Climate Science PDFs
Climate Science PDFs Collection
- Snowball Earth termination by destabilization of equatorial permafrost methane clathrate
- The start of the Ediacaran period is defined by one of the most severe climate change events recorded in Earth history—the recov- ery from the Marinoan ‘snowball’ ice age, ,635 Myr ago (ref. 1). Marinoan glacial-marine deposits occur at equatorial palaeolati- tudes2, and are sharply overlain by a thin interval of carbonate that preserves marine carbon and sulphur isotopic excursions of about 25 and 115 parts per thousand, respectively3–5; these deposits are thought to record widespread oceanic carbonate precipitation during postglacial sea level rise1,3,4. This abrupt transition records a climate system in profound disequilibrium3,6 and contrasts shar- ply with the cyclical stratigraphic signal imparted by the balanced feedbacks modulating Phanerozoic deglaciation. Hypotheses accounting for the abruptness of deglaciation include ice albedo feedback3, deep-ocean out-gassing during post-glacial oceanic overturn7 or methane hydrate destabilization8–10. Here we report the broadest range of oxygen isotope values yet measured in mar- ine sediments (225% to 112%) in methane seeps in Marinoan deglacial sediments underlying the cap carbonate. This range of values is likely to be the result of mixing between ice-sheet-derived meteoric waters and clathrate-derived fluids during the flushing and destabilization of a clathrate field by glacial meltwater. The equatorial palaeolatitude implies a highly volatile shelf permafrost pool that is an order of magnitude larger than that of the present day. A pool of this size could have provided a massive biogeochem- ical feedback capable of triggering deglaciation and accounting for the global postglacial marine carbon and sulphur isotopic excur- sions, abrupt unidirectional warming, cap carbonate deposition, and a marine oxygen crisis. Our findings suggest that methane released from low-latitude permafrost clathrates therefore acted as a trigger and/or strong positive feedback for deglaciation and warming. Methane hydrate destabilization is increasingly suspected as an important positive feedback to climate change11–13 that coincides with critical boundaries in the geological record14,15 and may represent one particularly important mechanism active during conditions of strong climate forcing.
- Strong effect of dispersal network structure on ecological dynamics
- A central question in ecology with great importance for management, conservation and biological control is how changing connectivity affects the persistence and dynamics of interacting species. Researchers in many disciplines have used large systems of coupled oscillators to model the behaviour of a diverse array of fluctuating systems in nature1–4. In the well-studied regime of weak coupling, synchronization is favoured by increases in coupling strength and large-scale network structures (for example ‘small worlds’) that produce short cuts and clustering5–9. Here we show that, by contrast, randomizing the structure of dispersal networks in a model of predators and prey tends to favour asyn- chrony and prolonged transient dynamics, with resulting effects on the amplitudes of population fluctuations. Our results focus on synchronization and dynamics of clusters in models, and on time- scales, more appropriate for ecology, namely smaller systems with strong interactions outside the weak-coupling regime, rather than the better-studied cases of large, weakly coupled systems. In these smaller systems, the dynamics of transients and the effects of changes in connectivity can be well understood using a set of methods including numerical reconstructions of phase dynamics, examinations of cluster formation and the consideration of important aspects of cyclic dynamics, such as amplitude.
- Using (brain)temperature to analyse temporal dynamics in the songbird motor pathway
- Here we address these issues by using temperature to manipulate the biophysical dynamics in different regions of the songbird forebrain involved in song production. We find that cooling the premotor nucleus HVC (formerly known as the high vocal centre) slows song speed across all timescales by up to 45 per cent but only slightly alters the acoustic structure, whereas cooling the downstream motor nucleus RA (robust nucleus of the arcopallium) has no observable effect on song timing. Our observations suggest that dynamics within HVC are involved in the control of song timing, perhaps through a chain-like organization. Local manipulation of brain temperature should be broadly applicable to the identification of neural circuitry that controls the timing of behavioural sequences and, more generally, to the study of the origin and role of oscillatory and other forms of brain dynamics in neural systems.
- vegetation controlled by tropical sea surface temperatures in the mid-Pleistocene period
- The dominant forcing factors for past large-scale changes in vegetation are widely debated. Changes in the distribution of C4 plants—adapted to warm, dry conditions and low atmospheric CO2 concentrations1—have been attributed to marked changes in environmental conditions, but the relative impacts of changes in aridity, temperature2,3 and CO2 concentration4,5 are not well understood. Here, we present a record of African C4 plant abundance between 1.2 and 0.45 million years ago, derived from compound-specific carbon isotope analyses of wind-trans- ported terrigenous plant waxes. We find that large-scale changes in African vegetation are linked closely to sea surface temperatures in the tropical Atlantic Ocean. We conclude that, in the mid- Pleistocene, changes in atmospheric moisture content—driven by tropical sea surface temperature changes and the strength of the African monsoon—controlled aridity on the African continent, and hence large-scale vegetation changes.
- Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model
- The continued increase in the atmospheric concentration of carbon dioxide due to anthropogenic emissions is predicted to lead to significant changes in climate1. About half of the current emissions are being absorbed by the ocean and by land ecosystems2, but this absorption is sensitive to climate3,4 as well as to atmospheric carbon dioxide concentrations5, creating a feedback loop. General circulation models have generally excluded the feedback between climate and the biosphere, using static vegetation distributions and CO2 concentrations from simple carbon-cycle models that do not include climate change6. Here we present results from a fully coupled, three-dimensional carbon±climate model, indicating that carbon-cycle feedbacks could signi®cantly accelerate climate change over the twenty-®rst century. We ®nd that under a `business as usual' scenario, the terrestrial biosphere acts as an overall carbon sink until about 2050, but turns into a source thereafter. By 2100, the ocean uptake rate of 5 Gt C yr-1 is balanced by the terrestrial carbon source, and atmospheric CO2 concentrations are 250 p.p.m.v. higher in our fully coupled simulation than in uncoupled carbon models2, resulting in a global-mean warming of 5.5 K, as compared to 4 K without the carbon-cycle feedback.
- CO2 emissions from forest loss
- Deforestation is the second largest anthropogenic source of carbon dioxide to the atmosphere, after fossil fuel combustion. Following a budget reanalysis, the contribution from deforestation is revised downwards, but tropical peatlands emerge as a notable carbon dioxide source.
- Global warmth with little extra co2
- Most climate models consider only short-term processes such as cloud and sea-ice formation when assessing Earth’s sensitivity to greenhouse-gas forcing. Mounting evidence indicates that the response could be stronger if boundary conditions change drastically.
- Importance of methane and nitrous oxide for Europe’s terrestrial greenhouse-gas balance
- Concluding sentence of the abstract: The trend towards more intensive agriculture and logging is likely to make Europe’s land surface a significant source of greenhouse gases. The development of land management policies which aim to reduce greenhouse-gas emissions should be a priority.
- Statistically derived contributions of diverse human influences to twentieth-century temperature changes
- The warming of the climate system is unequivocal as evidenced by an increase in global temperatures by 0.8 ◦ C over the past century. However, the attribution of the observed warming to human activities remains less clear, particularly because of the apparent slow-down in warming since the late 1990s. Here we analyse radiative forcing and temperature time series with state-of-the-art statistical methods to address this question without climate model simulations. We show that long-term trends in total radiative forcing and temperatures have largely been determined by atmospheric greenhouse gas concentrations, and modulated by other radiative factors. We identify a pronounced increase in the growth rates of both temperatures and radiative forcing around 1960, which marks the onset of sustained global warming. Our analyses also reveal a contribution of human interventions to two periods when global warming slowed down. Our statistical analysis suggests that the reduction in the emissions of ozone-depleting substances under the Montreal Protocol, as well as a reduction in methane emissions, contributed to the lower rate of warming since the 1990s. Furthermore, we identify a contribution from the two world wars and the Great Depression to the documented cooling in the mid-twentieth century, through lower carbon dioxide emissions. We conclude that reductions in greenhouse gas emissions are effective in slowing the rate of warming in the short term.
- Biodiversity and ecosystem multifunctionality
- Biodiversity loss can affect ecosystem functions and services1–4. Individual ecosystem functions generally show a positive asymptotic relationship with increasing biodiversity, suggesting that some species are redundant5–8. However, ecosystems are managed and conserved for multiple functions, which may require greater biodiversity. Here we present an analysis of published data from grassland biodiversity experiments9–11, and show that ecosystem multifunctionality does require greater numbers of species. We analysed each ecosystem function alone to identify species with desirable effects. We then calculated the number of species with positive effects for all possible combinations of functions. Our results show appreciable differences in the sets of species influ- encing different ecosystem functions, with average proportional overlap of about 0.2 to 0.5. Consequently, as more ecosystem pro- cesses were included in our analysis, more species were found to affect overall functioning. Specifically, for all of the analysed experiments, there was a positive saturating relationship between the number of ecosystem processes considered and the number of species influencing overall functioning. We conclude that because different species often influence different functions, studies focus- ing on individual processes in isolation will underestimate levels of biodiversity required to maintain multifunctional ecosystems.
- Subtropical to boreal convergence of tree-leaf temperatures
- The oxygen isotope ratio (d18O) of cellulose is thought to provide a record of ambient temperature and relative humidity during per- iods of carbon assimilation1,2. Here we introduce a method to resolve tree-canopy leaf temperature with the use of d18O of cellulose in 39 tree species. We show a remarkably constant leaf temperature of 21.4 6 2.2 6C across 506 of latitude, from subtropical to boreal biomes. This means that when carbon assimilation is maximal, the physiological and morphological properties of tree branches serve to raise leaf temperature above air temperature to a much greater extent in more northern latitudes. A main assumption underlying the use of d18O to reconstruct climate history is that the temperature and relative humidity of an actively photosynthesizing leaf are the same as those of the surrounding air3,4. Our data are contrary to that assumption and show that plant physiological ecology must be considered when reconstructing climate through isotope analysis. Furthermore, our results may explain why climate has only a modest effect on leaf economic traits5 in general.
- The effect of permafrost thaw on old carbon release and net carbon exchange from tundra
- Permafrost soils in boreal and Arctic ecosystems store almost twice as much carbon1,2 as is currently present in the atmosphere3. Permafrost thaw and the microbial decomposition of previously frozen organic carbon is considered one of the most likely positive climate feedbacks from terrestrial ecosystems to the atmosphere in a warmer world1,2,4–7. The rate of carbon release from permafrost soils is highly uncertain, but it is crucial for predicting the strength and timing of this carbon-cycle feedback effect, and thus how important permafrost thaw will be for climate change this century and beyond1,2,4–7. Sustained transfers of carbon to the atmosphere that could cause a significant positive feedback to climate change must come from old carbon, which forms the bulk of the perma- frost carbon pool that accumulated over thousands of years8–11. Here we measure net ecosystem carbon exchange and the radio- carbon age of ecosystem respiration in a tundra landscape under- going permafrost thaw12 to determine the influence of old carbon loss on ecosystem carbon balance. We find that areas that thawed over the past 15 years had 40 per cent more annual losses of old carbon than minimally thawed areas, but had overall net eco- system carbon uptake as increased plant growth offset these losses. In contrast, areas that thawed decades earlier lost even more old carbon, a 78 per cent increase over minimally thawed areas; this old carbon loss contributed to overall net ecosystem carbon release despite increased plant growth. Our data document significant losses of soil carbon with permafrost thaw that, over decadal timescales, overwhelms increased plant carbon uptake13–15 at rates that could make permafrost a large biospheric carbon source in a warmer world.
- The role of stomata in sensing and driving environmental change
- Stomata, the small pores on the surfaces of leaves and stalks, regulate the flow of gases in and out of leaves and thus plants as a whole. They adapt to local and global changes on all timescales from minutes to millennia. Recent data from diverse fields are establishing their central importance to plant physiology, evolution and global ecology. Stomatal morphology, distribution and behaviour respond to a spectrum of signals, from intracellular signalling to global climatic change. Such concerted adaptation results from a web of control systems, reminiscent of a ‘scale-free’ network, whose untangling requires integrated approaches beyond those currently used.
- rainfall preceded by air passage over forests
- Vegetation affects precipitation patterns by mediating moisture, energy and trace-gas fluxes between the surface and atmosphere1. When forests are replaced by pasture or crops, evapotranspiration of moisture from soil and vegetation is often diminished, leading to reduced atmospheric humidity and potentially suppressing precipitation2,3. Climate models predict that large-scale tropical deforestation causes reduced regional precipitation4–10, although the magnitude of the effect is model9,11 and resolution8 dependent. In contrast, observational studies have linked deforestation to increased precipitation locally12–14 but have been unable to explore the impact of large-scale deforestation. Here we use satellite remote-sensing data of tropical precipitation and vegetation, combined with simulated atmospheric transport patterns, to assess the pan-tropical effect of forests on tropical rainfall. We find that for more than 60 per cent of the tropical land surface (latitudes 30 degrees south to 30 degrees north), air that has passed over extens- ive vegetation in the preceding few days produces at least twice as much rain as air that has passed over little vegetation. We demonstrate that this empirical correlation is consistent with evapotranspiration maintaining atmospheric moisture in air that passes over extensive vegetation. We combine these empirical rela- tionships with current trends of Amazonian deforestation to estimate reductions of 12 and 21 per cent in wet-season and dry- season precipitation respectively across the Amazon basin by 2050, due to less-efficient moisture recycling. Our observation-based results complement similar estimates from climate models4–10, in which the physical mechanisms and feedbacks at work could be explored in more detail.
- New particle formation in forests inhibited by isoprene emissions
- 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.
- Successful range-expanding plants experience less above-ground and below-ground enemy impact
- 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.
- Attributing physical and biological impacts to anthropogenic climate change
- Significant changes in physical and biological systems are occurring on all continents and in most oceans, with a concentration of available data in Europe and North America. Most of these changes are in the direction expected with warming temperature. Here we show that these changes in natural systems since at least 1970 are occurring in regions of observed temperature increases, and that these temperature increases at continental scales cannot be explained by natural climate variations alone. Given the conclusions from the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report that most of the observed increase in global average temperatures since the mid-twentieth century is very likely to be due to the observed increase in anthropogenic greenhouse gas concentrations, and furthermore that it is likely that there has been significant anthropogenic warming over the past 50 years averaged over each continent except Antarctica, we conclude that anthropogenic climate change is having a significant impact on physical and biological systems globally and in some continents.
- Warming caused by cumulative carbon emissions towards the trillionth tonne
- 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.
- Greenhouse-gas emission targets for limiting global warming to 2 C
- 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.
- Increasing carbon storage in intact African tropical forests
- 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.
