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Relative humidity (RH) is projected to increase over ocean but decrease over land under anthropogenic warming. The land RH decrease was previously attributed to ocean influences on land changes. Here, we show that interactive soil moisture (SM) is necessary and may be sufficient for anthropogenic warming to reduce the land RH. The land RH decrease is absent in simulations with realistic land‐ocean geometry but fixed SM, and present in simulations with interactive SM but no oceans or changes in precipitation minus evaporation. The land RH decrease is coupled to the SM decline and the latter could be understood as a natural response of interactive SM to anthropogenic warming. Specifically, prior to adjustments in SM and RH, evaporation would increase by ∼6% K−1 following the Clausius–Clapeyron relation, outpacing the radiatively constrained ∼2% K−1 precipitation increase. This prior imbalance depletes SM and consequently reduces RH through the coupling between SM and RH. Plain Language Summary: As a measure of air wetness, relative humidity (RH) is projected to decrease over land under anthropogenic warming, contributing to increasing evaporative demand and intensifying droughts, heatwaves and wildfires. The land RH decrease was previously attributed to influences from ocean, but here we propose an alternative explanation. We first show that the land RH decrease is coupled with the soil moisture (SM) decline. Then, by excluding interactive SM and ocean influences respectively in warming simulations, we illustrate that interactive soil moisture (i.e., soil moisture is fed by precipitation minus evaporation and can in turn affect evaporation through moisture availability) is necessary and may be sufficient to produce a drier land under anthropogenic warming. A conceptual understanding of this declined SM and RH is provided based on theoretical evaporation equations. Key Points: We propose a new explanation for the projected decrease in land relative humidity under anthropogenic warmingComplex and idealized simulations show that interactive soil moisture (SM) is necessary and sufficient for anthropogenic warming to dry the landInteractive SM allows prior imbalance in evaporation and precipitation increases to dry the land independent of ocean influence [ABSTRACT FROM AUTHOR]

Short‐duration rainfall extremes are expected to intensify with warming at around 7%/K, following the Clausius‐Clapeyron (CC) relation, causing concomitant increases in flash floods. Observed hourly rainfall extremes show consistent CC‐scaling with dewpoint temperature across all land regions. Here, we use two global climate reanalysis products (ERA5 and MERRA‐2) to examine consistency in scaling with observations, in ungauged regions and over the oceans. We find that reanalyzes underestimate observed scaling, but ERA5 provides better estimates than MERRA‐2. Scaling rates at high latitudes and midlatitudes are similar to observations, at CC, while over the tropics mixed negative‐ and super‐CC scaling rates are seen. We show underestimation of scaling rates over the tropics is related to deficiencies in simulations of extreme rainfall over orographically complex and convection dominated regions. Importantly, both reanalyzes indicate scaling rates over the ocean are substantially higher than over land, with implications for extreme weather events originating over the ocean. Plain Language Summary: As temperatures rise, extreme rainfalls are expected to intensify by 7% per 1°C following the Clausius‐Clapeyron relation. Thus, flash floods are predicted to increase in magnitude and frequency with rising temperatures. In the areas where measurements are available, observations confirm the intensification of extreme hourly rainfall with increasing temperatures at the expected rate. Here, we use two global climate reanalysis products to examine the agreement in extreme rainfall‐temperature relation with observations, in ungauged regions and over the ocean. High‐latitude and midlatitude rainfall‐temperature rates obtained from the climate reanalysis are in accordance with observations. In the tropics, the rainfall‐temperature relation is a mix of negative and very high values (in the order of 14% per 1°C). Climate reanalyzes indicate extreme rainfall‐temperature rates are much higher over the ocean than on land. Key Points: Hourly precipitation‐temperature scaling was computed using MERRA‐2 and ERA‐5 at a global scaleScaling patterns in ERA‐5 are closer to those obtained from the Global Subdaily Rainfall (GSDR) data setScaling rates over ocean were found to be substantially higher than over land [ABSTRACT FROM AUTHOR]

Extreme precipitation events are intensifying with increasing temperatures. However, observed extreme precipitation‐temperature sensitivities have been found to vary significantly across the globe. Here we show that negative sensitivities found in previous studies are the result of limited consideration of within‐day temperature variations due to precipitation. We find that short‐duration extreme precipitation can be better described by subdaily atmospheric conditions before the start of storm events, resulting in positive sensitivities with increased consistency with the Clausius‐Clapeyron relation across a wide range of climatic regions. Contrary to previous studies that advocate that dew point temperature drives precipitation, dry‐bulb temperature is found to be a sufficient descriptor of precipitation variability. We argue that analysis methods for estimating extreme precipitation‐temperature sensitivities should account for the strong and prolonged cooling effect of intense precipitation, as well as for the intermittent nature of precipitation. Plain Language Summary: Increasing global temperatures are likely to result in the intensification of extreme precipitation events with resultant flooding of great societal concern. Understanding the relationship between extreme precipitation and temperature provides valuable information for the design, operation, and risk assessment of high‐hazard infrastructure. However, the observed precipitation‐temperature relationship has been found to vary significantly across the globe, with negative relationships found in warmer climatic regions. Using station‐based subdaily observations, we show that careful sampling of higher temperatures measured before the start of the storm events can result in positive extreme precipitation‐temperature relationships across a wide range of climatic regions. Dry‐bulb temperature is found to drive short‐duration precipitation extremes, contradicting previous studies promoting dew point temperature as the main driver of precipitation variability. Our results indicate the use of coarser daily‐scale observations in previous studies contributed to obtaining negative relationships by limiting consideration for within‐day temperature variation caused by the rainfall event itself. Key Points: Correct sampling of dry‐bulb temperature before storm events resolves inconsistencies in extreme precipitation‐temperature sensitivitiesPrevious underestimation of sensitivities is likely due to localized cooling and intermittent nature of precipitationDry‐bulb temperature drives short‐duration precipitation extremes, contrary to previous dew point temperature based findings [ABSTRACT FROM AUTHOR]

The future increase in extreme precipitation is likely to have a severe impact on society due to flooding. Previous research has shown the improved representation of precipitation in convection‐permitting models (CPMs), but until now it has not been possible to quantify uncertainties in future changes at convective (<5 km) scales. Here we analyze the first‐ever ensemble of convection‐permitting climate projections run within the UK Climate Projections project. We find that the CPM ensemble shows a significantly stronger intensification of summer hourly precipitation compared to the driving 12 km ensemble, with increases above the Clausius‐Clapeyron relation and natural variability dominating the ensemble spread for extremes. Results suggest that the climate change signal across different CPMs may converge thanks to the more realistic representation of the local storm dynamics. We conclude that CPMs offer the promise of reducing uncertainties for extreme summer precipitation projections due to the better and explicit representation of convection. Plain Language Summary: Accurate predictions of future changes in precipitation extremes are crucial for developing effective adaptation measures to avoid severe impacts on society due to flooding. State‐of‐the‐art kilometer‐scale models, called convection‐permitting models, give a much better simulation of how rainfall varies hour by hour and thus provide credible projections of future changes in hourly rainfall extremes. Up to now it was not possible to evaluate uncertainties in the future changes in extremes at convective (<5 km) scales since only single simulations were available due to the high computational costs. Thus, the ensemble of 12 local (2.2 km) projections, run within the UK Climate Projections project, represents a key step forward for climate prediction. We find that the 2.2 km ensemble shows a significantly stronger future increase of summer hourly precipitation compared to the coarser resolution (12 km) ensemble. The climate change signals are found to be consistent across the 2.2 km ensemble simulations, with differences largely explained just by natural variations in the climate from year to year. This means that convection‐permitting models could reduce the uncertainties in future changes in extreme summer precipitation and thus allow more robust and effective adaptation policy. Key Points: Convection‐permitting model (CPM) ensemble projects an intensification above the Clausius‐Clapeyron relation for extreme precipitationClimate change signal for extreme precipitation appears to converge in the CPM ensemble, with uncertainty dominated by natural variabilityThe correct representation of convection may lead to reduced model uncertainties in future changes in extreme precipitation [ABSTRACT FROM AUTHOR]

Ab initio molecular dynamics simulations predict that CaSiO3 perovskite melts at 5600 K at 136 GPa, and 6400 K at 300 GPa, significantly higher than MgSiO3 perovskite. The entropy of melting (1.8 kB per atom) is much larger than that of many silicates at ambient pressure and of simple liquids and varies little with pressure. The volume of melting decreases rapidly with increasing pressure, to 3 % at 136 GPa, producing a melting slope that diminishes rapidly with pressure. We determine the melting temperature via the ZW method, combining the Z method, for which we clarify the theoretical basis, with a waiting time analysis. The ZW method results are internally confirmed by integrating the Clausius‐Clapeyron equation, which also yields our results for the entropy and volume of melting. We find the eutectic composition on the MgSiO3‐CaSiO3 join to be xCa = 0.26 at 136 GPa and that metasilicate melt is denser than coexisting silicates. Plain Language Summary: Silicate melting is a major agent of thermal and chemical evolution of the Earth and other rocky planets. The melting temperature of Calcium silicate perovskite, a mineral that exists in Earth's lower mantle, is unknown over most of the pressure range that occurs in the mantle of Earth and super‐Earth exoplanets. We use advanced quantum mechanical simulations to predict the melting temperature of this material. We find that the melting temperature increases with increasing pressure but at a rate that diminishes continuously. The liquid and crystal have very similar volumes in the deep portions of planetary mantles, supporting the view that crystals may float at great depth. Key Points: Calcium silicate perovskite melts at 5600 K at the core mantle boundaryMetasilicate liquids are denser than coexisting crystalline silicates at the core‐mantle boundaryThe Z method allows precise ab initio melting temperature determination [ABSTRACT FROM AUTHOR]

We estimate the cooling flux from snow melting in the ocean through CloudSat satellite snowfall retrievals and reanalysis data. For snowfall events with less than 0.01 mm/hr, this flux is inconsequential. Melting snow begins to compete with other ocean surface heat fluxes as snowfall rates increase beyond 0.1 mm/hr, and it may often become the dominant heat flux as snowfall rates approach and exceed 1 mm/hr. The largest monthly average values of the melting snow cooling flux occur in winter months, approaching −10 W/m2 in both hemispheres. To determine the regional influence of melting snow on a seasonal basis, we calculate an impact metric that gauges the cooling flux of melting snow against the net flux in the ocean. This metric can be between 20% and 30% in the Northern Hemisphere during March, April, May; the Southern Ocean during March, April, May and September, October, November; and in high‐latitude polar oceans during sea ice freeze up seasons. Plain Language Summary: Climate scientists are very interested in understanding how much heat is entering and leaving the ocean, as this heat budget has important connections to the weather, ocean, and ice patterns that are crucial to life as we know it. Melting snow cools the ocean surface, but this effect has not been studied as much as heat exchanges from wind or sunlight. Using satellites and weather models, we determine how much melting snow cools the ocean. The cooling from snowmelt is most often inconsequential, since most snowfall is light. During heavy snowstorms, however, the cooling from melting snow can become quite powerful, perhaps becoming the most powerful cooling source in the ocean at times. Melting snow also has a large impact on the ocean heat budget in polar oceans during sea ice freeze up seasons. This is an important result, as the loss of sea ice is one of the most visible and concerning signs of climate change today. Ultimately, we find that melting snow can be a powerful and influential cooling force in the ocean but only under appropriate circumstances. Key Points: The cooling from melting snow in the ocean can be as strong as turbulent and radiative heat fluxes during heavy snow stormsAt high latitudes, monthly average cooling fluxes from melting snow vary between 0 and ‐10 W/m2, depending on the season and latitudeMelting snow has a large influence on the ocean heat budget in high‐latitude oceans, especially during sea ice freeze up seasons [ABSTRACT FROM AUTHOR]

The spatial extent and organization of extreme storm events has important practical implications for flood forecasting. Recently, conflicting evidence has been found on the observed changes of storm spatial extent with increasing temperatures. To further investigate this question, a regional climate model assessment is presented for the Greater Sydney region, in Australia. Two regional climate models were considered: the first a convection-resolving simulation at 2-km resolution, the second a resolution of 10 km with three different convection parameterizations. Both the 2- and the 10-km resolutions that used the Betts-Miller-Janjic convective scheme simulate decreasing storm spatial extent with increasing temperatures for 1-hr duration precipitation events, consistent with the observation-based study in Australia. However, other observed relationships of extreme rainfall with increasing temperature were not well represented by the models. Improved methods for considering storm organization are required to better understand potential future changes. [ABSTRACT FROM AUTHOR]

Global warming is known to substantially increase extreme daily precipitation, but there has been little focus on changes to the seasonal timing of these extreme events. We investigate this question using global and regional climate models from the Coupled Model Intercomparison Project phase 5 and the Coordinated Regional Downscaling Experiment, in 1871–1900, 1976–2005, and 2071–2100 for an extreme future emission scenario (Representative Concentration Pathways, 8.5 W/m2). Models reproduce the observed seasonal timing and indicate very little seasonality changes during the past hundred years. However, by the end of the 21st century, extreme precipitation could substantially shift later in the year, in most regions from summer and early fall toward fall and winter. This projected shift is not regionally homogeneous, and, among analyzed regions, is strongest in Northern Europe and Northeastern America (+12 and +17 days, respectively), although local changes of more than a month are also possible. Plain Language Summary: The intensification of extreme precipitation is a well‐known consequence of global warming, which could have devastating consequences on human societies and the environment. Using global and regional climate models simulations from the end of the nineteenth century to the end of the 21st century, we show that extreme future climate change could also change the seasonal timing of these extreme precipitation events. Models indicate that extreme precipitation could occur later in the year in a warmer climate, shifting in most regions from summer and early fall toward fall and winter. However, the magnitude of this change could also vary strongly between different regions. On the one hand, models indicate little change in Asia, Oceania, and the Tropics. On the other hand, Europe, Africa, South America, and high latitudes could show substantial changes. Among analyzed regions, these changes are particularly strong in Northern Europe and in Northeastern America (+12 and +17 days, respectively), but locally, the timing of extreme daily precipitation could shift by more than a month compared to present‐day conditions. Key Points: CMIP5 and CORDEX models reproduce the observed seasonal timing of daily precipitation extremesWith strong future warming, extremes could shift later in the year, by up to a month locallyFuture changes are regionally heterogeneous and are generally stronger at higher latitudes [ABSTRACT FROM AUTHOR]