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Opinion : No quick switch to low-carbon energy
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In the first of two pieces on reducing greenhouse-gas emissions, Gert Jan Kramer and Martin Haigh analyse historic growth in energy systems to explain why deploying alternative technologies will be a long haul.
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The late Precambrian greening of the Earth
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Many aspects of the carbon cycle can be assessed from temporal changes in the 13C/12C ratio of oceanic bicarbonate. 13C/12C can temporarily rise when large amounts of 13C-depleted photosyn- thetic organic matter are buried at enhanced rates1, and can decrease if phytomass is rapidly oxidized2 or if low 13C is rapidly released from methane clathrates3. Assuming that variations of the marine 13C/12C ratio are directly recorded in carbonate rocks, thousands of carbon isotope analyses of late Precambrian examples have been published to correlate these otherwise undatable strata and to document perturbations to the carbon cycle just before the great expansion of metazoan life. Low 13C/12C in some Neoproterozoic carbonates is considered evidence of carbon cycle perturbations unique to the Precambrian. These include complete oxidation of all organic matter in the ocean2 and complete produc- tivity collapse such that low-13C/12C hydrothermal CO2 becomes the main input of carbon4. Here we compile all published oxygen and carbon isotope data for Neoproterozoic marine carbonates, and consider them in terms of processes known to alter the isotopic composition during transformation of the initial precipitate into limestone/dolostone. We show that the combined oxygen and carbon isotope systematics are identical to those of well- understood Phanerozoic examples that lithified in coastal pore fluids, receiving a large groundwater influx of photosynthetic carbon from terrestrial phytomass. Rather than being perturba- tions to the carbon cycle, widely reported decreases in 13C/12C in Neoproterozoic carbonates are more easily interpreted in the same way as is done for Phanerozoic examples. This influx of terrestrial carbon is not apparent in carbonates older than 850 Myr, so we infer an explosion of photosynthesizing communities on late Precambrian land surfaces. As a result, biotically enhanced weathering generated carbon-bearing soils on a large scale and their detrital sedimentation sequestered carbon 5. This facilitated a rise in O2 necessary for the expansion of multicellular life.
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The El Nino with a difference
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Patterns of sea-surface warming and cooling in the tropical Pacific seem to be changing, as do the associated atmospheric effects. Increased global warming is implicated in these shifts in El Niño phenomena.
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Increase in Agulhas leakage due to poleward shift of Southern Hemisphere westerlies
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The transport of warm and salty Indian Ocean waters into the Atlantic Ocean—the Agulhas leakage—has a crucial role in the global oceanic circulation1 and thus the evolution of future climate. At present these waters provide the main source of heat and salt for the surface branch of the Atlantic meridional overturning circulation (MOC)2. There is evidence from past glacial-to-interglacial variations in foraminiferal assemblages3 and model studies4 that the amount of Agulhas leakage and its corresponding effect on the MOC has been subject to substantial change, potentially linked to latitudinal shifts in the Southern Hemisphere westerlies5. A pro- gressive poleward migration of the westerlies has been observed during the past two to three decades and linked to anthropogenic forcing6, but because of the sparse observational records it has not been possible to determine whether there has been a concomitant response of Agulhas leakage. Here we present the results of a high- resolution ocean general circulation model7,8 to show that the transport of Indian Ocean waters into the South Atlantic via the Agulhas leakage has increased during the past decades in response to the change in wind forcing. The increased leakage has contri- buted to the observed salinification 9 of South Atlantic thermocline waters. Both model and historic measurements off South America suggest that the additional Indian Ocean waters have begun to invade the North Atlantic, with potential implications for the future evolution of the MOC.
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Reconstruction of the history of anthropogenic CO2 concentrations in the ocean
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The release of fossil fuel CO2 to the atmosphere by human activity has been implicated as the predominant cause of recent global climate change1. The ocean plays a crucial role in mitigating the effects of this perturbation to the climate system, sequestering 20 to 35 per cent of anthropogenic CO2 emissions2–4. Although much progress has been made in recent years in understanding and quantifying this sink, considerable uncertainties remain as to the distribution of anthropogenic CO2 in the ocean, its rate of uptake over the industrial era, and the relative roles of the ocean and terrestrial biosphere in anthropogenic CO2 sequestration. Here we address these questions by presenting an observationally based reconstruction of the spatially resolved, time-dependent history of anthropogenic carbon in the ocean over the industrial era. Our approach is based on the recognition that the transport of tracers in the ocean can be described by a Green’s function, which we estimate from tracer data using a maximum entropy deconvo- lution technique. Our results indicate that ocean uptake of anthro- pogenic CO2 has increased sharply since the 1950s, with a small decline in the rate of increase in the last few decades. We estimate the inventory and uptake rate of anthropogenic CO2 in 2008 at 140 6 25 Pg C and 2.3 6 0.6 Pg C yr21, respectively. We find that the Southern Ocean is the primary conduit by which this CO2 enters the ocean (contributing over 40 per cent of the anthro- pogenic CO2 inventory in the ocean in 2008). Our results also suggest that the terrestrial biosphere was a source of CO2 until the 1940s, subsequently turning into a sink. Taken over the entire industrial period, and accounting for uncertainties, we estimate that the terrestrial biosphere has been anywhere from neutral to a net source of CO2, contributing up to half as much CO2 as has been taken up by the ocean over the same period.
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Increasing carbon storage in intact African tropical forests
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The response of terrestrial vegetation to a globally changing environment is central to predictions of future levels of atmospheric carbon dioxide1,2. The role of tropical forests is critical because they are carbon-dense and highly productive3,4. Inventory plots across Amazonia show that old-growth forests have increased in carbon storage over recent decades5–7, but the response of one-third of the world’s tropical forests in Africa8 is largely unknown owing to an absence of spatially extensive observation networks9,10. Here we report data from a ten-country network of long-term monitoring plots in African tropical forests. We find that across 79 plots (163ha) above-ground carbon storage in live trees increased by 0.63 Mg C ha21 yr21 between 1968 and 2007 (95% confidence inter- val (CI), 0.22–0.94; mean interval, 1987–96). Extrapolation to unmeasured forest components (live roots, small trees, necromass) and scaling to the continent implies a total increase in carbon storage in African tropical forest trees of 0.34 Pg C yr21 (CI, 0.15–0.43). These reported changes in carbon storage are similar to those reported for Amazonian forests per unit area6,7, providing evidence that increasing carbon storage in old-growth forests is a pan-tropical phenomenon. Indeed, combining all standardized inventory data from this study and from tropical America and Asia5,6,11 together yields a comparable figure of 0.49 Mg C ha21 yr21 (n 5 156; 562 ha; CI, 0.29–0.66; mean interval, 1987–97). This indicates a carbon sink of 1.3 Pg C yr21 (CI, 0.8–1.6) across all tropical forests during recent decades. Taxon-specific analyses of African inventory and other data12 suggest that widespread changes in resource availability, such as increasing atmospheric carbon dioxide concentrations, may be the cause of the increase in carbon stocks13, as some theory14 and models2,10,15 predict.
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Greenhouse-gas emission targets for limiting global warming to 2 C
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More than 100 countries have adopted a global warming limit of 2 6C or below (relative to pre-industrial levels) as a guiding principle for mitigation efforts to reduce climate change risks, impacts and damages1,2. However, the greenhouse gas (GHG) emissions corresponding to a specified maximum warming are poorly known owing to uncertainties in the carbon cycle and the climate response. Here we provide a comprehensive probabilistic analysis aimed at quantifying GHG emission budgets for the 2000–50 period that would limit warming throughout the twenty-first century to below 2 6C, based on a combination of published distributions of climate system properties and observational con- straints. We show that, for the chosen class of emission scenarios, both cumulative emissions up to 2050 and emission levels in 2050 are robust indicators of the probability that twenty-first century warming will not exceed 26C relative to pre-industrial temperatures. Limiting cumulative CO2 emissions over 2000–50 to 1,000Gt CO2 yields a 25% probability of warming exceeding 2 6C—and a limit of 1,440 Gt CO2 yields a 50% probability—given a representative estimate of the distri- bution of climate system properties. As known 2000–06 CO2 emissions3 were234 Gt CO2, less than half the proven economi-cally recoverable oil, gas and coal reserves 4–6 can still be emitted up to 2050 to achieve such a goal. Recent G8 Communique ́s7 envisage halved global GHG emissions by 2050, for which we estimate a 12– 45% probability of exceeding 2 6C—assuming 1990 as emission base year and a range of published climate sensitivity distributions. Emissions levels in 2020 are a less robust indicator, but for the scenarios considered, the probability of exceeding 26C rises to 53–87% if global GHG emissions are still more than 25% above 2000 levels in 2020.
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Warming caused by cumulative carbon emissions towards the trillionth tonne
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Global efforts to mitigate climate change are guided by projections of future temperatures1. But the eventual equilibrium global mean temperature associated with a given stabilization level of atmospheric greenhouse gas concentrations remains uncertain1–3, complicating the setting of stabilization targets to avoid poten- tially dangerous levels of global warming4–8. Similar problems apply to the carbon cycle: observations currently provide only a weak constraint on the response to future emissions9–11. Here we use ensemble simulations of simple climate-carbon-cycle models constrained by observations and projections from more compre- hensive models to simulate the temperature response to a broad range of carbon dioxide emission pathways. We find that the peak warming caused by a given cumulative carbon dioxide emission is better constrained than the warming response to a stabilization scenario. Furthermore, the relationship between cumulative emissions and peak warming is remarkably insensitive to the emis- sion pathway (timing of emissions or peak emission rate). Hence policy targets based on limiting cumulative emissions of carbon dioxide are likely to be more robust to scientific uncertainty than emission-rate or concentration targets. Total anthropogenic emissions of one trillion tonnes of carbon (3.67 trillion tonnes of CO2), about half of which has already been emitted since industrialization began, results in a most likely peak carbon-dioxide- induced warming of 2 6C above pre-industrial temperatures, with a 5–95% confidence interval of 1.3–3.9 6C.
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Successful range-expanding plants experience less above-ground and below-ground enemy impact
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Many species are currently moving to higher latitudes and altitudes1–3. However, little is known about the factors that influence the future performance of range-expanding species in their new habitats. Here we show that range-expanding plant species from a riverine area were better defended against shoot and root enemies than were related native plant species growing in the same area. We grew fifteen plant species with and without non-coevolved polyphagous locusts and cosmopolitan, polyphagous aphids. Contrary to our expectations, the locusts performed more poorly on the range-expanding plant species than on the congeneric native plant species, whereas the aphids showed no difference. The shoot herbivores reduced the biomass of the native plants more than they did that of the congeneric range expanders. Also, the range-expanding plants developed fewer pathogenic effects4,5 in their root-zone soil than did the related native species. Current predictions forecast biodiversity loss due to limitations in the ability of species to adjust to climate warming conditions in their range 6–8. Our results strongly suggest that the plants that shift ranges towards higher latitudes and altitudes may include potential invaders, as the successful range expanders may experience less control by above-ground or below- ground enemies than the natives.
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New particle formation in forests inhibited by isoprene emissions
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It has been suggested that volatile organic compounds (VOCs) are involved in organic aerosol formation, which in turn affects radiative forcing and climate1. The most abundant VOCs emitted by terrestrial vegetation are isoprene and its derivatives, such as monoterpenes and sesquiterpenes 2. New particle formation in boreal regions is related to monoterpene emissions3 and causes an estimated negative radiative forcing4 of about 20.2 to 20.9 W m22. The annual variation in aerosol growth rates during particle nucleation events correlates with the seasonality of mono- terpene emissions of the local vegetation, with a maximum during summer5. The frequency of nucleation events peaks, however, in spring and autumn5. Here we present evidence from simulation experiments conducted in a plant chamber that isoprene can sig- nificantly inhibit new particle formation. The process leading to the observed decrease in particle number concentration is linked to the high reactivity of isoprene with the hydroxyl radical (OH). The suppression is stronger with higher concentrations of iso- prene, but with little dependence on the specific VOC mixture emitted by trees. A parameterization of the observed suppression factor as a function of isoprene concentration suggests that the number of new particles produced depends on the OH concentra- tion and VOCs involved in the production of new particles undergo three to four steps of oxidation by OH. Our measure- ments simulate conditions that are typical for forested regions and may explain the observed seasonality in the frequency of aero- sol nucleation events, with a lower number of nucleation events during summer compared to autumn and spring5. Biogenic emissions of isoprene are controlled by temperature and light2, and if the relative isoprene abundance of biogenic VOC emissions increases in response to climate change or land use change, the new particle formation potential may decrease, thus damping the aerosol negative radiative forcing effect.
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