Inferred changes in El Niño–Southern Oscillation variance over the past six centuries

It is vital to understand how the El Niño–Southern Oscillation (ENSO) has responded to past changes in natural and anthropogenic forcings, in order to better understand and predict its response to future greenhouse warming. To date, however, the instrumental record is too brief to fully characterize natural ENSO variability, while large discrepancies exist amongst paleo-proxy reconstructions of ENSO. These paleo-proxy reconstructions have typically attempted to reconstruct ENSO's temporal evolution, rather than the variance of these temporal changes. Here a new approach is developed that synthesizes the variance changes from various proxy data sets to provide a unified and updated estimate of past ENSO variance. The method is tested using surrogate data from two coupled general circulation model (CGCM) simulations. It is shown that in the presence of dating uncertainties, synthesizing variance information provides a more robust estimate of ENSO variance than synthesizing the raw data and then identifying its running variance. We also examine whether good temporal correspondence between proxy data and instrumental ENSO records implies a good representation of ENSO variance. In the climate modeling framework we show that a significant improvement in reconstructing ENSO variance changes is found when combining information from diverse ENSO-teleconnected source regions, rather than by relying on a single well-correlated location. This suggests that ENSO variance estimates derived from a single site should be viewed with caution. Finally, synthesizing existing ENSO reconstructions to arrive at a better estimate of past ENSO variance changes, we find robust evidence that the ENSO variance for any 30 yr period during the interval 1590–1880 was considerably lower than that observed during 1979–2009

Description and validation of the Simple, Efficient, Dynamic, Global, Ecological Simulator (SEDGES v.1.0)

In this paper, we present a simple dynamic global vegetation model whose primary intended use is auxiliary to the land–atmosphere coupling scheme of a climate model, particularly one of intermediate complexity. The model simulates and provides important ecological-only variables but also some hydrological and surface energy variables that are typically either simulated by land surface schemes or else used as boundary data input for these schemes. The model formulations and their derivations are presented here, in detail. The model includes some realistic and useful features for its level of complexity, including a photosynthetic dependency on light, full coupling of photosynthesis and transpiration through an interactive canopy resistance, and a soil organic carbon dependence for bare-soil albedo. We evaluate the model's performance by running it as part of a simple land surface scheme that is driven by reanalysis data. The evaluation against observational data includes net primary productivity, leaf area index, surface albedo, and diagnosed variables relevant for the closure of the hydrological cycle. In this setup, we find that the model gives an adequate to good simulation of basic large-scale ecological and hydrological variables. Of the variables analyzed in this paper, gross primary productivity is particularly well simulated. The results also reveal the current limitations of the model. The most significant deficiency is the excessive simulation of evapotranspiration in mid- to high northern latitudes during their winter to spring transition. The model has a relative advantage in situations that require some combination of computational efficiency, model transparency and tractability, and the simulation of the large-scale vegetation and land surface characteristics under non-present-day conditions

The Holocene temperature conundrum

A recent temperature reconstruction of global annual temperature shows Early Holocene warmth followed by a cooling trend through the Middle to Late Holocene [Marcott SA, et al., 2013, Science 339(6124):1198–1201]. This global cooling is puzzling because it is opposite from the expected and simulated global warming trend due to the retreating ice sheets and rising atmospheric greenhouse gases. Our critical reexamination of this contradiction between the reconstructed cooling and the simulated warming points to potentially significant biases in both the seasonality of the proxy reconstruction and the climate sensitivity of current climate models

Climate and biogeochemical response to a rapid melting of the West-Antarctic Ice Sheet during interglacials and implications for future climate

We study the effects of a massive meltwater discharge from the West Antarctic Ice Sheet (WAIS) during interglacials onto the global climate-carbon cycle system using the Earth system model of intermediate complexity LOVECLIM. Prescribing a meltwater pulse in the Southern Ocean that mimics a rapid disintegration of the WAIS, a substantial cooling of the Southern Ocean is simulated that is accompanied by an equatorward expansion of the sea ice margin and an intensification of the Southern Hemispheric Westerlies. The strong halocline around Antarctica leads to suppression of Antarctic Bottom Water (AABW) formation and to subsurface warming in areas where under present-day conditions AABW is formed. This subsurface warming at depths between 500 and 1500 m leads to a thermal weathering of the WAIS grounding line and provides a positive feedback that accelerates the meltdown of the WAIS. Our model results further demonstrate that in response to the massive expansion of sea ice, marine productivity in the Southern Ocean reduces significantly. A retreat of the WAIS, however, does not lead to any significant changes in atmospheric CO2. The climate signature of a WAIS collapse is structurally consistent with available paleoproxy signals of the last interglacial MIS5e