High Latitudes Of Northern Hemisphere Research Articles

Assessing multi-site δ18O-climate calibrations of the coralline alga Clathromorphum across the high-latitude Northern Hemisphere

An increased number of climate proxy records and more refined interpretation of proxy data are crucial to improve projections of future climate at high latitudes, where internal feedbacks amplify warming and established high-resolution climate archives are especially sparse. Encrusting coralline algae are being developed as a mid- to high-latitude marine climate archive. These long-lived algae form a solid high-Mg calcite skeleton with annual growth bands similar to those of trees and tropical corals. The oxygen isotope ratio of the algal skeleton (δ18Oalg) records local environmental and climatic factors, notably sea surface temperature and seawater δ18O.Here we assess the δ18Oalg–climate relationship in diverse environments across the algal habitat range utilizing two species of coralline algae from the genus Clathromorphum. Clathromorphum is widely distributed from the cold-temperate North Atlantic and Pacific to the Arctic Ocean and has recently yielded numerous climate reconstructions of up to 650years in length. In this study, we calibrate δ18Oalg of four specimens to gridded temperature and salinity data, the latter a proxy for seawater δ18O. These specimens were collected from a variety of algal growth environments across the high-latitude Northern Hemisphere: two specimens from the Aleutian Archipelago, one from the Canadian Arctic, and one from the Gulf of Maine. Low winter temperatures and insolation restrict the months when algae record local climate in the δ18O of their skeletons; we therefore determine these response seasons by correlating monthly temperature and salinity anomalies with annual δ18Oalg anomalies at each site. We then average gridded data over months that correlate significantly (95% confidence interval) for regression with δ18Oalg. While the timing and nature of the climate signal vary across sites, we find significant relationships between δ18Oalg and either temperature or salinity averaged over the response season at three sites. Variation in local climatology among the four sites provides a physical explanation for calibration differences, compounded by uncertainties stemming from the proxy chronology, biological variability, temporal coverage, and sparse historical climate data. This work takes an essential step toward reconstructing high-latitude marine climate patterns with coralline algal δ18O and developing algae proxy system models.

Climate, herbivory, and fire controls on tropical African forest for the last 60ka

Abstract The Last Glacial Maximum (LGM) in Africa was drier than today and was followed by rapid step-wise climate changes during the last deglacial period. In much of Africa, these changes led to a drastic reduction of lowland forest area during the LGM, followed by recolonization of the lowlands by forest and woodland in concert with regional warming and wetting. However, the history of southeastern African vegetation contrasts with that observed further north. In particular, forest expansion appears to have occurred in southeastern Africa during episodes of high-latitude northern hemisphere cooling. Although vegetation history in Africa is generally assumed to relate purely to climate, previous studies have not addressed potential feedbacks between climate, vegetation, and disturbance regimes (fire, herbivory) that may create tipping points in ecosystems. This climate-vegetation history has profound implications for our understanding of the modern architecture of lowland and highland forests, both thought to be at risk from future climate change. Here we present analyses of fossil pollen, charcoal, and Sporormiella (dung fungus) on a continuous 60 kyr record from central Lake Tanganyika, Southeast Africa, that illustrates the interplay of climate and disturbance regimes in shaping vegetation composition and structure. We observe that extensive forests dominated the region during the last glacial period despite evidence of decreased rainfall. At the end of the LGM, forest opening at ∼17.5 ka followed warming temperatures but preceded rising precipitation, suggesting that temperature-induced water stress and disturbance from fire and herbivory affected initial landscape transformation. Our Sporormiella record indicates that mega-herbivore populations increased at the early Holocene. This higher animal density increased plant species richness and encouraged landscape heterogeneity until the mid-Holocene. At this time, regional drying followed by the onset of the Iron Age in the late Holocene resulted in expansion of thicket, more open woodland, and disturbance taxa that still characterize the landscape today. This climate-vegetation history has important implications for our understanding of the modern and future distribution of lowland and highland forests, which are at risk from future climate change.

Northern Hemisphere control of deglacial vegetation changes in the Rufiji uplands (Tanzania)

Abstract. In tropical eastern Africa, vegetation distribution is largely controlled by regional hydrology, which has varied over the past 20 000 years. Therefore, accurate reconstructions of past vegetation and hydrological changes are crucial for a better understanding of climate variability in the tropical southeastern African region. We present high-resolution pollen records from a marine sediment core recovered offshore of the Rufiji River delta. Our data document significant shifts in pollen assemblages during the last deglaciation, identifying, through changes in both upland and lowland vegetation, specific responses of plant communities to atmospheric (precipitation) and coastal (coastal dynamics and sea-level changes) alterations. Specifically, arid conditions reflected by a maximum pollen representation of dry and open vegetation occurred during the Northern Hemisphere cold Heinrich event 1 (H1), suggesting that the expansion of drier upland vegetation was synchronous with cold Northern Hemisphere conditions. This arid period is followed by an interval in which forest and humid woodlands expanded, indicating a hydrologic shift towards more humid conditions. Droughts during H1 and the shift to humid conditions around 14.8 kyr BP in the uplands are consistent with latitudinal shifts of the intertropical convergence zone (ITCZ) driven by high-latitude Northern Hemisphere climatic fluctuations. Additionally, our results show that the lowland vegetation, consisting of well-developed salt marshes and mangroves in a successional pattern typical for vegetation occurring in intertidal habitats, has responded mainly to local coastal dynamics related to marine inundation frequencies and soil salinity in the Rufiji Delta as well as to the local moisture availability. Lowland vegetation shows a substantial expansion of mangrove trees after ~ 14.8 kyr BP, suggesting an increased moisture availability and river runoff in the coastal area. The results of this study highlight the decoupled climatic and environmental processes to which the vegetation in the uplands and the Rufiji Delta has responded during the last deglaciation.

Equatorward phytoplankton migration during a cold spell within the Late Cretaceous super-greenhouse

Abstract. Oceanic Anoxic Event 2 (OAE2), a ∼ 600 kyr episode close to the Cenomanian–Turonian boundary (ca. 94 Ma), is characterized by relatively widespread marine anoxia and ranks amongst the warmest intervals of the Phanerozoic. The early stages of OAE2 are, however, marked by an episode of widespread transient cooling and bottom water oxygenation: the Plenus Cold Event. This cold spell has been linked to a decline in atmospheric pCO2, resulting from enhanced global organic carbon burial. To investigate the response of phytoplankton to this marked and rapid climate shift we examined the biogeographical response of dinoflagellates to the Plenus Cold Event. Our study is based on a newly generated geochemical and palynological data set from a high-latitude Northern Hemisphere site, Pratts Landing (western Alberta, Canada). We combine these data with a semi-quantitative global compilation of the stratigraphic distribution of dinoflagellate cyst taxa. The data show that dinoflagellate cysts grouped in the Cyclonephelium compactum–membraniphorum morphological plexus migrated from high to mid-latitudes during the Plenus Cold Event, making it the sole widely found (micro)fossil to mark this cold spell. In addition to earlier reports from regional metazoan migrations during the Plenus Cold Event, our findings illustrate the effect of rapid climate change on the global biogeographical dispersion of phytoplankton.

Temporal and Spatial Variability of Precipitation from Observations and Models*

Abstract Principal component analysis (PCA) is utilized to explore the temporal and spatial variability of precipitation from GPCP and a CAM5 simulation from 1979 to 2010. In the tropical region, the interannual variability of tropical precipitation is characterized by two dominant modes (El Niño and El Niño Modoki). The first and second modes of tropical GPCP precipitation capture 31.9% and 15.6% of the total variance, respectively. The first mode has positive precipitation anomalies over the western Pacific and negative precipitation anomalies over the central and eastern Pacific. The second mode has positive precipitation anomalies over the central Pacific and negative precipitation anomalies over the western and eastern Pacific. Similar variations are seen in the first two modes of tropical precipitation from a CAM5 simulation, although the magnitudes are slightly weaker than in the observations. Over the Northern Hemisphere (NH) high latitudes, the first mode, capturing 8.3% of the total variance of NH GPCP precipitation, is related to the northern annular mode (NAM). During the positive phase of NAM, there are negative precipitation anomalies over the Arctic and positive precipitation anomalies over the midlatitudes. Over the Southern Hemisphere (SH) high latitudes, the first mode, capturing 13.2% of the total variance of SH GPCP precipitation, is related to the southern annular mode (SAM). During the positive phase of the SAM, there are negative precipitation anomalies over the Antarctic and positive precipitation anomalies over the midlatitudes. The CAM5 precipitation simulation demonstrates similar results to those of the observations. However, they do not capture both the high precipitation anomalies over the northern Pacific Ocean or the position of the positive precipitation anomalies in the SH.

Sensitivity of Pliocene Arctic climate to orbital forcing, atmospheric CO2 and sea ice albedo parameterisation

General circulation model (GCM) simulations of the mid-Pliocene Warm Period (mPWP, 3.264 to 3.025 Myr ago) do not reproduce the magnitude of Northern Hemisphere high latitude surface air and sea surface temperature (SAT and SST) warming that proxy data indicate. There is also large uncertainty regarding the state of sea ice cover in the mPWP. Evidence for both perennial and seasonal mPWP Arctic sea ice is found through analyses of marine sediments, whilst in a multi-model ensemble of mPWP climate simulations, half of the ensemble simulated ice-free summer Arctic conditions. Given the strong influence that sea ice exerts on high latitude temperatures, an understanding of the nature of mPWP Arctic sea ice would be highly beneficial. Using the HadCM3 GCM, this paper explores the impact of various combinations of potential mPWP orbital forcing, atmospheric CO 2 concentrations and minimum sea ice albedo on sea ice extent and high latitude warming. The focus is on the Northern Hemisphere, due to availability of proxy data, and the large data–model discrepancies in this region. Changes in orbital forcings are demonstrated to be sufficient to alter the Arctic sea ice simulated by HadCM3 from perennial to seasonal. However, this occurs only when atmospheric CO 2 concentrations exceed 300 ppm. Reduction of the minimum sea ice albedo from 0.5 to 0.2 is also sufficient to simulate seasonal sea ice, with any of the combinations of atmospheric CO 2 and orbital forcing. Compared to a mPWP control simulation, monthly mean increases north of 60°N of up to 4.2 °C (SST) and 9.8 °C (SAT) are simulated. With varying CO 2 , orbit and sea ice albedo values we are able to reproduce proxy temperature records that lean towards modest levels of high latitude warming, but other proxy data showing greater warming remain beyond the reach of our model. This highlights the importance of additional proxy records at high latitudes and ongoing efforts to compare proxy signals between sites. • We run 30 simulations of the mid-Pliocene, altering orbit, CO 2 and sea ice albedo. • Changes in just one of these crucial to perennial or seasonal sea ice. • Warming from sea ice reduction does not resolve mid-Pliocene data–model discrepancy. • Greater spatial coverage and increased understanding of proxy signals is crucial.

Quantifying the impacts of land surface schemes and dynamic vegetation on the model dependency of projected changes in surface energy and water budgets

AbstractAssessing and quantifying the uncertainties in projected future changes of energy and water budgets over land surface are important steps toward improving our confidence in climate change projections. In this study, the contribution of land surface models to the inter‐GCM variation of projected future changes in land surface energy and water fluxes are assessed based on output from 19 global climate models (GCMs) and offline Community Land Model version 4 (CLM4) simulations driven by meteorological forcing from the 19 GCMs. Similar offline simulations using CLM4 with its dynamic vegetation submodel are also conducted to investigate how dynamic vegetation feedback, a process that is being added to more earth system models, may amplify or moderate the intermodel variations of projected future changes. Projected changes are quantified as the difference between the 2081–2100 period from the Representative Concentration Pathway 8.5 (RCP8.5) future experiment and the 1981–2000 period from the historical simulation. Under RCP8.5, projected changes in surface water and heat fluxes show a high degree of model dependency across the globe. Although precipitation is very likely to increase in the high latitudes of the Northern Hemisphere, a high degree of model‐related uncertainty exists for evapotranspiration, soil water content, and surface runoff, suggesting discrepancy among land surface models (LSMs) in simulating the surface hydrological processes and snow‐related processes. Large model‐related uncertainties for the surface water budget also exist in the Tropics including southeastern South America and Central Africa. These uncertainties would be reduced in the hypothetical scenario of a single near‐perfect land surface model being used across all GCMs, suggesting the potential to reduce uncertainties through the use of more consistent approaches toward land surface model development. Under such a scenario, the most significant reduction is likely to be seen in the Northern Hemisphere high latitudes. Including representation of vegetation dynamics is expected to further amplify the model‐related uncertainties in projected future changes in surface water and heat fluxes as well as soil moisture content. This is especially the case in the high latitudes of the Northern Hemisphere (e.g., northwestern North America and central North Asia) where the projected vegetation changes are uncertain and in the Tropics (e.g., the Amazon and Congo Basins) where dense vegetation exists. Findings from this study highlight the importance of improving land surface model parameterizations related to soil and snow processes, as well as the importance of improving the accuracy of dynamic vegetation models.

Evidence of high-elevation amplification versus Arctic amplification.

Elevation-dependent warming in high-elevation regions and Arctic amplification are of tremendous interest to many scientists who are engaged in studies in climate change. Here, using annual mean temperatures from 2781 global stations for the 1961–2010 period, we find that the warming for the world’s high-elevation stations (>500 m above sea level) is clearly stronger than their low-elevation counterparts; and the high-elevation amplification consists of not only an altitudinal amplification but also a latitudinal amplification. The warming for the high-elevation stations is linearly proportional to the temperature lapse rates along altitudinal and latitudinal gradients, as a result of the functional shape of Stefan-Boltzmann law in both vertical and latitudinal directions. In contrast, neither altitudinal amplification nor latitudinal amplification is found within the Arctic region despite its greater warming than lower latitudes. Further analysis shows that the Arctic amplification is an integrated part of the latitudinal amplification trend for the low-elevation stations (≤500 m above sea level) across the entire low- to high-latitude Northern Hemisphere, also a result of the mathematical shape of Stefan-Boltzmann law but only in latitudinal direction.

Simulating the mid-Pleistocene transition through regolith removal

Quaternary δ18O ice-volume proxy records show a transition from high frequency, small-amplitude glacial cycles to low frequency, large-amplitude glacial cycles. This reorganization of the climate system, termed the mid-Pleistocene transition (MPT), is thought to reflect a change in land-ice response to orbital forcing, despite no significant change in orbital cycles during this period. One potential explanation for the MPT proposes that gradual erosion of high-latitude northern hemisphere regolith by multiple cycles of glaciation caused a transition in ice sheet response to external forcing. Here, we explore this “regolith hypothesis” using a complex Earth system model. We show that simulating a transition from deformable sediment to crystalline bedrock produces an evolution in ice-volume response similar to proxy reconstructions of the MPT. The simulated change in ice-volume response is due to a combination of climate and ice-flow changes, with crystalline bedrock producing thicker, colder ice sheets that accumulate more snowfall and have a smaller ablation zone. Further, experiments that include transient eccentricity-amplifying CO2 forcing show only small differences in ice response compared to those with orbital forcing only, suggesting that cycles of CO2 were not the primary cause of the MPT.

Initiation of the last ice age in Canada by extreme precipitation resulting from a cascade of oceanic salinity increases

Numerical modeling has failed to confirm the classical Milankovitch hypothesis of initiation of the last ice age by Northern Hemisphere high latitude cooling due to decreasing summer insolation caused by orbital effects. The modeling failed to confirm ice sheet growth even with a widespread layer of glacial ice as an initial condition to embody positive feedback. The failure probably occurred because the initial conditions of the calculation did not include the actual effects of an altered climate in northeastern Canada that brought a sharp cooling to Europe and extreme amounts of precipitation to cloud-covered lands west of Greenland. In the conceptual model proposed here, diminishing orbital summer insolation in northern Africa is causally linked to this regional climate change by a cascade of oceanic salinity changes. The summer cooling in northern Africa weakened the monsoons, reduced the Nile River flow, and increased Mediterranean salinity and outflow at Gibraltar. The salt in the outflow contributes substantially to the salinities of the North Atlantic Drift and the Greenland Sea, to the formation rate of North Atlantic Deep Water (NADW) there, and to the northward flow of the Spitsbergen-Atlantic Current (SAC) that replaces the sinking NADW. When the increasing salt in the Mediterranean outflow made the SAC replacement flow sufficiently strong, the flow penetrated into the polar ocean along the north coast of Greenland. Denser and more saline Atlantic water then replaced the polar water flowing southward into Baffin Bay through the Nares Strait and Lancaster Sound, thus eliminating the stratification in the bay that enables freezing of winter sea ice. Without the southward flow of sea ice out of Baffin Bay, the Labrador Sea became much warmer. The warmer seas west of Greenland then triggered a persistent cyclonic circulation that caused large amounts of precipitation in eastern Canada and a much colder northern Europe. The resulting Canadian erosion yielded a 500yr-long deep-sea sediment record of the ice-free condition. Heavy snowfall started new ice sheet growth on Baffin Island, northern Quebec, Labrador, western Greenland, and the Barents Sea, causing world sea level to fall at a rate of 0.5cmyr-1. The modern increasing salinity of the Mediterranean Sea and extension of SAC flow into the polar ocean are now following the cascade steps toward an ice-free Baffin Bay and possible near term regional climate deterioration.

Uncertainties of satellite-derived surface skin temperatures in the polar oceans: MODIS, AIRS/AMSU, and AIRS only

Abstract. Uncertainties in the satellite-derived surface skin temperature (SST) data in the polar oceans during two periods (16–24 April and 15–23 September) 2003–2014 were investigated and the three data sets were intercompared as follows: MODerate Resolution Imaging Spectroradiometer Ice Surface Temperature (MODIS IST), the SST of the Atmospheric Infrared Sounder/Advanced Microwave Sounding Unit-A (AIRS/AMSU), and AIRS only. The AIRS only algorithm was developed in preparation for the degradation of the AMSU-A. MODIS IST was systematically warmer up to 1.65 K at the sea ice boundary and colder down to −2.04 K in the polar sea ice regions of both the Arctic and Antarctic than that of the AIRS/AMSU. This difference in the results could have been caused by the surface classification method. The spatial correlation coefficient of the AIRS only to the AIRS/AMSU (0.992–0.999) method was greater than that of the MODIS IST to the AIRS/AMSU (0.968–0.994). The SST of the AIRS only compared to that of the AIRS/AMSU had a bias of 0.168 K with a RMSE of 0.590 K over the Northern Hemisphere high latitudes and a bias of −0.109 K with a RMSE of 0.852 K over the Southern Hemisphere high latitudes. There was a systematic disagreement between the AIRS retrievals at the boundary of the sea ice, because the AIRS only algorithm utilized a less accurate GCM forecast over the seasonally varying frozen oceans than the microwave data. The three data sets (MODIS, AIRS/AMSU and AIRS only) showed significant warming rates (2.3 ± 1.7 ~ 2.8 ± 1.9 K decade−1) in the northern high regions (70–80° N) as expected from the ice-albedo feedback. The systematic temperature disagreement associated with surface type classification had an impact on the resulting temperature trends.

Multiscale monsoon variability during the last two climatic cycles revealed by spectral signals in Chinese loess and speleothem records

Abstract. The East Asian Monsoon (EAM) exhibits a significant variability on timescales ranging from tectonic to centennial as inferred from loess, speleothem and marine records. However, the relative contributions and plausible driving forces of the monsoon variability at different timescales remain controversial. Here, we spectrally explore time series of loess grain size and speleothem δ18O records and decompose the two proxies into intrinsic components using the empirical mode decomposition method. Spectral results of these two proxies display clear glacial and orbital periodicities corresponding to ice volume and solar cycles, and evident millennial signals which are in pace with Heinrich rhythm and Dansgaard–Oeschger (DO) cycles. Five intrinsic components are parsed out from loess grain size and six intrinsic components from speleothem δ18O records. Combined signals are correlated further with possible driving factors including the ice volume, insolation and North Atlantic cooling from a linear point of view. The relative contributions of components differ significantly between loess grain size and speleothem δ18O records. Coexistence of glacial and orbital components in the loess grain size implies that both ice volume and insolation have distinctive impacts on the winter monsoon variability, in contrast to the predominant precessional impact on the speleothem δ18O variability. Moreover, the millennial components are evident in loess grain size and speleothem δ18O records with variances of 13 and 17 %, respectively. A comparison of the millennial-scale signals of these two proxies reveals that abrupt changes in the winter and summer monsoons over the last 260 kyr share common features and similar driving forces linked to high-latitude Northern Hemisphere climate.

An Intercomparison of the Spatiotemporal Variability of Satellite- and Ground-Based Cloud Datasets Using Spectral Analysis Techniques

Abstract Because of the importance of clouds in modulating Earth’s energy budget, it is critical to understand their variability in space and time for climate and modeling studies. This study examines the consistency of the spatiotemporal variability of cloud amount (CA) and cloud-top pressure (CTP) represented by five 7-yr satellite datasets from the Global Energy and Water Cycle Experiment (GEWEX) cloud assessment project, and total cloud fraction observation from the Extended Edited Cloud Reports Archive (EECRA). Two spectral analysis techniques, namely combined maximum covariance analysis (CMCA) and combined principal component analysis (CPCA), are used to extract the dominant modes of variability from the combined datasets, and the resulting spatial patterns are compared in parallel. The results indicate that the datasets achieve overall excellent agreement on both seasonal and interannual scales of variability, with the correlations between the spatial patterns mostly above 0.6 and often above 0.8. For seasonal variability, the largest differences are found in the Northern Hemisphere high latitudes and near the South African coast for CA and in the Sahel region for CTP, where some differences in the phase and strength of the seasonal cycle are found. On interannual scales, global cloud variability is mostly associated with major climate modes, including El Niño–Southern Oscillation (ENSO), the Pacific decadal oscillation (PDO), and the Indian Ocean dipole mode (IODM), and the datasets also agree reasonably well. The good agreement across the datasets supports the conclusion that they are describing cloud variations with these climate modes.

Regional and global significance of Pliocene sea surface temperatures from the Gulf of Cadiz (Site U1387) and the Mediterranean

The Atlantic–Mediterranean water exchange is a component of global ocean circulation capable of influencing deep water formation in the North Atlantic, yet it is poorly constrained for the time period preceding the intensification of Northern Hemisphere Glaciation (NHG). The sea surface temperature (SST) gradient between the Atlantic and Mediterranean sides of the Strait of Gibraltar can shed light on the communication between the two basins. IODP Site U1387 in the Gulf of Cadiz provides the first alkenone based reconstruction of SST for the Atlantic waters that flowed into the Mediterranean Sea during the Pliocene. This site reflects open ocean North Atlantic subtropical temperature trends while the published SST records from the Rossello composite section (Sicily) in the Mediterranean reflect the addition of regional, continentally-influenced signals from Europe and Northern Africa. The Mediterranean, in particular, may be influenced by high latitude Northern hemisphere climatic evolution. In the modern regime the sites discussed in this work have comparable SST and uninhibited surface connection; however, change in local heat loss/gain over the Mediterranean due to variability in latent heat loss and obstructed connection can result in a gradient between the sites in the Pliocene. The Pliocene surface waters of the Gulf of Cadiz and the Mediterranean Sea were as much as 7°C warmer than the modern average of ~19–20°C. The reconstructed temperatures show a ~1°C cooling for the Atlantic side of the Strait of Gibraltar from ~6Ma to ~2.7Ma and increasingly cooler glacials. The long-term SST record from Site U1387 provides a basis for future studies into the hydrological balance of the Mediterranean and the temperature component of Mediterranean Outflow Water (MOW) density. We compared SST on either side of Gibraltar between ~3.4–2.7Ma and found that between ~2.7 and ~3.1Ma the Mediterranean and Atlantic surface waters show comparable average temperatures and comparable variance.

Projected changes in erythemal and vitamin D effective irradiance over northern-hemisphere high latitudes.

Simulations of the monthly mean noon UV index and the effective dose for the production of vitamin D in the human skin have been performed for local noon for the latitude band 55°N-85°N using a radiative transfer model. The magnitude and spatial distribution of the changes estimated for the two quantities between the past (1955-1965 mean), the present (2010-2020 mean) and the future (2085-2095 mean) are discussed and the main drivers for these changes are identified. The irradiance simulations are based on simulations and projections of total ozone, surface reflectivity and aerosol optical depth derived from models used in the fifth phase of the Coupled Model Intercomparison Project (CMIP-5). The cloud modification factor is also derived from the CMIP-5 models and used to account for the effects of cloudiness. Simulations have been derived for two socioeconomic scenarios: the moderate RCP 4.5 and the extreme RCP 8.5. For the future, the two quantities are projected to be generally lower than in the past and the present, mainly due to the projected super-recovery of stratospheric ozone and reduced surface reflectivity. Although the greatest changes are projected over the Arctic Ocean and do not directly affect humans, the changes over land are also important. Over land, the greatest changes are found in northern Asia, Greenland and the north-east shores of Canada and Alaska. The greatest reductions over land are projected for April under all skies, locally reaching ∼30% for the noon UV index and ∼50% for the noon effective UV dose for the production of vitamin D.

Testing the geocentric axial dipole hypothesis using regional paleomagnetic intensity records from 0 to 300 ka

Absolute and relative geomagnetic paleointensity records reveal variations in geomagnetic dipole strength, either via averaging time series of virtual axial dipole moments, or through formal inversion strategies like the penalized maximum likelihood (PML) method used for the PADM2M (Paleomagnetic Axial Dipole Moment for 0–2 Ma) model. However, departures from the most basic geocentric axial dipole (GAD) structure are obvious on centennial to millennial time scales, and paleomagnetic records from igneous rocks suggest small deviations persist on million year time scales. Spatial variations in heat flow at the core–mantle boundary (inferred from large low shear velocity provinces, LLSVPs) are widely suspected to influence both the average geomagnetic field and its regional secular variation. Long term departures from a GAD configuration should be visible from regional differences in paleointensity reconstructions. We use a PML method to construct time-varying models of regional axial dipole moment (RADMs) from a combined set of absolute and relative paleointensity data, and compare results from the last 300 kyr. RADMs are created from sediment records selected from specific latitude and longitude bands. We also test whether grouping records lying above each of the 2 major LLSVPs (centered on Africa and the Pacific) produce RADMs that are distinct from those above regions lacking anomalous seismic structure. Systematic differences appear in the various regional results. In the most recent part of the record regional differences are broadly similar to the Holocene, CALS10k.1b, time-varying geomagnetic field model spanning 0–10 ka. However, lack of Southern hemisphere records prevents direct confirmation of the hemispheric asymmetry present in CALS10k.1b in both average virtual axial dipole moment and its variability. As expected, the 300 kyr RADMs exhibit greater overall temporal field variability than is seen over 0–10 ka. Average RADM is higher in the Pacific and in Equatorial regions than in the Atlantic and in mid–high latitude northern hemisphere regions. Higher average RADMs are associated with lower overall field variability and less pronounced excursional signatures. Notably, the lower variability in the Pacific sector seen here (defined by either longitude band or LLSVP location) suggests that the modern low paleosecular variation there extends over at least the past few hundred thousand years. RADMs identified with LLSVPs show systematic deviations from the non-LLSVP group of records, with distinct characteristics for the African and Pacific provinces. The African LLSVP generates more pronounced RADM minima associated with geomagnetic excursions, and in general paleointensity decreases associated with excursions occur first in the Atlantic longitude sector and over the African LLSVP.

Northern Hemisphere control of deglacial vegetation changes in the Rufiji uplands (Tanzania)

Abstract. In tropical eastern Africa, vegetation distribution is largely controlled by regional hydrology, which has varied over the past 20 000 years. Therefore, accurate reconstructions of past vegetation and hydrological changes are crucial for a better understanding of climate variability in the tropical southeastern African region. We present high-resolution pollen records from a marine sediment core recovered offshore of the Rufiji River delta. Our data document significant shifts in pollen assemblages during the last deglaciation, identifying, through changes in both upland and lowland vegetation, specific responses of plant communities to atmospheric (precipitation) and coastal (coastal dynamics and sea-level changes) alterations. Specifically, arid conditions reflected by a maximum pollen representation of dry and open vegetation occurred during the Northern Hemisphere cold Heinrich event 1 (H1), suggesting that the expansion of drier upland vegetation was synchronous with cold Northern Hemisphere conditions. This arid period is followed by an interval in which forest and humid woodlands expanded, indicating a hydrologic shift towards more humid conditions. Droughts during H1 and the shift to humid conditions around 14.8 kyr BP in the uplands are consistent with latitudinal shifts of the intertropical convergence zone (ITCZ) driven by high-latitude Northern Hemisphere climatic fluctuations. Additionally, our results show that the lowland vegetation, consisting of well-developed salt marshes and mangroves in a successional pattern typical for vegetation occurring in intertidal habitats, has responded mainly to local coastal dynamics related to marine inundation frequencies and soil salinity in the Rufiji Delta as well as to the local moisture availability. Lowland vegetation shows a substantial expansion of mangrove trees after ~ 14.8 kyr BP, suggesting an increased moisture availability and river runoff in the coastal area. The results of this study highlight the decoupled climatic and environmental processes to which the vegetation in the uplands and the Rufiji Delta has responded during the last deglaciation.

Influence of methane sources in Northern Hemisphere high latitudes on the interhemispheric asymmetry of its atmospheric concentration and climate

Numerical experiments with the INMCM4 climate model have shown that the observed 10% difference in the lower tropospheric methane concentration between the Arctic and the Southern Hemisphere can be reproduced in a climate model in which only the present-day spatial distribution of anthropogenic methane emissions and calculated emissions from wet ecosystems are considered. In model runs with an additional 50- or 100-Mt/year methane emission in the Arctic, the difference in atmospheric methane concentrations between the Arctic and the Southern Hemisphere is higher than the observed one. An additional methane emission of 4000 Mt/year in the Arctic is shown to result in an increase of 1.5° in global mean surface air temperature. The spatial distribution of warming is similar to that induced by an increase in the carbon dioxide concentration, depends on the global mean methane concentration, and does not depend at all on where the additional methane source lies.

An active atmospheric methane sink in high Arctic mineral cryosols

Methane (CH4) emission by carbon-rich cryosols at the high latitudes in Northern Hemisphere has been studied extensively. In contrast, data on the CH4 emission potential of carbon-poor cryosols is limited, despite their spatial predominance. This work employs CH4 flux measurements in the field and under laboratory conditions to show that the mineral cryosols at Axel Heiberg Island in the Canadian high Arctic consistently consume atmospheric CH4. Omics analyses present the first molecular evidence of active atmospheric CH4-oxidizing bacteria (atmMOB) in permafrost-affected cryosols, with the prevalent atmMOB genotype in our acidic mineral cryosols being closely related to Upland Soil Cluster α. The atmospheric (atm) CH4 uptake at the study site increases with ground temperature between 0 °C and 18 °C. Consequently, the atm CH4 sink strength is predicted to increase by a factor of 5-30 as the Arctic warms by 5-15 °C over a century. We demonstrate that acidic mineral cryosols are a previously unrecognized potential of CH4 sink that requires further investigation to determine its potential impact on larger scales. This study also calls attention to the poleward distribution of atmMOB, as well as to the potential influence of microbial atm CH4 oxidation, in the context of regional CH4 flux models and global warming.

Contrasting sea surface temperature of summer and winter monsoon variability in the northern Arabian Sea over the last 25 ka

Abstract The seasonal monsoon cycle with winds from the southwest (SW) in summer and from the northeast (NE) in winter strongly impacts on modern regional sea surface temperature (SST) patterns in the Arabian Sea (northern Indian Ocean). To reconstruct the temporal and spatial variation in the dynamically coupled winter and summer monsoon strength over the last 25 ka, we analyzed alkenone-derived SST variations in one sediment core from the northwestern Arabian Sea, that is influenced by the summer monsoon (SST affected by upwelling processes), and in one core from the northeastern Arabian Sea, where SST is mainly governed by the winter monsoon (no upwelling). Comparison of the SST records reveals an antagonistic relationship of summer and winter monsoon strength throughout the late deglaciation and the Holocene. Upwelling along the Arabian Peninsula associated with peak SW monsoonal wind strength was strongest during the early Holocene climate optimum between 11 and 8 ka, and coincided with the northernmost position of the Intertropical Convergence Zone (ITCZ) marked by maximum precipitation over northern Oman. The SW monsoon weakened over the middle to late Holocene, while the NE monsoon gained strength. This different evolution was caused by the southward displacement of the ITCZ throughout the Holocene. Superimposed over the long-term trend are variations in northeast monsoon wind strength at time scales of centuries that were synchronous with late Holocene climate variations recorded on the Asian continent and in the high-latitude Northern Hemisphere. Their likely driving forces are insolation changes associated with sunspot cycles. Enhanced by feedback mechanisms (e.g. land-sea thermal contrast) they enforced centennial scale fluctuations in wind strength and temperature in the northern Arabian Sea monsoon system.