Large climate model ensembles are widely used to quantify changes in climate extremes. Here we demonstrate that model‐based estimates of changes in the probability of temperature extremes at 1.5 °C ...global warming regionally differ if quantified using prescribed sea surface temperatures (SSTs) instead of using a fully coupled climate model. Based on the identical climate model used in two experimental setups, we demonstrate that particularly over the tropics and Australia estimates of the changes in the odds of annual temperature extremes can be up to more than a factor of 5 to 10 larger using prescribed SSTs rather than a fully coupled model configuration. The two experimental designs imply a different perspective on framing projections. If experiments conditional on prescribed observed SSTs are interpreted as unconditional real‐world projections, they project changes in extremes that are systematically biased high and overconfident. Our results illustrate the importance of carefully considering experimental design when interpreting projections of extremes.
Plain Language Summary
There is great interest in understanding the likelihoods and associated risks of potential future climate extremes, especially at the Paris Agreement global warming targets of 1.5 and 2 °C warming above preindustrial conditions. In this study, we assess the implications of the model setup for the quantification of changes in the odds of temperature extremes between different global warming levels. Our analysis illustrates the strong sensitivity in the outcomes of such analyses related to the use of different model experiments. We demonstrate that despite using the exact same global climate model the projected changes in the probability of extreme annual temperature anomalies for a climate consistent with a 1.5 °C warming target are in some cases much larger if sea surface temperatures are prescribed over a decade rather than if the model is run in a fully coupled configuration. If prescribed sea surface temperature experiments are interpreted as a projection for the real world at the end of the 21st century independent of ocean variability, they regionally lead to estimates of changes in extremes that are systematically biased high and overconfident. Our results illustrate the importance of carefully considering experimental design when interpreting projected changes in extremes.
Key Points
Model‐based estimates of changes in probability of temperature extremes at 1.5 degrees Celsius global warming are sensitive to the experimental setup
Changes in odds of annual warm extremes in tropics more than 5 times larger in prescribed SST than fully coupled setup of same GCM
Experimental design needs to be taken into account when interpreting projected changes in probability of extremes
Abstract
Recent temperature extremes have shattered previously observed records, reaching intensities that were inconceivable before the events. Could the possibility of an event with such ...unprecedented intensity as the 2021 Pacific Northwest heatwave have been foreseen, based on climate model information available before the event? Could the scientific community have quantified its potential intensity based on the current generation of climate models? Here, we demonstrate how an ensemble boosting approach can be used to generate physically plausible storylines of a heatwave hotter than observed in the Pacific Northwest. We also show that heatwaves of much greater intensities than ever observed are possible in other locations like the Greater Chicago and Paris regions. In order to establish confidence in storylines of ‘black swan’-type events, different lines of evidence need to be combined along with process understanding to make this information robust and actionable for stakeholders.
Heavy precipitation and associated flooding during the cold season, such as the 1993 flood in central Europe (CEU), are a major threat to society and ecosystems. Due to the lack of long homogenous ...climate data and methodological frameworks, it is challenging to estimate how extreme precipitation could get and what the physical drivers are. This study presents two complementary strategies to extrapolate beyond the precipitation records: (a) statistical estimates based on fitting generalized extreme value distributions, providing their probabilistic information on return periods and, (b) ensemble boosting, a model‐based re‐initialization of heavy precipitation in large ensembles, providing a physical coherent storyline in space and time, however, with no direct quantification of its probability. Both show that 3‐day accumulated precipitation maxima can be substantially exceeded over CEU of around 30%–40%, but even higher magnitudes cannot be ruled out in the near future. An empirical orthogonal function analysis reveals that certain sea level pressure patterns, partly reminding of atmospheric rivers are more often associated with heavy precipitation than more moderate events. Additionally, ensemble boosting is a suitable tool for case studies to analyze how extreme heavy precipitation as for the event in 1993 can be simulated. By boosting a 1993‐analog, one‐quarter of the resulting storylines show increased rainfall than observed, due to a stronger north‐south pressure gradient that may have exacerbated the flooding. Overall, the precipitation estimates demonstrate that ensemble boosting is a complementary method to statistical tools and suitable for stress testing, for example, infrastructure protection measures against potentially unseen heavy precipitation events.
Key Points
Ensemble boosting is an efficient method to generate physical storylines of heavy precipitation events beyond the observational range
Statistical and storyline‐based methods estimate that observed 3‐day heavy precipitation events over central Europe can be substantially exceeded
Independent and related boosted heavy precipitation tends to be most extreme under atmospheric circulation patterns that remind of atmospheric river events
Noble-gas concentrations in ground water have been used as a proxy for
past air temperatures, but the accuracy of
this approach has been limited by the existence of a temperature-independent
...component of the noble gases in ground water, termed 'excess air',
whose origin and composition is poorly understood. In
particular, the evidence from noble gases in a Brazilian aquifer for a cooling
of more than 5 °C in tropical America during the Last Glacial Maximum has been called into question. Here we propose
a model for dissolved gases in ground water, which describes the formation
of excess air by equilibration of ground water with entrapped air in quasi-saturated soils. Our model predicts previously unexplained noble-gas data
sets, including the concentration of atmospheric helium, and yields consistent
results for the non-atmospheric helium isotopes that are used for dating ground
water. Using this model of excess air, we re-evaluate the use of noble gases
from ground water for reconstructing past temperatures. Our results corroborate
the inferred cooling in Brazil during the Last Glacial Maximum,
and indicate that even larger cooling took place at mid-latitudes.
Several studies have used the temperature dependence of gas solubilities in water to derive paleotemperatures from noble gases in groundwaters. We present a general method to infer environmental ...parameters from concentrations of dissolved atmospheric noble gases in water. Our approach incorporates statistical methods to quantify uncertainties of the deduced parameter values. The equilibration temperatures of water equilibrated with the atmosphere under controlled conditions could be inferred with a precision and accuracy of ±0.2°C. The equilibration temperatures of lake and river samples were determined with a similar precision. Most of these samples were in agreement with atmospheric equilibrium at the water temperature. In groundwaters either recharge temperature or altitude could be determined with high precision (±0.3°C and ±60 m, respectively) despite the presence of excess air. However, typical errors increase to ±3°C and ±700 m if both temperature and altitude are determined at the same time, because the two parameters are correlated. In some groundwater samples the composition of the excess air deviated significantly from atmospheric air, which was modeled by partial reequilibration. In this case the achievable precision of noble gas temperatures was significantly diminished (typical errors of ±1°C).
•Generating national climate scenarios need to be planned from a broad perspective.•Disseminating user-oriented products increases the uptake of climate projections.•A constant stakeholder dialogue ...is key in the production of climate scenarios.•The CH2018 climate scenarios come as a consolidated climate service in Switzerland.•Communication of climate scenarios should be targeted toward specific user types.
To make sound decisions in the face of climate change, government agencies, policymakers and private stakeholders require suitable climate information on local to regional scales. In Switzerland, the development of climate change scenarios is strongly linked to the climate adaptation strategy of the Confederation. The current climate scenarios for Switzerland CH2018 - released in form of six user-oriented products - were the result of an intensive collaboration between academia and administration under the umbrella of the National Centre for Climate Services (NCCS), accounting for user needs and stakeholder dialogues from the beginning. A rigorous scientific concept ensured consistency throughout the various analysis steps of the EURO-CORDEX projections and a common procedure on how to extract robust results and deal with associated uncertainties. The main results show that Switzerland’s climate will face dry summers, heavy precipitation, more hot days and snow-scarce winters. Approximately half of these changes could be alleviated by mid-century through strong global mitigation efforts. A comprehensive communication concept ensured that the results were rolled out and distilled in specific user-oriented communication measures to increase their uptake and to make them actionable. A narrative approach with four fictitious persons was used to communicate the key messages to the general public. Three years after the release, the climate scenarios have proven to be an indispensable information basis for users in climate adaptation and for downstream applications. Potential for extensions and updates has been identified since then and will shape the concept and planning of the next scenario generation in Switzerland.
The design, setup, and performance of a mass spectrometric system for the analysis of noble gas isotopes (3He, 4He, 20Ne, 21Ne, 22Ne, 36Ar, 40Ar, 84Kr, 136Xe) and tritium (3H) from water samples are ...described. The 3H concentration is measured indirectly by the 3He ingrowth from radioactive decay. After extraction, purification, and separation, the noble gases are measured in two noncommercial double-collector 90° magnetic sector mass spectrometers. We present a new approach for the analysis of the heavy noble gas isotopes that enables, in principle, simultaneous measurement of Ar, Kr, and Xe. Typical precisions of the measurements of 3H, He, Ne, Ar, Kr, and Xe concentrations are ±2.7%, ±0.3%, ±0.9%, ±0.3%, ±0.8%, and ±1.0%, respectively. For the isotopic ratios 3He/4He, 20Ne/22Ne, and 40Ar/36Ar the typical precisions are ±0.7%, ±0.3%, and ±0.2%. These values express the reproducibility of the measurement of an internal freshwater standard and include the overall stability of the system as well as of the extraction procedure. To verify the method, the noble gas concentrations of air-saturated water samples prepared under controlled conditions are compared with noble gas solubility data. The 20Ne/22Ne and 36Ar/40Ar fractionation during solution is estimated from 70 surface water samples to be −2.0 ± 0.2‰ and −1.3 ± 0.2‰, respectively.
Stable Cl isotope ratios (
37Cl/
35Cl) were measured in groundwater samples from the southwestern flow system of the Great Artesian Basin, Australia to gain a better understanding of the Cl
− sources ...and transport mechanisms.
δ
37Cl values range from 0‰ to −2.5‰ (SMOC), and are inversely correlated with Cl
− concentration along the inferred flow direction. The Cl isotopic compositions, in conjunction with other geochemical parameters, suggest that Cl
− in groundwaters is not derived from salt dissolution. Mixing of the recharge water with saline groundwater cannot explain the relationship between
δ
37Cl and Cl
− concentration measured. Marine aerosols deposited via rainfall and subsequent evapotranspiration appear to be responsible for the Cl
− concentrations observed in wells that are close to the recharge area, and in groundwaters sampled along the southern transect.
δ
37Cl values measured in the leachate of the Bulldog shale suggest that the aquitard is the subsurface source of Cl
− for the majority of groundwater samples studied. Diffusion is likely the mechanism through which Cl
− is transported from the pore water of the Bulldog shale to the aquifer. However, a more detailed study of the aquitard rocks is required to verify this hypothesis.
The decadal trend behavior of the Northern Hemisphere atmospheric circulation is investigated utilizing long-term simulations with different state-of-the-art coupled general circulation models (GCMs) ...for present-day climate conditions (1990), reconstructions of the past 500 yr, and observations. The multimodel simulations show that strong positive winter North Atlantic Oscillation (NAO) trends are connected with the underlying sea surface temperature (SST) and exhibit an SST tripole trend pattern and a northward shift of the storm-track tail. Strong negative winter trends of the Aleutian low are associated with SST changes in the El Niño–Southern Oscillation (ENSO) region and a westward shift of the storm track in the North Pacific. The observed simultaneous appearance of strong positive NAO and negative Aleutian low trends is very unlikely to occur by chance in the unforced simulations and reconstructions. The positive winter NAO trend of the last 50 yr is not statistically different from the level of internal atmosphere–ocean variability. The unforced simulations also show a strong link between positive SST trends in the ENSO region and negative Aleutian low trends. With much larger observed SST trends in the ENSO region, this suggests that the observed negative Aleutian low trend is possibly influenced by external forcing, for example, global warming, volcanism, and/or solar activity change.
We present elemental and isotopic measurements of noble gases (He, Ne, Ar, Kr, and Xe), oxygen and nitrogen of firn air from two sites. The first set of samples was taken in 1998 at the summit of the ...Devon Ice Cap in the eastern part of Devon Island. The second set was taken in 2001 at NGRIP location (North Greenland).
He and Ne are heavily enriched relative to Ar with respect to the atmosphere in the air near the close-off depth at around 50–70 m. The enrichment increases with depth and reaches the maximum value in the deepest samples just above the zone of impermeable ice where no free air could be extracted anymore. Similarly, elemental ratios of O
2
/
N
2, O
2
/
Ar and Ar
/
N
2 are increasing with depth. In contrast but in line with expectations, isotopic ratios of
15N
/
14N,
18O
/
16O, and
36Ar
/
40Ar show no significant enrichment near the close-off depth.
The observed isotopic ratios in the firn air column can be explained within the uncertainty ranges by the well-known processes of gravitational enrichment and thermal diffusion. To explain the elemental ratios, however, an additional fractionation process during bubble inclusion has to be considered. We implemented this additional process into our firn air model. The fractionation factors were found by fitting model profiles to the data. We found a very similar close-off fractionation behavior for the different molecules at both sites. For smaller gas species (mainly He and Ne) the fractionation factors are linearly correlated to the molecule size, whereas for diameters greater than about 3.6 Å the fractionation seems to be significantly smaller or even negligible. An explanation for this size dependent fractionation process could be gas diffusion through the ice lattice.
At Devon Island the enrichment at the bottom of the firn air column is about four times higher compared to NGRIP. We explain this by lower firn diffusivity at Devon Island, most probably due to melt layers, resulting in significantly reduced back diffusion of the excess gas near the close-off depth.
The results of this study considerably increase the understanding of the processes occurring during air bubble inclusion near the close-off depth in firn and can help to improve the interpretation of direct firn air measurements, as well as air bubble measurements in ice cores, which are used in numerous studies as paleo proxies.
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