Nature Reports: Climate Change989 (2007). doi:10.1038/446989a

Author: Philip W. Boyd

A huge phytoplankton bloom in the Southern Ocean yields estimates of how a continuous supply of iron affects oceanic carbon sequestration. But iron is not the only factor — nutrient supply is crucial too.

Nature Reports: Climate Change710 (2007). doi:10.1038/nature05829

Authors: Shannon L. LaDeau, A. Marm Kilpatrick & Peter P. Marra

Emerging infectious diseases present a formidable challenge to the conservation of native species in the twenty-first century. Diseases caused by introduced pathogens have had large impacts on species abundances, including the American chestnut, Hawaiian bird species and many amphibians. Changes in host population sizes can lead to marked shifts in community composition and ecosystem functioning. However, identifying the impacts of an introduced disease and distinguishing it from other forces that influence population dynamics (for example, climate) is challenging and requires abundance data that extend before and after the introduction. Here we use 26 yr of Breeding Bird Survey (BBS) data to determine the impact of West Nile virus (WNV) on 20 potential avian hosts across North America. We demonstrate significant changes in population trajectories for seven species from four families that concur with a priori predictions and the spatio-temporal intensity of pathogen transmission. The American crow population declined by up to 45% since WNV arrival, and only two of the seven species with documented impact recovered to pre-WNV levels by 2005. Our findings demonstrate the potential impacts of an invasive species on a diverse faunal assemblage across broad geographical scales, and underscore the complexity of subsequent community response.

Nature Reports: Climate Change652 (2007). doi:10.1038/nature05889

Author: Carsten Rahbek

A continent-wide analysis suggests that West Nile virus has severely affected bird populations associated with human habitats in North America. The declines parallel patterns of human disease caused by the virus.

Nature Reports: Climate Change8 (2007). doi:10.1038/climate.2007.11

Author: Matthew Lockwood

Will voluntary carbon markets genuinely tackle climate change or could they encourage further emissions?

Nature Reports: Climate Change12 (2007). doi:10.1038/climate.2007.12

Author: Amanda Leigh Haag

As discussions get underway over a global agreement to slash CO2 emissions beyond 2012, Amanda Leigh Haag looks at how the Kyoto Protocol has fared and the issues that will shape its successor.

Nature Reports: Climate Change16 (2007). doi:10.1038/climate.2007.13

Author: Kevin Vranes

This month, Kevin Vranes at the Center for Science and Technology Policy Research in Boulder, Colorado, reviews recent climate legislation passing through US Congress.

Nature Reports: Climate Change4 (2007). doi:10.1038/climate.2007.2

As the world's largest trial carbon storage project gets underway, some are questioning its necessity. Hannah Hoag reports from Australia.

Nature Reports: Climate Change6 (2007). doi:10.1038/climate.2007.3

Author: Jeffrey D. Sachs

Avoiding dangerous climate change will require considerable global efforts to reduce greenhouse gas emissions. A daunting challenge, but one that is practically and economically achievable, argues Jeffrey D. Sachs.

Nature Reports: Climate Change194 (2007). doi:10.1038/nature05832

Authors: Alberto C. Naveira Garabato, David P. Stevens, Andrew J. Watson & Wolfgang Roether

The oceanic overturning circulation has a central role in the Earth’s climate system and in biogeochemical cycling, as it transports heat, carbon and nutrients around the globe and regulates their storage in the deep ocean. Mixing processes in the Antarctic Circumpolar Current are key to this circulation, because they control the rate at which water sinking at high latitudes returns to the surface in the Southern Ocean. Yet estimates of the rates of these processes and of the upwelling that they induce are poorly constrained by observations. Here we take advantage of a natural tracer-release experiment—an injection of mantle helium from hydrothermal vents into the Circumpolar Current near Drake Passage—to measure the rates of mixing and upwelling in the current’s intermediate layers over a sector that spans nearly one-tenth of its circumpolar path. Dispersion of the tracer reveals rapid upwelling along density surfaces and intense mixing across density surfaces, both occurring at rates that are an order of magnitude greater than rates implicit in models of the average Southern Ocean overturning. These findings support the view that deep-water pathways along and across density surfaces intensify and intertwine as the Antarctic Circumpolar Current flows over complex ocean-floor topography, giving rise to a short circuit of the overturning circulation in these regions.

Nature Reports: Climate Change80 (2007). doi:10.1038/nature05747

Authors: Bettina M. J. Engelbrecht, Liza S. Comita, Richard Condit, Thomas A. Kursar, Melvin T. Tyree, Benjamin L. Turner & Stephen P. Hubbell

Although patterns of tree species distributions along environmental gradients have been amply documented in tropical forests, mechanisms causing these patterns are seldom known. Efforts to evaluate proposed mechanisms have been hampered by a lack of comparative data on species’ reactions to relevant axes of environmental variation. Here we show that differential drought sensitivity shapes plant distributions in tropical forests at both regional and local scales. Our analyses are based on experimental field assessments of drought sensitivity of 48 species of trees and shrubs, and on their local and regional distributions within a network of 122 inventory sites spanning a rainfall gradient across the Isthmus of Panama. Our results suggest that niche differentiation with respect to soil water availability is a direct determinant of both local- and regional-scale distributions of tropical trees. Changes in soil moisture availability caused by global climate change and forest fragmentation are therefore likely to alter tropical species distributions, community composition and diversity.

Nature Reports: Climate ChangeE1 (2007). doi:10.1038/nature05830

Authors: Graham A. Shields & James F. Kasting

Arising from: F. Robert & M. Chaussidon Nature443, 969–972 (2006); Robert & Chaussidon reply

Nature Reports: Climate Change991 (2007). doi:10.1038/446991a

Author: Euripides G. Stephanou

A sophisticated survey of certain volatile organic compounds in the air over forest ecosystems shows how such work can reveal varied emission patterns of different chiral, or mirror-image, forms of these compounds.

Nature Reports: Climate Change1070 (2007). doi:10.1038/nature05700

Authors: Stéphane Blain, Bernard Quéguiner, Leanne Armand, Sauveur Belviso, Bruno Bombled, Laurent Bopp, Andrew Bowie, Christian Brunet, Corina Brussaard, François Carlotti, Urania Christaki, Antoine Corbière, Isabelle Durand, Frederike Ebersbach, Jean-Luc Fuda, Nicole Garcia, Loes Gerringa, Brian Griffiths, Catherine Guigue, Christophe Guillerm, Stéphanie Jacquet, Catherine Jeandel, Patrick Laan, Dominique Lefèvre, Claire Lo Monaco, Andrea Malits, Julie Mosseri, Ingrid Obernosterer, Young-Hyang Park, Marc Picheral, Philippe Pondaven, Thomas Remenyi, Valérie Sandroni, Géraldine Sarthou, Nicolas Savoye, Lionel Scouarnec, Marc Souhaut, Doris Thuiller, Klaas Timmermans, Thomas Trull, Julia Uitz, Pieter van Beek, Marcel Veldhuis, Dorothée Vincent, Eric Viollier, Lilita Vong & Thibaut Wagener

The availability of iron limits primary productivity and the associated uptake of carbon over large areas of the ocean. Iron thus plays an important role in the carbon cycle, and changes in its supply to the surface ocean may have had a significant effect on atmospheric carbon dioxide concentrations over glacial–interglacial cycles. To date, the role of iron in carbon cycling has largely been assessed using short-term iron-addition experiments. It is difficult, however, to reliably assess the magnitude of carbon export to the ocean interior using such methods, and the short observational periods preclude extrapolation of the results to longer timescales. Here we report observations of a phytoplankton bloom induced by natural iron fertilization—an approach that offers the opportunity to overcome some of the limitations of short-term experiments. We found that a large phytoplankton bloom over the Kerguelen plateau in the Southern Ocean was sustained by the supply of iron and major nutrients to surface waters from iron-rich deep water below. The efficiency of fertilization, defined as the ratio of the carbon export to the amount of iron supplied, was at least ten times higher than previous estimates from short-term blooms induced by iron-addition experiments. This result sheds new light on the effect of long-term fertilization by iron and macronutrients on carbon sequestration, suggesting that changes in iron supply from below—as invoked in some palaeoclimatic and future climate change scenarios—may have a more significant effect on atmospheric carbon dioxide concentrations than previously thought.

Nature Reports: Climate Change530 (2007). doi:10.1038/nature05699

Authors: Dana L. Royer, Robert A. Berner & Jeffrey Park

A firm understanding of the relationship between atmospheric carbon dioxide concentration and temperature is critical for interpreting past climate change and for predicting future climate change. A recent synthesis suggests that the increase in global-mean surface temperature in response to a doubling of the atmospheric carbon dioxide concentration, termed ‘climate sensitivity’, is between 1.5 and 6.2 °C (5–95 per cent likelihood range), but some evidence is inconsistent with this range. Moreover, most estimates of climate sensitivity are based on records of climate change over the past few decades to thousands of years, when carbon dioxide concentrations and global temperatures were similar to or lower than today, so such calculations tend to underestimate the magnitude of large climate-change events and may not be applicable to climate change under warmer conditions in the future. Here we estimate long-term equilibrium climate sensitivity by modelling carbon dioxide concentrations over the past 420 million years and comparing our calculations with a proxy record. Our estimates are broadly consistent with estimates based on short-term climate records, and indicate that a weak radiative forcing by carbon dioxide is highly unlikely on multi-million-year timescales. We conclude that a climate sensitivity greater than 1.5 °C has probably been a robust feature of the Earth’s climate system over the past 420 million years, regardless of temporal scaling.

Nature Reports: Climate Change176 (2007). doi:10.1038/nature05591

Authors: James S. Eldrett, Ian C. Harding, Paul A. Wilson, Emily Butler & Andrew P. Roberts

The Eocene and Oligocene epochs (∼55 to 23 million years ago) comprise a critical phase in Earth history. An array of geological records supported by climate modelling indicates a profound shift in global climate during this interval, from a state that was largely free of polar ice caps to one in which ice sheets on Antarctica approached their modern size. However, the early glaciation history of the Northern Hemisphere is a subject of controversy. Here we report stratigraphically extensive ice-rafted debris, including macroscopic dropstones, in late Eocene to early Oligocene sediments from the Norwegian–Greenland Sea that were deposited between about 38 and 30 million years ago. Our data indicate sediment rafting by glacial ice, rather than sea ice, and point to East Greenland as the likely source. Records of this type from one site alone cannot be used to determine the extent of ice involved. However, our data suggest the existence of (at least) isolated glaciers on Greenland about 20 million years earlier than previously documented, at a time when temperatures and atmospheric carbon dioxide concentrations were substantially higher.

Nature Reports: Climate ChangeE1 (2007). doi:10.1038/nature05707

Author: Tapio Schneider

Arising from: G. C. Hegerl, T. J. Crowley, W. T. Hyde & D. J. Frame Nature440, 1029–1032 (2006); Hegerl et al. reply

Nature Reports: Climate Change908 (2007). doi:10.1038/nature05578

Authors: Guillaume Leduc, Laurence Vidal, Kazuyo Tachikawa, Frauke Rostek, Corinne Sonzogni, Luc Beaufort & Edouard Bard

Moisture transport from the Atlantic to the Pacific ocean across Central America leads to relatively high salinities in the North Atlantic Ocean and contributes to the formation of North Atlantic Deep Water. This deep water formation varied strongly between Dansgaard/Oeschger interstadials and Heinrich events—millennial-scale abrupt warm and cold events, respectively, during the last glacial period. Increases in the moisture transport across Central America have been proposed to coincide with northerly shifts of the Intertropical Convergence Zone and with Dansgaard/Oeschger interstadials, with opposite changes for Heinrich events. Here we reconstruct sea surface salinities in the eastern equatorial Pacific Ocean over the past 90,000 years by comparing palaeotemperature estimates from alkenones and Mg/Ca ratios with foraminiferal oxygen isotope ratios that vary with both temperature and salinity. We detect millennial-scale fluctuations of sea surface salinities in the eastern equatorial Pacific Ocean of up to two to four practical salinity units. High salinities are associated with the southward migration of the tropical Atlantic Intertropical Convergence Zone, coinciding with Heinrich events and with Greenland stadials. The amplitudes of these salinity variations are significantly larger on the Pacific side of the Panama isthmus, as inferred from a comparison of our data with a palaeoclimate record from the Caribbean basin. We conclude that millennial-scale fluctuations of moisture transport constitute an important feedback mechanism for abrupt climate changes, modulating the North Atlantic freshwater budget and hence North Atlantic Deep Water formation.

Nature Reports: Climate Change607 (2007). doi:10.1038/445607a

Author: Gabriel J. Bowen

As massive ice sheets grew on Antarctica during the first major glaciation of the Cenozoic era, the northern continents cooled and dried. The coincidence in timing implies that the cause was global rather than regional.

Nature Reports: Climate Change635 (2007). doi:10.1038/nature05516

Authors: Guillaume Dupont-Nivet, Wout Krijgsman, Cor G. Langereis, Hemmo A. Abels, Shuang Dai & Xiaomin Fang

Continental aridification and the intensification of the monsoons in Asia are generally attributed to uplift of the Tibetan plateau and to the land–sea redistributions associated with the continental collision of India and Asia, whereas some studies suggest that past changes in Asian environments are mainly governed by global climate. The most dramatic climate event since the onset of the collision of India and Asia is the Eocene–Oligocene transition, an abrupt cooling step associated with the onset of glaciation in Antarctica 34 million years ago. However, the influence of this global event on Asian environments is poorly understood. Here we use magnetostratigraphy and cyclostratigraphy to show that aridification, which is indicated by the disappearance of playa lake deposits in the northeastern Tibetan plateau, occurred precisely at the time of the Eocene–Oligocene transition. Our findings suggest that this global transition is linked to significant aridification and cooling in continental Asia recorded by palaeontological and palaeoenvironmental changes, and thus support the idea that global cooling is associated with the Eocene–Oligocene transition. We show that, with sufficient age control on the sedimentary records, global climate can be distinguished from tectonism and recognized as a major contributor to continental Asian environments.

Nature Reports: Climate Change639 (2007). doi:10.1038/nature05551

Authors: Alessandro Zanazzi, Matthew J. Kohn, Bruce J. MacFadden & Dennis O. Terry

The Eocene–Oligocene transition towards a cool climate (∼33.5 million years ago) was one of the most pronounced climate events during the Cenozoic era. The marine record of this transition has been extensively studied. However, significantly less research has focused on continental climate change at the time, yielding partly inconsistent results on the magnitude and timing of the changes. Here we use a combination of in vivo stable isotope compositions of fossil tooth enamel with diagenetic stable isotope compositions of fossil bone to derive a high-resolution (about 40,000 years) continental temperature record for the Eocene–Oligocene transition. We find a large drop in mean annual temperature of 8.2 ± 3.1 °C over about 400,000 years, the possibility of a small increase in temperature seasonality, and no resolvable change in aridity across the transition. The large change in mean annual temperature, exceeding changes in sea surface temperatures at comparable latitudes and possibly delayed in time with respect to marine changes by up to 400,000 years, explains the faunal turnover for gastropods, amphibians and reptiles, whereas most mammals in the region were unaffected. Our results are in agreement with modelling studies that attribute the climate cooling at the Eocene–Oligocene transition to a significant drop in atmospheric carbon dioxide concentrations.

Nature Reports: Climate Change495 (2007). doi:10.1038/445495a

Author: Thom Rahn

The behaviour of water in the atmosphere is a poorly understood part of the hydrological cycle. Applying the principles of isotope chemistry to satellite data provides a powerful approach for improving the situation.

Nature Reports: Climate Change528 (2007). doi:10.1038/nature05508

Authors: John Worden, David Noone & Kevin Bowman

Atmospheric moisture cycling is an important aspect of the Earth’s climate system, yet the processes determining atmospheric humidity are poorly understood. For example, direct evaporation of rain contributes significantly to the heat and moisture budgets of clouds, but few observations of these processes are available. Similarly, the relative contributions to atmospheric moisture over land from local evaporation and humidity from oceanic sources are uncertain. Lighter isotopes of water vapour preferentially evaporate whereas heavier isotopes preferentially condense and the isotopic composition of ocean water is known. Here we use this information combined with global measurements of the isotopic composition of tropospheric water vapour from the Tropospheric Emission Spectrometer (TES) aboard the Aura spacecraft, to investigate aspects of the atmospheric hydrological cycle that are not well constrained by observations of precipitation or atmospheric vapour content. Our measurements of the isotopic composition of water vapour near tropical clouds suggest that rainfall evaporation contributes significantly to lower troposphere humidity, with typically 20% and up to 50% of rainfall evaporating near convective clouds. Over the tropical continents the isotopic signature of tropospheric water vapour differs significantly from that of precipitation, suggesting that convection of vapour from both oceanic sources and evapotranspiration are the dominant moisture sources. Our measurements allow an assessment of the intensity of the present hydrological cycle and will help identify any future changes as they occur.

Nature Reports: Climate Change270 (2007). doi:10.1038/445270a

Authors: Jonathan T. Overpeck & Julia E. Cole

The burden of global warming falls most heavily on the developing world. A connection forged between the Indian Ocean climate, Asian monsoons and drought in Indonesia makes for an especially bleak outlook for that nation.

Nature Reports: Climate Change303 (2007). doi:10.1038/nature05463

Authors: Hailong Lu, Yu-taek Seo, Jong-won Lee, Igor Moudrakovski, John A. Ripmeester, N. Ross Chapman, Richard B. Coffin, Graeme Gardner & John Pohlman

Natural gas hydrates are a potential source of energy and may play a role in climate change and geological hazards. Most natural gas hydrate appears to be in the form of ‘structure I’, with methane as the trapped guest molecule, although ‘structure II’ hydrate has also been identified, with guest molecules such as isobutane and propane, as well as lighter hydrocarbons. A third hydrate structure, ‘structure H’, which is capable of trapping larger guest molecules, has been produced in the laboratory, but it has not been confirmed that it occurs in the natural environment. Here we characterize the structure, gas content and composition, and distribution of guest molecules in a complex natural hydrate sample recovered from Barkley canyon, on the northern Cascadia margin. We show that the sample contains structure H hydrate, and thus provides direct evidence for the natural occurrence of this hydrate structure. The structure H hydrate is intimately associated with structure II hydrate, and the two structures contain more than 13 different hydrocarbon guest molecules. We also demonstrate that the stability field of the complex gas hydrate lies between those of structure II and structure H hydrates, indicating that this form of hydrate is more stable than structure I and may thus potentially be found in a wider pressure–temperature regime than can methane hydrate deposits.

Nature Reports: Climate Change299 (2007). doi:10.1038/nature05477

Authors: Nerilie J. Abram, Michael K. Gagan, Zhengyu Liu, Wahyoe S. Hantoro, Malcolm T. McCulloch & Bambang W. Suwargadi

The Indian Ocean Dipole (IOD)—an oscillatory mode of coupled ocean–atmosphere variability—causes climatic extremes and socio-economic hardship throughout the tropical Indian Ocean region. There is much debate about how the IOD interacts with the El Niño/Southern Oscillation (ENSO) and the Asian monsoon, and recent changes in the historic ENSO–monsoon relationship raise the possibility that the properties of the IOD may also be evolving. Improving our understanding of IOD events and their climatic impacts thus requires the development of records defining IOD activity in different climatic settings, including prehistoric times when ENSO and the Asian monsoon behaved differently from the present day. Here we use coral geochemical records from the equatorial eastern Indian Ocean to reconstruct surface-ocean cooling and drought during individual IOD events over the past ∼6,500 years. We find that IOD events during the middle Holocene were characterized by a longer duration of strong surface ocean cooling, together with droughts that peaked later than those expected by El Niño forcing alone. Climate model simulations suggest that this enhanced cooling and drying was the result of strong cross-equatorial winds driven by the strengthened Asian monsoon of the middle Holocene. These IOD–monsoon connections imply that the socioeconomic impacts of projected future changes in Asian monsoon strength may extend throughout Australasia.

Nature Reports: Climate Change159 (2007). doi:10.1038/445159a

Authors: Douglas G. Capone & Angela N. Knapp

Some of our suppositions about the marine nitrogen cycle may be wrong. An innovative analysis of nutrients at the ocean's surface reveals a feedback mechanism that might hold the whole cycle in balance.

Nature Reports: Climate Change202 (2007). doi:10.1038/nature05429

Authors: Jason M. Tylianakis, Teja Tscharntke & Owen T. Lewis

Global conversion of natural habitats to agriculture has led to marked changes in species diversity and composition. However, it is less clear how habitat modification affects interactions among species. Networks of feeding interactions (food webs) describe the underlying structure of ecological communities, and might be crucially linked to their stability and function. Here, we analyse 48 quantitative food webs for cavity-nesting bees, wasps and their parasitoids across five tropical habitat types. We found marked changes in food-web structure across the modification gradient, despite little variation in species richness. The evenness of interaction frequencies declined with habitat modification, with most energy flowing along one or a few pathways in intensively managed agricultural habitats. In modified habitats there was a higher ratio of parasitoid to host species and increased parasitism rates, with implications for the important ecosystem services, such as pollination and biological control, that are performed by host bees and wasps. The most abundant parasitoid species was more specialized in modified habitats, with reduced attack rates on alternative hosts. Conventional community descriptors failed to discriminate adequately among habitats, indicating that perturbation of the structure and function of ecological communities might be overlooked in studies that do not document and quantify species interactions. Altered interaction structure therefore represents an insidious and functionally important hidden effect of habitat modification by humans.

Nature Reports: Climate Change74 (2007). doi:10.1038/nature05431

Authors: Gergana Yancheva, Norbert R. Nowaczyk, Jens Mingram, Peter Dulski, Georg Schettler, Jörg F. W. Negendank, Jiaqi Liu, Daniel M. Sigman, Larry C. Peterson & Gerald H. Haug

The Asian–Australian monsoon is an important component of the Earth’s climate system that influences the societal and economic activity of roughly half the world’s population. The past strength of the rain-bearing East Asian summer monsoon can be reconstructed with archives such as cave deposits, but the winter monsoon has no such signature in the hydrological cycle and has thus proved difficult to reconstruct. Here we present high-resolution records of the magnetic properties and the titanium content of the sediments of Lake Huguang Maar in coastal southeast China over the past 16,000 years, which we use as proxies for the strength of the winter monsoon winds. We find evidence for stronger winter monsoon winds before the Bølling–Allerød warming, during the Younger Dryas episode and during the middle and late Holocene, when cave stalagmites suggest weaker summer monsoons. We conclude that this anticorrelation is best explained by migrations in the intertropical convergence zone. Similar migrations of the intertropical convergence zone have been observed in Central America for the period ad 700 to 900 (refs 4–6), suggesting global climatic changes at that time. From the coincidence in timing, we suggest that these migrations in the tropical rain belt could have contributed to the declines of both the Tang dynasty in China and the Classic Maya in Central America.