David J. Beerling

TARGET ATMOSPHERIC CO2: WHERE SHOULD HUMANITY AIM?

Paleoclimate data show that climate sensitivity is ~3 deg-C for doubled CO2, including only fast feedback processes. Equilibrium sensitivity, including slower surface albedo feedbacks, is ~6 deg-C for doubled CO2 for the range of climate states between glacial conditions and ice-free Antarctica. Decreasing CO2 was the main cause of a cooling trend that began 50 million years ago, large scale glaciation occurring when CO2 fell to 450 +/- 100 ppm, a level that will be exceeded within decades, barring prompt policy changes. If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm. The largest uncertainty in the target arises from possible changes of non-CO2 forcings. An initial 350 ppm CO2 target may be achievable by phasing out coal use except where CO2 is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO2 is not brief, there is a possibility of seeding irreversible catastrophic effects.

The interdependence of mechanisms underlying climate-driven vegetation mortality

Climate-driven vegetation mortality is occurring globally and is predicted to increase in the near future. The expected climate feedbacks of regional-scale mortality events have intensified the need to improve the simple mortality algorithms used for future predictions, but uncertainty regarding mortality processes precludes mechanistic modeling. By integrating new evidence from a wide range of fields, we conclude that hydraulic function and carbohydrate and defense metabolism have numerous potential failure points, and that these processes are strongly interdependent, both with each other and with destructive pathogen and insect populations. Crucially, most of these mechanisms and their interdependencies are likely to become amplified under a warmer, drier climate. Here, we outline the observations and experiments needed to test this interdependence and to improve simulations of this emergent global phenomenon.

CO 2 as a primary driver of Phanerozoic climate

Royer et al. (2004) introduce a seawater pH correction to the Phanerozoic temperature reconstruction based on δO variations in marine fossils. Although this correction is a novel idea and it is likely to have played some role in offsetting the δO record, we show that (a) The correction cannot be as large as claimed by Royer et al. (b) Irrespective of the size of the correction, a CO2 signature cannot possibly be seen in the data. (c) Even though the CO2 signature cannot be seen, the pH correction implies only a somewhat higher global temperature sensitivity than that in Shaviv and Veizer (2003), a sensitivity that is consistent with a “black body Earth”, but only marginally with the lower limit of the IPCC range.

Maximum leaf conductance driven by CO2 effects on stomatal size and density over geologic time

Stomatal pores are microscopic structures on the epidermis of leaves formed by 2 specialized guard cells that control the exchange of water vapor and CO2 between plants and the atmosphere. Stomatal size (S) and density (D) determine maximum leaf diffusive (stomatal) conductance of CO2 (gcmax) to sites of assimilation. Although large variations in D observed in the fossil record have been correlated with atmospheric CO2, the crucial significance of similarly large variations in S has been overlooked. Here, we use physical diffusion theory to explain why large changes in S necessarily accompanied the changes in D and atmospheric CO2 over the last 400 million years. In particular, we show that high densities of small stomata are the only way to attain the highest gcmax values required to counter CO2“starvation” at low atmospheric CO2 concentrations. This explains cycles of increasing D and decreasing S evident in the fossil history of stomata under the CO2 impoverished atmospheres of the Permo-Carboniferous and Cenozoic glaciations. The pattern was reversed under rising atmospheric CO2 regimes. Selection for small S was crucial for attaining high gcmax under falling atmospheric CO2 and, therefore, may represent a mechanism linking CO2 and the increasing gas-exchange capacity of land plants over geologic time.

Plant development: Signals from mature to new leaves

Stomata are microscopic pores on the surfaces of leaves, the number and density of which vary in response to changes in environmental conditions, such as carbon dioxide concentration and light. We show here that mature leaves of Arabidopsis thaliana detect and transmit this external information to new leaves of the same plant, producing an appropriate adjustment of stomatal development. As CO2 concentration controls both stomatal opening and number, and stomatal numbers also increase with higher light intensity, the large gradients of CO2 and light found within plant communities have the potential to influence stomatal development.

Phanerozoic atmospheric CO2 change: evaluating geochemical and paleobiological approaches

Abstract The theory and use of geochemical modeling of the long-term carbon cycle and four paleo-PCO 2 proxies are reviewed and discussed in order to discern the best applications for each method. Geochemical models provide PCO 2 predictions for the entire Phanerozoic, but most existing models present 5–10 m.y. means, and so often do not resolve short-term excursions. Error estimates based on sensitivity analyses range from ±75–200 ppmV for the Tertiary to as much as ±3000 ppmV during the early Paleozoic. The δ 13 C of pedogenic carbonates provide the best proxy-based PCO 2 estimates for the pre-Tertiary, with error estimates ranging from ±500–1000 ppmV. Pre-Devonian estimates should be treated cautiously. Error estimates for Tertiary reconstructions via this proxy are higher than other proxies and models (±400–500 ppmV), and should not be solely relied upon. We also show the importance of measuring the δ 13 C of coexisting organic matter instead of inferring its value from marine carbonates. The δ 13 C of the organic remains of phytoplankton from sediment cores provide high temporal resolution (up to 10 3 –10 4 year), high precision (±25–100 ppmV for the Tertiary to ±150–200 ppmV for the Cretaceous) PCO 2 estimates that can be near continuous for most of the Tertiary. Its high temporal resolution and availability of continuous sequences is advantageous for studies aiming to discern short-term excursions. This method, however, must correct for changes in growth rate and oxygen level. At elevated PCO 2 (∼750–1250 ppmV), this proxy loses its sensitivity and is not useful. The stomatal density and stomatal index of land plants also provide high temporal resolution ( 2 year), high precision (±10–40 ppmV for the Tertiary and possibly Cretaceous) PCO 2 estimates, and so also is ideal for discerning short-term excursions. Unfortunately, this proxy also loses sensitivity at some level of PCO 2 above 350 ppmV (which, currently, is largely undetermined). Our analysis of the recently developed δ 11 B technique shows that it currently is not yet well constrained. Most importantly, it requires the assumption that the boron isotopic composition of the ocean remains nearly constant through time. In addition, it assumes that there are no biological or temperature effects and that diagenetic alteration of the boron isotopic composition does not occur. A fifth CO 2 proxy, based on the redox chemistry of marine cerium, has several fundamental flaws and is not discussed in detail here.

Assessing “dangerous climate change”: Required reduction of carbon emissions to protect young people, future generations and nature

We assess climate impacts of global warming using ongoing observations and paleoclimate data. We use Earth’s measured energy imbalance, paleoclimate data, and simple representations of the global carbon cycle and temperature to define emission reductions needed to stabilize climate and avoid potentially disastrous impacts on today’s young people, future generations, and nature. A cumulative industrial-era limit of ∼500 GtC fossil fuel emissions and 100 GtC storage in the biosphere and soil would keep climate close to the Holocene range to which humanity and other species are adapted. Cumulative emissions of ∼1000 GtC, sometimes associated with 2°C global warming, would spur “slow” feedbacks and eventual warming of 3–4°C with disastrous consequences. Rapid emissions reduction is required to restore Earth’s energy balance and avoid ocean heat uptake that would practically guarantee irreversible effects. Continuation of high fossil fuel emissions, given current knowledge of the consequences, would be an act of extraordinary witting intergenerational injustice. Responsible policymaking requires a rising price on carbon emissions that would preclude emissions from most remaining coal and unconventional fossil fuels and phase down emissions from conventional fossil fuels.

A humid climate state during the Palaeocene/Eocene thermal maximum.

An abrupt climate warming of 5 to 10 °C during the Palaeocene/Eocene boundary thermal maximum (PETM) 55 Myr ago is linked to the catastrophic release of ∼1,050–2,100 Gt of carbon from sea-floor methane hydrate reservoirs. Although atmospheric methane, and the carbon dioxide derived from its oxidation, probably contributed to PETM warming, neither the magnitude nor the timing of the climate change is consistent with direct greenhouse forcing by the carbon derived from methane hydrate. Here we demonstrate significant differences between marine and terrestrial carbon isotope records spanning the PETM. We use models of key carbon cycle processes to identify the cause of these differences. Our results provide evidence for a previously unrecognized discrete shift in the state of the climate system during the PETM, characterized by large increases in mid-latitude tropospheric humidity and enhanced cycling of carbon through terrestrial ecosystems. A more humid atmosphere helps to explain PETM temperatures, but the ultimate mechanisms underlying the shift remain unknown.

Past extreme warming events linked to massive carbon release from thawing permafrost

Between about 55.5 and 52 million years ago, Earth experienced a series of sudden and extreme global warming events (hyperthermals) superimposed on a long-term warming trend. The first and largest of these events, the Palaeocene–Eocene Thermal Maximum (PETM), is characterized by a massive input of carbon, ocean acidification and an increase in global temperature of about 5 °C within a few thousand years. Although various explanations for the PETM have been proposed, a satisfactory model that accounts for the source, magnitude and timing of carbon release at the PETM and successive hyperthermals remains elusive. Here we use a new astronomically calibrated cyclostratigraphic record from central Italy to show that the Early Eocene hyperthermals occurred during orbits with a combination of high eccentricity and high obliquity. Corresponding climate–ecosystem–soil simulations accounting for rising concentrations of background greenhouse gases and orbital forcing show that the magnitude and timing of the PETM and subsequent hyperthermals can be explained by the orbitally triggered decomposition of soil organic carbon in circum-Arctic and Antarctic terrestrial permafrost. This massive carbon reservoir had the potential to repeatedly release thousands of petagrams (1015 grams) of carbon to the atmosphere–ocean system, once a long-term warming threshold had been reached just before the PETM. Replenishment of permafrost soil carbon stocks following peak warming probably contributed to the rapid recovery from each event, while providing a sensitive carbon reservoir for the next hyperthermal. As background temperatures continued to rise following the PETM, the areal extent of permafrost steadily declined, resulting in an incrementally smaller available carbon pool and smaller hyperthermals at each successive orbital forcing maximum. A mechanism linking Earth’s orbital properties with release of soil carbon from permafrost provides a unifying model accounting for the salient features of the hyperthermals.

Nature's green revolution: the remarkable evolutionary rise of C4 plants

Plants with the C4 photosynthetic pathway dominate todays tropical savannahs and grasslands, and account for some 30% of global terrestrial carbon fixation. Their success stems from a physiological CO2-concentrating pump, which leads to high photosynthetic efficiency in warm climates and low atmospheric CO2 concentrations. Remarkably, their dominance of tropical environments was achieved in only the past 10 million years (Myr), less than 3% of the time that terrestrial plants have existed on Earth. We critically review the proposal that declining atmospheric CO2 triggered this tropical revolution via its effects on the photosynthetic efficiency of leaves. Our synthesis of the latest geological evidence from South Asia and North America suggests that this emphasis is misplaced. Instead, we find important roles for regional climate change and fire in South Asia, but no obvious environmental trigger for C4 success in North America. CO2-starvation is implicated in the origins of C4 plants 25–32 Myr ago, raising the possibility that the pathway evolved under more extreme atmospheric conditions experienced 10 times earlier. However, our geochemical analyses provide no evidence of the C4 mechanism at this time, although possible ancestral components of the C4 pathway are identified in ancient plant lineages. We suggest that future research must redress the substantial imbalance between experimental investigations and analyses of the geological record.