Current Warm Period Research Articles

Vegetation dynamics and its response to climate change during the past 2000 years along the Amur River Basin, Northeast China

Abstract The Amur River Basin is located in high latitudes, which is sensitive to the global climate change. However, only a few previous studies have tried to study the climate change in this region. To better understand the historical variation of vegetation and climate during the past 2000 years, we present a new palynological data from the Lesser Khingan Mountains, and compared the vegetation change along the Amur River Basin. It was possible to reconstruct climate based on vegetation composition before human disturbance. The results show that: the climate was moderately cold and humid during the 2000 to 700 cal yr BP, and the vegetation was mainly coniferous and broad-leaved mixed forest, and the abundance of Cyperaceae was relatively high. After that, the temperature and humidity declined, corresponding to the Little Ice Age. And the vegetation types also obviously changed that the conifers expanded, and the contents of broad-leaved trees and herbaceous Cyperaceae decreased. Since about 150 cal yr BP, the climate tended to be warm which belonged to Current Warm Period, and the main vegetation type was the secondary forests. The spread of the Han farming culture accompanied by the territorial expansion of the Tang Dynasty to the Sanjiang Plain at around 1300 cal yr BP caused human disturbance occurred earlier in the Sanjiang Plain than the Greater and Lesser Khingan Mountains. Additionally, the anthropogenic activities obviously intensified during the past hundred years along the Amur River Basin.

Influences of West Pacific Sea Surface Temperature on Covarying Eurasian Droughts Since the Little Ice Age

The Western Pacific Warm Pool (WP), with the highest sea surface temperature (SST) in the world, has strong impacts on the drought variations in Eurasia. However, since the little ice age (1250–1850, LIA), the co-climatic drought pattern due to WP warming in Eurasia remains unclear. This is a long-term warming background for the current warming period (CWP). In this paper, we use both instrumental data and 1625 tree-ring width records from Eurasia to investigate the drought patterns in both modern and historical periods. This study revealed two seesaw precipitation patterns, namely the Central Asia–Mongolia (CAMO) and Northern Europe–Southern Europe (NESE) patterns. When the Western Pacific Warm Pool sea surface temperature (WPSST) is high, precipitation increases in Central Asia and Northern Europe, and decreases in Mongolia and southern Europe. When the positive (negative) phase event of the El Niño–Southern Oscillation (ENSO) occurs, the WPSST is reduced (increased), and the decreases (increases) of precipitation in Central Asia and Northern Europe and the increases (decreases) in precipitation in Mongolia and southern Europe are more obvious. The CAMO dipole has been strengthened since the LIA. The CAMO dipole is positively correlated with solar radiation and Northern Hemisphere temperature, and negatively correlated with Pacific decadal oscillations (PDO).

Warming favors subtropical lake cyanobacterial biomass increasing

Subtropical lakes are increasingly subject to cyanobacterial blooms resulting from climate change and anthropogenic activities, but the lack of long-term historical data limits understanding of how climate changes have affected cyanobacterial growth in deep subtropical lakes. Using high-resolution DNA data derived from a sediment core from a deep lake in southwestern China, together with analysis of other sedimentary hydroclimatic proxies, we investigated cyanobacterial biomass and microbial biodiversity in relation to climate changes during the last millennium. Our results show that both cyanobacterial abundance and microbial biodiversity were higher during warmer periods, including the Medieval Warm Period (930–1350 CE) and the Current Warm Period (1900 CE–present), but lower during cold periods, including the Little Ice Age (1400–1850 CE). The significant increases in cyanobacterial abundance and microbial biodiversity during warmer intervals are probably because warm climate not only favors cyanobacterial growth but also concentrates lake water nutrients through water budgets between evaporation and precipitation. Furthermore, because rising temperatures result in greater vertical stratification in deep lakes, cyanobacteria may have exploited these stratified conditions and accumulated in dense surface blooms. We anticipate that under anthropogenic warming conditions, cyanobacterial biomass may continue to increase in subtropical deep lakes.

Hydrological variations in central China over the past millennium and their links to the tropical Pacific and North Atlantic oceans

Abstract. Variations of precipitation, also called the Meiyu rain, in the East Asian summer monsoon (EASM) domain during the last millennium could help enlighten the hydrological response to future global warming. Here we present a precisely dated and highly resolved stalagmite δ18O record from the Yongxing Cave, central China. Our new record, combined with a previously published one from the same cave, indicates that the Meiyu rain has changed dramatically in association with the global temperature change. In particular, our record shows that the Meiyu rain was weakened during the Medieval Climate Anomaly (MCA) but intensified during the Little Ice Age (LIA). During the Current Warm Period (CWP), our record indicates a similar weakening of the Meiyu rain. Furthermore, during the MCA and CWP, our records show that the atmospheric precipitation is similarly wet in northern China and similarly dry in central China, but relatively wet during the CWP in southern China. This spatial discrepancy indicates a complicated localized response of the regional precipitation to the anthropogenic forcing. The weakened (intensified) Meiyu rain during the MCA (LIA) matches well with the warm (cold) phases of Northern Hemisphere surface air temperature. This Meiyu rain pattern also corresponds well to the climatic conditions over the tropical Indo-Pacific warm pool. On the other hand, our record shows a strong association with the North Atlantic climate as well. The reduced (increased) Meiyu rain correlates well with positive (negative) phases of the North Atlantic Oscillation. In addition, our record links well to the strong (weak) Atlantic meridional overturning circulation during the MCA (LIA) period. All abovementioned localized correspondences and remote teleconnections on decadal to centennial timescales indicate that the Meiyu rain was coupled closely with oceanic processes in the tropical Pacific and North Atlantic oceans during the MCA and LIA.

Climate change on the northeastern Tibetan Plateau during the past ~ 600 years inferred from peat pollen records

A 600-year high-resolution pollen record from a peat core (FS-2010) on the Zoigê Plateau was obtained to reconstruct vegetation and climate changes on the northeastern Tibetan Plateau. Surface pollen data reveal that vegetation differences are reflected in different modern pollen assemblages. Fossil pollen data indicate that alpine meadow was the dominant vegetation type on the Zoigê Plateau during the Little Ice Age (LIA) from ~1400 to ~1900 AD. Pollen-based climatic reconstructions indicate climatic fluctuations during this period consistent with records from neighboring areas. Specifically, relatively “warm-wet” conditions persisted from ~1400 to ~1500 AD and ~1800 to ~1900 AD, but generally “cool-dry” conditions from ~1500 to ~1800 AD. The reconstructed temperature variations during the LIA (i.e., ~1400 to ~1900 AD) were attributed to variations in total solar irradiation. This study casts further doubt on the FG/MG (subalpine forest and shrub group/alpine meadow group) ratio as a reliable indicator of temperature reduction during the Current Warm Period (CWP; post ~1900 AD) due to significant anthropogenic impacts including deforestation and swamp drainage. The reconstructed patterns of precipitation in the Zoigê Plateau including Flower Swamp are shown to be contingent on variations in the strength of the Asian Monsoon.

South Pacific Subtropical High from the late Holocene to the end of the 21st century: insights from climate proxies and general circulation models

Abstract. The South Pacific Subtropical High (SPSH) is a predominant feature of the South American climate. The variability of this high-pressure center induces changes in the intensity of coastal alongshore winds and precipitation, among others, over southwestern South America. In recent decades, strengthening and expansion of the SPSH have been observed and attributed to the current global warming. These changes have led to an intensification of the southerly winds along the coast of northern to central Chile and a decrease in precipitation from central to southern Chile. Motivated by improving our understanding about the regional impacts of climate change in this part of the Southern Hemisphere, we analyzed SPSH changes during the two most extreme climate events of the last millennium, the Little Ice Age (LIA) and the Current Warm Period (CWP: 1970–2000), based on paleoclimate records and CMIP5/PMIP3 model simulations. In order to assess the level of agreement of general circulation models, we also compare them with ERA-Interim reanalysis data for the 1979–2009 period as a complementary analysis. Finally, with the aim of evaluating future SPSH behavior, we include 21st century projections under a Representative Concentration Pathway (RCP8.5) scenario in our analyses. Our results indicate that during the relative warm (cold) period, the SPSH expands (contracts). Together with this change, alongshore winds intensify (weaken) south (north) of ∼35∘ S; also, southern westerly winds become stronger (weaker) and shift southward (northward). Model results generally underestimate reanalysis data. These changes are in good agreement with paleoclimate records, which suggest that these variations could be related to tropical climate dynamics but also to extratropical phenomena. However, although models adequately represent most of the South American climate changes, they fail to represent the Intertropical Convergence Zone–Hadley cell system dynamics, emphasizing the importance of improving tropical system dynamics in simulations for a better understanding of its effects on South America. Climate model projections indicate that changes recently observed will continue during the next decades, highlighting the need to establish effective mitigation and adaptation strategies against their environmental and socioeconomic impacts.

Pollen‐recorded bioclimatic variations of the last ~2000 years retrieved from Bayan Nuur in the western Mongolian Plateau

The palaeoclimatic variations during the past ~2000 years in western Mongolia were reconstructed using a high‐resolution pollen sequence obtained from Bayan Nuur. The pollen‐based mean annual precipitation (MAP) from Bayan Nuur and the tree ring‐based temperature anomalies from the nearby Altai Mountains exhibit a ‘warm‐dry’ Roman Warm Period (RWP; ~1900 to ~1500 cal. a BP), a ‘cool‐wet’ Dark Age Cold Period (DACP; ~1500 to ~1150 cal. a BP), a ‘warm‐dry’ Medieval Warm Period (MWP; ~1150 to ~800 cal. a BP) and a ‘cool‐wet’ Little Ice Age (LIA; ~800 to ~100 cal. a BP). Nevertheless, the Current Warm Period (CWP; since ~100 cal. a BP) is an exception that fits a ‘warm‐wet’ mode. A more detailed comparison between the pollen‐based MAP curve and the North Atlantic Oscillation (NAO) index curve suggests that the higher‐precipitation episodes generally corresponded with more negative NAO episodes. The chronological correspondence between the higher‐precipitation episodes and the more negative NAO episodes means that more Atlantic storm tracks originated from the southern part of the North Atlantic Ocean and also from the Mediterranean Sea during negative NAO episodes. It implies that the precipitation variations during the past two millennia have been modulated primarily by NAO variations and negatively phased NAO settings contributed to more winter precipitation in Bayan Nuur basin.

Mangrove responses to droughts since the little ice age in the Colombian Caribbean

Mangroves are ecosystems that are seriously threatened by climate change, particularly sea level rise and an increase of droughts. They are resilient to short droughts but their response to longer ones remains arguable. In this work, we studied the responses of R. mangle dominated forests on a carbonate island (El Rosario Island) to climate anomalies and their drivers at inter-annual, decadal and inter-decadal scales during the last centuries. Palynological analysis of a sediment core retrieved from an R. mangle dominated forest was used to reconstruct vegetation changes during the last 400 years. Time series, Multi-taper spectral, Fourier and correlation analyses of PDO, AMO and ENSO series were performed to identify the matches between these series and mangrove changes during this time. The pollen record showed the influence of coastal dynamics on mangrove vegetation changes on a local scale and the influence of climate on a regional one. Changes in coastal vegetation and mangroves were influenced by beach ridge formations and lagoon closing, which were, in turn, determined by shifts in wave and wind intensity. The simultaneous diminution of both types of vegetation were related to both natural (earthquake and extreme droughts) and anthropogenic disturbances. Our results also showed the replacement of the Rhizophora dominated forest by an Avicennia dominated forest when droughts became longer. The peaks of Rhizophora pollen percentages identified along the pollen record showed a 64 year recurrence cycle that dominated during the Little Ice Age (1610–1862) and a 32 year cycle during the Current Warm Period (1866–1954). While the first cycle was positively correlated with the warm phase of PDO and AMO time series, the second cycle showed a negative significant correlation with PDO, AMO and ENSO time series. During warm periods, mangroves were stressed by droughts, but as these cycles became shorter, mangroves could recover more rapidly. From 1954 to the present, no significant correlations were observed between vegetation changes and climate drivers, which is likely related to the increased anthropogenic disturbance beyond that period of time. Our results show the high resilience of mangroves to cyclic climate changes that involve droughts, winds and sea level rises but low recovery and species turnover with extreme and longer droughts and strong anthropogenic disturbances.

Relative sea-level change and climate change in the Northeastern Adriatic during the last 1.5 ka (Istria, Croatia)

A new high-resolution relative sea-level (RSL) reconstruction is presented for the past 1500 years based on four bio-constructions formed by alga Lithophyllum byssoides (algal rims). Two algal structures have been studied on the southern (Premantura site) and two on the eastern Istrian coast (Uboka and Brseč sites) in the Northeastern Adriatic. The data from the algal rims (47 radiocarbon data points) enabled the distinction of four major phases of RSL change which corresponds to periods of climate change. RSL between AD 400 and 800 during the Dark Ages Cold Period (DACP), was almost stable. After AD 800, during the Medieval Climate Anomaly (MCA) the RSL increased up to ∼0.8 mm/yr. The following Little Ice Age period, (LIA) interval I (AD 1400 till 1600) is again characterised by RSL stability (RSL slowed down) which allows the rims at the southern coast to reach the width of ∼40–80 cm at their uppermost part and up to 20 cm for those along the eastern coast. Between AD ∼1600 and 1750, during the colder LIA II interval, algal rims do not form, as LIA II is assumed to be a period of RSL fall. Algal rims reveal that from the second part of the 19th century the RSL rose by 13–15 cm at the Premantura location and around 10 cm at the Brseč and Uboka areas, providing rates between 1 and 0.7 mm/yr respectively for the Current Warm Period (CWP).The sea-level trends were quantitatively defined using an Errors-In-Variables Integrated Gaussian Process (EIV-IGP) model, with full consideration of the available uncertainty. Following correction for the total land-level change (assumed to be around −0.4 mm/yr), four successive trends in sea-level change were confirmed. Sea-level dropped during the DACP at a mean rate of −0.4 mm/yr and increased to 0.5 mm/y as a consequence of Medieval warmth. Thereafter it was relatively stable during LIA I, fell up to −0.1 mm/yr during LIA II interval and has been slowly rising again during the CWP. Moreover, L. byssoides δ18O records show that these periods of sea-level changes are consistent with changes in temperature and thus with periods of rapid climate change.

Reply to Li & Yang's comments on “Comparing the current and early 20th century warm periods in China”

Reply to Li & Yang's comments on “Comparing the current and early 20th century warm periods in China”

Abrupt changes in Indian summer monsoon strength during the last ~900 years and their linkages to socio-economic conditions in the Indian subcontinent

Abrupt shifts in Indian summer monsoon (ISM) precipitation largely affected regional hydrometeorology, accelerating socio-economic and cultural adaptations of agrarian economies in South Asia in the last millennium. Linking ISM variability to human history in the Indian subcontinent is still limited by the scarcity of high-resolution paleoclimatic data for the period encompassing the last 1000 years or earlier. We present a new high resolution oxygen isotope proxy record from the Wah Shikar cave, northeastern India and identified abrupt shifts in the ISM that had profound impact on human societies during the last ~900 yrs. The ISM was strong during the Medieval Climate Anomaly (MCA) and the Current Warm Period (CWP), and show multiple shifts during the Little Ice Age (LIA). The abrupt shifts in the ISM were more frequent during warm intervals, showing a secular trend with few exceptions during the cold periods. Intervals of prolonged summer monsoon weakening are correlated to the decline of major dynasties in India, possibly due to decrease in agriculture productivity, declining economy and societal crisis. Changes in atmospheric temperature due to volcanic eruptions and sun spot activity have played major role in the onset of the LIA and other prolonged weak intervals of the ISM during the last ~900 yrs.

Rainfall variations in central Indo-Pacific over the past 2,700 y

Tropical rainfall variability is closely linked to meridional shifts of the Intertropical Convergence Zone (ITCZ) and zonal movements of the Walker circulation. The characteristics and mechanisms of tropical rainfall variations on centennial to decadal scales are, however, still unclear. Here, we reconstruct a replicated stalagmite-based 2,700-y-long, continuous record of rainfall for the deeply convective northern central Indo-Pacific (NCIP) region. Our record reveals decreasing rainfall in the NCIP over the past 2,700 y, similar to other records from the northern tropics. Notable centennial- to decadal-scale dry climate episodes occurred in both the NCIP and the southern central Indo-Pacific (SCIP) during the 20th century [Current Warm Period (CWP)] and the Medieval Warm Period (MWP), resembling enhanced El Niño-like conditions. Further, we developed a 2,000-y-long ITCZ shift index record that supports an overall southward ITCZ shift in the central Indo-Pacific and indicates southward mean ITCZ positions during the early MWP and the CWP. As a result, the drying trend since the 20th century in the northern tropics is similar to that observed during the past warm period, suggesting that a possible anthropogenic forcing of rainfall remains indistinguishable from natural variability.

Comments on “Comparing the current and early 20th century warm periods in China” by Soon W., R. Connolly, M. Connolly et al.

Comments on “Comparing the current and early 20th century warm periods in China” by Soon W., R. Connolly, M. Connolly et al.

Global warNing: what we know and what we should know about carabid beetles in high altitude habitats

Models that relate species distributions and climate to predict the future geographical range of species in response to forecast climate change have shown that species living at high altitudes are expected to be particularly affected. Presently, the global trend towards a rapid climate warming represents a major concern for high-altitude carabid beetles, specifically for the populations living in glacialised mountain areas. Most of the high altitude carabid species are endemic and cold-adapted, have low dispersal abilities and present small and/or isolated populations. These threats are triggering an increase of their extinction risk. Some researchers have demonstrated both local-scale extinctions and upward shift to higher altitudes. A key point is whether the losses determined by climate change could be mitigated by species’ survival in micro-refugium areas. Traditionally, the current species distribution in climate-limited ecosystems, like those at high altitude, have been described underlining the role of cold-stage refugia during the Last Glacial Maximum (c. 22000 years BP). On the other hand, no studies addressed the question if the present-day distribution of cold-adapted mountain species is driven by climate conditions occurring during the past and/or current warm periods. More recently, the potential role of some ice-related mountain landforms as warm-stage refugia was documented. It suggests that these landforms could be able to promote the long-term survival of cold-adapted species when the surrounding habitats become climatically unfavorable, thus more effort should be done to investigate the ecology these kind of harsh habitats.

Species-specific effects of climate change on the distribution of suitable baboon habitats – Ecological niche modeling of current and Last Glacial Maximum conditions

Baboons (genus Papio) have been proposed as a possible analogous phylogeographic model for intra-African dispersal of hominins during the Pleistocene. Previous studies of the genus reveal complex evolutionary dynamics including introgressive hybridization and, as for hominins, it has been hypothesized that past climate change has been a major driver in their evolutionary history. However, how historical climate changes affected the distribution and extent of baboon habitats is not clear. We therefore employed three ecological niche modeling algorithms (maximum entropy model: MaxEnt; general additive model: GAM; gradient boosting model: GBM) to map suitable habitat of baboons at both genus and species levels under two extreme late-Quaternary climates: current (warm period) and Last Glacial Maximum (LGM, cold period). The three model algorithms predicted habitat suitabilities for the baboon species with high accuracy, as indicated by AUC values of 0.83–0.85 at genus level and ≥0.90 for species. The results suggest that climate change from LGM to current affected the distribution and extent of suitable habitats for the genus Papio only slightly (>80% of the habitat remained suitable). However, and in contrast to our expectation for ecological generalists, individual species have been differentially affected. While P. ursinus and P. anubis lost some of their suitable habitats (net loss 25.5% and 13.3% respectively), P. kindae and P. papio gained large portions (net gain >62%), and P. cynocephalus and P. hamadryas smaller portions (net gain >20%). Overlap among the specific realized climate niches remained small, suggesting only slight overlap of suitable habitat among species. Results of our study further suggest that shifts of suitable habitats could have led to isolation and reconnection of populations which most likely affected gene flow among them. The impact of historic climate changes on baboon habitats might have been similar for other savanna living species, such as hominins.

Fish debris in sediments from the last 25 kyr in the Humboldt Current reveal the role of productivity and oxygen on small pelagic fishes

Upwelling of cold, nutrient-rich water from the oxygen minimum zone (OMZ) off Peru sustains the world’s highest production of forage fish, mostly composed of anchovy (Engraulis ringens). However, the potential impacts of climate change on upwelling dynamics and thus fish productivity in the near future are uncertain. Here, we reconstruct past changes in fish populations during the last 25,000 years to unravel their response to changes in OMZ intensity and productivity. We quantified and identified fish scales and bones deposited in laminated sediments from Pisco (Peru) with an average sampling resolution of 20.4 years (±7.1). The records span the Last Glacial Maximum to the recent Holocene and thus encompass a variety of combinations of productivity, oxygen, and global temperature. Our results reveal that productivity appears to be the main factor controlling small pelagic fish abundance, while sub-surface oxygenation affects mainly anchovy and likely sardine populations. Lower productivity and higher oxygen concentrations during the glacial resulted in lower total fish productivity, whereas higher productivity and a stronger OMZ in some time intervals during the Holocene resulted in higher fish abundances. A variety of different conditions between these two oceanographic end members indicate preferred environmental conditions for a variety of small pelagic fishes. There is no evidence in our record for an out of phase relationship between anchovy and sardine at the timescales examined in the present study. Anchovy have been the predominant small pelagic fish throughout the record, at least over centennial to millennial timescales. Its abundance reached a maximum during the Current Warm Period, an era characterized by high productivity and intense OMZ conditions. Thus, industrial fisheries developed during a period of exceptional productivity in relation to that of the last 25 kyr. The records reveal that dramatic decreases in pelagic fish abundances have occurred in response to past large-scale climate changes than those observed in the instrumental period, which suggests that future climate change may result in substantial changes in ecosystem structure.

Latitudinal variations in <i>δ</i><sup>30</sup>Si and <i>δ</i><sup>15</sup>N signatures along the Peruvian shelf: quantifying the effects of nutrient utilization versus denitrification over the past 600 years

Abstract. The stable sedimentary nitrogen isotope compositions of bulk organic matter (δ15Nbulk) and the silicon isotope composition of diatoms (δ30SiBSi) both mainly reflect the degree of past nutrient utilization by primary producers. However, in ocean areas where anoxic and suboxic conditions prevail, the δ15Nbulk signal ultimately recorded within the sediments is also influenced by water column denitrification, causing an increase in the subsurface δ15N signature of dissolved nitrate (δ15NO3-) upwelled to the surface. Such conditions are found in the oxygen minimum zone off the coast of Peru, where, at present, an increase in subsurface δ15NO3- from north to south along the shelf is observed due to ongoing denitrification within the poleward-flowing subsurface waters, while the δ30Si signature of silicic acid (δ30Si(OH)4) at the same time remains unchanged. Here, we present three new δ30SiBSi records between 11 and 15∘ S and compare these to previously published δ30SiBSi and δ15Nbulk records from Peru covering the past 600 years. We present a new approach to calculate past subsurface δ15NO3- signatures based on the direct comparison of δ30SiBSi and δ15Nbulk signatures at a latitudinal resolution for different time periods. Our results show that, during the Current Warm Period (CWP, since 1800 CE) and prior short-term arid events, source water δ15NO3- compositions have been close to modern values, increasing southward from 7 to 10 ‰ (between 11 and 15∘ S). In contrast, during the Little Ice Age (LIA) we calculate low δ15NO3- values between 6 ‰ and 7.5 ‰. Furthermore, the direct δ30SiBSi versus δ15Nbulk comparison also enables us to relate the short-term variability in both isotope compositions to changes in the ratio of nutrients (NO3-:Si(OH)4) taken up by different dominating phytoplankton groups (diatoms and non-siliceous phytoplankton) under the variable climatic conditions of the past 600 years. Accordingly, we estimate a shift from a 1:1 (or 1:2) ratio during the CWP and a 2:1 (up to 15:1) ratio during the LIA, associated with a shift from overall high nutrient utilization to NO3--dominated (and thus non-siliceous phytoplankton) utilization.

Episodic Reef Growth in the Northern South China Sea linked to Warm Climate During the Past 7,000 Years: Potential for Future Coral Refugia

AbstractHigh‐precision uranium‐thorium (U‐Th) dating of dead branching corals from Luhuitou reef, Sanya, northern South China Sea indicates that the reef framework grew episodically over the past 7,000 years. Episodes of coral reef growth (“switch‐on” phases) occurred in response to regional warming during the mid‐Holocene Climate Optimum, Medieval Warm Period, and Current Warm Period, when the East Asian summer monsoon was strong and the East Asian winter monsoon was weak. In contrast, episodes when reef growth dramatically slowed or ceased (“switch‐off” phases) occurred during comparatively cold periods (e.g., Dark Age Cold Period and Little Ice Age) and are linked to abrupt weakening of the East Asian summer monsoon and concurrent strengthening of the East Asian winter monsoon. In the context of global warming, the northern South China Sea may become an important refugium for coral reef growth of up to 2 °C above present, but only if local anthropogenic pressures are reduced.

Anthropogenic effects on tropical oceanic climate change and variability: An insight from the South China Sea over the past 2000 years

To better understand the effect of human activities on tropical oceanic climate change, existing coral-based paleoclimate data from the northern South China Sea (SCS) for several representative climate periods over the past 2000 years were synthesized and reanalyzed. The sea surface temperature (SST) during the current warm period (CWP), in which the climate has been influenced by human activities, exhibits larger change amplitude and higher oscillation frequency than during periods in which the climate was controlled by natural forcings. However, the variabilities of the sea water δ18O (δ18Osw) during the four climate periods over the past 2000 years examined here are somewhat complicated to decipher. The variation amplitudes are variable but the interannual oscillations are similar during different periods. Even so, high variability during the CWP may still suggest the anthropogenic effects on tropical oceanic hydroclimate. This may implies that the anthropogenic effects intensify the tropical SST and hydrological variation both in amplitude and frequency. The decoupling between coral δ13C and total solar insolation during the CWP may be a result of the oceanic δ13C Suess effect from the addition of anthropogenic 12CO2 to the surface ocean. This suggests that increased anthropogenic CO2 caused the main carbon source and the utilization process of coral reefs during calcification to change. To accurately assess the influence of human activities on oceanic climate and environmental change, the effects of anthropogenic CO2 should be monitored continuously over a longer time frame.

Peat δ13Ccelluose-signified moisture variations over the past ∼2200 years in the southern Altai Mountains, northwestern China

A ∼2200-year moisture history of the southern Altai Mountains (i.e., Chinese Altai) was established based on peat δ13Ccelluose data obtained at Yushenkule (YSKL) Peat. Generally speaking, the moisture record from YSKL Peat exhibits a “cool-wet” DACP (Dark Age Cold Period from ∼500 to ∼800 CE), a “warm-dry” MWP (Medieval Warm Period from ∼800 to ∼1200 CE), a “cool-wet” LIA (Little Ice Age from ∼1200 to ∼1900 CE), and a “warm-dry” CWP (Current Warm Period since ∼1900 CE). But, RWP (Roman Warm Period from prior to ∼100 CE to ∼500 CE) is an exception (i.e., “warm-wet”). If the heat-water combinations within those major temperature-change events are further examined, the “warm-dry/cool-wet” generalization collapses. The comparisons between the variations in the peat δ13Ccelluose-based moisture obtained from YSKL Peat and the variations of reconstructed climatic variables in the referenced sequences suggest that the moisture variations in the Altai Mountains have been basically controlled by the precipitation that was primarily modulated by the NAO variations.