Consistently constraining fNLf_{\rm NL} with the squeezed lensing bispectrum using consistency relations

We introduce a non-perturbative method to constrain the amplitude of local-type primordial non-Gaussianity ($f_{\rm NL}$) using squeezed configurations of the CMB lensing convergence and cosmic shear bispectra. First, we use cosmological consistency relations to derive a model for the squeezed limit of angular auto- and cross-bispectra of lensing convergence fields in the presence of $f_{\rm NL}$. Using this model, we perform a Fisher forecast with specifications expected for upcoming CMB lensing measurements from the Simons Observatory and CMB-S4, as well as cosmic shear measurements from a Rubin LSST/Euclid-like experiment. Assuming a minimum multipole $\ell_{\rm min}=10$ and maximum multipole $\ell_{\rm max}=1400$, we forecast $\sigma_{f_{\rm NL}}=175$ ($95$) for Simons Observatory (CMB-S4). Our forecasts improve considerably for an LSST/Euclid-like cosmic shear experiment with three tomographic bins and $\ell_{\rm min}=10$ and $\ell_{\rm max}=1400$ ($5000$) with $\sigma_{f_{\rm NL}}=31$ ($16$). A joint analysis of CMB-S4 lensing and LSST/Euclid-like shear yields little gain over the shear-only forecasts; however, we show that a joint analysis could be useful if the CMB lensing convergence can be reliably reconstructed at larger angular scales than the shear field. The method presented in this work is a novel and robust technique to constrain local primordial non-Gaussianity from upcoming large-scale structure surveys that is completely independent of the galaxy field (and therefore any nuisance parameters such as $b_\phi$), thus complementing existing techniques to constrain $f_{\rm NL}$ using the scale-dependent halo bias.Comment: 14 pages, 5 figures, comments welcome

The ecology of methane in streams and rivers: patterns, controls, and global significance

Streams and rivers can substantially modify organic carbon (OC) inputs from terrestrial landscapes, and much of this processing is the result of microbial respiration. While carbon dioxide (CO₂) is the major end‐product of ecosystem respiration, methane (CH₄) is also present in many fluvial environments even though methanogenesis typically requires anoxic conditions that may be scarce in these systems. Given recent recognition of the pervasiveness of this greenhouse gas in streams and rivers, we synthesized existing research and data to identify patterns and drivers of CH₄, knowledge gaps, and research opportunities. This included examining the history of lotic CH4 research, creating a database of concentrations and fluxes (MethDB) to generate a global‐scale estimate of fluvial CH₄ efflux, and developing a conceptual framework and using this framework to consider how human activities may modify fluvial CH₄ dynamics. Current understanding of CH₄ in streams and rivers has been strongly influenced by goals of understanding OC processing and quantifying the contribution of CH₄ to ecosystem C fluxes. Less effort has been directed towards investigating processes that dictate in situ CH₄ production and loss. CH₄ makes a meager contribution to watershed or landscape C budgets, but streams and rivers are often significant CH₄ sources to the atmosphere across these same spatial extents. Most fluvial systems are supersaturated with CH₄ and we estimate an annual global emission of 26.8 Tg CH₄, equivalent to ~15‐40% of wetland and lake effluxes, respectively. Less clear is the role of CH₄ oxidation, methanogenesis, and total anaerobic respiration to whole ecosystem production and respiration. Controls on CH₄ generation and persistence can be viewed in terms of proximate controls that influence methanogenesis (organic matter, temperature, alternative electron acceptors, nutrients) and distal geomorphic and hydrologic drivers. Multiple controls combined with its extreme redox status and low solubility result in high spatial and temporal variance of CH₄ in fluvial environments, which presents a substantial challenge for understanding its larger‐scale dynamics. Further understanding of CH₄ production and consumption, anaerobic metabolism, and ecosystem energetics in streams and rivers can be achieved through more directed studies and comparison with knowledge from terrestrial, wetland, and aquatic disciplines."Support for this paper was provided by funding from the North Temperate Lakes LTER program, NSF DEB‐0822700."https://esajournals.onlinelibrary.wiley.com/doi/full/10.1890/15-102