DOI: 10.5061/dryad.b5mkkwh9x
title: Data from: Extending the climatological concept of ‘Detection and Attribution’ to global change ecology in the Anthropocene

description: Research into global change ecology is motivated by the need to understand
 the role of humans in changing biotic systems. Mechanistic understanding
 of ecological responses requires the separation of different climatic
 parameters and processes that often operate on diverse spatiotemporal
 scales. Yet most environmental studies do not distinguish the effects of
 internal climate variability from those caused by external, natural (e.g.
 volcanic, solar, orbital) or anthropogenic (e.g. greenhouse gases, ozone,
 aerosols, land-use) forcing factors. We suggest extending the
 climatological concept of ‘Detection and Attribution’ (DA) to unravel
 abiotic drivers of ecological dynamics in the Anthropocene. We therefore
 apply DA to quantify the relative roles of natural versus industrial
 temperature change on elevational shifts in the outbreak epicentres of the
 larch budmoth (LBM; Zeiraphera diniana or griseana Gn.); the classic
 example of a cyclic forest defoliating insect. Our case study shows that
 anthropogenic warming shifts the epicentre of travelling LBM waves upward,
 which disrupts the intensity of population outbreaks that occurred
 regularly over the past millennium in the European Alps. Our findings
 demonstrate the ability of DA to detect ecological responses beyond
 internal system variability, to attribute them to specific external
 climate forcing factors, and to identify climate-induced ecological
 tipping points. In order to implement the climatological concept of
 ‘Detection and Attribution’ successfully into modern global change
 ecology, future studies should combine high-resolution paleoenvironmental
 reconstructions and state-of-the-art climate model simulations to inform
 inference-based ecosystem models.
description: To extend the above study beyond ‘Detection’ to ‘Attribution’, we
 assess the elevational dynamics of LBM outbreaks on simulated Alpine
 winter temperature fluctuations from ‘The Community Earth System
 Model-Last Millennium Ensemble’ (CESM-LME; Otto-Bliesner et al., 2016).
 Using ~2° resolution in the atmosphere and land components, CESM-LME
 expands on the CMIP5 and earlier LM model simulations by providing the
 largest ensemble of last millennium simulations with a single model to
 date. The CESM-LME uses the CMIP5 climate forcing reconstructions (Schmidt
 et al., 2011) and contains both ‘full forcing’ simulations containing all
 last millennium forcing agents, as well as ensembles of simulations with
 each forcing individually (greenhouse gases, ozone and aerosols, land-use,
 volcanic eruptions, solar variability, orbital changes). In using the most
 recent version of the comprehensive CESM (Hurrell et al., 2013), the
 herein employed state-of-the-art multi-model climate ensemble approach
 adds understanding into the role of internal variability versus external
 forcing in generating climate variations over the last 12 centuries
 (Otto-Bliesner et al., 2016). Annual winter temperatures of the monthly
 means from December–February and averaged from 12 grid points over the
 Alpine arc (6–16°E and 44–48°N) were extracted from control and forced
 model runs. With the exception of combined ozone and aerosol simulations
 (1851/52 to 2004/5 CE), all other model output reaches back to the winter
 851/52 CE. The climate simulations were coupled with a spatial model of
 LBM population dynamics (Johnson et al., 2010), in which population growth
 rates were a function of winter temperatures. The
 effects of anthropogenic climatic forcing on LBM population dynamics were
 explored by a discrete time tri-trophic model where the insect’s
 pre-dispersal population dynamics are coupled with pre-dispersal
 population dynamics of parasitoids and the quality of the larch host tree
 foliage. LBM population dynamics are most consistent with regulation by
 both parasitoids and host needle quality (Baltensweiler & Rubli,
 1999). Parameter values were empirically-derived and the model was
 selected based on the best fit to empirical population dynamics of LBM
 (Turchin et al., 2003). The model was spatially-extended by linking
 multiple populations with moth and parasitoid dispersal using exponential
 dispersal kernels. This model has been used to capture the recurrent
 west-east traveling waves of LBM outbreaks across the Alpine arc in the
 mid- to late-20<sup>th</sup> century (Johnson et al., 2004).
 The method was used later to demonstrate that the travelling LBM outbreak
 waves vary along an elevational gradient, and that the elevation of the
 outbreak epicentre is affected by winter temperatures (Johnson et al.,
 2010). Previous studies have shown that elevation has a unimodal impact on
 the growth rates of LBM populations (Baltensweiler & Rubli, 1999).
 The spatially-extended population model suggests that outbreak epicentres
 occur at or near elevations where the LBM growth rate is greatest (Johnson
 et al., 2010). In this study, we use the relationship
 between growth rate and elevation estimated by Johnson et al. (2010), and
 link the temperature component of the population model to the simulated
 temperature anomalies. From the simulations we calculate epicentre
 elevations using wavelet phase analysis (Grenfell et al., 2001). Phase
 angle values range from -180° to 180°. As a population cycles from a
 trough to a peak, the phase angle increases from -180° to 0°. As a
 population cycles from a peak to a trough, the phase angle continues to
 increase from 0° to 180° (Johnson et al., 2004). In this study, the
 elevation with the largest phase angle is identified as the location of
 the traveling wave epicentre. The magnitudes of LBM population cycles are
 estimated by identifying the maximum population density over 10-year
 intervals, which approximates the typical 8–9-year length of LBM cycles
 (Turchin et al., 2003). The population dynamics are simulated across a
 potential elevational range from 400–2200 m asl. The one-dimensional
 gradient is separated into 181 locations at 10 m intervals. Dispersal
 across elevation is modelled with isotropic movement. The effects of
 different climatic forcing agents on the elevation of epicentre and
 magnitude of cycles is explored by simulating LBM population dynamics
 under each scenario. In each simulation, LBM population dynamics are
 coupled with winter temperature anomalies from the respective forcing
 scenario. Epicentre elevations are estimated from the simulated LBM
 population dynamics using wavelet and phase angle analysis (Johnson et
 al., 2004). Model performance is assessed by comparing the likelihoods
 that empirical phase angle-elevation relationships (Johnson et al., 2010)
 would be produced by the population model under each of the various
 climate forcing scenarios. All analyses are performed in Matlab R2012b
 (see Johnson et al., 2004, 2010 for methodological details). Supplementary
 to the above, we refer to Otto-Bliesner et al. (2016) and Johnson et al.
 (2004, 2010) for gaining further insights into ‘The Community Earth System
 Model-Last Millennium Ensemble’ (CESM-LME) modelling project and the
 ecological LBM model, respectively.
description: See original publication for details.