Abstract
Climate warming is intensifying the global water cycle, including the rate of fresh water flux between the atmosphere and the surface, determined by precipitation minus evaporation (P−E). ...Surpluses or deficits of fresh water impact societies and ecosystems, so it is important to monitor and understand how and why P−E patterns and their seasonal range are changing across the globe. Here, annual maximum and minimum P−E and their changes are diagnosed globally over land and ocean using observation-based datasets and CMIP6 climate model experiments covering 1950–2100. Seasonal minimum P−E is negative across much of the globe, apart from the Arctic, mid-latitude oceans and the tropical warm pool. In the global mean, P−E maximum increases and P−E minimum decreases by around 3%–4% per
∘
C of global warming from 1995–2014 to 2080–2100 in the ensemble mean of an intermediate greenhouse gas emission scenario. Over land, there is less coherence across the 1960–2020 datasets, but an increase in the seasonal range in P−E emerges in future projections. Patterns of future changes in annual maximum and minimum P−E are qualitatively similar to present day trends with increases in maximum P−E in the equatorial belt and high-latitude regions and decreases in the subtropical subsidence zones. This adds confidence to future projections of a more variable and extreme water cycle but also highlights uncertainties in this response over land.
Changes in the seasonality of precipitation over Africa have high potential for detrimental socioeconomic impacts due to high societal dependence upon seasonal rainfall. Here, for the first time we ...conduct a continental-scale analysis of changes in wet season characteristics under the RCP4.5 and RCP8.5 climate projection scenarios across an ensemble of CMIP5 models using an objective methodology to determine the onset and cessation of the wet season. A delay in the wet season over West Africa and the Sahel of over 5–10 days on average, and later onset of the wet season over southern Africa, is identified and associated with increasing strength of the Saharan heat low in late boreal summer and a northward shift in the position of the tropical rain belt over August–December. Over the Horn of Africa rainfall during the “short rains” season is projected to increase by over 100 mm on average by the end of the twenty-first century under the RCP8.5 scenario. Average rainfall per rainy day is projected to increase, while the number of rainy days in the wet season declines in regions of stable or declining rainfall (western and southern Africa) and remains constant in central Africa, where rainfall is projected to increase. Adaptation strategies should account for shorter wet seasons, increasing rainfall intensity, and decreasing rainfall frequency, which will have implications for crop yields and surface water supplies.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
NASA Precipitation Measurement Mission observations are used to evaluate the diurnal cycle of precipitation from three CMIP6 models (NCAR-CESM2, CNRM-CM6.1, CNRM-ESM2.1) and the ERA5 reanalysis. ...NASA’s global-gridded IMERG product, which combines spaceborne microwave radiometer, infrared sensor, and ground-based gauge measurements, provides high-spatiotemporal-resolution (0.1° and half-hourly) estimates that are suitable for evaluating the diurnal cycle in models, as determined against the ground-based radar network over the conterminous United States. IMERG estimates are coarsened to the spatial and hourly resolution of the state-of-the-art CMIP6 and ERA5 products, and their diurnal cycles are compared across multiple decades of June–August in the 60°N–60°S domain (IMERG and ERA5: 2000–19; NCAR and CNRM: 1979–2008). Low-precipitation regions (and weak-amplitude regions when analyzing the diurnal phase) are excluded from analyses so as to assess only robust diurnal signals. Observations identify greater diurnal amplitudes over land (26%–134% of the precipitation mean; 5th–95th percentile) than over ocean (14%–66%). ERA5, NCAR, and CNRM underestimate amplitudes over ocean, and ERA5 overestimates over land. IMERG observes a distinct diurnal cycle only in certain regions, with precipitation peaking broadly between 1400 and 2100 LST over land (2100–0600 LST over mountainous and varying-terrain regions) and 0000 and 1200 LST over ocean. The simulated diurnal cycle is unrealistically early when compared with observations, particularly over land (NCAR-CESM2 AMIP:−1 h; ERA5: −2 h; CNRM-CM6.1 AMIP: −4 h on average) with nocturnal maxima not well represented over mountainous regions. Furthermore, ERA5’s representation of the diurnal cycle is too simplified, with less interannual variability in the time of maximum relative to observations over many regions.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Climate models suggest that extreme precipitation events will become more common in an anthropogenically warmed climate. However, observational limitations have hindered a direct evaluation of ...model-projected changes in extreme precipitation. We used satellite observations and model simulations to examine the response of tropical precipitation events to naturally driven changes in surface temperature and atmospheric moisture content. These observations reveal a distinct link between rainfall extremes and temperature, with heavy rain events increasing during warm periods and decreasing during cold periods. Furthermore, the observed amplification of rainfall extremes is found to be larger than that predicted by models, implying that projections of future changes in rainfall extremes in response to anthropogenic global warming may be underestimated.
The Atlantic Meridional Overturning Circulation (AMOC) has been, and will continue to be, a key factor in the modulation of climate change both locally and globally. However, there remains ...considerable uncertainty in recent AMOC evolution. Here, we show that the multimodel mean AMOC strengthened by approximately 10% from 1850–1985 in new simulations from the 6th Coupled Model Intercomparison Project (CMIP6), a larger change than was seen in CMIP5. Across the models, the strength of the AMOC trend up to 1985 is related to a proxy for the strength of the aerosol forcing. Therefore, the multimodel difference is a result of stronger anthropogenic aerosol forcing on average in CMIP6 than CMIP5, which is primarily due to more models including aerosol‐cloud interactions. However, observational constraints—including a historical sea surface temperature fingerprint and shortwave radiative forcing in recent decades—suggest that anthropogenic forcing and/or the AMOC response may be overestimated.
Key Points
The Atlantic Meridional Overturning Circulation (AMOC) is important in modulating climate change, but its past evolution is uncertain
In CMIP6 historical runs, the AMOC strengthens by ~10% as a response to increased forcing from aerosol indirect effects, unlike in CMIP5
Observational constraints suggest that anthropogenic forcing and/or the AMOC response may be overestimated
An even drier future for the arid lands Allan, Richard P; Douville, Hervé
Proceedings of the National Academy of Sciences - PNAS,
01/2024, Letnik:
121, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Recent heat extremes, wildfires, and droughts have affected various regions, highlighting the vulnerability of societies to climate change and their dependence on fresh water. A warming climate is ...expected to result in more intense wet and dry periods. Long-term continental drying has been observed, with decreasing aridity, soil moisture, and atmospheric relative humidity. The decline in water vapor over arid and semi-arid regions contradicts expectations and has implications for these vulnerable areas. The increase in water vapor in the atmosphere contributes to temperature rise through a feedback loop and can lead to more intense rainfall events and faster onset of dry spells, affecting flooding and drought. It is crucial to accurately represent water vapor changes in climate models. While there is evidence of a global increase in atmospheric moisture, the research finds deficiencies in how models represent moisture changes in arid regions. Ensuring accurate surface humidity records is challenging but necessary for assessing changes in surface water vapor. The study combines these records with a global reanalysis to improve the accuracy of the simulation.
Multiple observational data sets and atmosphere‐only simulations from the Coupled Model Intercomparison Project Phase 5 are analyzed to characterize recent rainfall variability and trends over Africa ...focusing on 1983–2010. Data sets exhibiting spurious variability, linked in part to a reduction in rain gauge density, were identified. The remaining observations display coherent increases in annual Sahel rainfall (29 to 43 mm yr−1 per decade), decreases in March–May East African rainfall (−14 to −65 mm yr−1 per decade), and increases in annual Southern Africa rainfall (32 to 41 mm yr−1 per decade). However, Central Africa annual rainfall trends vary in sign (−10 to +39 mm yr−1 per decade). For Southern Africa, observed and sea surface temperature (SST)‐forced model simulated rainfall variability are significantly correlated (r~0.5) and linked to SST patterns associated with recent strengthening of the Pacific Walker circulation.
Key Points
Rainfall trends from multiple observational data sets are not consistent in some regions
Spurious rainfall trends are linked to changes in gauge density over time
SST patterns play a strong role in determining Africa‐wide rainfall trends since 1983
Variation in the seasonal cycle of African rainfall is of key importance for agriculture. Here an objective method of determining the timing of onset and cessation is, for the first time, extended to ...the whole of Africa. The method is applied to five observational data sets and the ERA‐Interim reanalysis. Compatibility with known physical drivers of African rainfall, consistency with indigenous methods, and generally strong agreement between satellite‐based rainfall data sets confirm that the method is capturing the correct seasonal progression of African rainfall. The biannual rainfall regime is correctly identified over the coastal region of Ghana and the Ivory Coast. However, the ERA‐Interim reanalysis exhibits timing biases over areas with two rainy seasons, and both ERA‐Interim and the ARCv2 observational data set exhibit some inconsistent deviations over West Africa. The method can be used to analyze both seasonal‐interannual variability and long‐term change. Over East Africa, we find that failure of the rains and subsequent humanitarian disaster is associated with shorter as well as weaker rainy seasons, e.g., on average the long rains were 11 days shorter in 2011. Cessation of the short rains over this region is 7 days later in El Niño and 5 days earlier in La Niña years with only a small change in onset date. The methodology described in this paper is applicable to multiple data sets and to large regions, including those that experience multiple rainy seasons. As such, it provides a means for investigating variability and change in the seasonal cycle over the whole of Africa.
Key Points
Diagnosed patterns in onset/cessation of African rainy seasons, produced using an adapted method, are compatible with physical drivers
Consistent onset/cessation characteristics in satellite‐based rainfall data, ERA‐Interim deficient for biannual regime
Rainy season cessation over the Horn of Africa is 7 days later in El Nino years and 5 days earlier in La Nina
Global warming is expected to enhance fluxes of fresh water between the surface and atmosphere, causing wet regions to become wetter and dry regions drier, with serious implications for water ...resource management. Defining the wet and dry regions as the upper 30% and lower 70% of the precipitation totals across the tropics (30° S-30° N) each month we combine observations and climate model simulations to understand changes in the wet and dry regions over the period 1850-2100. Observed decreases in precipitation over dry tropical land (1950-2010) are also simulated by coupled atmosphere-ocean climate models (−0.3% decade) with trends projected to continue into the 21st century. Discrepancies between observations and simulations over wet land regions since 1950 exist, relating to decadal fluctuations in El Niño southern oscillation, the timing of which is not represented by the coupled simulations. When atmosphere-only simulations are instead driven by observed sea surface temperature they are able to adequately represent this variability over land. Global distributions of precipitation trends are dominated by spatial changes in atmospheric circulation. However, the tendency for already wet regions to become wetter (precipitation increases with warming by 3% K−1 over wet tropical oceans) and the driest regions drier (precipitation decreases of −2% K−1 over dry tropical land regions) emerges over the 21st century in response to the substantial surface warming.
Globally, thermodynamics explains an increase in atmospheric water vapor with warming of around 7%/°C near to the surface. In contrast, global precipitation and evaporation are constrained by the ...Earth's energy balance to increase at ∼2–3%/°C. However, this rate of increase is suppressed by rapid atmospheric adjustments in response to greenhouse gases and absorbing aerosols that directly alter the atmospheric energy budget. Rapid adjustments to forcings, cooling effects from scattering aerosol, and observational uncertainty can explain why observed global precipitation responses are currently difficult to detect but are expected to emerge and accelerate as warming increases and aerosol forcing diminishes. Precipitation increases with warming are expected to be smaller over land than ocean due to limitations on moisture convergence, exacerbated by feedbacks and affected by rapid adjustments. Thermodynamic increases in atmospheric moisture fluxes amplify wet and dry events, driving an intensification of precipitation extremes. The rate of intensification can deviate from a simple thermodynamic response due to in‐storm and larger‐scale feedback processes, while changes in large‐scale dynamics and catchment characteristics further modulate the frequency of flooding in response to precipitation increases. Changes in atmospheric circulation in response to radiative forcing and evolving surface temperature patterns are capable of dominating water cycle changes in some regions. Moreover, the direct impact of human activities on the water cycle through water ion, irrigation, and land use change is already a significant component of regional water cycle change and is expected to further increase in importance as water demand grows with global population.
Societies experience impacts through localized changes in water availability that are controlled by large‐scale atmospheric circulation as well as smaller‐scale physical processes. At regional to local scales, water cycle changes therefore result from the interplay between multiple drivers (CO2, aerosols, land use change and human water use). A primary focus here is on reviewing recent advances in understanding how these complex interactions are expected to determine responses in the global water cycle.
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