Theor Appl Climatol DOI 10.1007/s00704-016-1932-5
ORIGINAL PAPER
Proxy-based Northern Hemisphere temperature reconstruction for the mid-to-late Holocene Qing Pei 1 & David D. Zhang 2 & Jinbao Li 2 & Harry F. Lee 2
Received: 21 November 2015 / Accepted: 6 September 2016 # Springer-Verlag Wien 2016
Abstract The observed late twentieth century warming must be assessed in relation to natural long-term variations of the climatic system. Here, we present a Northern Hemisphere (NH) temperature reconstruction for the mid-to-late Holocene of the past 6000 years, based on a synthesis of existing paleo-temperature proxies that are capable of revealing centennial-scale variability. This includes 56 published temperature records across the NH land areas, with a sampling resolution ranging from 1 to 100 years and a time span of at least 1000 years. The composite plus scale (CPS) method is adopted with spatial weighting to develop the NH temperature reconstruction. Our reconstruction reveals abrupt cold epochs that match well the Bond events during the past 6000 years. The study further reveals two prominent cycles in NH temperature: 1700–2000-year cycle during the mid-to-late Holocene and 1200–1500-year cycle during the past 3500 years. Our reconstruction indicates that the late twentieth century NH temperature and its rate of warming are both unprecedentedly high over the past 5000 years. By comparing our reconstruction with the projected temperature increase scenarios, we find that temperature by the end of the twentyfirst century would likely exceed any peaks during the mid-tolate Holocene.
* Qing Pei
[email protected];
[email protected]
1
Department of Social Sciences, Education University of Hong Kong, Lo Ping Road, Tai Po, Hong Kong
2
Department of Geography and International Centre for China Development Studies, University of Hong Kong, Pokfulam Road, Pok Fu Lam, Hong Kong
1 Introduction Rapid warming in the late twentieth century has been observed at a global scale (Dai 2011; Hansen et al. 2012; IPCC 2013; Mann et al. 2008; Wheeler and Braun 2013). This warming must be assessed in relation to natural long-term variations of the climatic system in order to fully understand its causes and possible consequences (Bradley 2000). The proxy-based temperature reconstruction provides an important means to understand dynamics of the climatic system. Till now, there exist several large-scale temperature reconstructions of different time scales to reveal climate change during the Holocene period (Clegg et al. 2011; Davis et al. 2003; Marcott et al. 2013; Zhu et al. 2008). Yet, whether recent warming is anomalous when compared to temperature of the mid-to-late Holocene (i.e., the past 6000 year) has not been fully established, especially at the hemispheric scale (D’Arrigo et al. 2006). In particular, the work by Marcott et al. (2013) that reconstructed the temperature during the whole Holocene has only a millennial-scale resolution, failing to reveal the existence of the abrupt Bond events that are an prominent feature of Holocene climate (Wanner et al. 2008). Moreover, there is a lack of evidence revealing the cyclicity of climate variations at centennial time scale or the large-scale synchronicity of abrupt events such as the Bond events. To fill these knowledge gaps, we present a North Hemisphere (NH) temperature reconstruction of the past 6000 years by synthesizing existing paleo-proxies that are capable of revealing centennial-scale temperature variability.
2 Materials and methods Patterns of past climate change can be estimated through composite analysis of a network of spatially distributed proxy records (Jones et al. 1998; Mann et al. 2005). So far, there are
Pei Q. et al. Table 1
1
2
3 4 5 6
7 8
List of the 56 proxies considered
Location
Data type
Time span (years)
Resolution
Asia (Siberia) 67° 00′–67° 50′ N, 68° 30′–71° 00′ E Asia (China) 31° 24′–31° 64′ N, 109° 64′–110° 34′ E Asia (China)
Tree ring
4000
Annual to decadal
Pollen
16,000
Decadal to centennial
20041
Zhu et al. 2008
Historical documents Stalagmite
2000
10–30 years
1995
Ge et al. 2003
2650
Annual to decadal
1985
Tan et al. 2003
Asia (China) 115° 56′ E, 39° 47′ N Asia (China) Asia (China) 38° 31′ 12″ N, 93° 45′ 0″ E and 37° 4′ 48″ N, 97° 18′ 36″ E Asia (China) 25–40° N, 85–105° E Asia (China)
Base year AD 1996
Source
Hantemirov and Shiyatov 2002
Multi-proxies
2000
Decadal
1990
Yang et al. 2002
Lake sediment
2000
Annual to decadal
2006
He et al. 2013
Tree ring
1000
Annual
2000
Wang et al. 2015
Multi-proxies
2000
Decadal
2005
Ge et al. 2013
9
Asia (China) 36–37° N, 98–100° E 10 Asia (China)
Tree ring
2485
Annual
2000
Liu et al. 2009
Multi-proxies
1000
Annual
1996
Shi et al. 2012
11 Europe
Pollen
12,000
Centennial
1950
Davis et al. 2003
2
12 Europe (Sweden) 68° 22′ N, 18° 42′ E 13 Europe (Spain) 42° 59′ 20″ N, 4° 26′ 07″ W 14 Europe (Greenland) 72° 36′ N, 38° 30′ W 15 Europe (Greenland) 72° 35′ N, 37° 38′ W and 72° 36′ N, 38° 30′ W 16 Europe (Finland) 67° 20′ N, 37° 0′ 0″ E 17 Europe
Lake sediment
10,000
Centennial
Stalagmite
3950
Decadal
1950
Martín-Chivelet et al. 2011
Ice core
4000
Annual to decadal
1993
Kobashi et al. 2011
Ice core
50,000
Decadal to centennial
1950
Alley 2000
Pollen
2000
Decadal
1996
Bjune et al. 2009
Tree ring
2500
Annual to decadal
2005
Büntgen et al. 2011
18 Europe (Alps) 47° 04′ 49″ N, 11° 40′ 18″ E 19 Europe (Fennoscandia) 68° 28′–68° 31′ N, 27° 16′–27° 24′ E 20 Central Europe
Stalagmite
2000
Decadal
1935
Mangini et al. 2005
Tree ring
1257
Annual to decadal
2007
Lindholm et al. 2010
Historical documents Ice core
996
Annual to decadal
2001
Glaser and Riemann 2009
994
1993
Kobashi et al. 2010
21 Europe (Greenland) 72° 36′ N, 38° 30′ W 22 Europe 23 Europe (Switzerland) 46° 26′ 56″ N, 9° 47′ 33″ E 24 Europe (northern Sweden) 68° 13′–68° 19′ N, 19° 27′–19° 48′ E 25 Europe (Alps) 45–50° N, 5–15° E 26 North America (Alaska)3 61° 54′ N, 145° 40′ W 27 North America (Alaska) 61° 22′ 27″ N, 143° 35′ 56″ W 28 North America 29 North America (Beringia) 60° 6′ 12″ N, 143° 27′ 7″ W 30 North America (Canada) 50–70° N, 50–140° W 31 North America (Alaska)
1949
Larocque and Hall 2004
Multi-proxies
1401
Decadal to centennial Annual to decadal
2007
Guiot et al. 2010
Lake sediment
944
Decadal
1975
Larocque-Tobler et al. 2010
Tree ring
1501
2004
Grudd 2008
Multi-proxies
944
Decadal to centennial Annual to decadal
1996
Trachsel et al. 2012
Lake sediment
13,500
1950
Clegg et al. 2011
Lake sediment
6000
1950
Clegg et al. 2010
Pollen
14,000
Decadal to centennial Decadal to centennial Centennial
Pollen
25,000
Pollen Lake sediment
20004
Viau et al. 2006
Centennial
1950
Viau et al. 2008
12,000
Centennial
1950
Viau and Gajewski 2009
2000
Decadal
1962
Hu et al. 2001
Proxy-based Northern Hemisphere temperature reconstruction Table 1 (continued) Location
Data type
Time span (years)
Resolution
Base year AD
Source
Lake sediment
2000
Decadal
1955
MacDonald et al. 2009
Tree ring
2000
Decadal
1996
Salzer and Kipfmueller 2005
Lake sediment
1241
1992
Moore et al. 2001
Lake sediment
1030
Decadal to centennial Decadal
2003
Thomas and Briner 2008
Tree ring
1049
Centennial
1998
Luckman and Wilson 2005
Tree ring
1276
Decadal
1999
Wilson et al. 2007
62° 33′ N, 153° 38′ W 32 North America (Canada) 63° 43′ 7″ N, 109° 19′ 4″ W 33 North America (Colorado Plateau) 37° 0′ 1″ N, 110° 0′ 3″ W 34 North America (Baffin Island, Canada) 66.15′ N, 62° 0′ W 35 North America (Big Round Lake, Canada) 69° 52′ N, 68° 50′ W 36 North America (Canadian Rockies) 51° 45′–52° 45′ N, 116° 23′–117° 52′ W 37 North America (Gulf of Alaska) 58° 28′ 38″ N, 145° 25′ 56″ W 38 North America (Iceberg Lake, Alaska) 60° 47′ N, 142° 57′ W 39 Northern Hemisphere
Multi-proxies
1557
Centennial
1998
Loso 2009
Multi-proxies
2000
Annual to decadal
1973
Christiansen and Ljungqvist 2011
40 Northern Hemisphere
Multi-proxies
2000
1979
Moberg et al. 2005
41 Northern Hemisphere
Multi-proxies
2000
2006
Mann et al. 2008
42 Northern Hemisphere
Multi-proxies
992
Decadal to centennial Decadal to centennial Annual to decadal
1991
Jones et al. 1998
43 Northern Hemisphere
Multi-proxies
1500
Annual to decadal
2006
Mann et al. 2009
44 Northern Hemisphere
Multi-proxies
1200
Annual to decadal
1995
Osborn and Briffa 2006
45 Northern Hemisphere
Multi-proxies
1000
Annual to decadal
1975
46 Northern Hemisphere
Tree ring
1403
Annual to decadal
1960
Christiansen and Ljungqvist 2011 Hegerl et al. 2007
47 Northern Hemisphere
Tree ring
1162
Annual to decadal
1992
Esper et al. 2002
48 Northern Hemisphere
Tree ring
1283
Annual to decadal
1995
D’Arrigo et al. 2006
49 Northern Hemisphere
Multi-proxies
994
Decadal
1993
Crowley 2000
50 Northern Hemisphere 51 Northern Hemisphere
Multi-proxies Multi-proxies
981 981
Decadal Decadal
1980 1980
Ammann and Wahl 2007 Mann et al. 1999
52 Northern Hemisphere
Ice core
993
Decadal
1992
Frank et al. 2010
53 Global 54 Global
Multi-proxies Multi-proxies
2000 2000
55 Global
Multi-proxies
2000
56 Global
Multi-proxies
2000
Decadal Decadal to centennial Decadal to centennial Decadal to centennial
1995
Ljungqvist 2010
2006
Mann et al. 2008
1995
Jones and Mann 2004
1980
Mann and Jones 2003
1
The present year is the starting year of the project
2
The starting year of the project is AD 1999, and the data series is from −50 BP. Therefore, we set the base year as AD 1949
3
There are three sites in the dataset. Each site has July temperature. We average the temperature of the three sites
4
The data used for the reconstruction includes the newly added data which is published in AD 2000. Therefore, the base year is set as AD 2000
dozens of temperature reconstructions with varied time spans and resolutions. To ensure the reliability of the records, we only selected the reconstructed temperature series that are published in the internationally refereed journals, and that are well verified with local instrumental data. Moreover, the records must have a sampling resolution ranging from 1 to 100 years and a
time span of at least 1000 years. Based on these criteria, we found 56 published temperature records across the NH (Table 1). Among them, 36 series are based on a single proxy, including tree rings (12 curves), lake sediments (9 curves), pollen (6 curves), ice core (4 curves), stalagmites (3 curves), and historical documents (2 curves). The rest of the temperature
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Fig. 1 Geographic locations of the employed proxy records in this study. The bar charts show the breakdown (number and type) of the employed proxy records in each of the continents and macro regions. Only those
proxy records with clearly specified locational information are shown in the map.
series (20 curves) are reconstructed from multi-proxy records. Notably, because Europe and China are the two major regions with abundant historical records, there are two reconstructions using historical documents from the regions (Ge et al. 2003; Glaser and Riemann 2009). Meanwhile, the reconstructions from lake sediments are mostly from North America. In general, the reconstructions from multi-proxy records have higher resolution than those by a single proxy. The spatial distribution of temperature records is shown in Fig. 1. The composite plus scale (CPS) method is widely adopted to reconstruct paleo-climatic variations (Esper et al. 2002; Mann and Jones 2003). The CPS method has been adopted with spatial weighting to develop the NH temperature reconstruction (Mann et al. 2005). Likewise, we adopted the area-weighting method, with the weighting of each continent as follows: Asia 45.7 %, Africa (NH part) 18.6 %, North America 25.1 %, and Europe 10.6 %. In the study, before the CPS calculation, missing values in all series were replaced with linear interpolation to derive a series with annual resolution. As all series cover the period of AD 1200–1800, each series was normalized to have zero mean and unit standard deviation during this period. Then, according to the weighting of each continent, the temperature reconstruction was developed by applying the CPS method. The base period is set to AD 1961– 1990. Correlation coefficient between our reconstruction and instrumental records during AD 1961–1990 is 0.911 (p < 0.001). As the original temperature series was linearly interpolated to replace the missing data, the final CPS reconstruction was smoothed with a 200-year Butterworth low-pass filter to highlight the centennialscale variations during the mid-to-late Holocene, which is more interpretable given that all the original data have a resolution of 1–100 years. Hence, the comparisons and
discussion hereafter are primarily based on the 200-year low-pass filtered values of our NH temperature reconstruction.
3 Results and discussion The reconstructed mid-to-late Holocene NH temperature is shown in Fig. 2a. We conducted a sensitivity test to assess possible uncertainty brought by the overlapping use of proxy records in some NH/global temperature reconstructions during the past two millennia. For the test, we only used one of the NH/global temperature records that have data replication and then pooled it with all the other proxy records to do the reconstruction. The above procedure is repeated for all the NH/global temperature records that have data replication. Our test results show that the resulting NH temperature reconstructions are very consistent among themselves and to our final NH temperature reconstruction as well in terms of centennial to millennial variability, except for two that show modest disagreement during AD 500–1000 (Fig. 3). This sensitivity test shows that the inclusion of those NH/global temperature records that have proxy data replication during the past two millennia does not affect our results and conclusion. Instead, including all the records could increase the comprehensiveness of our sampled proxies. To further verify the reliability of our NH reconstruction, we compared it with the existing hemispheric or global temperature reconstructions (Fig. 2b, c). For the NH temperature reconstruction of the past 2000 years, ours is highly consistent with one of the most recent reconstruction at centennial time scale (PAGES-2kConsortium 2013), with a correlation of 0.76 (p < 0.001) (Fig. 2b). Compared with a recent global Holocene temperature reconstruction (Marcott et al.
Proxy-based Northern Hemisphere temperature reconstruction Fig. 2 The NH temperature reconstruction in comparison with the Bond events and recent temperature reconstructions. a Reconstructed NH temperature anomaly (°C) 4000 BC–2000 AD. The red line denotes its 200year Butterworth low-pass filtered values. b Past 2000-year NH temperature change (°C) (PAGES-2k-Consortium 2013). c Global temperature anomaly from 73 records (°C) (Marcott et al. 2013). d δ18O from Dongge Cave, China, representing changes in the Asian monsoon (low δ18O corresponds to strong Asian monsoon and vice versa for high values) (Wang et al. 2005). Shadow area B0–B4 denotes the Bond event within error ± 100 years (Bond et al. 2001). e Curve illustrating the number of proxy records over time.
2013), ours shows a similar trend during the mid-to-late Holocene period, with improved variations at centennial time scale (Fig. 2c). Overall, the two time series correlate at 0.84 (p < 0.001). These comparisons validate our NH temperature reconstruction. Meanwhile, our reconstructed NH temperature shows a better centennial-scale resolution than that by Marcott et al. (2013). Although the reconstruction by PAGES-2k-Consortium (2013) also shows centennial-scale variation, its temporal coverage is generally less than 2000 years. With a higher resolution and longer time span, our reconstruction allows the
assessment of the NH temperature and its relationship with various climate forcings during the mid-to-late Holocene. The summer Asian monsoon strength was closely associated with the NH thermal conditions during the mid-to-late Holocene (Gasse 2000). Model simulation also indicates that warmer large-scale NH temperatures favor a stronger monsoon (Fan and Mann 2009). Therefore, the Asian monsoon records are further adopted to verify our NH temperature reconstruction. As shown in Fig. 2d, the reconstructed NH temperature and the Asian monsoon show a similar trend during
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Fig. 3 Sensitivity test for the uncertainty brought by the overlapping use of proxy records in some NH/global temperature reconstructions during the past two millennia. The left y-axis is for all the results of sensitivity test and right y-axis is for NH temperature reconstruction in the study. Each line below shows one of the NH/global temperature records which
has been used to pool with all the other proxy records to do the reconstruction in the sensitivity test. The blue line of Mann et al. (2008) is the reconstruction by the method of “error-in-variables” (EIV) and the green line of Mann et al. (2008) is the reconstruction by the method of CPS.
the past 6000 years, especially at the millennial time scale. Because low δ18O corresponds to strong Asian Monsoon and vice versa for weak monsoon (Wang et al. 2005), the correlation of the two series is negative (−0.72; p < 0.001), suggestive of good correspondence between the Asian monsoon and the NH temperature variations during the mid-to-late Holocene. Furthermore, since our temperature is reconstructed at the centennial scale, the Bond events are adopted as an important benchmark (Wanner et al. 2008) to validate the reconstruction. As shown in Fig. 2a, our reconstructed NH temperature anomalies show a good match with the Bond Events. No. B1–B4 events all correspond well to periods of decreased NH temperature. Although B0 event mismatches the coldest period of the Little Ice Age (AD 1600–1700), it coincides well with the decreasing period of temperature in all the reconstructions (Fig. 2a–c). Overall, our NH temperature reconstruction shows a better agreement with the Bond events during the mid-to-late Holocene than the previous Holocene temperature reconstruction (Fig. 2b, c), suggestive of improved reconstruction skills. Our reconstructed NH temperature clearly shows several distinct periods during the past 2000 years, including the Medieval Climate Anomalies (AD 900–1200), Little Ice Age (AD 1500–1700), and Current Warming Period (the recent decades), consistent with the previous reconstructions (e.g., Mann et al. (2008); PAGES-2k-Consortium 2013). Moreover, our temperature reconstruction indicates that recent warming is not only unprecedented for the past two millennia
but also probably higher than any other periods during the past 5000 years at the centennial time scale. Variations of solar activities are a key external forcing of global climate. The paleo-climatic records have provided evidence for a linkage between climate change and solar activities (Bond et al. 2001). Climate variations at the centennial time scale may also reflect the influence of solar activities (Ault et al. 2013). In fact, many studies found that the solar cycle was the main cause of millennial-scale climate oscillations during the Holocene (Geel et al. 1999), which was included in climate models to examine millennial-scale climate (Ganopolski and Rahmstorf 2002). We used our reconstructed NH temperature to investigate the relationship between solar forcing and temperature change. The atmospheric Δ14C record was adopted to indicate the solar irradiation variations (Wang et al. 2005). As shown in Fig. 4b, before 700 BC, the correspondence between solar activity and the NH temperature was not high, perhaps because their connection was mediated by other factors. After 700 BC, the match between solar activity and the NH temperature was rather clear. Overall, the reconstructed NH temperature shows a similar trend to solar radiation changes during the past 6000 years at the millennial scale, with a correlation of 0.75 (p < 0.001). The North Atlantic is considered as the starting point of thermohaline circulation (Debret et al. 2009). It is also a major source of climate variability at inter-decadal or even longer
Proxy-based Northern Hemisphere temperature reconstruction Fig. 4 The NH temperature reconstruction in comparison with the solar and North Atlantic forcing. a The reconstructed NH temperature anomaly (°C) 4000 BC–2000 AD. The red line denotes the Butterworth low-pass filter of 200 years. b The solar irradiance from the atmospheric Δ14C record (Stuiver et al. 1998). c NAO SST (°C) from Foraminifera in the Nordic Seas (Risebrobakken et al. 2011). d NH temperature (red) and solar forcing (blue) records after a 200– 2000-year band-pass filter. e NH temperature (red) and North Atlantic SST (blue) records after a 200–2000-year band-pass filter.
time scales (Thompson and Wallace 2001). Therefore, North Atlantic sea surface temperatures (SSTs) could represent a major mode of NH climate variability (Wanner et al. 2008). The correlation between our reconstructed NH temperature and North Atlantic SST record (Risebrobakken et al. 2011) is −0.38 (p < 0.001) (Fig. 4c), in spite of the opposite multimillennia trends of two data series. The differentiated Holocene long-term trend in foraminifera-based SST temperatures has been previously noted on the Vøring Plateau (Jansen et al. 2008). Therefore, the data series are detrended with a 200–2000-year band-pass filter, as variability longer than 2000 years is not fully resolved in previous temperature reconstruction (Marcott et al. 2013). The correlation of the NH
temperature with solar forcing is 0.17 (p < 0.001) (Fig. 4d), and its correlation with the North Atlantic SST is 0.36 (p < 0.001) at the multi-centennial time scale (Fig. 4e). These results suggest that the impact of solar forcing on NH temperature is more pronounced at multi-millennial scales, whereas the impact of North Atlantic SSTs on NH temperature is more prominent at multi-centennial time scales. A close relationship between the Asian monsoon and the NH temperature over the last glacial period has been previously demonstrated (Wang et al. 2005). Our results suggest that the Asian monsoon variability is closely related to the NH temperature during the mid-to-late Holocene (Fig. 2a, d), with a correlation of −0.72 (p < 0.001). However, due to the lack of
Pei Q. et al.
centennial-scale NH temperature record, the Asian monsoon variability was only discussed in the context of solar forcing in
previous studies (Wang et al. 2005; Zhang et al. 2008). Here, we use the wavelet analysis to further verify the close association between the NH temperature and the Asian monsoon. As shown in Fig. 5a, a continuous cycle of 1700–2000 years during the reconstruction period is identified. The band of this frequency is significant at the 0.05. Besides, there is another continuous band of 1200–1500 years during the past 3500 years. Notably, the period of 1200–1500 years is not as significant as the period of 1700–2000 years during the past 6000 years. These wavelet results are consistent with those from the sea ice cover records from the North Atlantic area (Debret et al. 2009). The millennial-scale cycles in hemispheric (Clemens 2005; Rohling et al. 2003) and regional (Bond et al. 1997; Si et al. 2013) climate were noted in previous paleo-climate studies. Furthermore, the millennial-scale cycles have been detected in the Asian monsoon system (Wang et al. 2008). As shown in Fig. 5b, coherency analysis between the NH temperature and the Asian monsoon shows that there is a consistent band of 1700–2000 years during the past 6000 years and a consistent band of 1200–1500 years during the past 3500 years. Moreover, the NH temperature and the Asian monsoon records show similar fluctuations at the millennial scale if they were smoothed by a 1000–2000-year band-pass filter (Fig. 5c). Our reconstruction indicates that temperature of the twentieth century increased around 0.51 °C (Fig. 6c), consistent with the range of temperature increase of 0.49 to 0.89 °C during AD 1951–2012 as reported by the latest IPCC report (IPCC 2013). Our reconstruction further reveals that the rate of the twentieth century warming is probably the highest at least over the past 5000 years (Fig. 6a). The warming trend since the end of the nineteenth century reversed a persistent long-term global cooling trend during most the mid-to-late Holocene (PAGES-2k-Consortium 2013). To link temperature change in the past with that in the future, we compared our reconstruction with the latest temperature projections for the twenty-first century from four different Representative Concentration Pathways (RCP) scenarios
Fig. 6 a Reconstructed NH temperature (°C) during 4000 BC to 2100 AD with connection of the projected temperature under four emission scenarios of RCP2.6, RCP4.5, RCP6, and RCP8.5 (IPCC 2013). Results are smoothed with a 200-year Butterworth low-pass filter. b Reconstructed NH temperature during AD 2000–2100 with connection
of projected temperature under scenario of RCP2.6, RCP4.5, RCP6, and RCP8.5 (IPCC 2013). Results are smoothed with a 200-year Butterworth low-pass filter. c The raw data of reconstructed temperature series during AD 1900–2000. The black line is the rate of warming in the twentieth century.
Fig. 5 Wavelet analysis of NH temperature and the Asian monsoon. a The continuous wavelet power spectra of reconstructed NH temperature. The left panel (a1) indicates the 200–2500-year frequency band on the vertical axis and the frequency distribution (a2) in the study period (4000 BC–2000 AD) on the horizontal axis. In the left panel (a1), the spectra values vary from dark blue, indicating low values, to dark red, indicating high values. b Wavelet coherence between the NH temperature and Asian monsoon in the 200–2500-year frequency band shown on the vertical axis, and the coherency distribution in the study period (4000 BC–2000 AD) shown on the horizontal axis. The coherence values are color-coded from dark blue (low values) to dark red (high values). c NH temperature (red) and the Asian monsoon (blue) records after a 1000–2000-year band-pass filter.
Proxy-based Northern Hemisphere temperature reconstruction
(IPCC 2013). These four RCP scenarios, namely, RCP2.6, RCP4.5, RCP6.0, and RCP8.5, consider different emission levels to project possible temperature change during AD 2000–2100. Among them, RCP2.6 represents the most stringent mitigation scenario, while RCP8.5 the least. Thus, projected temperature increase is the lowest for RCP2.6 and the highest for RCP8.5 scenario (Fig. 6). These four scenarios are linked with the temperature reconstruction through the instrumental records during AD 1961–1990. As shown in Fig. 6a and b, even considering the lowest temperature increase as projected by the RCP2.6 scenario, both the warming rate and the temperature itself by the end of the twenty-first century will likely be the highest as compared to the mid-tolate Holocene.
4 Conclusions We developed a NH temperature reconstruction for the midto-late Holocene period from a synthesis of 56 published temperature records across the NH land areas. Our reconstruction reveals centennial-scale NH temperature variations during the past 6000 years, which matches well the Bond events. The late twentieth century NH temperature is likely the highest at least over the past 5000 years, and that the twentieth century experienced the highest warming rate of the past 5000 years. Our study also reveals two prominent cycles in NH temperature: 1700–2000 years during the mid-to-late Holocene and 1200– 1500 years for the past 3500 years, which are also found in the Asian monsoon variations. If it continues to increase as projected by the IPCC report, temperature by the end of the twenty-first century will be likely the highest for the whole mid-to-late Holocene period. Acknowledgements The research has been generously supported by Start-up Research Grant for Newly Recruited Assistant Professors from Education University of Hong Kong (Project Code R3744), Department Small Scale Research Grant from Department of Social Sciences, Education University of Hong Kong, Hui Oi-Chow Trust Fund (201502172003 and 201602172006), Research Grants Council of The Government of the Hong Kong Special Administrative Region of the People’s Republic of China (HKU745113H and 17610715), and the CAS/SAFEA International Partnership Program for Creative Research Teams.
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