We present a new 3‐dimensional 1° × 1° gridded data set for the annual mean seawater oxygen isotope ratio (δ18O) to use in oceanographic and paleoceanographic applications. It is constructed from a ...large set of observations made over the last 50 years combined with estimates from regional δ18O to salinity relationships in areas of sparse data. We use ocean fronts and water mass tracer concentrations to help define distinct water masses over which consistent local relationships are valid. The resulting data set compares well to the GEOSECS data (where available); however, in certain regions, particularly where sea ice is present, significant seasonality may bias the results. As an example application of this data set, we use the resulting surface δ18O as a boundary condition for isotope‐enabled GISS ModelE to yield a more realistic comparison to the isotopic composition of precipitation data, thus quantifying the ‘source effect’ of δ18O on the isotopic composition of precipitation.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
We use numerical climate simulations, paleoclimate data, and modern observations to study the effect of growing ice melt from Antarctica and Greenland. Meltwater tends to stabilize the ocean column, ...inducing amplifying feedbacks that increase subsurface ocean warming and ice shelf melting. Cold meltwater and induced dynamical effects cause ocean surface cooling in the Southern Ocean and North Atlantic, thus increasing Earth's energy imbalance and heat flux into most of the global ocean's surface. Southern Ocean surface cooling, while lower latitudes are warming, increases precipitation on the Southern Ocean, increasing ocean stratification, slowing deepwater formation, and increasing ice sheet mass loss. These feedbacks make ice sheets in contact with the ocean vulnerable to accelerating disintegration. We hypothesize that ice mass loss from the most vulnerable ice, sufficient to raise sea level several meters, is better approximated as exponential than by a more linear response. Doubling times of 10, 20 or 40 years yield multi-meter sea level rise in about 50, 100 or 200 years. Recent ice melt doubling times are near the lower end of the 10–40-year range, but the record is too short to confirm the nature of the response. The feedbacks, including subsurface ocean warming, help explain paleoclimate data and point to a dominant Southern Ocean role in controlling atmospheric CO2, which in turn exercised tight control on global temperature and sea level. The millennial (500–2000-year) timescale of deep-ocean ventilation affects the timescale for natural CO2 change and thus the timescale for paleo-global climate, ice sheet, and sea level changes, but this paleo-millennial timescale should not be misinterpreted as the timescale for ice sheet response to a rapid, large, human-made climate forcing. These climate feedbacks aid interpretation of events late in the prior interglacial, when sea level rose to +6–9 m with evidence of extreme storms while Earth was less than 1 °C warmer than today. Ice melt cooling of the North Atlantic and Southern oceans increases atmospheric temperature gradients, eddy kinetic energy and baroclinicity, thus driving more powerful storms. The modeling, paleoclimate evidence, and ongoing observations together imply that 2 °C global warming above the preindustrial level could be dangerous. Continued high fossil fuel emissions this century are predicted to yield (1) cooling of the Southern Ocean, especially in the Western Hemisphere; (2) slowing of the Southern Ocean overturning circulation, warming of the ice shelves, and growing ice sheet mass loss; (3) slowdown and eventual shutdown of the Atlantic overturning circulation with cooling of the North Atlantic region; (4) increasingly powerful storms; and (5) nonlinearly growing sea level rise, reaching several meters over a timescale of 50–150 years. These predictions, especially the cooling in the Southern Ocean and North Atlantic with markedly reduced warming or even cooling in Europe, differ fundamentally from existing climate change assessments. We discuss observations and modeling studies needed to refute or clarify these assertions.
Understanding the atmospheric moisture budget of Greenland is critical for predicting future Greenland climate. However, past attempts to quantify the provenance of Greenland precipitation have been ...limited to certain seasons. Here we present an analysis of Greenland moisture sources using water tracers in the Goddard Institute for Space Studies climate model, which can provide source estimates for all time periods. The North Atlantic is found to be the dominant moisture source, except during summer when continental sources increase substantially. The variability is also found to be partially correlated with the Greenland Blocking Index. The key finding, however, is a long‐term trend in the moisture source location for Northwest Greenland, with an increase in more locally sourced moisture over time during nonsummer months. This is at least partially related to sea ice loss in the Baffin Bay region, along with a larger increase in sea surface temperatures for the region relative to other locales.
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It is still not fully clear where the precipitation that falls over Greenland actually evaporates from, and how these evaporation locations change with the climate, with previous studies being limited in terms of season or transport time. Here we use a climate model that can trace water through the atmosphere from evaporation to rainout to determine the sources of moisture for Greenland precipitation. It is found that the North Atlantic is the largest source for every season except summer, when land‐based moisture sources become larger. These sources vary over time, particularly in response to the Greenland Blocking climate index. Finally, a long‐term change in the moisture source for Northwest Greenland was found, with more water evaporating from locations closer to Greenland itself. This change is found to be at least partially related to the loss of sea ice and a warming in sea surface temperatures in the oceans and seas closest to Greenland, resulting in more evaporation and thus an increase in local moisture sources.
Key Points
Precipitation moisture source varies substantially with season and location in Greenland
Large interannual variability exists for moisture source, which is partially explained by indices like the GBI
A significant long‐term trend toward more local moisture sources exists for Northwest Greenland
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Observations show that all recent large tropical volcanic eruptions (1850 to Present) were followed by surface winter warming in the first Northern Hemisphere (NH) winter after the eruption. Recent ...studies show that climate models produce a surface winter warming response in the first winter after the largest eruptions but require a large ensemble of simulations to see significant changes. It is also generally required that the eruption be very large, and only two such eruptions occurred in the historical period: Krakatau in 1883 and Pinatubo in 1991. Here we examine surface winter warming patterns after the 10 largest volcanic eruptions between 850 and 1850 in the Paleoclimate Modeling Intercomparison Project 3 last millennium simulations and in the Community Earth System Model Last Millennium Ensemble. These eruptions were all larger than those since 1850. Though the results depend on both the individual models and the forcing data set used, we have found that models produce a surface winter warming signal in the first winter after large volcanic eruptions, with higher temperatures over NH continents and a stronger polar vortex in the lower stratosphere. We also examined NH summer precipitation responses in the first year after the eruptions and find clear reductions of summer Asian and African monsoon rainfall.
Key Points
Last millennium climate model simulations produce robust Northern Hemisphere surface winter warming response after large tropical volcanic eruptions
Last millennium climate model simulations show significant reduction of the summer monsoon after large tropical volcanic eruptions
The simulated responses depend strongly on the model and choice of the forcing data set
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Strong, strato-volcanic eruptions are a substantial, intermittent source of natural climate variability. Initial atmospheric and oceanic conditions, such as El Niño Southern Oscillation (ENSO) and ...the North Atlantic Oscillation (NAO), also naturally impact climate on interannual timescales. We examine how initial conditions of ENSO and NAO contribute to the evolution of climate in the period following a Pinatubo-type eruption using a large (81-member) ensemble of model simulations in GISS model E2.1-G. Simulations are initialized from sampled conditions of ENSO and NAO using the protocol of the coordinated CMIP6 Volcanic Model Intercomparison Project (VolMIP) – where aerosols are forced with respect to time, latitude, and height. We analyze paired anomalous variations (perturbed – control) to understand changes in global and regional climate responses under positive, negative, and neutral ENSO and NAO conditions. In particular, we find that for paired anomalies there is a high probability of strong (∼1.5 °C) warming of northern Eurasia surface air temperature in the first winter after the volcanic eruption for negative NAO ensembles coincident with decreased lower stratospheric temperature at the poles, decreased geopotential height, and strengthening of the stratospheric polar vortex. Climate anomalies (relative to average conditions across the control period), however, show no mean warming and suggest that the strength of this response is impacted by conditions present in the selected period of the control run. Again using paired anomalies, we also observe that under both +ENSO and −ENSO ensembles sea surface temperature decreases in the first post-eruptive boreal winter coinciding with surface cooling from volcanic aerosols. Neutral ENSO ensembles, on the other hand, show variability in their response with no clear trend in post-eruptive warming or cooling. In general, paired anomalies from unperturbed simulations give insight into the evolution of the climate response to volcanic forcing; however, when compared with anomalies from climatological conditions, it is clear that paired anomalies are significantly affected by sampled initial conditions occurring at the time of the volcanic eruption.
Water isotopes provide a clear record of past climate variability but establishing their precise relationship to local or regional climate changes is the key to quantitative interpretations. We have ...incorporated water isotope tracers within the complete hydrological cycle of Goddard Institute for Space Studies coupled ocean‐atmosphere model (ModelE) in order to assess these relationships. Using multicentennial simulations of the modern (preindustrial) and mid‐Holocene (6 kyr BP) climate, we examine the internal variability and the forced response to orbital and greenhouse gas forcing. Modelled isotopic anomalies clearly reflect climatic changes and, particularly in the tropics, are more regionally coherent than the precipitation anomalies. Matches to observations at the mid‐Holocene and over the instrumental period are good. We calculate water isotope‐climate relationships for many patterns of intrinsic and for forced variability relevant to the Holocene, and we show that in general, calibrations depend on the nature of the climate change. Specifically, we examine relationships between isotopes in precipitation and local temperatures and precipitation amounts in the principal ice coring regions (Greenland, Antarctica, and the tropical Andes) and the seawater isotope‐salinity gradients in the ocean. We suggest that isotope‐based climate reconstructions based on spatial patterns and nonlocal calibrations will be more robust than interpretations based on local relationships.
The enhancement of the stratospheric aerosol layer by volcanic eruptions induces a complex set of responses causing global and regional climate effects on a broad range of timescales. Uncertainties ...exist regarding the climatic response to strong volcanic forcing identified in coupled climate simulations that contributed to the fifth phase of the Coupled Model Intercomparison Project (CMIP5). In order to better understand the sources of these model diversities, the Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP) has defined a coordinated set of idealized volcanic perturbation experiments to be carried out in alignment with the CMIP6 protocol. VolMIP provides a common stratospheric aerosol data set for each experiment to minimize differences in the applied volcanic forcing. It defines a set of initial conditions to assess how internal climate variability contributes to determining the response. VolMIP will assess to what extent volcanically forced responses of the coupled ocean-atmosphere system are robustly simulated by state-of-the-art coupled climate models and identify the causes that limit robust simulated behavior, especially differences in the treatment of physical processes. This paper illustrates the design of the idealized volcanic perturbation experiments in the VolMIP protocol and describes the common aerosol forcing input data sets to be used.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
The eruption of Mt. Tambora in 1815 was the largest volcanic eruption of the past 500 years. The eruption had significant climatic impacts, leading to the 1816 year without a summer, and remains ...a valuable event from which to understand the climatic effects of large stratospheric volcanic sulfur dioxide injections. The eruption also resulted in one of the strongest and most easily identifiable volcanic sulfate signals in polar ice cores, which are widely used to reconstruct the timing and atmospheric sulfate loading of past eruptions. As part of the Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP), five state-of-the-art global aerosol models simulated this eruption. We analyse both simulated background (no Tambora) and volcanic (with Tambora) sulfate deposition to polar regions and compare to ice core records. The models simulate overall similar patterns of background sulfate deposition, although there are differences in regional details and magnitude. However, the volcanic sulfate deposition varies considerably between the models with differences in timing, spatial pattern and magnitude. Mean simulated deposited sulfate on Antarctica ranges from 19 to 264 kg km−2 and on Greenland from 31 to 194 kg km−2, as compared to the mean ice-core-derived estimates of roughly 50 kg km−2 for both Greenland and Antarctica. The ratio of the hemispheric atmospheric sulfate aerosol burden after the eruption to the average ice sheet deposited sulfate varies between models by up to a factor of 15. Sources of this inter-model variability include differences in both the formation and the transport of sulfate aerosol. Our results suggest that deriving relationships between sulfate deposited on ice sheets and atmospheric sulfate burdens from model simulations may be associated with greater uncertainties than previously thought.
Paleosalinity reconstructions are a goal of paleoceanographic study because of their potential to provide insight into past ocean circulation. While temperature reconstructions have been assessed by ...using multiple independent proxies, the skill of existing salinity reconstructions remains poorly quantified. We examine the applicability of two different approaches using a set of coupled water isotope–enabled general circulation model experiments as a numerical analog for the real climate system. These simulations for the Holocene, at roughly 1000 year time steps, explicitly track variability in both the water isotopologues and salinity. Our simulations suggest that quantitative reconstructions of past salinity variability based solely on inferred δ18Osw variability have very large errors and uncertainties. However, we find that paired δ18Osw and δD variability (from combining biomarker and calcite proxies) holds promise for providing better quantitative estimates of salinity variability.
Key Points
The d18Osw‐Sal method for paleosalinity reconstruction has large errors
Paleosalinity reconstruction is improved by using paired dD and d18Osw
Percent biases for the d18Osw‐salinity method are provided
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Early Holocene summer warmth drove dramatic Greenland ice sheet (GIS) retreat. Subsequent insolation‐driven cooling caused GIS margin readvance to late Holocene maxima, from which ice margins are now ...retreating. We use 10Be surface exposure ages from four locations between 69.4°N and 61.2°N to date when in the early Holocene south to west GIS margins retreated to within these late Holocene maximum extents. We find that this occurred at 11.1 ± 0.2 ka to 10.6 ± 0.5 ka in south Greenland, significantly earlier than previous estimates, and 6.8 ± 0.1 ka to 7.9 ± 0.1 ka in southwest to west Greenland, consistent with existing 10Be ages. At least in south Greenland, these 10Be ages likely provide a minimum constraint for when on a multicentury timescale summer temperatures after the last deglaciation warmed above late Holocene temperatures in the early Holocene. Current south Greenland ice margin retreat suggests that south Greenland may have now warmed to or above earliest Holocene summer temperatures.
Key Points
South Greenland ice retreated within its near‐present margins 11.1‐10.6 ka
South Greenland ice receded inboard of modern extent 3‐4 ka earlier than west
Current south Greenland summer climate may be similar to that of the early Holocene
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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