Climate sciences

  • Review Article |

    Global climate and the atmospheric partial pressure of carbon dioxide are correlated over recent glacial cycles, with lower partial pressure of carbon dioxide during ice ages, but the causes of the changes in the partial pressure of carbon dioxide are unknown. Here the authors review the evidence in support of the hypothesis that the Southern Ocean is an important driver of glacial/interglacial changes in the partial pressure of carbon dioxide.

    • Daniel M. Sigman
    • , Mathis P. Hain
    •  & Gerald H. Haug
  • Letter |

    It remains uncertain whether added nitrogen enhances total plant productivity in response to CO2-fertilisation in natural ecosystems. Here the authors show that nitrogen addition initially enhances the CO2-stimulation of plant productivity but also promotes the encroachment of plant species that respond less strongly to elevated CO2 concentrations. Overall, the observed shift in the plant community ultimately suppresses the CO2-stimulation of plant productivity.

    • J. Adam Langley
    •  & J. Patrick Megonigal
  • Letter |

    A pinwheel array of deep troughs has been one of the most perplexing features of the north polar layered deposits on Mars. Many ideas have been put forward about how it formed, but there is as yet no consensus. Here, penetrating radar has been used to rule out erosional cutting as a mechanism for the formation of the array. Instead, it is concluded that the troughs are largely depositional in origin, and have migrated to the poles and upwards in elevation over the past two million years or so.

    • Isaac B. Smith
    •  & John W. Holt
  • Letter |

    Many large mammals became extinct worldwide at the end of the Pleistocene epoch, around 12,000 years ago. Here, it is shown that smaller mammals, which often provide much more comprehensive fossil records than large mammals, were much less likely to respond to the Pleistocene–Holocene transition by becoming extinct. Instead, diversity and evenness suffered, so that less abundant species became rarer, with more generalist 'weedy' species becoming more common.

    • Jessica L. Blois
    • , Jenny L. McGuire
    •  & Elizabeth A. Hadly
  • Letter |

    The upper 300 m of the world's oceans act as a giant heat sink and have absorbed the majority of the excess energy generated by anthropogenic greenhouse gases. But the magnitude of the oceanic heat uptake is uncertain, and differing estimates have led to questions regarding the closure of the global energy budget. Here, a comparison of ocean heat content estimates is presented; the conclusion is that a robust warming of 0.64 W m−2 occurred from 1993 to 2008.

    • John M. Lyman
    • , Simon A. Good
    •  & Josh K. Willis
  • Letter |

    Accelerated uplift and global cooling have been suggested as possible causes for a fourfold increase in global sedimentation rates, and by inference in erosion rates or weathering fluxes, during the past 5 million years. Here, proxy evidence is provided that indicates stable weathering fluxes in the late-Cenozoic era. It is proposed that processes different from an increase in denudation caused Cenozoic global cooling.

    • Jane K. Willenbring
    •  & Friedhelm von Blanckenburg
  • Letter |

    To examine the effect of increased livestock numbers on nitrous oxide emissions the authors report year-round nitrous oxide flux measurements at ten steppe grassland sites in Inner Mongolia. They find that nitrous oxide emission is much higher during spring thaw and is highest in ungrazed steppe, decreasing with increasing stocking rate, which suggests that grazing decreases rather than increases nitrous oxide emissions.

    • Benjamin Wolf
    • , Xunhua Zheng
    •  & Klaus Butterbach-Bahl
  • Letter |

    Climate change does not occur symmetrically; instead, in a process called polar amplification, polar areas warm faster than the tropics. Recent work indicated that transport processes in the upper atmosphere account for much of the recent polar amplification, but this conclusion proved controversial. Here, updated reanalysis data have been used to show that reductions in sea ice are instead responsible.

    • James A. Screen
    •  & Ian Simmonds
  • Letter |

    Our current concepts of abrupt climate change are influenced by palaeoclimate evidence for events such as the Younger Dryas cold interval, in which massive climate changes occurred essentially instantaneously. It is thought that an injection of fresh water from the retreating Laurentide Ice Sheet altered the Atlantic meridional overturning circulation and triggered the Younger Dryas, but convincing geological evidence has been elusive. Here, a major flood event that is chronologically consistent with the Younger Dryas has been identified—through the MacKenzie River into the Arctic Ocean.

    • Julian B. Murton
    • , Mark D. Bateman
    •  & Zhirong Yang
  • Letter |

    It has been inferred that, during the Archaean eon, there must have been a high concentration of atmospheric CO2 and/or CH4, causing a greenhouse effect that would have compensated for the lower solar luminosity at the time and allowed liquid water to be stable in the hydrosphere. Here it is shown, however, that the mineralogy of Archaean sediments is inconsistent with such high concentrations of greenhouse gases. Instead it is proposed that a lower albedo on the Earth helped to moderate surface temperature.

    • Minik T. Rosing
    • , Dennis K. Bird
    •  & Christian J. Bjerrum
  • Letter |

    Palaeoclimate data show that 3–5 million years ago in the early Pliocene the equatorial Pacific experienced persistent warm, El Niño conditions. Here a hurricane model and a coupled climate model show a feedback between sea surface temperature and frequent hurricanes that could account for such conditions.

    • Alexey V. Fedorov
    • , Christopher M. Brierley
    •  & Kerry Emanuel
  • Letter |

    Anthropogenic global warming is likely to be amplified by positive feedback from the global carbon cycle; however, the magnitude of the climate sensitivity of the global carbon cycle, and thus of its positive feedback strength, is under debate. By combining a probabilistic approach with an ensemble of proxy-based temperature reconstructions and pre-industrial CO2 data from three ice cores, this climate sensitivity is now shown to be much smaller than previously thought.

    • David C. Frank
    • , Jan Esper
    •  & Fortunat Joos
  • Letter |

    High concentrations of ozone in the troposphere are toxic and act as a greenhouse gas. Anthropogenic emissions of ozone precursors have caused widespread increases in ozone concentrations since the late 1800s, with the fastest-growing ozone precursor emissions currently coming out of east Asia. Much of the springtime east Asian pollution is exported towards western North America; a strong increase in springtime ozone mixing ratios is now found in the free troposphere over this region.

    • O. R. Cooper
    • , D. D. Parrish
    •  & M. A. Avery
  • Letter |

    The elevation of the Tibetan plateau is thought to cause its surface to serve as a heat source that drives the South Asian summer monsoon, potentially coupling uplift of the plateau to climate changes on geologic timescales. Here, however, an atmospheric model is used to show that flattening of the Tibetan plateau has little effect on the monsoon, provided that the narrow orography of the Himalayas and adjacent mountain ranges is preserved.

    • William R. Boos
    •  & Zhiming Kuang
  • Letter |

    The Southern Ocean is potentially a substantial sink of anthropogenic carbon dioxide; however, the regulation of this carbon sink by the wind-driven Ekman flow, mesoscale eddies and their interaction is under debate. Here, a high-resolution ocean circulation and carbon cycle model is used to study intra-annual variability in anthropogenic carbon dioxide over a two-year time period; the Ekman flow is found to be the primary mechanism of anthropogenic carbon dioxide transport across the Antarctic polar front.

    • T. Ito
    • , M. Woloszyn
    •  & M. Mazloff