carbon sinks for the 1980s are close to IPCC central estimates. The ocean sink is reduced by ~0.3 GtC yr-1 when the ocean surface is assumed to warm at the same rate as global surface temperature. Land and oceanic carbon sinks are predicted to be growing at present and hence buffering the rate of rise of atmospheric CO2. In the basic model, the current land carbon sink is assumed to be due to CO2 fertilisation of photosynthesis. The slight warming that has occurred enhances soil respiration (carbon loss) and net primary productivity (carbon uptake) by similar amounts. When the model is forced with a “business as usual”(IS92a) emissions scenario for 1990–2100 followed by a linear decline in emissions to zero at 2200, CO2 reaches a peak of 985 ppmv in 2170 and temperature peaks at +5.5 K in 2180. Peak CO2 is ~135 ppmv higher than suggested by IPCC for the same forcing, principally because global warming first suppresses the land carbon sink then generates a land carbon source. When warming exceeds ~4.5 K, soil respiration “overtakes” the CO2 fertilisation of NPP, triggering a release of ~70 GtC from terrestrial ecosystems over ~100 years. When the effects of temperature on photosynthesis, respiration and soil respiration are removed, peak levels of CO2 are reduced by ~100 ppmv and peak temperature by ~0.5 K. Distinguishing separate soil carbon pools with different residence times does not significantly alter the timing of the switch to a land carbon source or its effect on peak CO2, but it causes the source to persist for longer. If forest re-growth or nitrogen deposition are assumed to contribute to the current land carbon sink, this implies a weaker CO2 fertilisation effect on photosynthesis and generates a larger future carbon source. Peak CO2 levels are also sensitive by about ±80 ppmv to upper and lower limits on the temperature responses of photosynthesis, plant respiration and soil respiration. By forcing the model with a range of future emission scenarios it is found that the creation of a significant land carbon source requires rapid warming, exceeding ~4.5 K, and its magnitude increases with the rate of forcing. The carbon source is greatest for the most rapid burning of the largest reserve of fossil fuel. It is concluded that carbon loss from terrestrial ecosystems may significantly (~10%) amplify global warming under “business as usual” or more extreme scenarios.

Ocean acidification and the Permo-Triassic mass extinction

Ocean acidification and mass extinction The largest mass extinction in Earths history occurred at the Permian-Triassic boundary 252 million years ago. Several ideas have been proposed for what devastated marine life, but scant direct evidence exists. Clarkson et al. measured boron isotopes across this period as a highly sensitive proxy for seawater pH. It appears that, although the oceans buffered the acidifiying effects of carbon release from contemporary pulses of volcanism, buffering failed when volcanism increased during the formation of the Siberian Traps. The result was a widespread drop in ocean pH and the elimination of shell-forming organisms. Science, this issue p. 229 A rapid injection of massive amounts of carbon into the atmosphere acidified the oceans, causing mass extinction. Ocean acidification triggered by Siberian Trap volcanism was a possible kill mechanism for the Permo-Triassic Boundary mass extinction, but direct evidence for an acidification event is lacking. We present a high-resolution seawater pH record across this interval, using boron isotope data combined with a quantitative modeling approach. In the latest Permian, increased ocean alkalinity primed the Earth system with a low level of atmospheric CO2 and a high ocean buffering capacity. The first phase of extinction was coincident with a slow injection of carbon into the atmosphere, and ocean pH remained stable. During the second extinction pulse, however, a rapid and large injection of carbon caused an abrupt acidification event that drove the preferential loss of heavily calcified marine biota.