Dalhousie University

About: Dalhousie University is a education organization based out in Halifax, Nova Scotia, Canada. It is known for research contribution in the topics: Population & Health care. The organization has 25660 authors who have published 58465 publications receiving 2082403 citations. The organization is also known as: Dalhousie College & The Governors of Dalhousie College and University.

Tropical Rainfall Trends and the Indirect Aerosol Effect

Abstract: An atmospheric global climate model coupled to a mixed layer ocean model is used to study changes in tropical rainfall due to the indirect effects of anthropogenic sulfate aerosol. The model is run to equilibrium for present-day (PD) and preindustrial (PI) sulfur emission scenarios. As in two other recent studies, the model generally gives a southward shift of tropical rainfall in the PD run relative to the PI run. This is largely due to a hemispheric asymmetry in the reduction of sea surface temperature (SST) induced by the perturbation of cloud albedo and lifetime. Observed precipitation trends over land for the period 1900‐98 show a complex pattern in the Tropics, but when zonally averaged, a southward shift similar to (but weaker than) the modeled shift is clearly evident. The zonally averaged tropical trends are significant at the 5% level in several latitude bands. The modeled presentday hemispheric contrast in cloud droplet effective radius (which affects cloud albedo) is well supported by one long-term satellite retrieval, but not by another. A third satellite retrieval, which only covers an 8-month period, does show a marked hemispheric contrast in effective radius. Both in the modeled changes and the observed trends, a prominent feature is the drying of the Sahel in North Africa. Modeled dynamical changes in this region are similar to observed changes that have been associated with Sahelian drought. Previous work has identified a near-global, quasi-hemispheric pattern of contrasting SST anomalies (cool in the Northern Hemisphere and warm in the Southern Hemisphere) associated with dry conditions in the Sahel. The present results, combined with this earlier finding, suggest that the indirect effects of anthropogenic sulfate may have contributed to the Sahelian drying trend. More generally, it is concluded that spatially varying aerosol-related forcing (both direct and indirect) can substantially alter low-latitude circulation and rainfall.

The origins of joint visual attention in infants.

Abstract: Two experiments examined the origins of joint visual attention with a training procedure. In Experiment 1, infants aged 6-11 months were tested for a gaze-following (joint visual attention) response under feedback and no feedback conditions. In Experiment 2, infants 8-9 months received feedback for either following the experimenter's gaze (natural group) or looking to the opposite side (unnatural group). Results of the 2 experiments indicate that (a) joint visual attention does not reliably appear prior to 10 months of age, (b) from about 8 months of age, a gaze-following response can be learned, and (c) simple learning is not sufficient as the mechanism through which joint attention cues acquire their signal value.

Eutrophication causes speciation reversal in whitefish adaptive radiations

Abstract: Species diversity can be lost through two different but potentially interacting extinction processes: demographic decline and speciation reversal through introgressive hybridization. To investigate the relative contribution of these processes, we analysed historical and contemporary data of replicate whitefish radiations from 17 pre-alpine European lakes and reconstructed changes in genetic species differentiation through time using historical samples. Here we provide evidence that species diversity evolved in response to ecological opportunity, and that eutrophication, by diminishing this opportunity, has driven extinctions through speciation reversal and demographic decline. Across the radiations, the magnitude of eutrophication explains the pattern of species loss and levels of genetic and functional distinctiveness among remaining species. We argue that extinction by speciation reversal may be more widespread than currently appreciated. Preventing such extinctions will require that conservation efforts not only target existing species but identify and protect the ecological and evolutionary processes that generate and maintain species.