The social cost of carbon dioxide (SC-CO2) measures the monetized value of the damages to society caused by an incremental metric tonne of CO2 emissions and is a key metric informing climate policy. Used by governments and other decision-makers in benefit–cost analysis for over a decade, SC-CO2 estimates draw on climate science, economics, demography and other disciplines. However, a 2017 report by the US National Academies of Sciences, Engineering, and Medicine1 (NASEM) highlighted that current SC-CO2 estimates no longer reflect the latest research. The report provided a series of recommendations for improving the scientific basis, transparency and uncertainty characterization of SC-CO2 estimates. Here we show that improved probabilistic socioeconomic projections, climate models, damage functions, and discounting methods that collectively reflect theoretically consistent valuation of risk, substantially increase estimates of the SC-CO2. Our preferred mean SC-CO2 estimate is $185 per tonne of CO2 ($44–$413 per tCO2: 5%–95% range, 2020 US dollars) at a near-term risk-free discount rate of 2%, a value 3.6 times higher than the US government’s current value of $51 per tCO2. Our estimates incorporate updated scientific understanding throughout all components of SC-CO2 estimation in the new open-source Greenhouse Gas Impact Value Estimator (GIVE) model, in a manner fully responsive to the near-term NASEM recommendations. Our higher SC-CO2 values, compared with estimates currently used in policy evaluation, substantially increase the estimated benefits of greenhouse gas mitigation and thereby increase the expected net benefits of more stringent climate policies.

Rain on snow (ROS) events are rare in most parts of the circumpolar Arctic, but have been shown to have great impact on soil surface temperatures and serve as triggers for avalanches in the midlatitudes, and they have been implicated in catastrophic die-offs of ungulates. The study of ROS is inherently challenging due to the difficulty of both measuring rain and snow in the Arctic and representing ROS events in numerical weather predictions and climate models. In this paper these challenges are addressed, and the occurrence of these events is characterized across the Arctic. Incidents of ROS in Canadian meteorological station data and in the 40-yr ECMWF Re-Analysis (ERA-40) are compared to evaluate the suitability of these datasets for characterizing ROS. The ERA-40 adequately represents the large-scale synoptic fields of ROS, but too often has a tendency toward drizzle. Using the ERA-40, a climatology of ROS events is created for thresholds that impact ungulate populations and permafrost. It is found that ROS events with the potential to harm ungulate mammals are widespread, but the large events required to impact permafrost are limited to the coastal margins of Beringia and the island of Svalbard. The synoptic conditions that led to ROS events on Banks Island in October of 2003, which killed an estimated 20 000 musk oxen, and on Svalbard, which led to significant permafrost warming in December of 1995, are examined. Compositing analyses are used to show the prevailing synoptic conditions that lead to ROS in four disparate parts of the Arctic. Analysis of ROS in the daily output of a fully coupled GCM under a future climate change scenario finds an increase in the frequency and areal extent of these events for many parts of the Arctic over the next 50 yr and that expanded regions of permafrost become vulnerable to ROS.

Variability in daily wintertime [December–February (DJF)] 500-hPa heights on low [L: <(30 day)−1], intermediate [M: (6–30 day)−1], and high [H: >(6 day)−1] frequencies is examined using 40-yr ECMWF Re-Analysis (ERA-40) data. Leading EOFs of L correspond to planetary-scale teleconnection patterns; those of M to retrograding, eastward-dispersing long waves oriented along great circle routes; and those of H to baroclinic waves in the climatological-mean storm tracks. In the Atlantic sector, EOF 1 of M appears to be embedded in EOF 1 of L. Cross-frequency coupling between L and M exhibits distinctive patterns. In the Atlantic sector the negative polarity of the North Atlantic Oscillation (NAO) with above-normal heights over Greenland is associated with enhanced M variability over Greenland. An analogous relationship is observed in the Pacific sector between an NAO-like pattern and the variance of M over Alaska. Cross-frequency coupling between L and H in both sectors is indicative of a reinforcement of the background flow by the baroclinic waves. Cross-frequency coupling between L and M is responsible for most of the skewness of the anomalies in the 500-hPa height field. Linear wave dynamics evidently play an important role in M. Composites of high amplitude anomalies of contrasting signs over Baffin Bay exhibit similar spatial structures (apart from the sign reversal) and they exhibit a similar evolution, with westward phase propagation and downstream development characteristic of the behavior of Rossby waves. It is argued that teleconnection patterns exhibit memories much longer than the 7–10-day decorrelation time of daily indices formed by projecting unfiltered daily fields onto their spatial patterns.

In October 2003, a severe rain‐on‐snow (ROS) event killed approximately 20,000 musk‐oxen (Figure 1) on Banks Island, which is the westernmost of the Canadian Arctic islands (approximately 380 kilometers by 290 kilometers in size). The event reduced the isolated herd by 25% and significantly affected the people dependent on the herd's well‐being. Because of the sparsity of weather stations in the Arctic and the lack of routinely deployed weather equipment that was capable of accurately sensing the ROS event, its detection largely was based on reports from hunters who were in the affected areas at the time. Such events can significantly alter a frozen ecosystem—with changes that often persist for the remainder of a winter—by creating ice layers at the surface of, within, or below the snowpack. The water and ice layers are known to facilitate the growth of toxic fungi, significantly warm the soil surface under thick snowpack, and deter large grazing mammals. Although ROS events of the magnitude that was experienced in Banks Island in 2003 likely have reverberations throughout the entire Arctic and subarctic ecosystem, little is presently known about them and their impacts. As understanding of ROS events expands, many ROS‐related aspects of the Arctic ecology and hydrology are likely to be discovered. They may include topics such as the fate of small mammals under the snowpack at the iced soil surface, the difficulty of ptarmigans to burrow into the iced snow, the limited infiltration of spring snowmelt into the iced over soil, and the changing drifting patterns of ice‐crusted snowpack.

The social cost of carbon (SCC) is a crucial metric for informing climate policy, most notably for guiding climate regulations issued by the US government. Characterization of uncertainty and transparency of assumptions are critical for supporting such an influential metric. Challenges inherent to SCC estimation push the boundaries of typical analytical techniques and require augmented approaches to assess uncertainty, raising important considerations for discounting. This paper addresses the challenges of projecting very long-term