The Paleogene atmospheric CO2 concentrations reconstructed using stomatal analysis of fossil Metasequoia needles

During the Paleogene, the Earth experienced a global greenhouse climate, which was much warmer and more humid than the present climate. The present global warming is ascribed to increasing levels of atmospheric CO₂ caused by human activity since the industrial revolution; therefore, knowledge of the role of atmospheric CO₂ in the thermal climate during the Paleogene will be helpful for understanding current and future climate. However, unlike for the late Cenozoic, atmospheric CO₂ reconstructions for the Paleogene are still inconsistent and vary between preindustrial-level to values over 4000 ppmv. In this study, we reconstructed the levels of atmospheric CO₂ in the early and middle Paleocene and middle Eocene based on the stomatal index of fossil Metasequoia needles collected from four fossil sites in Canada and Japan. We found the atmospheric CO₂ levels during the early and middle Paleocene to be similar to that of the present, and up to twice the present atmospheric CO₂ level was found during the middle Eocene. Our estimated atmospheric CO₂ level supports the hypothesis that the climate changes during the Paleogene cannot be explained merely by atmospheric CO₂ variations, which suggests that atmospheric CO₂ might not have always played a critical role in climate change during these ancient epochs and therefore cannot be a direct analogy for the current global warming.     

Warming effects on greenhouse gas fluxes in peatlands are modulated by vegetation composition

Understanding the effects of warming on greenhouse gas feedbacks to climate change represents a major global challenge. Most research has focused on direct effects of warming, without considering how concurrent changes in plant communities may alter such effects. Here, we combined vegetation manipulations with warming to investigate their interactive effects on greenhouse gas emissions from peatland. We found that although warming consistently increased respiration, the effect on net ecosystem CO₂ exchange depended on vegetation composition. The greatest increase in CO₂ sink strength after warming was when shrubs were present, and the greatest decrease when graminoids were present. CH₄ was more strongly controlled by vegetation composition than by warming, with largest emissions from graminoid communities. Our results show that plant community composition is a significant modulator of greenhouse gas emissions and their response to warming, and suggest that vegetation change could alter peatland carbon sink strength under future climate change.     

Climate change reduces the net sink of CH4 and N2O in a semiarid grassland

Atmospheric concentrations of methane (CH₄) and nitrous oxide (N₂O) have increased over the last 150 years because of human activity. Soils are important sources and sinks of both potent greenhouse gases where their production and consumption are largely regulated by biological processes. Climate change could alter these processes thereby affecting both rate and direction of their exchange with the atmosphere. We examined how a rise in atmospheric CO₂ and temperature affected CH₄ and N₂O fluxes in a well‐drained upland soil (volumetric water content ranging between 6% and 23%) in a semiarid grassland during five growing seasons. We hypothesized that responses of CH₄ and N₂O fluxes to elevated CO₂ and warming would be driven primarily by treatment effects on soil moisture. Previously we showed that elevated CO₂ increased and warming decreased soil moisture in this grassland. We therefore expected that elevated CO₂ and warming would have opposing effects on CH₄ and N₂O fluxes. Methane was taken up throughout the growing season in all 5 years. A bell‐shaped relationship was observed with soil moisture with highest CH₄ uptake at intermediate soil moisture. Both N₂O emission and uptake occurred at our site with some years showing cumulative N₂O emission and other years showing cumulative N₂O uptake. Nitrous oxide exchange switched from net uptake to net emission with increasing soil moisture. In contrast to our hypothesis, both elevated CO₂ and warming reduced the sink of CH₄ and N₂O expressed in CO₂ equivalents (across 5 years by 7% and 11% for elevated CO₂ and warming respectively) suggesting that soil moisture changes were not solely responsible for this reduction. We conclude that in a future climate this semiarid grassland may become a smaller sink for atmospheric CH₄ and N₂O expressed in CO₂‐equivalents.     

Vegetation-climate feedbacks in a greenhouse world

The potential for feedbacks between terrestrial vegetation, climate, and the atmospheric CO₂ partial pressure have been addressed by modelling. Previous research has established that under global warming and CO₂ enrichment, the stomatal conductance of vegetation tends to decrease, causing a warming effect on top of the driving change in greenhouse warming. At the global scale, this positive feedback is ultimately changed to a negative feedback through changes in vegetation structure. In spatial terms this structural feedback has a variable geographical pattern in terms of magnitude and sign. At high latitudes, increases in vegetation leaf area index (LAI) and vegetation height cause a positive feedback, and warming through reductions in the winter snow-cover albedo. At lower latitudes when vegetation becomes more sparse with warming, the higher albedo of the underlying soil leads to cooling. However, the largest area effects are of negative feedbacks caused by increased evaporative cooling with increasing LAI. These effects do not include feedbacks on the atmospheric CO₂ concentration, through changes in the carbon cycle of the vegetation. Modelling experiments, with biogeochemical, physiological and structural feedbacks on atmospheric CO₂, but with no changes in precipitation, ocean activity or sea ice formation, have shown that a consequence of the CO₂ fertilization effect on vegetation will be a reduction of atmospheric CO₂ concentration, in the order of 12% by the year 2100 and a reduced global warming by 0.7°C, in a total greenhouse warming of 3.9°C.     

Response of the terrestrial biosphere to global climate change and human perturbation

Despite 20 years of intensive effort to understand the global carbon cycle, the budget for carbon dioxide in the atmosphere is unbalanced. To explain why atmospheric CO₂ is not increasing as rapidly as it should be, various workers have suggested that land vegetation acts as a sink for carbon dioxide. Here, I examine various possibilities and find that the evidence for a sink of sufficient magnitude on land is poor. Moreover, it is unlikely that the land vegetation will act as a sink in the postulated warmer global climates of the future. In response to rapid human population growth, destruction of natural ecosystems in the tropics remains a large net source of CO₂ for the atmosphere, which is only partially compensated by the potential for carbon storage in temperate and boreal regions. Direct and inadvertent human effects on land vegetation might increase the magnitude of regional CO₂ storage on land, but they are unlikely to play a significant role in moderating the potential rate of greenhouse warming in the future.     

Interacting effects of elevated atmospheric CO<Subscript>2</Subscript> and hydrology on the growth and carbon sequestration of <Emphasis Type="Italic">Sphagnum</Emphasis> moss

Peatlands are a critical carbon store comprising 30% of the Earth’s terrestrial soil carbon. Sphagnum mosses comprise up to 90% of peat in the northern hemisphere but impacts of climate change on Sphagnum mosses are poorly understood, limiting development of sustainable peatland management and restoration. This study investigates the effects of elevated atmospheric CO₂ (eCO₂) (800 ppm) and hydrology on the growth of Sphagnum fallax, Sphagnum capillifolium and Sphagnum papillosum and greenhouse gas fluxes from moss–peat mesocosms. Elevated CO₂ levels increased Sphagnum height and dry weight but the magnitude of the response differed among species. The most responsive species, S. fallax, yielded the most biomass compared to S. papillosum and S. capillifolium. Water levels and the CO₂ treatment were found to interact, with the highest water level (1 cm below the surface) seeing the largest increase in dry weight under eCO₂ compared to ambient (400 ppm) concentrations. Initially, CO₂ flux rates were similar between CO₂ treatments. After week 9 there was a consistent three-fold increase of the CO₂ sink strength under eCO₂. At the end of the experiment, S. papillosum and S. fallax were greater sinks of CO₂ than S. capillifolium and the ? 7 cm water level treatment showed the strongest CO₂ sink strength. The mesocosms were net sources of CH₄ but the source strength varied with species, specifically S. fallax produced more CH₄ than S. papillosum and S. capillifolium. Our findings demonstrate the importance of species selection on the outcomes of peatland restoration with regards to Sphagnum’s growth and GHG exchange.     

Effects of Experimental Water Table and Temperature Manipulations on Ecosystem CO2 Fluxes in an Alaskan Rich Fen

Peatlands store 30% of the world’s terrestrial soil carbon (C) and those located at northern latitudes are expected to experience rapid climate warming. We monitored growing season carbon dioxide (CO₂) fluxes across a factorial design of in situ water table (control, drought, and flooded plots) and soil warming (control vs. warming via open top chambers) treatments for 2 years in a rich fen located just outside the Bonanza Creek Experimental Forest in interior Alaska. The drought (lowered water table position) treatment was a weak sink or small source of atmospheric CO₂ compared to the moderate atmospheric CO₂ sink at our control. This change in net ecosystem exchange was due to lower gross primary production and light-saturated photosynthesis rather than increased ecosystem respiration. The flooded (raised water table position) treatment was a greater CO₂ sink in 2006 due largely to increased early season gross primary production and higher light-saturated photosynthesis. Although flooding did not have substantial effects on rates of ecosystem respiration, this water table treatment had lower maximum respiration rates and a higher temperature sensitivity of ecosystem respiration than the control plot. Surface soil warming increased both ecosystem respiration and gross primary production by approximately 16% compared to control (ambient temperature) plots, with no net effect on net ecosystem exchange. Results from this rich fen manipulation suggest that fast responses to drought will include reduced ecosystem C storage driven by plant stress, whereas inundation will increase ecosystem C storage by stimulating plant growth.     

Moss Responses to Elevated CO2 and Variation in Hydrology in a Temperate Lowland Peatland

We studied the effects of elevated CO₂ (180–200 ppmv above ambient) on growth and chemistry of three moss species (Sphagnum palustre, S. recurvum and Polytrichum commune) in a lowland peatland in the Netherlands. Thereto, we conducted both a greenhouse experiment with both Sphagnum species and a field experiment with all three species using MiniFACE (Free Air CO₂ Enrichment) technology during 3 years. The greenhouse experiment showed that Sphagnum growth was stimulated by elevated CO₂ in the short term, but that in the longer term (≥1 year) growth was probably inhibited by low water tables and/or down-regulation of photosynthesis. In the field experiment, we did not find significant changes in moss abundance in response to elevated CO₂, although CO₂ enrichment appeared to reduce S. recurvum abundance. Both Sphagnum species showed stronger responses to spatial variation in hydrology than to increased atmospheric CO₂ concentrations. Polytrichum was insensitive to changes in hydrology. Apart from the confounding effects of hydrology, the relative lack of growth response of the moss species may also have been due to the relatively small increase in assimilated CO₂ as achieved by the experimentally added CO₂. We calculated that the added CO₂ contributed at most 32% to the carbon assimilation of the mosses, while our estimates based on stable C isotope data even suggest lower contributions for Sphagnum (24–27%). Chemical analyses of the mosses showed only small elevated CO₂ effects on living tissue N concentration and C/N ratio of the mosses, but the C/N ratio of Polytrichum was substantially lower than those of the Sphagnum species. Continuing expansion of Polytrichum at the expense of Sphagnum could reduce the C sink function of this lowland Sphagnum peatland, and similar ones elsewhere, as litter decomposition rates would probably be enhanced. Such a reduction in sink function would be driven mostly by increased atmospheric N deposition, water table regulation for agricultural purposes and land management to preserve the early successional stage (mowing, tree and shrub removal), since these anthropogenic factors will probably exert a greater control on competition between Polytrichum and Sphagnum than increased atmospheric CO₂ concentrations.     

Magnitude and biophysical regulators of methane emission and consumption in the Australian agricultural, forest, and submerged landscapes: a review

Increases in the concentrations of atmospheric greenhouse gases, carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O) due to human activities are associated with global climate change. CO₂ concentration in the atmosphere has increased by 33% (to 380 ppm) since 1750 ad, whilst CH₄ concentration has increased by 75% (to 1,750 ppb), and as the global warming potential (GWP) of CH₄ is 25 fold greater than CO₂ it represents about 20% of the global warming effect. The purpose of this review is to: (a) address recent findings regarding biophysical factors governing production and consumption of CH₄, (b) identify the current level of knowledge regarding the main sources and sinks of CH₄ in Australia, and (c) identify CH₄ mitigation options and their potential application in Australian ecosystems. Almost one-third of CH₄ emissions are from natural sources such as wetlands and lake sediments, which is poorly documented in Australia. For Australia, the major anthropogenic sources of CH₄ emissions include energy production from fossil fuels (~24%), enteric fermentation in the guts of ruminant animals (~59%), landfills, animal wastes and domestic sewage (~15%), and biomass burning (~5%), with minor contributions from manure management (1.7%), land use, land-use change and forestry (1.6%), and rice cultivation (0.2%). A significant sink exists for CH₄ (~6%) in aerobic soils, including agricultural and forestry soils, and potentially large areas of arid soils, however, due to limited information available in Australia, it is not accounted for in the Australian National Greenhouse Gas Inventory. CH₄ emission rates from submerged soils vary greatly, but mean values ≤10 mg m^?2 h^?1 are common. Landfill sites may emit CH₄ at one to three orders of magnitude greater than submerged soils. CH₄ consumption rates in non-flooded, aerobic agricultural, pastoral and forest soils also vary greatly, but mean values are restricted to ≤100 μg m^?2 h^?1, and generally greatest in forest soils and least in agricultural soils, and decrease from temperate to tropical regions. Mitigation options for soil CH₄ production primarily relate to enhancing soil oxygen diffusion through water management, land use change, minimised compaction and soil fertility management. Improved management of animal manure could include biogas capture for energy production or arable composting as opposed to open stockpiling or pond storage. Balanced fertiliser use may increase soil CH₄ uptake, reduce soil N₂O emissions whilst improving nutrient and water use efficiency, with a positive net greenhouse gas (CO₂-e) effect. Similarly, the conversion of agricultural land to pasture, and pastoral land to forestry should increase soil CH₄ sink. Conservation of native forests and afforestation of degraded agricultural land would effectively mitigate CH₄ emissions by maintaining and enhancing CH₄ consumption in these soils, but also by reducing N₂O emissions and increasing C sequestration. The overall impact of climate change on methanogenesis and methanotrophy is poorly understood in Australia, with a lack of data highlighting the need for long-term research and process understanding in this area. For policy addressing land-based greenhouse gas mitigation, all three major greenhouse gases (CO₂, CH₄ and N₂O) should be monitored simultaneously, combined with improved understanding at process-level.     

Effect of increased pCO2 on phytoplankton–virus interactions

Atmospheric carbon dioxide (CO₂) has increased since the pre-industrial period and is predicted to continue to increase throughout the twenty-first century. The ocean is a sink for atmospheric CO₂ and increased CO₂ concentration will change the carbonate equilibrium of seawater and result in lower carbonate ion concentration and lower pH. This may affect the entire marine biota but in particular calcifying organisms. In this study we investigated the effect of increased CO₂ on the virus host interaction of Emiliania huxleyi as a calcifying organism and of Phaeocystis poucheti as a non- calcifying organism. Both algae were grown in laboratory controlled conditions under past (280 ppmv), present (350 ppmv) and future (700 ppmv) CO₂ concentrations with and without added virus. Increased CO₂ had a negative effect on the growth rate of P. pouchetii, but not of E. huxleyi. No impact was found on viral lysis of P. pouchetii while increased burst size and slightly delayed lysis was observed for E. huxleyi with increased CO₂. We conclude that this short time study could not confirm earlier reports and our hypothesis of a negative effect of high CO₂ on E. huxleyi growth and E. huxleyi virus production.     

Bio-Energy Retains Its Mitigation Potential Under Elevated CO2

Background

If biofuels are to be a viable substitute for fossil fuels, it is essential that they retain their potential to mitigate climate change under future atmospheric conditions. Elevated atmospheric CO₂ concentration [CO₂] stimulates plant biomass production; however, the beneficial effects of increased production may be offset by higher energy costs in crop management.

Methodology/Main Findings

We maintained full size poplar short rotation coppice (SRC) systems under both current ambient and future elevated [CO₂] (550 ppm) and estimated their net energy and greenhouse gas balance. We show that a poplar SRC system is energy efficient and produces more energy than required for coppice management. Even more, elevated [CO₂] will increase the net energy production and greenhouse gas balance of a SRC system with 18%. Managing the trees in shorter rotation cycles (i.e., 2 year cycles instead of 3 year cycles) will further enhance the benefits from elevated [CO₂] on both the net energy and greenhouse gas balance.

Conclusions/Significance

Adapting coppice management to the future atmospheric [CO₂] is necessary to fully benefit from the climate mitigation potential of bio-energy systems. Further, a future increase in potential biomass production due to elevated [CO₂] outweighs the increased production costs resulting in a northward extension of the area where SRC is greenhouse gas neutral. Currently, the main part of the European terrestrial carbon sink is found in forest biomass and attributed to harvesting less than the annual growth in wood. Because SRC is intensively managed, with a higher turnover in wood production than conventional forest, northward expansion of SRC is likely to erode the European terrestrial carbon sink.     

Net greenhouse gas balance in response to nitrogen enrichment: perspectives from a coupled biogeochemical model

Increasing reactive nitrogen (N) input has been recognized as one of the important factors influencing climate system through affecting the uptake and emission of greenhouse gases (GHG). However, the magnitude and spatiotemporal variations of N‐induced GHG fluxes at regional and global scales remain far from certain. Here we selected China as an example, and used a coupled biogeochemical model in conjunction with spatially explicit data sets (including climate, atmospheric CO₂, O₃, N deposition, land use, and land cover changes, and N fertilizer application) to simulate the concurrent impacts of increasing atmospheric and fertilized N inputs on balance of three major GHGs (CO₂, CH₄, and N₂O). Our simulations showed that these two N enrichment sources in China decreased global warming potential (GWP) through stimulating CO₂ sink and suppressing CH₄ emission. However, direct N₂O emission was estimated to offset 39% of N‐induced carbon (C) benefit, with a net GWP of three GHGs averaging ?376.3 ± 146.4 Tg CO₂ eq yr^?1 (the standard deviation is interannual variability of GWP) during 2000–2008. The chemical N fertilizer uses were estimated to increase GWP by 45.6 ± 34.3 Tg CO₂ eq yr^?1 in the same period, and C sink was offset by 136%. The largest C sink offset ratio due to increasing N input was found in Southeast and Central mainland of China, where rapid industrial development and intensively managed crop system are located. Although exposed to the rapidly increasing N deposition, most of the natural vegetation covers were still showing decreasing GWP. However, due to extensive overuse of N fertilizer, China's cropland was found to show the least negative GWP, or even positive GWP in recent decade. From both scientific and policy perspectives, it is essential to incorporate multiple GHGs into a coupled biogeochemical framework for fully assessing N impacts on climate changes.     

Seed germination and seedling establishment of an invasive tropical tree species under different climate change scenarios

Increasing air temperature and atmospheric CO₂ levels may affect the distribution of invasive species. Whereas there is wide knowledge on the effect of global change on temperate species, responses of tropical invasive species to these two global change drivers are largely unknown. We conducted a greenhouse experiment on Terminalia catappa L. (Combretaceae), an invasive tree species on Brazilian coastal areas, to evaluate the effects of increased air temperature and CO₂ concentration on seed germination and seedling growth on the island of Santa Catarina (Florianópolis, Brazil). Seeds of the invasive tree were subjected to two temperature levels (ambient and +1.6 °C) and two CO₂ levels (ambient and ~650 ppmv) with a factorial design. Increased temperature enhanced germination rate and shortened germination time of T. catappa seeds. It also increased plant height, number of leaves and above‐ground biomass. By contrast, increased atmospheric CO₂ concentration had no significant effects, and the interaction between temperature and CO₂ concentration did not affect any of the measured traits. Terminalia catappa adapts to a relatively broad range of environmental conditions, being able to tolerate cooler temperatures in its invasive range. As T. catappa is native to tropical areas, global warming might favour its establishment along the coast of subtropical South America, while increased CO₂ levels seem not to have significant effects on seed germination or seedling growth.     

Effects of UV-B radiation on terrestrial plants and ecosystems: interaction with CO2 enrichment

UV-B radiation is just one of the environmental factors, that affect plant growth. It is now widely accepted that realistic assessment of plant responses to enhanced UV-B should be performed at sufficiently high Photosynthetically Active Radiation (PAR), preferably under field conditions. This will often imply, that responses of plants to enhanced UV-B in the field will be assessed under simultaneous water shortage, nutrient deficiency and variation of temperature. Since atmospheric CO₂ enrichment, global warming and increasing UV-B radiation represent components of global climatic change, interactions of UV-B with CO₂ enrichment and temperature are particularly relevant. Only few relevant UV-B× CO₂ interaction studies have been published. Most of these studies refer to greenhouse experiments. We report a significant CO₂ × UV-B interaction for the total plant dry weight and root dry weight of the C₃-grass Elymus athericus. At elevated CO₂ (720 mol mol^-1, plant growth was much less reduced by enhanced UV-B than at ambient atmospheric CO₂ although there were significant (positive) CO₂ effects and (negative) UV-B effects on plant growth. Most other CO₂ × UV-B studies do not report significant interactions on total plant biomass. This lack of CO₂ × UV-B interactions may result from the fact that primary metabolic targets for CO₂ and UVB are different. UV-B and CO₂ may differentially affect plant morphogenetic parameters: biomass allocation, branching, flowering, leaf thickness, emergence and senescence. Such more subtle interactions between CO₂ and UV-B need careful and long term experimentation to be detected. In the case of no significant CO₂× UV-B interactions, combined CO₂ and UV-B effects will be additive. Plants differ in their response to CO₂ and UV-B, they respond in general positively to elevated CO₂ and negatively to enhanced UV-B. Moreover, plant species differ in their responsiveness to CO₂ and UV-B. Therefore, even in case of additive CO₂ and UV-B effects, plant competitive relationships may change markedly under current climatic change with simultaneous enhanced atmospheric CO₂ and solar UV-B radiation.     

Combined global change effects on ecosystem processes in nine U.S. topographically complex areas

Concurrent changes in climate, atmospheric nitrogen (N) deposition, and increasing levels of atmospheric carbon dioxide (CO₂) affect ecosystems in complex ways. The DayCent-Chem model was used to investigate the combined effects of these human-caused drivers of change over the period 1980–2075 at seven forested montane and two alpine watersheds in the United States. Net ecosystem production (NEP) increased linearly with increasing N deposition for six out of seven forested watersheds; warming directly increased NEP at only two of these sites. Warming reduced soil organic carbon storage at all sites by increasing heterotrophic respiration. At most sites, warming together with high N deposition increased nitrous oxide (N₂O) emissions enough to negate the greenhouse benefit of soil carbon sequestration alone, though there was a net greenhouse gas sink across nearly all sites mainly due to the effect of CO₂ fertilization and associated sequestration by plants. Over the simulation period, an increase in atmospheric CO₂ from 350 to 600 ppm was the main driver of change in net ecosystem greenhouse gas sequestration at all forested sites and one of two alpine sites, but an additional increase in CO₂ from 600 to 760 ppm produced smaller effects. Warming either increased or decreased net greenhouse gas sequestration, depending on the site. The N contribution to net ecosystem greenhouse gas sequestration averaged across forest sites was only 5–7 % and was negligible for the alpine. Stream nitrate (NO₃ ^?) fluxes increased sharply with N-loading, primarily at three watersheds where initial N deposition values were high relative to terrestrial N uptake capacity. The simulated results displayed fewer synergistic responses to warming, N-loading, and CO₂ fertilization than expected. Overall, simulations with DayCent-Chem suggest individual site characteristics and historical patterns of N deposition are important determinants of forest or alpine ecosystem responses to global change.     

Including CO2 implications of land occupation in LCAs—method and example for livestock products

Purpose

Until recently, life cycle assessments (LCAs) have only addressed the direct greenhouse gas emissions along a process chain, but ignored the CO₂ emissions of land-use. However, for agricultural products, these emissions can be substantial. Here, we present a new methodology for including the implications of land occupation for CO₂ emissions to realistically reflect the consequences of consumers?? decisions.

Method

In principle, one can distinguish five different approaches of addressing the CO₂ consequences of land occupation: (1) assuming constant land cover, (2) land-use change related to additional production of the product under consideration, (3) historic land-use change, assuming historical relations between existing area and area expansion (4) land-use change related to less production of the product under consideration (??missed potential carbon sink?? of land occupation), and (5) an approach of integrating land conversion emissions and delayed uptake due to land occupation. Approach (4) is presented in this paper, using LCA data on land occupation, and carbon dynamics from the IMAGE model. Typically, if less production occurs, agricultural land will be abandoned, leading to a carbon sink when vegetation is regrowing. This carbon sink, which does not occur if the product would still be consumed, is thus attributed to the product as ??missed potential carbon sink??, to reflect the CO₂ implications of land occupations.

Results

We analyze the missed potential carbon sink by relating land occupation data from LCA studies to the potential carbon sink as calculated by an Integrated Global Assessment Model and its process-based, spatially explicit carbon cycle model. Thereby, we account for regional differences, heterogeneity in land-use, and different time horizons. Example calculations for several livestock products show that the CO₂ consequences of land occupation can be in the same order of magnitude as the other process related greenhouse gas emissions of the LCA, and depend largely on the production system. The highest CO₂ implications of land occupation are calculated for beef and lamb, with beef production in Brazil having a missed potential carbon sink more than twice as high as the other GHG emissions.

Conclusions

Given the significant contribution of land occupation to the total GHG balance of agricultural products, they need to be included in life cycle assessments in a realistic way. The new methodology presented here reflects the consequences of producing or not producing a certain commodity, and thereby it is suited to inform consumers fully about the consequences of their choices.     

Enhanced terrestrial carbon uptake in the Northern High Latitudes in the 21st century from the Coupled Carbon Cycle Climate Model Intercomparison Project model projections

The ongoing and projected warming in the northern high latitudes (NHL; poleward of 60 °N) may lead to dramatic changes in the terrestrial carbon cycle. On the one hand, warming and increasing atmospheric CO₂ concentration stimulate vegetation productivity, taking up CO₂. On the other hand, warming accelerates the decomposition of soil organic matter (SOM), releasing carbon into the atmosphere. Here, the NHL terrestrial carbon storage is investigated based on 10 models from the Coupled Carbon Cycle Climate Model Intercomparison Project. Our analysis suggests that the NHL will be a carbon sink of 0.3 ± 0.3 Pg C yr^?1 by 2100. The cumulative land organic carbon storage is modeled to increase by 38 ± 20 Pg C over 1901 levels, of which 17 ± 8 Pg C comes from vegetation (43%) and 21 ± 16 Pg C from the soil (8%). Both CO₂ fertilization and warming enhance vegetation growth in the NHL. Although the intense warming there enhances SOM decomposition, soil organic carbon (SOC) storage continues to increase in the 21st century. This is because higher vegetation productivity leads to more turnover (litterfall) into the soil, a process that has received relatively little attention. However, the projected growth rate of SOC begins to level off after 2060 when SOM decomposition accelerates at high temperature and then catches up with the increasing input from vegetation turnover. Such competing mechanisms may lead to a switch of the NHL SOC pool from a sink to a source after 2100 under more intense warming, but large uncertainty exists due to our incomplete understanding of processes such as the strength of the CO₂ fertilization effect, permafrost, and the role of soil moisture. Unlike the CO₂ fertilization effect that enhances vegetation productivity across the world, global warming increases the productivity at high latitudes but tends to reduce it in the tropics and mid‐latitudes. These effects are further enhanced as a result of positive carbon cycle–climate feedbacks due to additional CO₂ and warming.     

Cassava about‐FACE: Greater than expected yield stimulation of cassava (Manihot esculenta) by future CO2 levels

Globally, cassava is the second most important root crop after potatoes and the fifth most important crop overall in terms of human caloric intake. In addition to its growing global importance for feed, fuel, and starch, cassava has long been vital to food security in Sub‐Saharan Africa. Climate change is expected to have its most severe impact on crops in food insecure regions, yet little is known about how cassava productivity will respond to climate change. The most important driver of climate change is globally increasing atmospheric CO₂ concentration ([CO₂]). However, the potential for cassava to enhance food security in an elevated [CO₂] world is uncertain as greenhouse and open top chamber (OTC) study reports are ambiguous. Studies have yielded misleading results in the past regarding the effect of elevated [CO₂] on crop productivity, particularly in cases where pots restricted sink growth. To resolve these conflicting results, we compare the response of cassava to growth at ambient (ca. 385 ppm) and elevated [CO₂] (585 ppm) under field conditions and fully open air [CO₂] elevation. After three and half months of growth at elevated [CO₂], above ground biomass was 30% greater and cassava root tuber dry mass increased over 100% (fresh weight increased 89%). High photosynthetic rates and photosynthetic stimulation by elevated [CO₂], larger canopies, and a large sink capacity all contributed to cassava's growth and yield stimulation. Cassava exhibited photosynthetic acclimation via decreased Rubisco capacity early in the season prior to root tuber initiation when sink capacity was smaller. Importantly, and in contrast to a greenhouse study, we found no evidence of increased leaf N or total cyanide concentration in elevated [CO₂]. All of our results are consistent with theoretical expectations; however, the magnitude of the yield increase reported here surpasses all other C₃ crops and thus exceeds expectations.     

Feasibility of atmospheric methane removal using methanotrophic biotrickling filters

Methane is a potent greenhouse gas with a global warming potential ~23 times that of carbon dioxide. Here, we describe the modeling of a biotrickling filtration system composed of methane-consuming bacteria, i.e., methanotrophs, to assess the utility of these systems in removing methane from the atmosphere. Model results indicate that assuming the global average atmospheric concentration of methane, 1.7 ppmv, methane removal is ineffective using these methanotrophic biofilters as the methane concentration is too low to enable cell survival. If the concentration is increased to 500–6,000 ppmv, however, similar to that found above landfills and in concentrated animal feeding operations (factory farms), 4.98–35.7 tons of methane can be removed per biofilter per year assuming biotrickling filters of typical size (3.66 m in diameter and 11.5 m in height). Using reported ranges of capital, operational, and maintenance costs, the cost of the equivalent ton of CO₂ removal using these systems is $90–$910 ($2,070–$20,900 per ton of methane), depending on the influent concentration of methane and if heating is required. The use of methanotrophic biofilters for controlling methane emissions is technically feasible and, provided that either the costs of biofilter construction and operation are reduced or the value of CO₂ credits is increased, can also be economically attractive.     

Time-adjusted global warming potentials for LCA and carbon footprints

Purpose

The common practice of summing greenhouse gas (GHG) emissions and applying global warming potentials (GWPs) to calculate CO₂ equivalents misrepresents the global warming effects of emissions that occur over a product or system??s life cycle at a particular time in the future. The two primary purposes of this work are to develop an approach to correct for this distortion that can (1) be feasibly implemented by life cycle assessment and carbon footprint practitioners and (2) results in units of CO₂ equivalent. Units of CO₂ equilavent allow for easy integration in current reporting and policy frameworks.

Methods

CO₂ equivalency is typically calculated using GWPs from the Intergovernmental Panel on Climate Change. GWPs are calculated by dividing a GHG??s global warming effect, as measured by cumulative radiative forcing, over a prescribed time horizon by the global warming effect of CO₂ over that same time horizon. Current methods distort the actual effect of GHG emissions at a particular time in the future by summing emissions released at different times and applying GWPs; modeling them as if they occur at the beginning of the analytical time horizon. The method proposed here develops time-adjusted warming potentials (TAWPs), which use the reference gas CO₂, and a reference time of zero. Thus, application of TAWPs results in units of CO₂ equivalent today.

Results and discussion

A GWP for a given GHG only requires that a practitioner select an analytical time horizon. The TAWP, however, contains an additional independent variable; the year in which an emission occurs. Thus, for each GHG and each analytical time horizon, TAWPs require a simple software tool (TAWPv1.0) or an equation to estimate their value. Application of 100-year TAWPs to a commercial building??s life cycle emissions showed a 30?% reduction in CO₂ equivalent compared to typical practice using 100-year GWPs. As the analytical time horizon is extended the effect of emissions timing is less pronounced. For example, at a 500-year analytical time horizon the difference is only 5?%.

Conclusions and recommendations

TAWPs are one of many alternatives to traditional accounting methods, and are envisioned to be used as one of multiple characterizations in carbon accounting or life cycle impact assessment methods to assist in interpretation of a study??s outcome.