The Leaf Annals

The recent publication of the summary for policy makers by Working Group I of the Intergovernmental Panel on Climate Change (IPCC) [1] has injected a renewed sense of urgency to address climate change. It is therefore timely to review the notion of preventing 'dangerous anthropogenic interference with the climate system' as put forward in the United Nations Framework Convention on Climate Change (UNFCCC). The article by Danny Harvey in this issue [2] offers a fresh perspective by rephrasing the concept of 'dangerous interference' as a problem of risk assessment. As Harvey points out, identification of 'dangerous interference' does not require us to know with certainty that future climate change will be dangerous—an impossible task given that our knowledge about future climate change includes uncertainty. Rather, it requires the assertion that interference would lead to a significant probability of dangerous climate change beyond some risk tolerance, and therefore would pose an unacceptable risk.

Carbon assimilation at low carbon dioxide levels was measured on three Oryza specics (O. sativa L. cv. Toyonishiki, O. officinalis Wall, and O. meyriana Baill.), Brassica napus L. cv. Michinokunatane and Triticum aestivum L. cv. Konosu No.25. Measurements were made at two different oxygen concentrations; 140% and 21% (atmospheric pressure). An improvement in measurement device was made for ensuring an accuracy of the meter readings. That is, a recorder with a modulator was used to enlarge the differences in the carbon dioxide concentration; two- and five-fold for carbon dioxide levels above and below carbon dioxidc compensation point (gamma), respectively. It seems that HEATH and ORCHARD (1968) and HOLMGREN and JARVIS (1967) Changed the carbon dioxide concentration at large intervals, resulting in 3 to 5 measurements below gamma. Such a few measurements would obscure the statistics of the carbon dioxide exchangc rate at low carbon dioxide levels. The changes of carbon dioxide concentration in this experiment, however, were made at intervals of about 2 or 4 ppm from 0 ppm to gamma, resulting in 7 to 14 mean values below it. As the characteristics of carbon dioxide absorption at low carbon dioxide levels, GABRIELSEN (1948) proposed the 'threshold hyPothesis' in which gamma was regarded as a threshold value below which no assimilaton occurred, while HEATH and 0RCHARD (1968) postulated the existence of a 'third process', in addition to ordinary (dark) respiration and assimilation, which could be expected to have a different balance between respiration and assimilation. They denied the adoptation of the threshold hypothesis. From the prescnt experiment in which the carbon dioxide exchange rates were traced by Changing the Carbon dioxide concentrations at very small intervals, hwever, it appeared that the rate of carbon dioxide uptake at low carbon dioxide levels and atmospheric oxygen pressure tended to decrease toward 1/2 gamma carbon dioxide level, and the carbon dioxide uptake seems to cease and only the carbon dioxide release secms to occur below it. In case of measurements at 14% oxygen concentration the situation was similar to those at 2l% oxygen concentration, but a considerable decrease of the value of gamma. Thus, the process of carbon dioxide uptake at low carbon dioxide levels seems to imply the threshold hypothesis and 1/2 gamma seems to be an approximation of the threshold value. The assimilation rate is estimated as the ratio of carbon dioxide concentration differences between ambient air and assimilation center to the sum of diffusion resistances.As an estimate of carbon dioxide levels in the assimilation center in this formula, GAASTRA (1959) proposed zero, while BIERHUIZEN and SLAYTER (1964) adopted the Practise of using gamma to estimate it. From the results mentioned above, we could propose to use 1/2 gamma as its primary approximation because the photosynthetic center would be exposed to this carbon dioxide level but not absorb it.

BackgroundChanges in aboveground plant species diversity as well as variations of environmental conditions such as exposure of ecosystems to elevated concentrations of atmospheric carbon dioxide may lead to changes in metabolic activity, composition and diversity of belowground microbial communities, both bacterial and fungal.ResultsWe examined soil samples taken from a biodiversity × CO2 grassland experiment where replicate plots harboring 5, 12, or 31 different plant species had been exposed to ambient or elevated (600 ppm) levels of carbon dioxide for 5 years. Analysis of soil bacterial communities in these plots by temporal temperature gradient gel electrophoresis (TTGE) showed that dominant soil bacterial populations varied only very little between different experimental treatments. These populations seem to be ubiquitous. Likewise, screening of samples on a high-resolution level by terminal restriction fragment length polymorphism (T-RFLP) showed that increased levels of carbon dioxide had no significant influence on both soil bacterial community composition (appearance and frequency of operational taxonomic units, OTUs) and on bacterial richness (total number of different OTUs). In contrast, differences in plant diversity levels had a significant effect on bacterial composition but no influence on bacterial richness. Regarding species level, several bacterial species were found only in specific plots and were related to elevated carbon dioxide or varying plant diversity levels. For example, analysis of T-RFLP showed that the occurrence of Salmonella typhimurium was significantly increased in plots exposed to elevated CO2 (P < 0.05).ConclusionPlant diversity levels are affecting bacterial composition (bacterial types and their frequency of occurrence). Elevated carbon dioxide does not lead to quantitative alteration (bacterial richness), whereas plant diversity is responsible for qualitative changes (bacterial diversity).

Climate Change About 60 million years ago, carbon dioxide concentration in Earth's atmosphere rose sharply, resulting in severe climatic changes that may in some ways mirror Earth's future climate. In a Perspective, Alley explains that the consequences of these climatic changes were severe. Consequences included dwarfing of land mammals, ecosystem disruptions, ocean acidification, and loss of coral reefs. The carbon dioxide rise during the Paleocene-Eocene Thermal Maximum (PETM) occurred over a longer time scale than that caused by fossil fuel burning today, which gave ecosystems more time to adjust to climate changes. Thus, the consequences of current and future global warming may be even more severe than those seen in this distant mirror. Science , this issue p. [151][1] [1]: /lookup/doi/10.1126/science.aaf4837

There is compelling evidence that warming of the climate system due to human influence is taking place. The mechanisms for these processes include increasing levels of greenhouse gases (GHGs) in the atmosphere. In May 2013, carbon dioxide levels in the atmosphere exceeded 400 ppm for the first time in several hundred millennia. Levels of carbon dioxide in the remote atmosphere have increased by 0.28 and 2.93 parts per million (ppm) per year since 1960. Ongoing increases in levels of GHGs can be expected to result in a rise in global temperatures, which could trigger a wide range of risk scenarios. A level of 450 ppm is considered to be the atmospheric concentration of carbon dioxide at which temperature change can still be limited to 2 °C. This is a benchmark of climate change specified so as to limit the dangerous effects of climate change. The international community has initiated a series of programmes to address GHG emissions, underpinned by a significant effort to develop robust emission inventories: however, these initiatives have had limited success in limiting the rising trends in GHG emissions. The key issues encountered during these programmes include: population and economic growth; scientific credibility; political priorities; outsourcing of emissions; and difficulties in implementing renewable energy technologies. In this context, the scientific community has an ongoing role to produce data and analysis to link individual, corporate and state actions and policies to evaluations of GHG emissions, and analysis of evidence for changes in the global climate. The scientific and policy communities should continue to engage with the public, to provide reliable information about the vitally important issues associated with global climate change. Public support for positive action and investment to mitigate climate change is essential to support the decisions which will need to be taken in the coming decades.

The global cycles of carbon and nitrogen are being perturbed by human activities that increase the transfer from large pools of non‐reactive forms of the elements to reactive forms that are essential to the functioning of the terrestrial biosphere. The cycles are closely linked at all scales, and global change analyses must consider C and N cycles together. The increasing amount of N originating from fossil fuel combustion and deposited to terrestrial ecosystems as nitrogen oxides could increase the capacity of ecosystems to sequester C, thereby removing some of the excess carbon dioxide from the atmosphere and slowing the development of greenhouse warming. Several global and ecosystem models have calculated the amount of C sequestration that can be attributed to N deposition, based on assumptions about the allocation of N among ecosystem components with different C∶N ratios. They support the premise that, since industrialization began, N deposition has been responsible for an increasing terrestrial C sink, but there is great uncertainty whether ecosystems will continue to retain exogenous N. Whether terrestrial ecosystems continue to sequester additional C will depend in part on their response to increasing concentrations of atmospheric carbon dioxide, widely thought to be constrained by limited N availability. Ecosystem models generally support the conclusion that responses to increasing concentrations of carbon dioxide will be greater, and the range of possible responses will be wider, in ecosystems where increased N inputs originate as atmospheric deposition. The interactions between N deposition and increasing carbon dioxide concentrations could be altered considerably, however, by additional factors, including N saturation of ecosystems, changes in community composition, and climate change. Nitrogen deposition is also linked to global change issues through the volatile losses of nitrous oxide, which is a potent greenhouse gas, and the role of nitrogen oxides in the production of tropospheric ozone, which could interact with plant responses to elevated carbon dioxide. Any consideration of the role of N deposition in global change issues must also balance the projected responses against the serious detrimental impact of excess N on the environment.

As the global carbon dioxide concentration rises, we need to understand the combination of direct stress effects of this gas and the anticipated effects of climatic change, including drought, on the physiology and growth of all crops [1]. The current increase in the atmospheric carbon dioxide concentration along with predictions of possible future increases in global air temperatures have stimulated interest in the effects of CO2 and temperature on the growth and yield of food crops [2] 2 has been documented continuously since 1958 by Keeling et al. [3], and currently the concentration of CO2 in air is about 360 μL L . The concentration could increase to about 670–760 μL L 1 by the year 2075 mainly because of the burning of fossil fuels [4,5]. General circulation models predict that global warming will result from rising CO2 and other greenhouse gases [6–11]. The stress effects of rising CO2 and elevated temperatures on tropical plants have received less attention than the effects on temperate species [12]. Because both CO2 and temperature have large effects on plants, especially those with the C3 photosynthetic pathway, it is important to quantify the effects of these climatic variables on C3 food crops [10]. Concern over the well-documented increase in the concentration of carbon dioxide in the earth’s atmosphere has stimulated research on the response of plants to this aspect of global change. Much of this research has focused on the response of photosynthetic carbon dioxide fixation, because the process is often dramatically and directly affected by the carbon dioxide concentration, and it is of fundamental importance both to plant growth and to ecosystem carbon storage. The concentration of carbon dioxide in the atmo-

The oxygen production of a photosynthetic gas exchanger containing Chlorella pyrenoidosa (1% packed cell volume) was measured when various concentrations of carbon dioxide were present within the culture unit. The internal carbon dioxide concentrations were obtained by manipulating the entrance gas concentration and the flow rate. Carbon dioxide percentages were monitored by means of electrodes placed directly in the nutrient medium. The concentration of carbon dioxide in the nutrient medium which produced maximal photosynthesis was in the range of 1.5 to 2.5% by volume. Results were unaffected by either the level of carbon dioxide in the entrance gas or the rate of gas flow. Entrance gases containing 2% carbon dioxide flowing at 320 ml/min, 3% carbon dioxide at 135 ml/min, and 4% carbon dioxide at 55 ml/min yielded optimal carbon dioxide concentrations in the particular unit studied. By using carbon dioxide electrodes implanted directly in the gas exchanger to optimize the carbon dioxide concentration throughout the culture medium, it should be possible to design more efficient large-scale units.

For all intents and purposes, climate change has resulted in a massive, accidental experiment involving the entire planet and requiring concerted action around the world. In February 2015, scientists at the National Oceanic and Atmospheric Administration’s Mauna Loa Observatory recorded an atmospheric concentration of carbon dioxide at 399 parts per million,1 the highest it has been in hundreds of millions of years. Concentrations of carbon dioxide in the atmosphere could exceed 1,000 parts per million by volume by the year 2050 if trends continue.2

This study employs regression analysis to investigate the relationships between carbon dioxide levels, sunspot occurrences, and global temperatures, encompassing both land and sea. By uncovering these connections, the study contributes to our understanding of climate change and solar phenomena interactions. The primary objective is to reveal the intricate associations between these elements, potentially influencing climate change and solar activity. The study's outcomes have significant implications for climate change research and solar activity monitoring. The positive correlation between carbon dioxide concentration and ocean temperatures emphasizes the impact of atmospheric carbon dioxide on sea temperature fluctuations. Conversely, the inverse correlation between sunspot numbers and land/global temperatures suggests solar activity's potential role in shaping Earth's temperature oscillations. This research introduces novelty by concurrently investigating the interconnectedness of these factors. The study establishes substantial connections between carbon dioxide concentration, sunspot numbers, and global temperatures. While the models shed light on some variability, the complexity of climate change and solar activity calls for further exploration of additional factors. This underscores the need to consider multiple variables for a comprehensive understanding. Further research is recommended to enhance the precision of these models.

Increased storage of carbon in the oceans has been proposed as a mechanism to explain lower concentrations of atmospheric carbon dioxide during ice ages; however, unequivocal signatures of this storage have not been found1. In seawater, the dissolved gases oxygen and carbon dioxide are linked via the production and decay of organic material, with reconstructions of low oxygen concentrations in the past indicating an increase in biologically mediated carbon storage. Marine sediment proxy records have suggested that oxygen concentrations in the deep ocean were indeed lower during the last ice age, but that near-surface and intermediate waters of the Pacific Ocean-a large fraction of which are poorly oxygenated at present-were generally better oxygenated during the glacial1-3. This vertical opposition could suggest a minimal net basin-integrated change in carbon storage. Here we apply a dual-proxy approach, incorporating qualitative upper-water-column and quantitative bottom-water oxygen reconstructions4,5, to constrain changes in the vertical extent of low-oxygen waters in the eastern tropical Pacific since the last ice age. Our tandem proxy reconstructions provide evidence of a downward expansion of oxygen depletion in the eastern Pacific during the last glacial, with no indication of greater oxygenation in the upper reaches of the water column. We extrapolate our quantitative deep-water oxygen reconstructions to show that the respired carbon reservoir of the glacial Pacific was substantially increased, establishing it as an important component of the coupled mechanism that led to low levels of atmospheric carbon dioxide during the glacial.

Assessing human impacts on climate and biodiversity requires an understanding of the relationship between the concentration of carbon dioxide (CO2) in the Earth’s atmosphere and global temperature (T). Here I explore this relationship empirically using comprehensive, recently-compiled databases of stable-isotope proxies from the Phanerozoic Eon (~540 to 0 years before the present) and through complementary modeling using the atmospheric absorption/transmittance code MODTRAN. Atmospheric CO2 concentration is correlated weakly but negatively with linearly-detrended T proxies over the last 425 million years. Of 68 correlation coefficients (half non-parametric) between CO2 and T proxies encompassing all known major Phanerozoic climate transitions, 77.9% are non-discernible (p &gt; 0.05) and 60.0% of discernible correlations are negative. Marginal radiative forcing (ΔRFCO2), the change in forcing at the top of the troposphere associated with a unit increase in atmospheric CO2 concentration, was computed using MODTRAN. The correlation between ΔRFCO2 and linearly-detrended T across the Phanerozoic Eon is positive and discernible, but only 2.6% of variance in T is attributable to variance in ΔRFCO2. Of 68 correlation coefficients (half non-parametric) between ΔRFCO2 and T proxies encompassing all known major Phanerozoic climate transitions, 75.0% are non-discernible and 41.2% of discernible correlations are negative. Spectral analysis, auto- and cross-correlation show that proxies for T, atmospheric CO2 concentration and ΔRFCO2 oscillate across the Phanerozoic, and cycles of CO2 and ΔRFCO2 are antiphasic. A prominent 15 million-year CO2 cycle coincides closely with identified mass extinctions of the past, suggesting a pressing need for research on the relationship between CO2, biodiversity extinction, and related carbon policies. This study demonstrates that changes in atmospheric CO2 concentration did not cause temperature change in the ancient climate.

It has been recently recognized that increases in carbon dioxide concentration such as are anticipated for the earth's atmosphere in the next century often reduce plant respiration. There can be both a short‐term reversible effect of unknown cause, and long‐term acclimation, which may reflect the synthesis and maintenance of less metabolically expensive materials in plants grown at elevated carbon dioxide concentrations. Because respiration provides energy and carbon intermediates for growth and maintenance, reductions in respiration by increasing carbon dioxide concentrations may have effects on physiology beyond an improvement in plant carbon balance. As atmospheric carbon dioxide concentration increases, reduced respiration could be as important as increased photosynthesis in improving the ability of terrestrial vegetation to act as a sink for carbon, but it could also have other consequences.

Terrestrial carbon fluxes are an important factor in regulating concentrations of atmospheric carbon dioxide (CO2). In this study, we use a coupled climate model with interactive biogeochemistry to benchmark the simulation of net primary productivity (NPP) and its response to elevated atmospheric CO2. Short-term field experiments such as Free-Air Carbon Dioxide Enrichment (FACE) studies have examined this phenomenon but it is difficult to infer trends from only a few years of field data. Here, we employ the University of Victoria's Earth System Climate Model (UVic ESCM) version 2.8 to compare simulated changes in NPP due to an elevated atmospheric CO2 concentration of 550 ppm to observed increases in NPP of 23% ±2% from four temperate forest FACE studies between 1997 and 2002. We further compare two scenarios: elevated CO2 with climate change, and elevated CO2 without climate change, the latter being consistent with FACE methodology. In the climate change scenario global terrestrial and forest-only NPP increased by 24.5% and 27.9%, respectively, while these increases were 21.0% and 17.2%, respectively, in the latitude band most representative of the location of the FACE studies. In the scenario without climate change, terrestrial and forest-only NPP increased instead by 28.3% and 30.6%, respectively, while these increases were 24.3% and 14.4%, respectively, in the FACE latitudes. This suggests that the model may underestimate temperate forest NPP increases when compared to results from temperate forest FACE studies and highlights the need for both increased experimental study of other forest biomes and further model development.

Geological and palaeomagnetic studies indicate that ice sheets may have reached the Equator at the end of the Proterozoic eon, 800 to 550 million years ago, leading to the suggestion of a fully ice-covered 'snowball Earth'. Climate model simulations indicate that such a snowball state for the Earth depends on anomalously low atmospheric carbon dioxide concentrations, in addition to the Sun being 6 per cent fainter than it is today. However, the mechanisms producing such low carbon dioxide concentrations remain controversial. Here we assess the effect of the palaeogeographic changes preceding the Sturtian glacial period, 750 million years ago, on the long-term evolution of atmospheric carbon dioxide levels using the coupled climate-geochemical model GEOCLIM. In our simulation, the continental break-up of Rodinia leads to an increase in runoff and hence consumption of carbon dioxide through continental weathering that decreases atmospheric carbon dioxide concentrations by 1,320 p.p.m. This indicates that tectonic changes could have triggered a progressive transition from a 'greenhouse' to an 'icehouse' climate during the Neoproterozoic era. When we combine these results with the concomitant weathering effect of the voluminous basaltic traps erupted throughout the break-up of Rodinia, our simulation results in a snowball glaciation.