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Climate over land—where humans live and the majority of food is produced—is changing rapidly, driving severe impacts through extreme heat, wildfires, drought and flooding. Our ability to monitor and model this changing climate is being transformed through new observational systems and increasingly complex Earth system models. But fundamental understanding of the processes governing land climate has not kept pace, weakening our ability to interpret and utilize data from these advanced tools. Here we argue that for land-climate science to accelerate forwards, an alternative approach is needed. We advocate a parallel scientific effort, one emphasizing robust theories, that aims to inspire current and future land-climate scientists to better comprehend the processes governing land climate, its variability and extremes and its sensitivity to global warming. Such an effort, we believe, is essential to better understand the risks people face, where they live, in an era of climate change.

A key implication of the well known wet‐get‐wetter/dry‐get‐drier (WGW) scaling is that model biases in the representation of precipitation and evaporation in the present climate lead to spurious projected changes under global warming. Here we estimate the extent of such spurious changes in projections by 60 models participating in phases 5 and 6 of the Coupled Model Intercomparison Project. Utilizing known thermodynamic constraints on evaporation, we show that the WGW scaling can be applied to precipitation and evaporation separately (specific WGW scaling), which we use to correct for spurious projected changes in precipitation and evaporation over tropical oceans. The spurious changes in precipitation can be of comparable amplitude to projected changes, but are generally small for evaporation. The spurious changes may increase the uncertainty in projections of tropical precipitation and evaporation by up to 30% and 15% respectively. It is well established that if winds are assumed unchanged, the response of the hydrological cycle to global warming will follow the simple rule that wet regions get wetter and dry regions get drier. A key implication of this rule is that climate model errors in the representation of precipitation and evaporation in the present climate lead to artificial simulated responses in a warming climate. Here we show that this simple rule can be applied separately to precipitation and evaporation over tropical oceans. We then use this rule to estimate the extent of the artificial response over tropical oceans to global warming in modern climate models, caused by errors in the representation of precipitation and evaporation. Overall, the artificial changes may increase the uncertainty in projections by up to 30% for precipitation and 15% for evaporation. Biases in the representation of the present hydroclimate lead to spurious but correctable projected changes in precipitation and evaporationA wet‐get‐wetter/dry‐get‐drier scaling bias adjustment can be applied separately to precipitation and evaporationThe spurious changes increase uncertainty in precipitation and evaporation projections by up to 30% and 15% respectively Biases in the representation of the present hydroclimate lead to spurious but correctable projected changes in precipitation and evaporation A wet‐get‐wetter/dry‐get‐drier scaling bias adjustment can be applied separately to precipitation and evaporation The spurious changes increase uncertainty in precipitation and evaporation projections by up to 30% and 15% respectively

The Hadley circulation (HC) extent and strength are analyzed in a wide range of simulated climates from the Last Glacial Maximum to global warming scenarios. Motivated by HC theories, we analyze how the HC is influenced by the subtropical stability, the near‐surface meridional potential temperature gradient, and the tropical tropopause level. The subtropical static stability accounts for the bulk of the HC changes across the simulations. However, since it correlates strongly with global mean surface temperature, most HC changes can be attributed to global mean surface temperature changes. The HC widens as the climate warms, and it also weakens, but only robustly so in the Northern Hemisphere. On the other hand, the Southern Hemisphere strength response is uncertain, in part because subtropical static stability changes counteract meridional potential temperature gradient changes to various degrees in different models, with no consensus on the response of the latter to global warming. The Hadley circulation extent and strength are analyzed in simulations spanning the Last Glacial Maximum to global warming scenariosThe Hadley circulation generally widens and weakens as the climate warmsChanges in meridional temperature gradients have a modest modulating effect, principally on the Southern Hemisphere Hadley circulation

Regions along the edges of the tropics host vast populations and ecosystems which are sensitive to climate change. Here we examine the extent of tropical land areas in the ERA5 and MERRA‐2 reanalyses and in high‐emission scenarios of 45 models participating in phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5/6). Based on the definition of tropical climate land areas as regions where the diurnal temperature range exceeds the seasonal temperature range, we find a net reduction of tropical land area with global warming. This change is primarily due to an increased seasonal temperature range driven by enhanced summer warming, which in turn is largely driven by reduced evaporative cooling. The reduction of tropical climate area is consistent with a narrowing of the tropical rain belt and with an equatorward and poleward expansion of the subtropical dry zones. Understanding the links between these trends requires further study. Tropical climate land areas host about 40% of the world’s population and 80% of the world’s biodiversity. Changes in the extent of tropical climate land areas, which generally border semi‐arid climate zones, can therefore carry vast ecological and socio‐economic implications. Tropical climate land areas are generally defined as regions where the daily temperature range exceeds the seasonal temperature range. Based on this definition we find a net decrease in tropical climate land area in climate model projections of greenhouse‐gas‐induced global warming. The net reduction in tropical land area is driven primarily by increased seasonal temperature range, due to enhanced summer warming, which in turn is largely driven by drying. The reduction in tropical climate land area in a warming climate agrees with a narrowing of the tropical rain belt and with “Subtropical widening”, that is, a poleward and equatorward expansion of the subtropical dry zones. However, understanding the links between these trends requires further study. Net tropical climate land area, defined by smaller seasonal than diurnal temperature range, is projected to decrease in a warming climateThe decrease is primarily due to enhanced summer warming, leading to an elevated seasonal temperature range in the tropicsThe decrease agrees with narrowing of the tropical rain belt and expansion of the subtropical dry zone at its equatorward and poleward sides Net tropical climate land area, defined by smaller seasonal than diurnal temperature range, is projected to decrease in a warming climate The decrease is primarily due to enhanced summer warming, leading to an elevated seasonal temperature range in the tropics The decrease agrees with narrowing of the tropical rain belt and expansion of the subtropical dry zone at its equatorward and poleward sides

We examine how tropical zonal mean precipitation biases in current climate models relate to the atmospheric energy budget. Both hemispherically symmetric and antisymmetric tropical precipitation biases contribute to the well‐known double‐Intertropical Convergence Zone (ITCZ) bias; however, they have distinct signatures in the energy budget. Hemispherically symmetric biases in tropical precipitation are proportional to biases in the equatorial net energy input; hemispherically antisymmetric biases are proportional to the atmospheric energy transport across the equator. Both relations can be understood within the framework of recently developed theories. Atmospheric net energy input biases in the deep tropics shape both the symmetric and antisymmetric components of the double‐ITCZ bias. Potential causes of these energetic biases and their variation across climate models are discussed. CMIP5 precipitation biases have significant hemispherically symmetric and antisymmetric componentsHemispherically antisymmetric biases are related to the cross‐equatorial atmospheric energy fluxHemispherically symmetric biases are related to the atmospheric net energy input near the equator

End of century projections from Coupled Model Intercomparison Project (CMIP) models show a decrease in precipitation over subtropical oceans that often extends into surrounding land areas, but with substantial intermodel spread. Changes in precipitation are controlled by both thermodynamical and dynamical processes, though the importance of these processes for regional scales and for intermodel spread is not well understood. The contribution of dynamic and thermodynamic processes to the model spread in regional precipitation minus evaporation (P − E) is computed for 48 CMIP models. The intermodel spread is dominated essentially everywhere by the change of the dynamic term, including in most regions where thermodynamic changes drive the multi‐model mean response. The dominant role of dynamic changes is insensitive to zonal averaging which removes any influence of stationary wave changes, and is also evident in subtropical oceanic regions. Relatedly, intermodel spread in P − E is generally unrelated to climate sensitivity. Earth's hydroclimate will change in response to increased greenhouse gas concentrations. However, the physical process(es) whereby climate change leads to these hydroclimate change is still uncertain, especially on regional scales. Furthermore, models disagree as to the magnitude of these hydroclimate changes, and the causes of this intermodel spread are also not well understood. We demonstrate that uncertainty in future changes is driven almost everywhere by changes in the large scale winds, while the precise amount of warming simulated by a given model is largely irrelevant. This highlights that reducing uncertainty in future hydroclimate changes requires primarily narrowing uncertainties in the circulation response. Even in regions where thermodynamic changes drive the multi‐model mean hydrologic changes, dynamic changes drive uncertaintyDynamic changes more important for intermodel spread even after zonal averaging and even over subtropical oceansNarrowing climate sensitivity will not help constrain future hydroclimate changes in most regions Even in regions where thermodynamic changes drive the multi‐model mean hydrologic changes, dynamic changes drive uncertainty Dynamic changes more important for intermodel spread even after zonal averaging and even over subtropical oceans Narrowing climate sensitivity will not help constrain future hydroclimate changes in most regions

A basin‐wide strong shallow bias in the thermocline of the southern tropical Pacific is found in an ensemble of models from the coupled model intercomparison project phases 5 and 6 (CMIP5/6). The bias is present in over 90% of models examined and is equally represented in CMIP5 and CMIP6 models. In contrast to observations, where the southern thermocline is far deeper than its northern counterpart, models have a hemispherically symmetric tropical thermocline. The shallow thermocline bias is closely linked to the well known double intertropical convergence zone (ITCZ) bias; more so for the physical thermocline (i.e., depth of maximal vertical thermal gradient) than the commonly used 20°C isotherm thermocline proxy. A shallow thermocline bias is further found to be associated with climatic conditions including wider separation of double ITCZ peaks, a pronounced cold tongue, and a spurious south equatorial counter current. In this study we find an error common to nearly all comprehensive climate models used over the past decade: the depth of the tropical south Pacific thermocline is significantly shallower than in reality. The thermocline is the layer that separates the warm upper ocean from its cold depths. When the thermocline is shallower, the capacity of the upper ocean to absorb heat from the atmosphere is reduced. This means that the error in the modeled depth of the thermocline could alter the modeled energy balance between the ocean and atmosphere, as well as the energy balance between the northern and southern hemispheres. We further find that a well known error of excessive rain in the tropical south Pacific, which has been a major problem in models for the past three decades and remains unsolved, is worse in models that have this thermocline simulation problem. The persistence of these errors hinders the reliability of global scale model predictions. We show climatic aspects linking the two model inaccuracies, which can facilitate progress in understanding their hitherto elusive cause. Shallow thermocline bias across the southern tropical Pacific in CMIP5/6 modelsDouble‐ITCZ and shallow thermocline biases are closely relatedPhysical thermocline depth characterizes tropical coupled dynamic biases better than 20°C isotherm Shallow thermocline bias across the southern tropical Pacific in CMIP5/6 models Double‐ITCZ and shallow thermocline biases are closely related Physical thermocline depth characterizes tropical coupled dynamic biases better than 20°C isotherm

A moist General Circulation Model is used to investigate the forcing of the Asian monsoon and the associated upper level anticyclone by land‐sea contrast, net horizontal oceanic heat transport, and topography. The monsoonal pattern is not simply the linear additive sum of the response to each forcing; only when all three forcings are included simultaneously does the monsoonal circulation extend westward to India. This nonadditivity impacts the location of the upper level anticyclone, which is shifted eastward and weaker if the forcings are imposed individually. Sahelian precipitation, and also austral summer precipitation over Australia, southern Africa, and South America, are likewise stronger if all forcings are imposed simultaneously. The source of the nonlinearity can be diagnosed using gross moist stability, but appears inconsistent with the land‐sea breeze paradigm. This non‐additivity implies that the question of which forcing is most important may be ill‐posed in many regions. Monsoons dominate the climate in many regions of the tropics and subtropics, and are characterized by rainy summer and drier winter seasons. This rainfall is crucial for agriculture, among other societal implications, in heavily populated regions of the world. The processes leading to regional confinement of the monsoons are investigated using an intermediate complexity moist General Circulation Model. We find that a linear perspective cannot account for the monsoons; only when land‐sea contrast, orography, and ocean heat transport are included simultaneously does the monsoonal circulation extend westward to India. A similar nonlinearity is evident for Sahelian precipitation, and also for austral summer precipitation over Australia, southern Africa, and South America. Monsoons are due to a non‐additive response to three forcings: land‐sea contrast, orography, and ocean heat transportThe non‐additivity in precipitation leads to a non‐additive response of the upper level stationary waves and Asian monsoon anticycloneGross moist stability can account for this effect, but not the land‐sea breeze paradigm Monsoons are due to a non‐additive response to three forcings: land‐sea contrast, orography, and ocean heat transport The non‐additivity in precipitation leads to a non‐additive response of the upper level stationary waves and Asian monsoon anticyclone Gross moist stability can account for this effect, but not the land‐sea breeze paradigm

Projections of extreme precipitation based on modern climate models suffer from large uncertainties. Specifically, unresolved physics and natural variability limit the ability of climate models to provide actionable information on impacts and risks at the regional, watershed and city scales relevant for practical applications. Here, we show that the interaction of precipitating systems with local features can constrain the statistical description of extreme precipitation. These observational constraints can be used to project local extremes of low yearly exceedance probability (e.g., 100‐year events) using synoptic‐scale information from climate models, which is generally represented more accurately than the local scales, and without requiring climate models to explicitly resolve extremes. The novel approach, demonstrated here over the south‐eastern Mediterranean, offers a path for improving the predictability of local statistics of extremes in a changing climate, independent of pending improvements in climate models at regional and local scales. Climate change impact studies are currently restrained by the limited accuracy of climate models in resolving precipitation extremes, by the computational power required to run high‐resolution models, and by the uncertainties characterizing their analysis. Here, we present a novel approach which permits to project extreme precipitation for future climatic scenarios based on the combination of coarse‐scale information from climate models with local observations. Focusing on the south‐eastern Mediterranean, we provide projections of precipitation extremes which could not yet be derived using traditional methods, such as the events occurring on average once in 100 years. The combined effect of changes in intensity and occurrence of two dominant synoptic systems is projected to increase the intensity of the 100‐year events in the coast and in the desert areas of the region and to decrease it elsewhere. The novel approach offers a path for improving the predictability of extremes in a changing climate, independent of pending improvements in climate models. Local observations can be used to constrain the intensity distribution of precipitation events associated with different synoptic systemsThe constraints allow projecting extreme return levels at scales relevant for impact studies from synoptic information from climate modelsThe approach improves the predictability of local extremes, independent of improvements in climate models at regional and local scales Local observations can be used to constrain the intensity distribution of precipitation events associated with different synoptic systems The constraints allow projecting extreme return levels at scales relevant for impact studies from synoptic information from climate models The approach improves the predictability of local extremes, independent of improvements in climate models at regional and local scales

Current techniques for measuring absolute lung volumes rely on bulky and expensive equipment and are complicated to use for the operator and the patient. A novel method for measurement of absolute lung volumes, the minibox method, is presented.

The role of tropical wind‐driven ocean circulation in setting the position of the Intertropical Convergence Zone (ITCZ) is studied using an atmospheric model coupled to a slab ocean with parameterized Ekman energy transport. Unlike previous studies, the parameterized cross‐equatorial ocean energy transport, which is critical for the coupled response, is explicitly represented as the curl of the wind stress at the equator. A nonlinear sensitivity of the ITCZ position to hemispherically asymmetric heating and ocean stratification is seen, which is primarily linked to the partitioning of cross‐equatorial energy transport between (i) the atmosphere and ocean and (ii) atmospheric mean and transient eddy components. Due to the nonmonotonic response of transient eddies to the asymmetric heating, amplified or damped response is possible. The common assumption of a linear relation between the ITCZ position and hemispherically asymmetric heating is therefore found to be inadequate when large climatic variations are considered. A novel parameterization of Ekman energy transport with explicit cross‐equatorial fluxes is presentedITCZ migrations depend nonlinearly on extratropical heating and ocean stratificationPartitioning of atmospheric energy transport between eddy and mean components can lead to either damped or amplified response