Transient twenty-first century changes in daily-scale temperature extremes in the United States

A key question for climate mitigation and adaptation decisions is how quickly significant changes in temperature extremes will emerge as greenhouse gas concentrations increase, and whether that emergence will be uniform between hot and cold extremes and across different geographic areas. We use a high-resolution, multi-member ensemble climate model experiment over the United States (U.S.) to investigate the transient response of the annual frequency, duration and magnitude of 8 daily-scale extreme temperature indices during the twenty-first century of the A1B emissions scenario. We evaluate the time of emergence of a permanent exceedance (PE) above the colder part of the historical (1980-2009) extremes distribution, and the time of emergence of a new norm (NN) centered on the historical maxima (for hot extremes) or minima (for cold extremes). We find that during the twenty-first century, hot extremes permanently exceed the historical distribution's colder half over large areas of the U.S., and that the hot extremes distribution also becomes centered on or above the historical distribution's maxima. The changes are particularly robust for the exceedance of the annual 95th percentile of daily maximum temperature over the West and the Northeast (with the earliest emergence of a PE by 2030 and of a NN by 2040), for warm days over the Southwest (with the earliest emergence of a PE by 2020 and of a NN by 2030), and tropical nights over the eastern U.S. (with the earliest emergence of a PE by 2020 and of a NN by 2030). Conversely, no widespread emergence of a PE or a NN is found for most cold extremes. Exceptions include frost day frequency (with a widespread emergence of a PE below the historical median frequency by 2030 and of a NN by 2040 over the western U.S.), and cold night frequency (with an emergence of a PE below the historical median frequency by 2040 and of a NN by 2060 in virtually the entire U.S.). Our analysis implies a transition over the next half century to a climate of recently unprecedented heat stress in many parts of the U.S., along with cold extremes that, although less frequent, remain at times as long and as severe as are found in the current climate.