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Precipitation extreme changes are often assumed to scale with, or are constrained by, the change in atmospheric moisture content. Studies have generally confirmed the scaling based on moisture content for the midlatitudes but identified deviations for the tropics. In fact half of the twelve selected Intergovernmental Panel on Climate Change (IPCC) models exhibit increases faster than the climatological-mean precipitable water change for high percentiles of tropical daily precipitation, albeit with significant intermodel scatter. Decomposition of the precipitation extreme changes reveals that the variations among models can be attributed primarily to the differences in the upward velocity. Both the amplitude and vertical profile of vertical motion are found to affect precipitation extremes. A recently proposed scaling that incorporates these dynamical effects can capture the basic features of precipitation changes in both the tropics and midlatitudes. In particular, the increases in tropical precipitation extremes significantly exceed the precipitable water change in Model for Interdisciplinary Research on Climate (MIROC), a coupled general circulation model with the highest resolution among IPCC climate models whose precipitation characteristics have been shown to reasonably match those of observations. The expected intensification of tropical disturbances points to the possibility of precipitation extreme increases beyond the moisture content increase as is found in MIROC and some of IPCC models.

A new version of the atmosphere–ocean general circulation model cooperatively produced by the Japanese research community, known as the Model for Interdisciplinary Research on Climate (MIROC), has recently been developed. A century-long control experiment was performed using the new version (MIROC5) with the standard resolution of the T85 atmosphere and 1° ocean models. The climatological mean state and variability are then compared with observations and those in a previous version (MIROC3.2) with two different resolutions (medres, hires), coarser and finer than the resolution of MIROC5. A few aspects of the mean fields in MIROC5 are similar to or slightly worse than MIROC3.2, but otherwise the climatological features are considerably better. In particular, improvements are found in precipitation, zonal mean atmospheric fields, equatorial ocean subsurface fields, and the simulation of El Niño–Southern Oscillation. The difference between MIROC5 and the previous model is larger than that between the two MIROC3.2 versions, indicating a greater effect of updating parameterization schemes on the model climate than increasing the model resolution. The mean cloud property obtained from the sophisticated prognostic schemes in MIROC5 shows good agreement with satellite measurements. MIROC5 reveals an equilibrium climate sensitivity of 2.6 K, which is lower than that in MIROC3.2 by 1 K. This is probably due to the negative feedback of low clouds to the increasing concentration of CO₂, which is opposite to that in MIROC3.2.

Climate change and coronavirus pandemic are the twin crises in the Anthropocene, the era in which unsustainable growth of human activities has led to a significant change in the global environment. The two crises have also exposed a chronic social illness of our time—a deep, widespread inequality in society. Whilst the circumstances are unfortunate, the pandemic can provide an opportunity for sustainability scientists to focus more on human society and its inequalities, rather than a sole focus on the natural environment. It opens the way for a new normative commitment of science in a time of crises. We suggest three agendas for future climate and sustainability research after the pandemic: (1) focus on health and well-being, (2) moral engagement through empathy, and (3) science of loss for managing grief.

This study evaluates the tropical intraseasonal variability, especially the fidelity of Madden–Julian oscillation (MJO)simulations, in 14 coupled general circulation models (GCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Eight years of daily precipitation from each model’s twentieth-century climate simulation are analyzed and compared with daily satellite-retrieved precipitation. Space–time spectral analysis is used to obtain the variance and phase speed of dominant convectively coupled equatorial waves, including the MJO, Kelvin, equatorial Rossby (ER), mixed Rossby–gravity (MRG), and eastward inertio–gravity (EIG) and westward inertio–gravity (WIG) waves. The variance and propagation of the MJO, defined as the eastward wavenumbers 1–6, 30–70-day mode, are examined in detail. The results show that current state-of-the-art GCMs still have significant problems and display a wide range of skill in simulating the tropical intraseasonal variability. The total intraseasonal (2–128 day) variance of precipitation is too weak in most of the models. About half of the models have signals of convectively coupled equatorial waves, with Kelvin and MRG–EIG waves especially prominent. However, the variances are generally too weak for all wave modes except the EIG wave, and the phase speeds are generally too fast, being scaled to excessively deep equivalent depths. An interesting result isthat this scaling is consistent within a given model across modes, in that both the symmetric and antisymmetric modes scale similarly to a certain equivalent depth. Excessively deep equivalent depths suggest that these models may not have a large enough reduction in their “effective static stability” by diabatic heating. The MJO variance approaches the observed value in only 2 of the 14 models, but is less than half of the observed value in the other 12 models. The ratio between the eastward MJO variance and the variance of its westward counterpart is too small in most of the models, which is consistent with the lack of highly coherent eastward propagation of the MJO in many models. Moreover, the MJO variance in 13 of the 14 models does not come from a pronounced spectral peak, but usually comes from part of an overreddened spectrum, which in turn is associated with too strong persistence of equatorial precipitation.The two models that arguably do best at simulating the MJO are the only ones having convective closures/triggers linked insome way to moisture convergence.

Climate warming due to human activities will be accompanied by hydrological cycle changes. Economies, societies and ecosystems in South America are vulnerable to such water resource changes. Hence, water resource impact assessments for South America, and corresponding adaptation and mitigation policies, have attracted increased attention. However, substantial uncertainties remain in the current water resource assessments that are based on multiple coupled Atmosphere Ocean General Circulation models. This uncertainty varies from significant wetting to catastrophic drying. By applying a statistical method, we characterized the uncertainty and identified global-scale metrics for measuring the reliability of water resource assessments in South America. Here, we show that, although the ensemble mean assessment suggested wetting across most of South America, the observational constraints indicate a higher probability of drying in the Amazon basin. Thus, over-reliance on the consensus of models can lead to inappropriate decision making. Assessments of future water availability in South America are uncertain based on multiple coupled general circulation models. Shiogama et al. identify global-scale metrics for measuring the reliability of water resource assessments, and indicate a higher probability of drying in the Amazon basin.

This study utilized a new method for future surface temperature changes constrained by model performance in the late 20th century (20C) climate. We applied the singular value decomposition method to investigate covariance between future and present climates in a multimodel ensemble. We established a transfer function between the expansion coefficients of the present and future climate modes. By projecting the observations onto the present modes and using the transfer function, we obtained the best estimates of the future projection. In this study, we extracted the first two significant leading modes. The first mode showed inter‐model variation in spatial patterns of temperature change, with Arctic amplification in the future, and the second mode showed variability in temperature change occurring in the southern marginal regions of sea ice in the Arctic for the 20C simulation. Our evaluation suggests that the future warming of the ensemble mean projection underestimated, particularly in the Arctic region. The estimated temperature change depends mainly on the 20C state of the sea ice and surface temperature in the northern North Atlantic Ocean, which are strongly expressed in the first mode of the 20C climate simulation. The leave‐one‐out cross‐validation indicated that our estimation can improve multimodel mean estimates of temperature change in the higher‐latitudes of the Northern Hemisphere. Key Points Constraint of future surface temperature change using the SVD Relationship between sea ice in the 20C and future temperature change Test of our method using the leave‐one‐out cross validation

Clarifying characteristics of hazards and risks of climate change at 2 °C and 1.5 °C global warming is important for understanding the implications of the Paris Agreement. We perform and analyze large ensembles of 2 °C and 1.5 °C warming simulations. In the 2 °C runs, we find substantial increases in extreme hot days, heavy rainfalls, high streamflow and labor capacity reduction related to heat stress. For example, about half of the world’s population is projected to experience a present day 1-in-10 year hot day event every other year at 2 °C warming. The regions with relatively large increases of these four hazard indicators coincide with countries characterized by small CO2 emissions, low-income and high vulnerability. Limiting global warming to 1.5 °C, compared to 2 °C, is projected to lower increases in the four hazard indicators especially in those regions.

We conducted sensitivity experiments using a coupled atmosphere–ocean general circulation model to examine the asymmetry between the hydrological responses to instantaneous positive and negative CO₂ forcing and the impact of the CO₂ physiological effects (CDPEs) on these responses. This study focuses on the fast response occurring on time scales shorter than 1 year after imposing CO₂ forcing. Experiments investigating the CO₂ physiological effect show that the fast response of precipitation to positive CO₂ forcing is a decrease in the global and annual mean, whereas that of negative forcing is an increase the global and annual mean precipitation. The fast global precipitation response to negative forcing is stronger than the response to positive forcing. In contrast, the experiments without the CDPE reveal similar magnitudes of the fast global precipitation responses to negative and positive CO₂ forcing. Significant differences in the magnitudes of the fast precipitation response due to the CDPE are found in tropical regions such as the Amazon Basin, the Maritime Continents, and tropical Africa, where C3-type plants are common. The stomatal conductance of plant leaves is decreased by both positive and negative CO₂ forcing, which suppress the transpiration from the leaves. Consequently, the CDPE enhances the asymmetry of the fast precipitation responses to positive and negative CO₂ forcing. The asymmetric impact of CDPE requires a careful evaluation of future hydrological changes which is constrained by paleoclimate evidence.

We have assessed the risks associated with setting 1.5, 2.0, or 2.5 °C temperature goals and ways to manage them in a systematic manner and discussed their implications. The results suggest that, given the uncertainties in climate sensitivity, “net zero emissions of anthropogenic greenhouse gases in the second half of this century” is a more actionable goal for society than the 2 or 1.5 °C temperature goals themselves. If the climate sensitivity is proven to be relatively high and the temperature goals are not met even when the net zero emission goal is achieved, the options left are: (A) accepting/adapting to a warmer world, (B) boosting mitigation, and (C) climate geoengineering, or any combination of these. This decision should be made based on a deeper discussion of risks associated with each option. We also suggest the need to consider a wider range of policies: not only climate policies, but also broader “sustainability policies”, and to envisage more innovative solutions than what integrated assessment models can currently illustrate. Finally, based on a consideration of social aspects of risk decisions, we recommend the establishment of a panel of “intermediate layer” experts, who support decision-making by citizens as well as social and ethical thinking by policy makers.

We investigated the feasibilities of 2.0°C and 1.5°C climate targets by considering the abatement potentials of a full suite of greenhouse gases, pollutants, and aerosols. We revised the inter‐temporal dynamic optimization model DICE‐2013R by introducing three features as follows. First, we applied a new marginal abatement cost curve derived under moderate assumptions regarding future socioeconomic development—the Shared Socioeconomic Pathways 2 (SSP2) scenario. Second, we addressed emission abatement for not only industrial CO2 but also land‐use CO2, CH4, N2O, halogenated gases, CO, volatile organic compounds, SOx, NOx, black carbon and organic carbon. Third, we improved the treatment of the non‐CO2 components in the climate module based on MAGICC 6.0. We obtained the following findings: (1) It is important to address the individual emissions in an analysis of low stabilization scenarios because abating land‐use CO2, non‐CO2 and aerosol emissions also contributes to maintaining a low level of radiative forcing and substantially affects the climate costs. (2) The 2.0°C target can be efficiently reached under the assumptions of the SSP2 scenario. (3) The 1.5°C target can be met with early deep cuts under the assumption of a temperature overshoot, and it will triple the carbon price and double the mitigation cost compared with the 2.0°C case. Key Points A dynamic optimization model with a full suite of GHGs, pollutants, and aerosols was developed based on DICE‐2013R and MAGICC 6.0. Addressing individual emissions in an analysis of low stabilization scenarios is important. 2.0°C can be efficiently achieved under SSP2; 1.5°C is achieved using an overshoot, tripled carbon price, and doubled mitigation cost.