Long-term, high-resolution temperature records which combine an unambiguous proxy and precise dating are rare in China. In addition, the societal implications of past temperature change on a regional scale have not been sufficiently assessed. Here, based on the modern relationship between chironomids and temperature, we use fossil chironomid assemblages in a precisely dated sediment core from Gonghai Lake to explore temperature variability during the past 4000 years in northern China. Subsequently, we address the possible regional societal implications of temperature change through a statistical analysis of the occurrence of wars. Our results show the following. (1) The mean annual temperature (TANN) was relatively high during 4000–2700 cal yr BP, decreased gradually during 2700–1270 cal yr BP and then fluctuated during the last 1270 years. (2) A cold event in the Period of Disunity, the Sui-Tang Warm Period (STWP), the Medieval Warm Period (MWP) and the Little Ice Age (LIA) can all be recognized in the paleotemperature record, as well as in many other temperature reconstructions in China. This suggests that our chironomid-inferred temperature record for the Gonghai Lake region is representative. (3) Local wars in Shanxi Province, documented in the historical literature during the past 2700 years, are statistically significantly correlated with changes in temperature, and the relationship is a good example of the potential societal implications of temperature change on a regional scale.
Climate change presents new and significant challenges for human society, including the need to understand and respond to the possible dangers (Stocker et al., 2013). Since the past is the key to the present and the future, the study of past temperature changes is becoming increasingly important for improving our ability to predict the long-term trends of regional and global climate change, and to explore the relationship between climate change and human society.
East Asia, a densely populated region, has attracted much research attention focused on documenting the frequency and amplitude of past climate changes. While the Holocene variability of the precipitation associated with the East Asian summer monsoon (EASM) has been discussed in detail (e.g. Hu et al., 2008; Cai et al., 2010; Chen et al., 2015; Liu et al., 2015, 2017; Chen et al., 2016), studies of temperature change on different temporal and spatial scales may provide deeper insights to past climate fluctuations and facilitate the prediction of future climate change. During the past few decades, various studies have reconstructed temperature change on different timescales in northern China, using for example pollen (e.g. Wen et al., 2010; Xu et al., 2010), glycerol dialkyl glycerol tetraethers (GDGTs; e.g. Gao et al., 2012; Jia et al., 2013; Peterse et al., 2014), stalagmites (Tan et al., 2003) and historical archives (Ge et al., 2003). However, many of these temperature records have significant limitations; for example, pollen assemblages are regarded as a precipitation indicator in many records in northern China (e.g. Zhao et al., 2010; Chen et al., 2015), the resolution of GDGTs records is too low (although their environmental significance is relatively unambiguous) and the timescales of the stalagmite records from Shihua Cave and of historical documents from eastern China are too short, even if they are accurately dated. All these factors impede our understanding of paleotemperature variability during the Holocene, and in addition there is a mismatch between model simulations of a cooler-than-baseline annual temperature series during the late Holocene compared to the present climate (Jiang et al., 2012) and multi-proxy reconstructions of the mid-Holocene megathermal in China (e.g. Shi et al., 1993; Wang et al., 2001; Peterse et al., 2011; Huang et al., 2013). Thus, a long-term, high-resolution paleotemperature reconstruction, using an unequivocal proxy with a robust chronology, is needed.
Chironomids, benthic invertebrates, are recognized as a reliable paleotemperature proxy because of their stenotopic and environmentally sensitive characteristics (Walker et al., 1991; Levesque et al., 1997; Brooks et al., 2007, 2012a). Many modern chironomid training sets have been established and used for paleoenvironmental reconstruction (especially paleotemperature) worldwide (e.g. Walker and Cwynar, 2006; Rees et al., 2008; Eggermont et al., 2010; Heiri et al., 2011; Nazarova et al., 2011; Massaferro and Larocque-Tobler, 2013). The paleoenvironmental application of chironomid analysis is relatively recent in China, and studies have concentrated mainly on lake ecology, including analysis of total phosphorus in the middle and lower reaches of the Yangtze River (Zhang et al., 2006), salinity on the Tibetan Plateau (Zhang et al., 2007; Chen et al., 2009), lake water depth in the arid region of northwest China (Chen et al., 2014) and precipitation near the EASM boundary (Wang et al., 2016). Currently, there is only one chironomid-based temperature record, which was obtained from the southeastern Tibetan Plateau (Zhang et al., 2017a, b).
Here, we present the results of a study of chironomid assemblages in a
sediment core from Gonghai Lake in northern China, with the aim of
reconstructing regional temperature variability during the past 4000 years in
northern China. Gonghai Lake, a freshwater closed-basin lake in Shanxi
Province (Fig. 1a), was previously shown to be suitable for chironomid
studies (Wang et al., 2016). A modern calibration data set consisting of 44
fresh water bodies in the area has been developed by Wang et al. (2016).
Although this data set suggested that chironomid assemblages in the region
responded significantly to fluctuations in water depth since the last
deglaciation (Wang et al., 2016), the existence of several typical
stenothermal species (e.g.
Gonghai Lake (38
The age–depth model for Gonghai Lake core GH09B (Chen et al., 2015) was
used in this study. Figure 2 shows the chronology for the last 4000 years.
In the age–depth model for core GH09B, 25 accelerator mass spectrometry (AMS)
Age–depth model for core GH09B (Chen et al., 2015).
For each sample, chironomid remains were extracted from 1–5 g of
freeze-dried sediment. The preparation procedure followed the standard
techniques described in Brooks et al. (2007). The sediments were
deflocculated in warm 10 % KOH for about 15 min, and then sieved with
212 and 90
The modern calibration set from around Gonghai Lake obtained by Wang et al. (2016) was used to identify temperature-sensitive chironomid taxa in the region. The data set comprises 44 water bodies in northern China (Fig. 3a), samples from only 30 of which contained sufficient chironomid head capsules for analysis.
Mean annual temperature (TANN), mean summer (from June to August) temperature (summer temp), and the mean temperatures for June (June temp), July (July temp) and August (August temp) were interpolated from meteorological data from 2001 to 2011 (Dataset of monthly values of climate data from Chinese surface station, 2017; Zhao et al., unpublished data). It should be noted that the surface of Gonghai Lake freezes in winter which disrupts the linear relationship between water temperature and air temperature. Moreover, the winter season is not the growing season of chironomids (Armitage et al., 1995), and therefore the mean temperature of the winter months was not included in the numerical analysis.
A large amount of detailed documentary evidence is available for China. This
material documents a wide range of human activities and it provides a
valuable reference for the present study. Information pertaining to wars was
obtained from the
Only taxa which were present in at least two samples with an abundance of > 2 % were selected for analysis. A chironomid percentage diagram was plotted using Tilia 2.0.2 (Grimm, 2004). Zonation of the chironomid assemblages was accomplished using stratigraphically constrained cluster analysis (CONISS) in Tilia 2.0.2 (Grimm, 2004). Both redundancy analysis (RDA) and detrended correspondence analysis (DCA) were performed using R 3.2.1 (R Core Team, 2014) to explore the relationship between modern chironomid taxa and temperature variables, and to analyse the distribution characteristics of fossil assemblages, respectively. In addition, Pearson correlation and Granger causality analysis were performed to explore the relationship between climate change and the occurrence of wars.
Air temperature is widely assumed to play a key role in controlling the
abundance and composition of chironomid taxa in freshwater bodies (e.g. Walker,
2001; Brooks, 2003; Walker and Cwynar, 2006). RDA of the chironomid taxa and
temperature variables shows that TANN tends to be more significant in
influencing the chironomid assemblages than summer temp, June temp, July temp and August temp (Fig. 3b). This result also passed the Monte Carlo
permutation test (
The following criteria were used to identify temperature-sensitive species:
(1) those located at the ends of Fig. 3c and (2) those species previously
reported as warm or cold stenotherms. On the left side of the diagram,
Information about the modern calibration data set obtained from the
Gonghai Lake area.
We identified 44 major taxa within 25 genera and 4 subfamilies (Tanypodinae, Chironomini, Tanytarsini and Orthocladiinae), and three chironomid assemblage zones were recognized (Fig. 4). Of the chironomid head capsules, 95.7 % were identified to genus or species morphotype. Due to poor preservation, the remaining 4.3 % were only identified to subfamily level; this was especially applicable to the head capsules of the tribe Tanypodinae because the key identification segments of fragmented subfossils were often covered by other material. The concentration of chironomid head capsules appeared to follow variations in the organic matter content of the samples. The concentration was high before 1500 cal yr BP and then decreased to very low values until the present (Fig. 4). The chironomid assemblage zones are described below.
Relative abundance of the main chironomid taxa from Gonghai Lake during the past 4000 years. Taxa are plotted from left to right in order of their DCA 1 scores. Loss-on-ignition (LOI) values, chironomid concentration and percentages of warm- and cold-preferring taxa are plotted as black line with squares, black bars and red and blue patterns, respectively. Three chironomid assemblage zones were defined by CONISS results.
Based on the definition of warm- and cold-preferring taxa given above, their totals were calculated to reconstruct temperature changes during the past 4000 years (Fig. 4). The results indicate an overall trend of decreasing temperature; furthermore, fluctuations in the abundance of cold-preference taxa indicate that the temperature was high in zone 1, decreased sharply around 2700 cal yr BP but remained relatively high in zone 2 and fluctuated significantly and reached a minimum in zone 3. It should be noted that the abundance of warm-preferring taxa is much less than that of cold-preferring taxa, and the former were often absent during the past 2700 years (Fig. 4). To avoid the potential limitations of presence/absence data, the changes in abundance of cold-preferring taxa (which provide more detailed information about temperature variations on a centennial timescale) were primarily used to investigate temperature changes.
We calculated a total of 418 wars from 718 BC to AD 1911. Given that the
resolution of the Gonghai Lake samples ranges from 50 to 100 years, the
incidences of wars were summed to produce a 50-year resolution. The record of
chironomid-inferred temperature variability (Fig. 5a) and the pollen-based
precipitation reconstruction for Gonghai Lake (Fig. 5b; Chen et al., 2015)
were compared with the cumulative frequency of these events (Fig. 5c). The
distribution of wars reveals that they occurred more frequently when
temperature and precipitation decreased abruptly, and they also lasted for a
relatively long time (Fig. 5c). For example, these events were the most
severe during the Little Ice Age (LIA), which lasted for nearly 350 years,
when both the temperature and precipitation decreased significantly. The
results of Pearson correlation and Granger causality analysis show that the
change in abundance of the cold-preferring taxa are significantly related to
the incidence of wars (
Only 19 records of population size in Shanxi Province from 340 BC onwards were mentioned in Lu and Teng (2006), and they are used in the present study. These data are evenly distributed within each dynasty (Fig. 5d). Although the population size fluctuated significantly, an overall increasing trend is evident, together with frequent population collapses following intervals with a significant number of wars.
Comparison of
Relevant physical, chemical and climatic variables were all included in the
investigation of the relationships between chironomid assemblages and
environmental parameters in the Gonghai Lake region (Wang et al., 2016).
Although previous analyses indicated that the fossil chironomids mainly
responded to changes in precipitation through water depth since the last
deglaciation (Wang et al., 2016), the existence of certain typical
stenothermic taxa provides high potential for extracting a temperature
signal. To further verify whether the stenothermic taxa (based on the
published literature) also have a thermal significance in the Gonghai Lake
region, the temperature variables (TANN, summer temp, June temp, July temp
and August temp) were used as the only variables to constrain the changes in
the abundance of the taxa in the calibration set. The results of RDA of
modern chironomid assemblages and temperature variables (Fig. 3b), as well as
the Monte Carlo permutation test, demonstrate that TANN was a significant
environmental variable influencing the modern chironomid taxa. In addition,
TANN has a higher score on the first axes than the other variables in
Fig. 3b; furthermore, TANN was the only variable selected in the
interactive-forward-selection (
Results of Pearson correlation analysis of cold-preferring chironomid taxa percentages and reconstructed precipitation to the incidence of war.
Granger causality analysis of cold-preferring chironomid taxa percentages, reconstructed precipitation and incidence of war.
Temperature variability in the Gonghai Lake region during the past 4000 years is revealed by changes in the abundance of the warm-preferring and cold-preferring chironomid taxa (Fig. 4). As described in Sect. 5.3, the variations in the abundance of the cold-preferring taxa were primarily used to investigate the temperature changes. An explanation for the more resolved temperature signal carried by the cold-preferring taxa may be that they were easily able to become dominant in Gonghai Lake and respond quickly to temperature fluctuations due to the lake's relatively high elevation (1860 m a.s.l.) and the decreasing trend of late Holocene temperature.
In addition to the warm- and cold-preferring taxa, there are other chironomid taxa in the Gonghai Lake record which could also be regarded as temperature indicators (although to a significantly lesser extent), and it is worth investigating whether they exhibit a similar trend of temperature change to the warm- and cold-preferring taxa. Details of the faunistics and inferred environmental change for each of the three intervals of the record are given below.
The foregoing analysis indicates that the temperature variability inferred from typical chironomid temperature indicators is in accordance with that inferred from most of the other taxa in the Gonghai Lake sediments, which supports our reconstruction.
In addition, a recent study indicated that the organic matter content of the Gonghai Lake sediments was dominated by the authigenic fraction during the past 4000 years (Chen et al., 2018). This suggests that most of the organic matter is of within-lake origin (Birks and Birks, 2006) and thus that variations in its content probably reflect past regional temperature changes. The variation of the organic content of the Gonghai Lake sediments (Fig. 4; Chen et al., 2018) is consistent with the decreasing trend of chironomid-inferred temperature, validating the reliability of our temperature reconstruction.
As mentioned previously, climate model simulation results indicate that TANN
in China was higher in the late Holocene than in the mid-Holocene (Jiang et
al., 2012). In addition, even the global TANN indicates a warming trend from
the early Holocene onwards, due to the retreating ice sheets and rising
atmospheric greenhouse gas concentrations (Liu et al., 2014), in
contradiction to the cooling trend inferred from various proxy records for
30–90
To validate our chironomid-inferred temperature record (Fig. 6a), all the Holocene temperature reconstructions for China were collected. However, as mentioned in the Introduction, many of the records are problematic in that they have large dating uncertainty, low resolution or are environmentally ambiguous; for these reasons, they were excluded. Only two unambiguous and high-resolution temperature reconstructions were finally chosen for further comparison due to their precise high-quality dating which was the most important selection criterion used in this study. The first record is based on stalagmite layer thickness at Shihua Cave, close to Gonghai Lake (Tan et al., 2003; Fig. 6b); the second is based on historical documents pertaining to winter temperature changes in eastern China (Ge et al., 2003; Fig. 6c). The three (Fig. 6a, b, c) records exhibit a consistent pattern of temperature change on both millennial and shorter scales: cold intervals of 1650–1350, 1150–950 and 650–300 cal yr BP (LIA); and warm intervals of 1350–1150 cal yr BP (STWP) and 950–650 cal yr BP (MWP). In addition, an integrated temperature record for the whole of China, produced by combining multiple paleoclimate proxy records from ice cores, tree rings, lake sediments and historical documents (Fig. 6d, Yang et al., 2002), was compared with the chironomid-inferred temperature record from Gonghai Lake. Both records show the same pattern of warm and cold intervals during the past 2000 years: for example, the cold intervals of 1650–1350 and 1150–950 cal yr BP, and the LIA, STWP, MWP and modern warm periods.
Comparison of
In addition to the consistency of the records described above, the trend of
generally decreasing temperature during the past 4000 years is also evident
in several other recent proxy-based reconstructions: for example, the
The foregoing analysis demonstrates that our chironomid-based temperature reconstruction is reasonable and representative and that the approach can be extended to longer timescales. The success of our approach can be attributed to the following factors: (i) chironomids are sensitive to temperature changes; (ii) the precise, high-resolution chronology increases the usefulness of the temperature reconstruction; and (iii) the 60-year resolution enables the results to be compared with other high-quality temperature reconstructions and with documentary evidence. Furthermore, our results, combined with a pollen-based precipitation reconstruction from the same core, enable the identification of trends in both temperature and humidity (Fig. 5b; Chen et al., 2015). Generally, there were pronounced changes in warm and humid/cool and dry climatic patterns both on millennial and centennial scales in the Gonghai Lake region, consistent with a previous synthesis study (Tan et al., 2011b). However, this pattern is not evident during the last 300 years (Fig. 5). Given the general consistency between our temperature reconstruction and other records in this period (such as the rapid warming at the end of LIA; Fig. 6), more high-quality precipitation records are needed to further validate this warm and dry configuration.
Although past wars in China were often the consequence of social–geopolitical factors, including territorial disputes (Zhao, 2006), nomadic invasions and agricultural expansion (Di Cosmo, 2002), the impact of climate change should also be considered when analysing societal evolution (Ge, 2011). Traditionally, China was an agricultural society, the productivity of which was very low during most of its history. When temperature or precipitation decreased abruptly, or fluctuated significantly, there tended to be an increase in the incidence of natural disasters such as floods and droughts (Zhang et al., 2008), which seriously affected agricultural production. The combination of a large population and a poor grain harvest often resulted in high rice prices and famines, which generated large numbers of homeless refugees and outbreaks of plague. These factors would finally trigger wars and social unrest which acted to reduce the population size. To analyse the societal response in Shanxi Province to climate change, the occurrence of wars (Fig. 5c) and changes in population size (Fig. 5d) were summarized for comparison with the chironomid-inferred temperature record (Fig. 5a) and the pollen-based precipitation reconstruction (Fig. 5b; Chen et al., 2015) from Gonghai Lake.
Although both temperature and precipitation in the Gonghai Lake region
exhibit a decreasing trend during the last 4000 years, temperature changes
were not always in phase with precipitation changes. For example, four cold
events can be recognized from the chironomid-inferred temperature record
(Fig. 5a), which occurred during
The demise of the Ming dynasty provides an example of how climatic
deterioration, as well as the related socioeconomic impacts, severely
undermined an empire in historical China. The late Ming (390–306 cal yr BP)
coincided with the Little Ice Age, when temperatures decreased significantly
(Fig. 5a). During this cold period, the incidence of natural disasters such
as flood and droughts was the highest in Shanxi history (Chen, 1939).
Rapid cooling accompanied by large-scale desertification began in the
1620s
and had a devastating effect on agricultural production (Wang et al.,
2010; Yin et al., 2015). Zheng et al. (2014) noted that the total grain yield in
Shanxi in the 1630s ranged from
Together with a precise high-resolution chronology and a modern calibration
set, we have used chironomid assemblages from the sediments of Gonghai Lake
to reconstruct temperature variations during the past 4000 years in northern
China. Combined with historical documents, the temperature record was used
to explore the relationship between climate change and human societal
changes at the regional scale. The principal conclusions are as follows:
The chironomid-inferred temperature record shows a stepwise
decreasing trend since 4000 cal yr BP. Temperature remained high during
4000–2700 cal yr BP, decreased abruptly around 2700 cal yr BP, decreased
gradually during 2700–1270 cal yr BP and reached a minimum, accompanied by
frequent fluctuations, during the last 1270 years. In addition, the cold
event, corresponding to the Period of Disunity in China, and the STWP, MWP and LIA,
revealed in the chironomid record from Gonghai Lake, were also recorded in
numerous other multi-proxy records, validating the reliability of our
temperature reconstruction. The frequency of wars in Shanxi Province during the last 2700 years is
significantly correlated with the chironomid-inferred temperature record
from Gonghai Lake. Reductions in population size, associated with warfare
and famine, are also correlated with the temperature fluctuations. We
suggest that the impacts of temperature and precipitation on human society
should be further studied in the future.
Data are not publicly available.
The authors declare that they have no conflict of interest.
We thank Qiang Wang and Haichao Xie for help with fieldwork, Guanghui Dong and Harry F. Lee for helpful discussions and Jan Bloemendal for improving the English writing. This research was jointly supported by the National Natural Science Foundation of China (grants 41471162, 41790421, 41721091), the National Key R&D Program of China (grant 2017YFA0603402) and Fundamental Research Funds for the Central Universities (grant lzujbky-2016-271). Edited by: Denis-Didier Rousseau Reviewed by: two anonymous referees