Abstract. The finite response time of alpine glaciers means that glaciers will be in a state of disequilibrium in the presence of a climate trend. Using a simple model of glacier dynamics, we use metrics of glacier geometry to evaluate the present-day disequilibrium for a population of 5600 alpine glaciers in Alaska. Our results indicate that glaciers throughout the region are in a severe state of disequilibrium. We estimate that the median glacier has only undergone 27 % of the retreat necessary to achieve equilibrium with the present-day climate. In general, glaciers with smaller areas have smaller response times, and so are closer to equilibrium than large glaciers. Because much of Alaska’s glacier area is contained in a few large glaciers that are far from equilibrium, and because the rate of warming has increased in the last ~50 years, the median equilibration weighted by area is only 16 %. Our estimates are sensitive to uncertainty in response time and to the shape of the warming trend. Uncertainty is greatest for intermediate glacier response times but is small for glaciers with the smallest and largest response times. Finally, we demonstrate that accounting for the increased rate of warming in the late-20th century is important for estimating glacier disequilibrium, whereas the shape of the warming trend in the early-20th century is less relevant. Our results imply substantial future glacier retreat is already guaranteed regardless of the trajectory of future warming.
Critical wedge theory provides a direct link between the form of an orogen, the rate of orogen evolution, and the accretionary and erosional fluxes that promote orogen growth and decay, respectively. We explore several fundamental characteristics of an eroding critical orogen: (1) the sensitivity of steady-state orogen size to tectonic and climatic forcing, (2) the response time of a critical orogen to perturbations in forcing, and (3) the behavior of surface topography and the rock uplift field in a system in which they are not prescribed. To do this, we develop a numerical model that couples a two-dimensional, planform surface erosion model with a two-dimensional, plane-strain finite element model of deformation. We first present a base model in which a critical orogen evolves to a steady-state under boundary conditions similar to those of analog sandbox experiments. We find that mean topography and tectonic uplift reach steady states, whereas planform topography remains dynamic throughout the simulation. From a suite of simulations, we determine the steady-state scaling relationship between orogen size and tectonic and climatic forcing and find good agreement with predictions from one-dimensional models. In addition, we examine the response of the steady-state orogen to climatic and tectonic perturbation with four simulations in which changes in tectonic and climatic conditions lead to either growth or contraction of the orogen to a new steady state. We show that the response time to perturbation agrees well with predictions from a one-dimensional semi-analytical model. We find that the transient evolution of erosion rate and erosional flux is potentially useful for distinguishing between tectonic and climatic forcing mechanisms.
La Niña winters exhibit significant local enhancement of heavy rainfall in the southwest United States, relative to El Niño. This contrasts with average daily rainfall intensity, which is instead increased during El Niño winters. The present study explores the relationship between heavy rainfall and associated atmospheric circulation patterns. Using composite analysis, we find that heavy rainfall events in the southwest arise from the presence of a persistent offshore trough and simultaneous emplacement of a strong source of subtropical water vapor. Greater intensity of these storms during La Niña is consistent with a deeper offshore trough leading to strengthened moisture fluxes. Composite circulation patterns survive amongst a large degree of synoptic variability, highlighting the importance of understanding this variability when making regional climate predictions.
Uncertainties in projections of future climate change have not lessened substantially in past decades. Both models and observations yield broad probability distributions for long-term increases in global mean temperature expected from the doubling of atmospheric carbon dioxide, with small but finite probabilities of very large increases. We show that the shape of these probability distributions is an inevitable and general consequence of the nature of the climate system, and we derive a simple analytic form for the shape that fits recent published distributions very well. We show that the breadth of the distribution and, in particular, the probability of large temperature increases are relatively insensitive to decreases in uncertainties associated with the underlying climate processes.
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