Intermittent convection, mixed boundary conditions and the stability of the thermohaline circulation

 Intermittent convection and its consequences on the stability of the thermohaline circulation are investigated with an oceanic global circulation model (OGCM) and simple box models. A two-box model shows that intermittency is a consequence of the non-linearity of the equation of state and of the ratio of heat and freshwater fluxes at surface versus the fluxes at depth. Moreover, it only occurs in areas, where the instability of the water column is caused by temperature or by salinity. Intermittency is not necessarily suppressed by long restoring times. Because intermittent convection causes temporal variations of the ocean-atmosphere fluxes, an OGCM cannot reach an exact equilibrium. After a switch to mixed boundary conditions, changes of the convective activity occur in areas where intermittency is observed. Intermittent convection becomes either continuous or is stopped depending on the method used for calculating the freshwater fluxes. Advective and diffusive fluxes between these regions and their surroundings change in order to balance the altered convective fluxes. A comparison between the OGCM and a six-box model illustrates that this may lead to an alteration of adjacent deep convection and of the related deep water formation. Received: 4 November 1997 / Accepted: 5 November 1998     

Glacial and deglacial climatic patterns in Australia and surrounding regions from 35 000 to 10 000 years ago reconstructed from terrestrial and near-shore proxy data

This study forms part of a wider investigation of late Quaternary environments in the Southern Hemisphere. We here review the terrestrial and near-shore proxy data from Australia, Indonesia, Papua New Guinea (PNG), New Zealand and surrounding oceans during 35–10 ka, an interval spanning the lead-up to the Last Glacial Maximum (LGM), the LGM proper (21 ± 2 ka), and the ensuing deglaciation. Sites selected for detailed discussion have a continuous or near continuous sedimentary record for this time interval, a stratigraphically consistent chronology, and one or more sources of proxy climatic data. Tropical Australia, Indonesia and PNG had LGM mean annual temperatures 3–7 °C below present values and summer precipitation reduced by at least 30%, consistent with a weaker summer monsoon and a northward displacement of the Intertropical Convergence Zone. The summer monsoon was re-established in northwest Australia by 14 ka. Precipitation in northeast Australia was reduced to less than 50% of present values until warmer and wetter conditions resumed at 17–16 ka, followed by a second warmer, wetter phase at 15–14 ka. LGM temperatures were up to 8 °C lower than today in mainland southeast Australia and up to 4 °C cooler in Tasmania. Winter rainfall was much reduced throughout much of southern Australia although periodic extreme flood events are evident in the fluvial record. Glacial advances in southeast Australia are dated to 32 ± 2.5, 19.1 ± 1.6 and 16.8 ± 1.4 ka, with periglacial activity concentrated towards 23–16 ka. Deglaciation was rapid in the Snowy Mountains, which were ice-free by 15.8 ka. Minimum effective precipitation in southern Australia was from 14 to 12 ka. In New Zealand the glacial advances date to ～28, 21.5 and 19 ka, with the onset of major cooling at ～28 ka, or well before the LGM. There is no convincing evidence for a Younger Dryas cooling event in or around New Zealand, but there are signs of the Antarctic Cold Reversal in and around New Zealand and off southern Australia. There remain unresolved discrepancies between the climates inferred from pollen and those inferred from the beetle and chironomid fauna at a number of New Zealand sites. One explanation may be that pollen provides a generalised regional climatic signal in contrast to the finer local resolution offered by beetles and chironomids. Sea surface temperatures (SSTs) were up to 5 °C cooler during the LGM with rapid warming after 20 ka to attain present values by 15 ka. The increase in summer monsoonal precipitation at or before 15 ka reflects higher insolation, warmer SSTs and steeper thermal gradients between land and sea. The postglacial increase in winter rainfall in southern Australia is probably related to the southward displacement of the westerlies as SSTs around Antarctica became warmer and the winter pack ice and Antarctic Convergence Zone retreated to the south.     

Trimlines as evidence for palaeo-tsunamis

As seen in many of the satellite images from the tsunami in the Indian Ocean which struck in 2004, there is a distinctive limit between an area with sand coverage, vegetation destruction, and soil erosion on the one hand, and the unaffected natural vegetation on the other. This distinction provides a good landmark to map the inundation width, delimited on the landward side by a trimline. In this study, older trimlines, dating back about 300?years, from tsunamis that occurred throughout the world were documented. We discuss the origin and chronology of trimline modification and extinction, both of which depend on local topography, rock type, and climate.     

Beach scraping — Is it damaging to beach stability?

Beach scraping is removal of material from the lower part of the beach for deposition on the higher part of the beach or at the dune toe. Beach scraping has become a controversial subject, as it is sometimes claimed that it does little good to the beach in question, and is harmful to adjacent beaches. This brief article is a discussion on the subject. The conclusion is that undertaken in a practical and modest manner, scraping is not harmful but rather it is beneficial in protecting dunes and dykes against erosion on a short-term basis.     

Identification of data-driven Dutch dietary patterns that benefit the environment and are healthy

More sustainable dietary patterns are needed to mitigate global warming. This study aims to identify data-driven healthy dietary patterns that benefit the environment. In EPIC-NL, diet was assessed using a 178-item FFQ in 36,203 participants aged 20–70 years between 1993 and 1997. The Dutch Healthy Diet index 2015 (DHD15-index) was used to score healthiness of the diet. As proxy for environmental impact, greenhouse gas (GHG) emissions were calculated using life cycle analysis. To determine patterns that are both healthy and environmentally friendly, reduced rank regression was applied. A “Plant-based Pattern” characterized by high consumption of fruits, vegetables, and legumes, and low consumption of fries, red meat, and processed meat and a “Dairy-based Pattern” characterized by high consumption of dairy, and nuts and seeds and low consumption of coffee and tea, sugar-containing sodas, low-fiber bread, and savory sauces were derived. At equal energy intake, the diet of adherents (highest quartile) to the “Plant-based Pattern” were significantly healthier (89.8 points on the DHD15-index, p?<?0.0001) and more sustainable (3.96 kg C0₂-eq/day, p?<?0.0001) compared to the average diet (76.2 points, 4.06 kg C0₂-eq/day), whereas the “Dairy-based Pattern” was somewhat healthier (77.9 points, p?<?0.0001), but less sustainable (4.43 kg C0₂-eq/day, p?<?0.0001). When deriving dietary patterns based on health and environmental aspects of the diets, a “Plant-based” and a “Dairy-based” pattern were observed in our study population. Of these, the plant-based diet benefits health as well as the environment.     

Spatial modelling of rice yield losses in Tanzania due to bacterial leaf blight and leaf blast in a changing climate

Rice is the most rapidly growing staple food in Africa and although rice production is steadily increasing, the consumption is still out-pacing the production. In Tanzania, two important diseases in rice production are leaf blast caused by Magnaporthe oryzae and bacterial leaf blight caused by Xanthomonas oryzae pv. oryzae. The objective of this study was to quantify rice yield losses due to these two important diseases under a changing climate. We found that bacterial leaf blight is predicted to increase causing greater losses than leaf blast in the future, with losses due to leaf blast declining. The results of this study indicate that the effects of climate change on plant disease can not only be expected to be uneven across diseases but also across geographies, as in some geographic areas losses increase but decrease in others for the same disease.