Century-scale shifts in early holocene atmospheric CO2 concentration

How Much Human-Caused Global Warming Should We Expect with Business-As-Usual (BAU) Climate Policies? A Semi-Empirical Assessment

In order to assess the merits of national climate change mitigation policies, it is important to have a reasonable benchmark for how much human-caused global warming would occur over the coming century with “Business-As-Usual” (BAU) conditions. However, currently, policymakers are limited to making assessments by comparing the Global Climate Model (GCM) projections of future climate change under various different “scenarios”, none of which are explicitly defined as BAU. Moreover, all of these estimates are ab initio computer model projections, and policymakers do not currently have equivalent empirically derived estimates for comparison. Therefore, estimates of the total future human-caused global warming from the three main greenhouse gases of concern (CO2, CH4, and N2O) up to 2100 are here derived for BAU conditions. A semi-empirical approach is used that allows direct comparisons between GCM-based estimates and empirically derived estimates. If the climate sensitivity to greenhouse gases implies a Transient Climate Response (TCR) of ≥ 2.5 °C or an Equilibrium Climate Sensitivity (ECS) of ≥ 5.0 °C then the 2015 Paris Agreement’s target of keeping human-caused global warming below 2.0 °C will have been broken by the middle of the century under BAU. However, for a TCR < 1.5 °C or ECS < 2.0 °C, the target would not be broken under BAU until the 22nd century or later. Therefore, the current Intergovernmental Panel on Climate Change (IPCC) “likely” range estimates for TCR of 1.0 to 2.5 °C and ECS of 1.5 to 4.5 °C have not yet established if human-caused global warming is a 21st century problem.

Plant carbon assimilation rates in atmospheric CO2 reconstructions

Fossil plant gas-exchange-based CO₂ reconstructions use carbon (C) assimilation rates of extant plant species as substitutes for assimilation rates of fossil plants. However, assumptions in model species adoption can lead to systematic error propagation. We used a dataset of c. 2500 extant species to investigate the role of phylogenetic relatedness and ecology in determining C assimilation, an essential variable in gas-exchange-based CO₂ models. We evaluated the effect on random and systematic error propagation in atmospheric CO₂ caused by adopting different model species. Phylogenetic relatedness, growth form, and solar exposure are important predictors of C assimilation rate. CO₂ reconstructions that apply C assimilation rates from modern species based solely on phylogenetic relatedness to fossil species can result in CO₂ estimates that are systematically biased by a factor of > 2. C assimilation rates used in CO₂ reconstructions should be determined by averaging assimilation rates of modern plant species that are (1) in the same family and (2) have a similar habit and habitat as the fossil plant. In addition, systematic bias potential and random error propagation are greatly reduced when CO₂ is reconstructed from multiple fossil plant species with different modern relatives at the same site.

Multi-Year Leaf-Level Response to Sub-Ambient and Elevated Experimental CO2 in Betula nana

The strong link between stomatal frequency and CO₂ in woody plants is key for understanding past CO₂ dynamics, predicting future change, and evaluating the significant role of vegetation in the hydrological cycle. Experimental validation is required to evaluate the long-term adaptive leaf response of C3 plants to CO₂ conditions; however, studies to date have only focused on short-term single-season experiments and may not capture (1) the full ontogeny of leaves to experimental CO₂ exposure or (2) the true adjustment of structural stomatal properties to CO₂, which we postulate is likely to occur over several growing seasons. We conducted controlled growth chamber experiments at 150 ppmv, 450 ppmv and 800 ppmv CO₂ with woody C3 shrub Betula nana (dwarf birch) over two successive annual growing seasons and evaluated the structural stomatal response to atmospheric CO₂ conditions. We find that while some adjustment of leaf morphological and stomatal parameters occurred in the first growing season where plants are exposed to experimental CO₂ conditions, amplified adjustment of non-plastic stomatal properties such as stomatal conductance occurred in the second year of experimental CO₂ exposure. We postulate that the species response limit to CO₂ of B. nana may occur around 400–450 ppmv. Our findings strongly support the necessity for multi-annual experiments in C3 perennials in order to evaluate the effects of environmental conditions and provide a likely explanation of the contradictory results between historical and palaeobotanical records and experimental data.

Evolutionary History of Atmospheric CO2 during the Late Cenozoic from Fossilized Metasequoia Needles

The change in ancient atmospheric CO2 concentrations provides important clues for understanding the relationship between the atmospheric CO2 concentration and global temperature. However, the lack of CO2 evolution curves estimated from a single terrestrial proxy prevents the understanding of climatic and environmental impacts due to variations in data. Thus, based on the stomatal index of fossilized Metasequoia needles, we reconstructed a history of atmospheric CO2 concentrations from middle Miocene to late Early Pleistocene when the climate changed dramatically. According to this research, atmospheric CO2 concentration was stabile around 330-350 ppmv in the middle and late Miocene, then it decreased to 278-284 ppmv during the Late Pliocene and to 277-279 ppmv during the Early Pleistocene, which was almost the same range as in preindustrial time. According to former research, this is a time when global temperature decreased sharply. Our results also indicated that from middle Miocene to Pleistocene, global CO2 level decreased by more than 50 ppmv, which may suggest that CO2 decrease and temperature decrease are coupled.

Drought stress variability in ancient Near Eastern agricultural systems evidenced by δ13C in barley grain

The collapse and resilience of political systems in the ancient Near East and their relationship with agricultural development have been of wide interest in archaeology and anthropology. Despite attempts to link the archaeological evidence to local paleoclimate data, the precise role of environmental conditions in ancient agricultural production remains poorly understood. Recently, stable isotope analysis has been used for reconstructing site-specific ancient growing conditions for crop species in semiarid and arid landscapes. To open the discussion of the role of regional diversity in past agricultural production as a factor in societal development, we present 1.037 new stable carbon isotope measurements from 33 archaeological sites and modern fields in the geographic area of the Fertile Crescent, spanning the Aceramic Neolithic [10,000 calibrated years (cal) B.C.] to the later Iron Age (500 cal B.C.), alongside modern data from 13 locations. Our data show that drought stress was an issue in many agricultural settlements in the ancient Near East, particularly in correlation with the major Holocene climatic fluctuations, but its regional impact was diverse and influenced by geographic factors. Although cereals growing in the coastal areas of the northern Levant were relatively unaffected by Holocene climatic fluctuations, farmers of regions further inland had to apply irrigation to cope with increased water stress. However, inland agricultural strategies showed a high degree of variability. Our findings suggest that regional differences in climatic effects led to diversified strategies in ancient subsistence and economy even within spatially limited cultural units.

A critical framework for the assessment of biological palaeoproxies: predicting past climate and levels of atmospheric CO(2) from fossil leaves

This review uses proxies of past temperature and atmospheric CO(2) composition based on fossil leaves to illustrate the uncertainties in biologically based proxies of past environments. Most leaf-based proxies are geographically local or genetically restricted and therefore can be confounded by evolution, extinction, changes in local environment or immigration of species. Stomatal frequency proxies illustrate how genetically restricted proxies can be particularly vulnerable to evolutionary change. High predictive power in the modern world resulting from the use of a very narrow calibration cannot be confidently extrapolated into the past (the Ginkgo paradox). Many foliar physiognomic proxies of climate are geographically local and use traits that are more or less fixed for individual species. Such proxies can therefore be confounded by floristic turnover and biome shifts in the region of calibration. Uncertainty in proxies tends to be greater for more ancient fossils. I present a set of questions that should be considered before using a proxy. Good proxies should be relatively protected from environmental and genetic change, particularly through having high information content and being founded on biomechanical or biochemical principles. Some current and potential developments are discussed, including those that involve more mechanistically sound proxies and better use of multivariate approaches.

Stomatal index responses of Agrostis canina to CO2 and sulphur dioxide: implications for palaeo-[CO2] using the stomatal proxy

• Stomatal index values of fossil plants are widely used in reconstructing palaeo-[CO(2)]. This depends upon the assumption that the stomatal index is determined by the atmospheric concentration of CO(2) ([CO(2)]). This study investigates whether fumigation with, and resistance to, sulphur dioxide (SO(2)) induces a reduction in the stomatal index that may affect stomatal reconstructions of palaeo-[CO(2)] coinciding with episodes of global-scale volcanism. • Agrostis canina from Mefite di Ansanto, Italy, grow in atmospheres of elevated-[CO(2)], SO(2) and hydrogen sulphide (H(2)S). Mefite A. canina were compared with a control population in a 'common-garden' experiment and a controlled-environment study under elevated-[CO(2)] and SO(2) fumigation. • In A. canina, resistance to toxic volcanic gases is not associated with reduced stomatal index, and fumigation with SO(2) does not cause a decrease in stomatal initiation. The two populations of A. canina analyzed in this study exhibit different stomatal index-[CO(2)] 'responses', with control plants showing a reduction in stomatal index and Mefite plants showing no response. • Stomatal reconstructions of palaeo-[CO(2)] during past episodes of global-scale volcanism probably reflect atmospheric [CO(2)] and not [SO(2)]. The lack of a reduction in the stomatal index in response to elevated [CO(2)] in the Mefite plants, suggests that resistance to toxic gases and/or long-term growth at high [CO(2)] reduces, or negates, sensitivity of the stomatal index-[CO(2)] relationship, or that stomatal index-[CO(2)] in the Mefite plants is attuned to [CO(2)] fluctuations at much higher concentrations.

Drought stress signals in modern and subfossil Quercus laurifolia (Fagaceae) leaves reflect winter precipitation in southern Florida tied to El Nino-Southern Oscillation activity

In the present study, structural xeromorphic features in modern and subfossil Quercus laurifolia leaves from southern Florida were quantified to reconstruct past precipitation changes in sensitive terrestrial settings. Absolute cell numbers/mm(2), quantified as epidermal cell density (ED) have been analyzed on leaves from herbarium collections as well as the leaves accumulated during the past 125 years in peat deposits. The results reveal a common principal correlation between the measured ED and winter precipitation (November through March, NDJFM: Herbarium r = -0.74; peat profiles FAK98 r = -0.72, FAK02 r = -0.53) providing a measure of seasonal drought stress. In Florida, the amount of winter precipitation depends on El Niño-Southern Oscillation (ENSO) activity, where El Niño years produce wet and cold winters, while La Niña winters are dry and warm. The negative correlation between cell numbers and winter precipitation has the potential to record precipitation variability from subfossil leaves on near-annual to decadal time scales. In subtropical, terrestrial environments, where traditional paleo-proxies are limited, systematic analysis of leaf morphological characteristics can provide important information on precipitation changes through time.

Differences in the response sensitivity of stomatal index to atmospheric CO2 among four genera of Cupressaceae conifers

BACKGROUND AND AIMS

The inverse relationship between stomatal density (SD: number of stomata per mm(2) leaf area) and atmospheric concentration of CO2 ([CO2]) permits the use of plants as proxies of palaeo-atmospheric CO2. Many stomatal reconstructions of palaeo-[CO2] are based upon multiple fossil species. However, it is unclear how plants respond to [CO2] across genus, family or ecotype in terms of SD or stomatal index (SI: ratio of stomata to epidermal cells). This study analysed the stomatal numbers of conifers from the ancient family Cupressaceae, in order to examine the nature of the SI-[CO2] relationship, and potential implications for stomatal reconstructions of palaeo-[CO2]. Methods Stomatal frequency measurements were taken from historical herbarium specimens of Athrotaxis cupressoides, Tetraclinis articulata and four Callitris species, and live A. cupressoides grown under CO2-enrichment (370, 470, 570 and 670 p.p.m. CO2).

KEY RESULTS

T. articulata, C. columnaris and C. rhomboidea displayed significant reductions in SI with rising [CO2]; by contrast, A. cupressoides, C. preissii and C. oblonga show no response in SI. However, A. cupressoides does reduce SI to increases in [CO2] above current ambient (approx. 380 p.p.m. CO2). This dataset suggests that a shared consistent SI-[CO2] relationship is not apparent across the genus Callitris. Conclusions The present findings suggest that it is not possible to generalize how conifer species respond to fluctuations in [CO2] based upon taxonomic relatedness or habitat. This apparent lack of a consistent response, in conjunction with high variability in SI, indicates that reconstructions of absolute palaeo-[CO2] based at the genus level, or upon multiple species for discrete intervals of time are not as reliable as those based on a single or multiple temporally overlapping species.

Global warming and carbon dioxide through sciences

Increased atmospheric CO(2)-concentration is widely being considered as the main driving factor that causes the phenomenon of global warming. This paper attempts to shed more light on the role of atmospheric CO(2) in relation to temperature-increase and, more generally, in relation to Earth's life through the geological aeons, based on a review-assessment of existing related studies. It is pointed out that there has been a debate on the accuracy of temperature reconstructions as well as on the exact impact that CO(2) has on global warming. Moreover, using three independent sets of data (collected from ice-cores and chemistry) we perform a specific regression analysis which concludes that forecasts about the correlation between CO(2)-concentration and temperature rely heavily on the choice of data used, and one cannot be positive that indeed such a correlation exists (for chemistry data) or even, if existing (for ice-cores data), whether it leads to a "severe" or a "gentle" global warming. A very recent development on the greenhouse phenomenon is a validated adiabatic model, based on laws of physics, forecasting a maximum temperature-increase of 0.01-0.03 degrees C for a value doubling the present concentration of atmospheric CO(2). Through a further review of related studies and facts from disciplines like biology and geology, where CO(2)-change is viewed from a different perspective, it is suggested that CO(2)-change is not necessarily always a negative factor for the environment. In fact it is shown that CO(2)-increase has stimulated the growth of plants, while the CO(2)-change history has altered the physiology of plants. Moreover, data from palaeoclimatology show that the CO(2)-content in the atmosphere is at a minimum in this geological aeon. Finally it is stressed that the understanding of the functioning of Earth's complex climate system (especially for water, solar radiation and so forth) is still poor and, hence, scientific knowledge is not at a level to give definite and precise answers for the causes of global warming.

A role for atmospheric CO2 in preindustrial climate forcing

Complementary to measurements in Antarctic ice cores, stomatal frequency analysis of leaves of land plants preserved in peat and lake deposits can provide a proxy record of preindustrial atmospheric CO(2) concentration. CO(2) trends based on leaf remains of Quercus robur (English oak) from the Netherlands support the presence of significant CO(2) variability during the first half of the last millennium. The amplitude of the reconstructed multidecadal fluctuations, up to 34 parts per million by volume, considerably exceeds maximum shifts measured in Antarctic ice. Inferred changes in CO(2) radiative forcing are of a magnitude similar to variations ascribed to other mechanisms, particularly solar irradiance and volcanic activity, and may therefore call into question the concept of the Intergovernmental Panel on Climate Change, which assumes an insignificant role of CO(2) as a preindustrial climate-forcing factor. The stomata-based CO(2) trends correlate with coeval sea-surface temperature trends in the North Atlantic Ocean, suggesting the possibility of an oceanic source/sink mechanism for the recorded CO(2) changes.

The impact of Miocene atmospheric carbon dioxide fluctuations on climate and the evolution of terrestrial ecosystems

The Miocene is characterized by a series of key climatic events that led to the founding of the late Cenozoic icehouse mode and the dawn of modern biota. The processes that caused these developments, and particularly the role of atmospheric CO2 as a forcing factor, are poorly understood. Here we present a CO2 record based on stomatal frequency data from multiple tree species. Our data show striking CO2 fluctuations of approximately 600-300 parts per million by volume (ppmv). Periods of low CO2 are contemporaneous with major glaciations, whereas elevated CO2 of 500 ppmv coincides with the climatic optimum in the Miocene. Our data point to a long-term coupling between atmospheric CO2 and climate. Major changes in Miocene terrestrial ecosystems, such as the expansion of grasslands and radiations among terrestrial herbivores such as horses, can be linked to these marked fluctuations in CO2.

Rapid atmospheric CO2 changes associated with the 8,200-years-B.P. cooling event

By applying the inverse relation between numbers of leaf stomata and atmospheric CO2 concentration, stomatal frequency analysis of fossil birch leaves from lake deposits in Denmark reveals a century-scale CO2 change during the prominent Holocene cooling event that occurred in the North Atlantic region between 8,400 and 8,100 years B.P. In contrast to conventional CO2 reconstructions based on ice cores from Antarctica, quantification of the stomatal frequency signal corroborates a distinctive temperature-CO2 correlation. Results indicate a global CO2 decline of approximately 25 ppm by volume over approximately 300 years. This reduction is in harmony with observed and modeled lowering of North Atlantic sea-surface temperatures associated with a short-term weakening of thermohaline circulation.

Lateglacial and Early Holocene vegetation history of the northern Wetterau and the Amöneburger Basin (Hessen), central-west Germany

The Lateglacial and Early Holocene vegetation history of the northern Wetterau and Amöneburger Basin, two intra-montane basins in Hessen, central-west Germany, is reconstructed by means of pollen and macrofossil analyses. Regional pollen assemblage zones are defined for the Lateglacial and Early Holocene. After calibration of the radiocarbon dates and establishment of age/depth relationships, the ages of the pollen zone boundaries are calculated. The regional vegetation changes correlate closely with the major fluctuations in the delta18O curve of the Greenland ice cores spanning the same time period. During the early Lateglacial, the open herbaceous vegetation with dwarf shrubs in the northern Wetterau was replaced by woodlands. Initially these woodlands consisted of birch, but after the immigration of pine, mixed forests of pine and birch developed. Soon after its immigration pine became dominant and pine woodlands largely replaced the birch forests. Only on the locally wetter substrates of the river floodplain did Betula stands persist. Gradually the importance of herbaceous communities declined and the pine woodlands lost their open character. During the Lateglacial two regressive phases in the vegetation succession are reflected in the data which are equated with the Older- and Younger Dryas biozones. At the beginning of the Younger Dryas, the forest-limit was lowered and the importance of herbaceous communities increased. Later, pine woodlands thinned and Ericales became part of the vegetation, indicating the development of more acid, nutrient-poor soils. A subdivision of the Younger Dryas biozone into a wetter, colder first part and a drier, warmer second part is suggested. At the beginning of the Early Holocene, pine woodlands became more closed and soils more stabilised. The transition between the Younger Dryas and Preboreal biozones is indicated by a lithological change to organic (-rich) deposits. Betula stands persisted on the locally wetter substrates of the fluvial plains. At the end of the early Preboreal, there was an extension of Betula stands on the river floodplains. This is interpreted as a phase in which climate was temporarily wetter, perhaps a reflection of a short-term climatic oscillation, the so-called Preboreal oscillation. During the Preboreal biozone, ferns became more important in the local vegetation of the mires, and later during the Boreal they formed the undergrowth of swamp forests. During the Boreal biozone, a major expansion of hazel took place in both areas. On the hills and slopes of the northern Wetterau and the Amöneburger Basin, open pine forests and hazel scrub developed with grasses and Pteridium in the field layer. In the river valleys, pine forests were replaced by deciduous mixed forests with oak, elm and later also with lime.

High altitude C(4) grasslands in the northern Andes: relicts from glacial conditions?

The altitudinal vegetation distribution in the northern Andes during glacial time differed from the present-day conditions as a result of temperature and precipitation change. New evidence indicate that as a response to a reduced atmospheric partial CO(2) pressure (pCO(2)), the competitive balance between C(3) and C(4) plants have changed. Effects may have remained virtually undetected in pollen records, but can be observed using a stable carbon isotope analysis. Vegetation dominated by C(4) taxa, belonging to the families Cyperaceae (e.g. Bulbostylis and Cyperus) and Poaceae (e.g. Muhlenbergia, Paspalum and Sporobolus), may have been able to replace for a significant part the modern type C(3) taxa (e.g. species belonging to Carex, Rhynchospora, Aciachne, Agrostis, Calamagrostis, and Chusquea). Impact of reduced glacial atmospheric pCO(2) levels and lower glacial temperatures on the composition and the elevational distribution of the vegetation types is discussed. The present high Andean vegetation communities may differ from the glacial equivalents (non-modern analogue situation). We identified dry Sporobolus lasiophyllus tussock grassland and Arcytophyllum nitidum dwarfshrub paramo as the possible relict communities from glacial time. The effect on previous estimates of paleo-temperatures is estimated to be small.

Sun and shade leaves? Cuticle ultrastructure of Jurassic Komlopteris nordenskioeldii (Nathorst) Barbacka

An ultrastructural transmission electron microscope (TEM) study of fossil leaf cuticles from the Jurassic pteridosperm Komlopteris nordenskioeldii (Nathorst) Barbacka from the Mecsek Mountains (South Hungary) was conducted. Remnants of cuticles of leaves originating from so-called "sun and shade" environments were sectioned with a diamond knife, transversally as well as longitudinally. Although the present study showed a simple type of cuticle in this pteridosperm, differences were observed in the occurrence of its components, such as electron lucent amorphous material and various densities of granules, which give rise to different zones. The included fibrilous elements appeared to be made of aggregated and aligned granules, equivalent in size and electron density to nearby non-fibrilous granular regions. The combinations of these ultrastructural features allow distinctions between four types of cuticle: sun upper, sun lower, shade upper and shade lower. Considering the distinction made earlier in two types of cuticle and supposed to be related to sun and shade on the basis of macroscopical and microscopical features, four types only on the basis of differences in thickness, the present study reinforces the distinctions with ultrastructural microcharacteristics. As this study shows the variations in ultrastructure of cuticle among the four types, the differences observed may reveal the great sensitivity of some plants to environment. At the same time, it points out the importance, in ultrastructural studies of cuticles, of studying a number of samples for one taxon.

Stomatal density and stomatal index as indicators of paleoatmospheric CO(2) concentration

A growing number of studies use the plant species-specific inverse relationship between atmospheric CO(2) concentration and stomatal density (SD) or stomatal index (SI) as a proxy for paleo-CO(2) levels. A total of 285 previously published SD and 145 SI responses to variable CO(2) concentrations from a pool of 176 C(3) plant species are analyzed here to test the reliability of this method. The percentage of responses inversely responding to CO(2) rises from 40 and 36% (for SD and SI, respectively) in experimental studies to 88 and 94% (for SD and SI, respectively) in fossil studies. The inconsistent experimental responses verify previous concerns involving this method, however the high percentage of fossil responses showing an inverse relationship clearly validates the method when applied over time scales of similar length. Furthermore, for all groups of observations, a positive relationship between CO(2) and SD/SI is found in only </=12% of cases. Thus, CO(2) appears to inversely affect stomatal initiation, although the mechanism may involve genetic adaptation and therefore is often not clearly expressed under short CO(2) exposure times.Experimental responses of SD and SI based on open-top chambers (OTCs) inversely relate to CO(2) less often than greenhouse-based responses (P<0.01 for both SD and SI), and should be avoided when experimental responses are required for CO(2) reconstructions. In the combined data set, hypostomatous species follow the inverse relationship more often than amphistomatous species (56 vs. 44% for SD; 69 vs. 32% for SI; P<0.03 for both comparisons). Both the SD and SI of fossil responses are equally likely to inversely relate to CO(2) when exposed to elevated versus subambient CO(2) concentrations (relative to today). This result casts doubt on previous claims that stomata cannot respond to CO(2) concentrations above present-day levels. Although the proportion of SD and SI responses inversely relating to CO(2) are similar, SD is more strongly affected by various environmental stresses, and thus SI-based CO(2) reconstructions are probably more accurate.