Climate Model Intercomparison Project Phase Research Articles

Substantial Warming of the Atlantic Ocean in CMIP6 Models

Abstract The storage of anthropogenic heat in oceans is geographically inhomogeneous, leading to differential warming rates among major ocean basins with notable regional climate impacts. Our analyses of observation-based datasets show that the average warming rate of 0–2000-m Atlantic Ocean since 1960 is nearly threefold stronger than that of the Indo-Pacific Oceans. This feature is robustly captured by historical simulations of phase 6 of Coupled Model Intercomparison Project (CMIP6) and is projected to persist into the future. In CMIP6 simulations, the ocean heat uptake through surface heat fluxes plays a central role in shaping the interbasin warming contrasts. In addition to the slowdown of the Atlantic meridional overturning circulation as stressed in some existing studies, alterations of atmospheric conditions under greenhouse warming are also essential for the increased surface heat flux into the North Atlantic. Specifically, the reduced anthropogenic aerosol concentration in the North Atlantic since the 1980s has been favorable for the enhanced Atlantic Ocean heat uptake in CMIP6 models. Another previously overlooked factor is the geographic shape of the Atlantic Ocean which is relatively wide in midlatitudes and narrow in low latitudes, in contrast to that of the Indo-Pacific Oceans. Combined with the poleward migration of atmospheric circulations, which leads to the meridional pattern of surface heat uptake with broadly enhanced heat uptake in midlatitude oceans due to reduced surface wind speed and cloud cover, the geographic shape effect renders a higher basin-average heat uptake in the Atlantic.

Seasonal and Monthly Climate Variability in South Korea’s River Basins: Insights from a Multi-Model Ensemble Approach

This study conducts a comprehensive analysis of the impacts of climate change on South Korea’s climate and hydrology, utilizing a Multi-Model Ensemble (MME) approach with thirteen Climate Model Intercomparison Project Phase 5 (CMIP5) models under two Representative Concentration Pathways, RCP4.5 and RCP8.5. We observed an average temperature increase of up to 3.5 °C under RCP8.5 and around 2.0 °C under RCP4.5. Precipitation patterns showed an overall increase, particularly during the summer months, with increases up to 20% under RCP8.5 and 15% under RCP4.5, characterized by more intense and frequent rainfall events. Evapotranspiration rates are projected to rise by approximately 5–10% under RCP8.5 and 3–7% under RCP4.5. Runoff is expected to increase significantly, particularly in the summer and autumn months, with increases up to 25% under RCP8.5 and 18% under RCP4.5. This research focuses on employing the Precipitation Runoff Modeling System (PRMS) to project future streamflow across South Korea, with an emphasis on both monthly and seasonal scales to understand the varying impacts of climate change on different river basins. These climatic changes have profound implications for agriculture, urban water management, and ecosystem sustainability, stressing the need for dynamic and region-specific adaptation measures. This study emphasizes the critical role of localized factors, such as topography, land use, and basin-specific characteristics, in influencing the hydrological cycle under changing climatic conditions.

Projections of central Arctic summer sea surface temperatures in CMIP6

One of the most dramatic climate responses to future global warming is the near-disappearance of the perennial sea ice cover in the central Arctic Ocean, a phenomenon known as the ice-free summer Arctic. The immediate consequence of an ice-free Arctic would be the surface warming of the central Arctic Ocean, where sea surface temperatures (SSTs) used to be at freezing levels. Through an analysis of climate models participating in the climate model intercomparison project phase-6, this study demonstrates a wide range of responses in the central Arctic SSTs in August–September as a result of the projected ice-free summer Arctic. These responses vary from 0.7 to 8 °C in the shared socioeconomic pathway 2 (SSP2-4.5), referred to as the ‘middle of the road’ scenario, in which socioeconomic and technological trends do not significantly deviate from historical patterns. The extent of the central Arctic sea surface warming in August–September is found to have a loose correlation with the September sea ice extent (SIE), but a stronger connection to the SIE during spring to early summer (May–July), when incoming shortwave radiation is most intense. In certain climate models, the perennial sea ice cover disappears completely in September, causing central Arctic SSTs to rise by 5 °C–8 °C by the end of the 21st century. This leads to a bimodal distribution of annual SSTs. Further analysis reveals a close relationship between mid-summer SSTs in the central Arctic Ocean and preceding winter sea ice thickness (SIT). This underscores the significance of winter SIT in predicting future Arctic surface warming and marine heatwaves.

Assessing the impact of climate change on future extreme temperature events in major South African cities

In this era of climate change, extreme weather events are expected to become more intense and frequent. This study analysed the long-term future climate data from the mean of five climate model intercomparison project phase 6 (CMIP6) global climate models (GCMs) to examine the impacts of climate change on extreme temperature in four major South African cities. The bias correction was successfully conducted using the CMhyd software program. The analysis of extreme temperatures was conducted using indices developed by the World Meteorological Organization’s Expert Team on Sector-specific Climate Indices (ET-SCI) and calculated using the R-based Climpact2 software. All statistical metrics (mean, R2 and RMSE) show that bias correction was fairly good, and further analysis and conclusions could also be drawn using the adjusted dataset. The overall result shows that annual trends of all temperature indices analysed in this study are significantly increasing for both scenarios (SSP2-4.5 and SSP5-8.5) except for some lower extreme temperature indices (i.e., number of cool days, cold nights and cold spells). In the historical time scale, however, some indices showed no trend for some stations. The study also found that coastal cities had a slower increase in higher extreme weather indices as compared to inland cities. However, for lower extreme indices (such as number of cool days, cold nights, cool day’s temperature and cold spells), the opposite was true. This information is important for policymakers, development agents and disaster prevention workers to make informed decisions about adapting to and mitigating extreme weather events.

Decadal variability of oxygen uptake, export, and storage in the Labrador Sea from observations and CMIP6 models

The uptake of dissolved oxygen from the atmosphere via air-sea gas exchange and its physical transport away from the region of uptake are crucial for supplying oxygen to the deep ocean. This process takes place in a few key regions that feature intense oxygen uptake, deep water formation, and physical oxygen export. In this study we analyze one such region, the Labrador Sea, utilizing the World Ocean Database (WOD) to construct a 65–year oxygen content time series in the Labrador Sea Water (LSW) layer (0–2200 m). The data reveal decadal variability associated with the strength of deep convection, with a maximum anomaly of 27 mol m–2 in 1992. There is no long-term trend in the time series, suggesting that the mean oxygen uptake is balanced by oxygen export out of the region. We compared the time series with output from nine models of the Ocean Model Intercomparison Project phase 1 in the Climate Model Intercomparison Project phase 6, (CMIP6-OMIP1), and constructed a “model score” to evaluate how well they match oxygen observations. Most CMIP6-OMIP1 models score around 50/100 points and the highest score is 57/100 for the ensemble mean, suggesting that improvements are needed. All of the models underestimate the maximum oxygen content anomaly in the 1990s. One possible cause for this is the representation of air-sea gas exchange for oxygen, with all models underestimating the mean uptake by a factor of two or more. Unrealistically deep convection and biased mean oxygen profiles may also contribute to the mismatch. Refining the representation of these processes in climate models could be vital for enhanced predictions of deoxygenation. In the CMIP6-OMIP1 multi-model mean, oxygen uptake has its maximum in 1980–1992, followed by a decrease in 1994–2006. There is a concurrent decrease in export, but oxygen storage also changes between the two periods, with oxygen accumulated in the first period and drained out in the second. Consequently, the change in oxygen export (5%) is much less than that in uptake (28%), suggesting that newly ventilated LSW which remains in the formation region acts to buffer the linkage between air-sea gas exchange and oxygen export.

Atmospheric Response to a Collapse of the North Atlantic Circulation under a Mid-Range Future Climate Scenario: A Regime Shift in Northern Hemisphere Dynamics

Abstract Climate models project a future weakening of the Atlantic meridional overturning circulation (AMOC), but the impacts of this weakening on climate remain highly uncertain. A key challenge in quantifying the impact of an AMOC decline is in isolating its influence on climate, relative to other changes associated with increased greenhouse gases. Here we isolate the climate impacts of a weakened AMOC in the broader context of a warming climate using a unique ensemble of Shared Socioeconomic Pathway (SSP) 2–4.5 integrations that was performed using the Climate Model Intercomparison Project phase 6 (CMIP6) version of the NASA Goddard Institute for Space Studies ModelE (E2.1). In these runs internal variability alone results in a spontaneous bifurcation of the ocean flow, wherein 2 out of 10 ensemble members exhibit an entire AMOC collapse, while the other 8 members recover at various stages despite identical forcing of each ensemble member and with no externally prescribed freshwater perturbation. We show that an AMOC collapse results in an abrupt northward shift and strengthening of the Northern Hemisphere (NH) Hadley cell (HC) and intensification of the northern midlatitude eddy-driven jet. We then use a set of coupled atmosphere–ocean abrupt CO2 experiments spanning the range 1 times to 5 times CO2 (1x to 5xCO2) to show that this response to an AMOC collapse results in a nonlinear shift in the NH circulation moving from 2xCO2 to 3xCO2. Slab-ocean versions of these experiments, by comparison, do not capture this nonlinear behavior. Our results suggest that changes in ocean heat flux convergences associated with an AMOC collapse—while highly uncertain—can result in profound changes in the NH circulation and continued efforts to constrain the AMOC response to future climate change are needed.

The global impact of the transport sectors on the atmospheric aerosol and the resulting climate effects under the Shared Socioeconomic Pathways (SSPs)

Abstract. A global aerosol–climate model is applied to quantify the impact of the transport sectors (land transport, shipping, and aviation) on aerosol and climate. Global simulations are performed for the present day (2015), based on the emission inventory of the Climate Model Intercomparison Project Phase 6 (CMIP6), and for near-term (2030) and mid-term (2050) future projections, under the Shared Socioeconomic Pathways (SSPs). The results for the present day show that land transport emissions have a large impact on near-surface concentrations of black carbon and aerosol nitrate over the most populated areas of the globe, but with contrasting patterns in terms of relative contributions between developed and developing countries. In spite of the recently introduced regulations to limit the fuel sulfur content in the shipping sector, shipping emissions are still responsible for a considerable impact on aerosol sulfate near-surface concentrations, about 0.5 to 1 µg m−3 in the most travelled regions, with significant effects on continental air pollution and in the northern polar regions as well. Aviation impacts on aerosol mass are found to be quite small, of the order of a few nanograms per cubic metre, while this sector considerably affects particle number concentrations, contributing up to 20 %–30 % of the upper-tropospheric particle number concentration at the northern mid-latitudes. The transport-induced impacts on aerosol mass and number concentrations result in a present-day radiative forcing of −164, −145, and −64 mW m−2 for land transport, shipping, and aviation, respectively, with a dominating contribution by aerosol–cloud interactions. These forcings represent a marked offset to the CO2 warming from the transport sectors and are therefore very relevant for climate policy. The projections under the SSPs show that the impact of the transport sectors on aerosol and climate are generally consistent with the narratives underlying these scenarios: the lowest impacts of transport on both aerosol and climate are simulated under SSP1, especially for the land transport sector, while SSP3 is generally characterized by the largest effects. Notable exceptions to this picture, however, exist, as the emissions of other anthropogenic sectors also contribute to the overall aerosol concentrations and thus modulate the relevance of the transport sectors in the different scenarios, not always consistently with their underlying storyline. On a qualitative level, the results for the present day mostly confirm the findings of our previous assessment for the year 2000, which used a predecessor version of the same model and the CMIP5 emission data. Some important quantitative differences are found, which can mostly be ascribed to the improved representation of aerosol background concentrations in the present study.

Evaluating the Performance of CMIP6 GCMs to Simulate Precipitation and Temperature Over the Vietnamese Mekong Delta

This study evaluates the performance of simulated precipitation and maximum and minimum temperatures in the historical runs of the Climate Model Intercomparison Project Phase 6 (CMIP6) for the Vietnamese Mekong Delta (VMD). The precipitation, as well as maximum and minimum temperatures outputs from 16 general circulation models (GCMs), were compared with observations from 12 stations for the period 1980–2014, using a set of statistical metrics, namely, normalised root mean square error (NRMSE), percentage of bias (PBIAS), Nash–Sutcliffe efficiency (NSE), coefficient of determination (R2), and volumetric efficiency (VE). Finally, ranking (total score - TS) was carried out and the probability distribution function (PDF) and Taylor diagram were used to confirm rankings. The results show that different statistical indicators reveal variation ranking order of the 16 GCMs. Based on RS ranking, it is indicated that each simulation GCM performed differently under the different metrics and no single model performed best for all metrics. The top five highest ranked GCMs based on TS were HadGEM3-GC31-LL, ACCESS-CM2, CanESM5, NESM3 and CanESM5-CanOE for precipitation; and CNRM-CM6-1, CNRM-ESM2-1, GFDL-ESM4, NESM3 and INM-CM5-0 for the maximum; and CNRM-CM6-1, CNRM-ESM2-1, GFDL-ESM4, NESM3 and INM-CM5-0 for minimum temperatures, respectively. We also observed an underestimation of precipitation and an overestimation of temperature over the study area. The TS method demonstrates efficiency to aggregate the multi-model ensemble GCMs based on different statistical indicators which were sometimes contradictory. The findings from this study provide useful guidance in the selection of GCMs for climate change applications in the VMD.

Modelling the impacts of changing land use and climate on sediment and nutrient retention in Lake Tana Basin, Upper Blue Nile River Basin, Ethiopia

Natural ecosystems provide many different services such as carbon sequestration, regulation of climate change, biodiversity, and aesthetic value. Ecosystem functions are significantly impacted by both natural and man-made processes, such as sediment and nutrient deposition. The Lake Tana basin faces significant problems with soil depletion, sedimentation, and degradation. Thus, this study is designed to assess the effects of climate and land use change on the export of sediment and nutrients as well as their retention in the basin. This study used different data sources such as satellite data for land use and land cover and watershed preparation, climate data from Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS) and Climate Model Intercomparison Project phase five (CMIP5), soil data from Ministry of Water, Irrigation and Electricity (MoWIE), and field data for model calibration. Five different Global Climate Models (GCMs) were employed to produce climate projections using the Long Ashton Research Station Weather Generator (LARS-WG) for the basin. The cellular automata (CA) and Markov chain has then been employed for future projections of changes in land use. Finally, the Integrated Valuation of Ecosystem Services and Tradeoff (InVEST) sediment delivery ratio (SDR) and nutrient delivery ratio (NDR) models have been applied to assess the changes in sediment and nutrient export between the baseline and future time periods. The findings indicated that between 1990 and 2019, there was a change in land use that led to an increase in sediment exports of 60.5% and a decrease in sediment retention of 12.8%. Projecting into the future, the amount of exported and retained sediment increases under the RCP4.5 and RCP8.5 scenarios in the 2040s by 8.4% and 17.8%, respectively due to the combined effects of climate and land use change. Equally, nitrogen and phosphorus exports rose under land use change scenarios but are projected to decrease under climate and integrated climate and land use change scenarios. Out of the 12 sub-watersheds that were designated, five were found to have a high mean annual rate of sediment and nutrient export and were then ranked in terms of priority level for conservation planning. Hence, this research shows that the integration of spatial data with the InVEST model can be used to aid land use planning and decision-making to combat nutrient and sediment exports and prioritize watersheds.

Updating the intensity-duration-frequency curves in major Canadian cities under changing climate using CMIP5 and CMIP6 model projections

Characteristics of extreme rainfall events are often presented using the Intensity-Duration-Frequency (IDF) curves. As Canada gets warmer, the credibility of using historical IDF curves for flood risk analysis and design of various urban infrastructure becomes questionable. This study aims at updating and intercomparing the IDF curves in 30 Canadian cities using the simulations of Climate Model Intercomparison Project Phase 6 (CMIP6) under Shared Socioeconomic Pathways 2–4.5 and 5–8.5 and their predecessor CMIP5 under Representative Concentration Pathways 4.5 and 8.5. Quantile-Quantile Downscaling is utilized to estimate extreme rainfall values at fine spatiotemporal scales. Overall, more frequent storms with an average increase of around 30%-40% in their intensity are projected in most cities based on both CMIP5 and CMIP6 models during 2071–2100. For instance, the fast-growing cities of Vancouver, Toronto, and Montreal could expect around 4–11 times more frequent 5-min storms with a 100-year return period during 2071–2100. Moreover, although IDF curves based on CMIP5 and CMIP6 individual models diverge, the expected IDF curves based on their multi-model ensemble mean are somewhat similar if models with a high climate sensitivity based on CMIP6 are excluded from the analysis. This study underlines the need to update IDF curves in Canadian cities.

Soybean response under climatic scenarios with changed mean and variability under rainfed and irrigated conditions in major soybean-growing states of the USA

AbstractClimate change has an impact on soybean production in the USA, necessitating thorough impact studies across broad geographic areas and extended periods to develop appropriate coping strategies. This study investigates the simulated response of soybean in ten major soybean-growing states of the USA under Climate Model Intercomparison Project Phase 5 based on multiple global climate models, two representative concentration pathways [RCP8.5 and RCP4.5] under rainfed and irrigated conditions for 2013–2039, 2043–2069, 2063–2099. The future climate series was developed using Agricultural Model Intercomparison and Improvement Project protocol by applying mean and variability, and CROPGRO-soybean model was explored for soybean simulation under 400 ppm CO2 level and a set of management. Under future climate, intense changes in temperature, precipitation amount and variability are anticipated under RCP8.5 than RCP4.5. As a result, a shorter life cycle, more evapotranspiration, lower grain production, higher water consumption and water productivity were expected under RCP8.5 than RCP4.5 scenarios. A higher reduction in grain yield and water productivity is expected under rainfed than irrigated conditions and intensity increases with advancement towards end of the century. Irrigation tends to decrease adverse climate change effects; however, the marginal economy for irrigation water must be assessed. Since the northern states under study are likely to experience increased grain yields or lower negative impacts, these areas could be the major production zones for soybean production in the future if only climate change is taken into account. Before reaching a convincing conclusion, different adaptation strategies must be thoroughly investigated.

Probabilistic projections of El Niño Southern Oscillation properties accounting for model dependence and skill

The El Niño – Southern Oscillation (ENSO) is a dominant mode of global climate variability. Nevertheless, future multi-model probabilistic projections of ENSO properties have not yet been made. Main roadblocks that have been hindering making these projections are climate model dependence and difficulty in quantifying historical model performance. Dependence is broadly defined as similarity between climate model output, assumptions, or physical parameterizations. Here, we propose a unifying metric of relative model performance, based on the probability density function (PDF) of ENSO paths. This metric is applied to assess the overall skill of Climate Model Intercomparison Project phase 6 (CMIP6) climate models at capturing ENSO. We then perform future multi-model probabilistic projections of changes in ENSO properties (from years 1850–1949 to 2040–2099) under the shared socioeconomic pathway scenario SSP585, accounting for model skill and dependence. We find that future ENSO will likely be more seasonally locked (89% chance), and have a longer period (67% chance). Yet, the jury is still out on future ENSO amplification. Our method reduces uncertainty by up to 37% compared to a simple approach ignoring model dependence and skill.

Northern Hemisphere Stratosphere‐Troposphere Circulation Change in CMIP6 Models: 1. Inter‐Model Spread and Scenario Sensitivity

AbstractProjected changes in the Northern Hemisphere stratospheric polar vortex are analyzed using Climate Model Intercomparison Project Phase 6 experiments. Previous studies showed that projections of the wintertime zonally averaged polar vortex strength diverge widely between climate models with no agreement on the sign of change, and that this uncertainty contributes to the regional climate change uncertainty. Here, we show that there remains large uncertainty in the projected strength of the polar vortex in experiments with global warming levels ranging from moderate (SSP245 runs) to large (Abrupt‐4xCO2 runs), and that the uncertainty maximizes in winter. Partitioning of the uncertainty in wintertime polar vortex strength projections reveals that, by the end of the 21st century, model uncertainty contributes half of the total uncertainty, with scenario uncertainty contributing only 10%. Regression analysis shows that up to 20% of the intermodel spread in projected precipitation over the Iberian Peninsula and northwestern US, and 20%–30% in near‐surface temperature over western US and northern Eurasian, can be associated with the spread in vortex strength projections after accounting for global warming. While changes in the magnitude and sign of the zonally averaged vortex strength are uncertain, most models (>95%) predict an eastward shift of the vortex by 8°–20° degrees in longitude relative to its historical location with the magnitude of the shift increasing for larger global warming levels. There is less agreement across models on a latitudinal shift, whose direction and magnitude correlate with changes in the zonally averaged vortex strength so that vortex weakening/strengthening corresponds to a southward/poleward shift.

Future Changes in Precipitation Over Northern Europe Based on a Multi-model Ensemble from CMIP6: Focus on Tana River Basin

Accurate climate projections help policymakers mitigate the negative effects of climatic changes and prioritize environmental issues based on scientific evidences. These projections rely heavily on the outputs of GCMs (General Circulation Models), but the large number of GCMs and their different outputs in each region confuses researchers in their selection. In this paper, we analyzed the performance of a CMIP6 (Climate Model Intercomparison Project Phase 6) multi-model ensemble for Pr (precipitation) data over NE (Northern Europe). First of all, we evaluated the overall performance of 12 CMIP6 models from GCMs in 30 years of 1985–2014. Furthermore, future projections were analyzed between 2071 and 2100 using SSP1-2.6 and SSP5-8.5 (Shared Socioeconomic Pathways). Then, simulations were statistically improved using an ensemble method to correct the systematic error of the CMIP6 models and then the capacity of postprocessed data to reproduce historical trends of climate events was investigated. Finally, the possible spatio-temporal changes of future Pr data were explored in Tana River Basin. The results of this study show that different CMIP6 models do not have the same accuracy in estimating Pr in the study area. However, the ensemble method can be effective in increasing the accuracy of the projections. The results of this study projected a change in the monthly Pr data over Tana River Basin by 2.46% and 2.06% from 2071 to 2100 compared to the historical period, based on SSP1-2.6 and SSP5-8.5, respectively.

Towards reconstructing the Arctic atmospheric methane history over the 20th century: measurement and modelling results for the North Greenland Ice Core Project firn

Abstract. Systematic measurements of atmospheric methane (CH4) mole fractions at the northern high latitudes only began in the early 1980s. Although CH4 measurements from Greenland ice cores consistently covered the period before ∼ 1900, no reliable observational record is available for the intermediate period. We newly report a data set of trace gases from the air trapped in firn (an intermediate stage between snow and glacial ice formation) collected at the NGRIP (North Greenland Ice Core Project) site in 2001. We also use a set of published firn air data at the NEEM (North Greenland Eemian ice Drilling) site. The two Arctic firn air data sets are analysed with a firn air transport model, which translates historical variations to depth profiles of trace gases in firn. We examine a variety of possible firn diffusivity profiles, using a suite of measured trace gases, and reconstruct the CH4 mole fraction by an iterative dating method. Although the reconstructions of the Arctic CH4 mole fraction before the mid-1970s still has large uncertainties (> 30 ppb – parts per billion), we find a relatively narrow range of atmospheric CH4 history that is consistent with both depth profiles of NGRIP and NEEM. The atmospheric CH4 history inferred by this study is more consistent with the atmospheric CH4 scenario prepared for the NEEM firn modelling than that for the CMIP6 (Climate Model Intercomparison Project Phase 6) experiments. Our study shows that the atmospheric CH4 scenario used for the NEEM firn modelling is considered to be the current best choice for the Arctic CH4 history, but it should not be used to tune firn air transport models until being verified by further measurements from sources such as the Arctic ice cores. Given the current difficulty in reconstructing the CH4 history with low uncertainty from the firn air data sets from Greenland, future sampling and measurements of ice cores at a high-accumulation site may be the only way to accurately reconstruct the atmospheric CH4 trend over the 20th century.

East African population exposure to precipitation extremes under 1.5 °C and 2.0 °C warming levels based on CMIP6 models

Understanding population exposure to precipitation-related extreme events is important for effective climate change adaptation and mitigation measures. We analyze extreme precipitation using indices (EPIs), including consecutive dry days (CDD), annual total precipitation, simple daily intensity, and the number of extremely wet days, under the past and future climatic conditions over East Africa. The exposure of the East African population to these extreme events at 1.5 °C and 2.0 °C global warming levels (GWLs) is analyzed based on Climate Model Intercomparison Project phase 6 models. Exposure is computed from extremely wet and dry days (R95p and CDD, respectively). Under both GWLs, EPIs (except CDD) averaged over East Africa are projected to increase under the Shared Socio-economic Pathways (SSP)2-4.5 and SSP5-8.5 scenarios. The largest increase in wet events will likely occur in eastern and northern Kenya. The results also reveal an intensification of precipitation extremes over Burundi, Rwanda, and some parts of Uganda. However, small changes are expected over most parts of Kenya and Tanzania. Examination of population exposure to EPIs shows that the most prominent and net intense occurrence is over Burundi, Rwanda, and some parts of Uganda. In contrast, less change is noted to occur over vast parts of Kenya and Tanzania. Meanwhile, limiting the warming target to less than 1.5 °C but not more than 2.0 °C has 37% (44.2%) and 92% (4%) less impact on the occurrence of EPIs for R95p (CDD) under SSP2-4.5 (SSP5-8.5) scenarios, respectively. The study establishes that future exposure is predominantly driven by changes in population compared to other factors such as climate or concurrent changes in climate and population (the nonlinear interaction effect). For instance, climate effects are anticipated to contribute ∼10.6% (12.6%) of the total change in population exposure under 1.5 °C (2.0 °C) warming levels, while population and interaction effects are expected to contribute ∼77.4% (71.9%) and 12% (15.5%), respectively, under 1.5 °C (2.0 °C) scenarios. Interestingly, the projected changes in regional exposure due to the interaction effects under SSP2-4.5 are greater than the climate effect, while the reverse pattern is observed under SSP5-8.5. For example, under SSP5-8.5, climate effects for 1.5 °C and 2.0 °C are larger (after population effect) with ∼3.8 × 105 (15.7%) and ∼6.1 × 105 (17.5%) billion person-mm, respectively. The high exposure noted over East Africa calls for a shift in policies to instate suitable adaptation measures to cushion the already vulnerable population.

Observed poleward freshwater transport since 1970.

Warming-induced global water cycle changes pose a significant challenge to global ecosystems and human society. However, quantifying historical water cycle change is difficult owing to a dearth of direct observations, particularly over the ocean, where 77% and 85% of global precipitation and evaporation occur, respectively1-3. Air-sea fluxes of freshwater imprint on ocean salinity such that mean salinity is lowest in the warmest and coldest parts of the ocean, and is highest at intermediate temperatures4. Here we track salinity trends in the warm, salty fraction of the ocean, and quantify the observed net poleward transport of freshwater in the Earth system from 1970 to 2014. Over this period, poleward freshwater transport from warm to cold ocean regions has occurred at a rate of 34-62 milli-sverdrups (mSv = 103 m3 s-1), a rate that is not replicated in the current generation of climate models (the Climate Model Intercomparison Project Phase 6 (CMIP6)). In CMIP6 models, surface freshwater flux intensification in warm ocean regions leads to an approximately equivalent change in ocean freshwater content, with little impact from ocean mixing and circulation. Should this partition of processes hold for the real world, the implication is that the historical surface flux amplification is weaker (0.3-4.6%) in CMIP6 compared with observations (3.0-7.4%). These results establish a historical constraint on poleward freshwater transport that will assist in addressing biases in climate models.

A storyline view of the projected role of remote drivers on summer air stagnation in Europe and the United States

Storylines of atmospheric circulation change, or physically self-consistent narratives of plausible future events, have recently been proposed as a non-probabilistic means to represent uncertainties in climate change projections. Here, we apply the storyline approach to 21st century projections of summer air stagnation over Europe and the United States. We use a Climate Model Intercomparison Project Phase 6 (CMIP6) ensemble to generate stagnation storylines based on the forced response of three remote drivers of the Northern Hemisphere mid-latitude atmospheric circulation: North Atlantic warming, North Pacific warming, and tropical versus Arctic warming. Under a high radiative forcing scenario (SSP5-8.5), models consistently project increases in stagnation over Europe and the U.S., but the magnitude and spatial distribution of changes vary substantially across CMIP6 ensemble members, suggesting that future projections are not well-constrained when using the ensemble mean alone. We find that the diversity of projected stagnation changes depends on the forced response of remote drivers in individual models. This is especially true in Europe, where differences of ∼2 summer stagnant days per degree of global warming are found amongst the different storyline combinations. For example, the greatest projected increase in stagnation for most European regions leads to the smallest increase in stagnation for southwestern Europe; i.e. limited North Atlantic warming combined with near-equitable tropical and Arctic warming. In the U.S., only the atmosphere over the northern Rocky Mountain states demonstrates comparable stagnation projection uncertainty, due to opposite influences of remote drivers on the meteorological conditions that lead to stagnation.

Climate change projections and trends simulated from the CMIP5 models for the Lake Tana sub-basin, the Upper Blue Nile (Abay) River Basin, Ethiopia

Climate change is mainly refers to the long-term fluctuations in the weather parameters of a large area with statistical significance. Earth's climate change is either due to natural variability or human activity, in combination. The Lake Tana basin has been studied to project future climate change using the Statistical DownSclaing Model (SDSM). The SDSM was used to downscale both temperature and precipitation from Canadian Earth System Model version 2 (CanESM2) Global Climate Models (GCMs) for Bahir Dar, Dangila, Debre Tabor and Gonder synoptic weather stations. The Manna Kendall non-parametric test was then used to examine both the baseline and projected precipitation and temperature trends. The historical or baseline data for downscaling purpose was obtained from National Meteorological Agency, Ethiopia (NMA) while large-scale predictor variables were obtained under Climate Model Intercomparison Project Phase 5 (CMIP5). Representative Concentration Pathways (RCPs) 2.6, 4.5 and 8.5 radiative forcing scenarios have been derived from the CanESM2 model. The performance of SDSM have been validated by using various statistical methods, such as RMSE, NSE, R and R2. The SDSM results are not reliablefor projecting the precipitation as compared to maximum and minimum temperatures. These findings indicated that maximum temperature tend to increase from 1.38 °C to 3.59 °C under RCP4.5 radiative forcing scenario by the year 2080s, while minimum temperature is projected to increase up to 5.92 °C under RCP8.5 by the end of the 21st century in the Lake Tana sub-basin. On the other hand, precipitation projection did not show consistent pattern over the basin. For instance, precipitation is projected to increase up to 255 mm in the northern and central parts of the basin, however, but relatively lower result (up to 200 mm) from the RCP8.5 radiative forcing scenarios by year 2080s. By considering Mann-Kendal trend test, the baseline and projected mean annual maximum and minimum temperatures show increasing trend, while baseline rainfall shows increasing trend, but projected rainfall shows a decreasing trend at Dangila and Debre Tabor stations. Whereas, an increasing trend noticed at Bahir Dar and Gonder stations. Therefore, basin or watershed scale climate change adaptation and mitigation strategies have be developed to minimize the negative impacts of climate change.

An appraisal of seasonal precipitation dynamics over the North‐West Himalayan region under future warming scenarios

ABSTRACTIn the context of widely reported climate change over the mountainous region, the present work addresses the notable changes in the seasonal precipitation dynamics over the NWH region under two different future warming scenarios [socioeconomic pathway 245 (SSP245) and socio economic pathway 585 (SSP585)]. Using three coupled climate models which participated in the Climate Model Inter‐comparison Project Phase 6 (CMIP6), we focus on the alterations of the spatio‐temporal pattern of solid and liquid precipitation in respective seasons under future warming (2066–2100) scenarios with respect to the historical time period (1981–2014). Intercomparative validation of the CMIP6 data sets has been carried out against the GPCP and ERA‐5 reanalysis datasets where the multimodel mean (MMM) product has performed well (r value = 0.63) compared with the individual model outputs in the historical scenario for last 35 years (1981–2014). Therefore, MMM has been used for analysis. Statistically robust K–S test suggests a remarkable variation in the cumulative distribution of the precipitation across the NWH region at 95% confidence level in both of the future warming scenarios as compared with the historical time period. Furthermore, significant increase in the JJAS rainfall (~25–30%) and decrease in the DJF snowfall (~20–25%) has been observed for ssp585 scenario whereas no significant results are noted under ssp245 scenario. Remarkable variations in the surface temperature distribution, large scale wind circulation pattern and mesoscale hydrometeorological balance under the warmest scenario (SSP585) elucidate the reason behind the noteworthy changes in the precipitation dynamics reflected from the normalized precipitation index (NPI) and changes in annual cycle and therefore illustrates the possibility of more extreme precipitation years in future. The overall investigation reflects the possible effect of climate change over the NWH region pertaining to the dynamics of the South West Monsoon and Western Disturbances.