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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

Pei Q. et al.

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

Pei Q. et al.

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.

References Alley RB (2000) The younger Dryas cold interval as viewed from Central Greenland. Quat Sci Rev 19:213–226 Ammann CM, Wahl ER (2007) The importance of the geophysical context in statistical evaluations of climate reconstruction procedures. Clim Chang 85:71–88 Ault TR, Deser C, Newman M, Emile-Geay J (2013) Characterizing decadal to centennial variability in the equatorial Pacific during the last millennium. Geophys Res Lett 40:3450–3456

Bjune AE, Seppä H, Birks HJB (2009) Quantitative summer-temperature reconstructions for the last 2000 years based on pollenstratigraphical data from northern Fennoscandia. J Paleolimnol 41: 43–56 Bond G et al. (1997) A pervasive millennial-scale cycle in North Atlantic Holocene and glacial climates. Science 278:1257–1265 Bond G et al. (2001) Persistent solar influence on North Atlantic climate during the Holocene. Science 294:2130–2136 Bradley RS (2000) Past global changes and their significance for the future. Quat Sci Rev 19:391–402 Büntgen U et al. (2011) 2500 years of European climate variability and human susceptibility. Science 331:578–582 Christiansen B, Ljungqvist FC (2011) Reconstruction of the extratropical NH mean temperature over the last millennium with a method that preserves low-frequency variability. J Clim 24:6013–6034 Clegg BF, Clarke GH, Chipman ML, Chou M, Walker IR, Tinner W, Hu FS (2010) Six millennia of summer temperature variation based on midge analysis of lake sediments from Alaska. Quat Sci Rev 29: 3308–3316 Clegg BF, Kelly R, Clarke GH, Walker IR, Hu FS (2011) Nonlinear response of summer temperature to Holocene insolation forcing in Alaska. Proc Natl Acad Sci 108:19299–19304 Clemens SC (2005) Millennial-band climate spectrum resolved and linked to centennial-scale solar cycles. Quat Sci Rev 24:521–531 Crowley TJ (2000) Causes of climate change over the past 1000 years. Science 289:270–277 Dai A (2011) Drought under global warming: a review. Wiley Interdiscip Rev Clim Chang 2:45–65 D’Arrigo R, Wilson R, Jacoby G (2006) On the long-term context for late twentieth century warming. J Geophys Res 111:D03103 Davis BAS, Brewer S, Stevenson AC, Guiot J (2003) The temperature of Europe during the Holocene reconstructed from pollen data. Quat Sci Rev 22:1701–1716 Debret M, Sebag D, Crosta X, Massei N, Petit J-R, Chapron E, BoutRoumazeilles V (2009) Evidence from wavelet analysis for a midHolocene transition in global climate forcing. Quat Sci Rev 28: 2675–2688 Esper J, Cook ER, Schweingruber FH (2002) Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability. Science 295:2250–2253 Fan F, Mann ME (2009) Understanding changes in the Asian summer monsoon over the past millennium: insights from a long-term coupled model simulation. J Clim 22:1736–1748 Frank DC, Esper J, Raible CC, Büntgen U, Trouet V, Stocker B, Joos F (2010) Ensemble reconstruction constraints on the global carbon cycle sensitivity to climate. Nature 463:527–530 Ganopolski A, Rahmstorf S (2002) Abrupt glacial climate changes due to stochastic resonance. Phys Rev Lett 88 Gasse F (2000) Hydrological changes in the African tropics since the last glacial maximum. Quat Sci Rev 19:189–211 Ge Q, Zheng J, Fang X, Man Z, Zhang X, Zhang P, Wang W-C (2003) Winter half-year temperature reconstruction for the middle and lower reaches of the Yellow River and Yangtze River, China, during the past 2000 years. The Holocene 13:933–940 Ge Q, Hao Z, Zheng J, Shao X (2013) Temperature changes over the past 2000 yr in China and comparison with the Northern Hemisphere. Clim Past 9:1153–1160 Geel BV, Raspopov OM, Renssen H, Plicht J, Dergachev VA, Meijer HAJ (1999) The role of solar forcing upon climate change. Quat Sci Rev 18:331–338 Glaser R, Riemann D (2009) A thousand-year record of temperature variations for Germany and Central Europe based on documentary data. J Quat Sci 24:437–449 Grudd H (2008) Torneträsk tree-ring width and density AD 500-2004: a test of climatic sensitivity and a new 1500-year reconstruction of north Fennoscandian summers. Clim Dyn 31:843–857

Pei Q. et al. Guiot J, Corona C, members E (2010) Growing season temperatures in Europe and climate forcings over the past 1400 years. PLoS One 5: e9972 Hansen J, Sato M, Ruedy R (2012) Perception of climate change. Proceedings of national. Acad Sci 109:E2415–E2423 Hantemirov RM, Shiyatov SG (2002) A continuous multimillennial ringwidth chronology in Yamal, northwestern Siberia. The Holocene 12: 717–726 He Y et al. (2013) Solar influenced late Holocene temperature changes on the northern Tibetan Plateau. Chin Sci Bull 58:1053–1059 Hegerl GC, Crowley TJ, Allen M, Hyde WT, Pollack HN, Smerdon J, Zorita E (2007) Detection of human influence on a new, validated 1500-year temperature reconstruction. J Clim 20:650–666 Hu FS, Ito E, Brown TA, Curry BB, Engstrom DR (2001) Pronounced climatic variations in Alaska during the last two millennia. Proc Natl Acad Sci 98:10552–10556 IPCC (2013) Climate change 2013: the physical science basis. Working group I contribution to the IPCC 5th assessment report—changes to the underlying scientific/technical assessment. Intergovernmental panel on climate change, Stockholm. doi:http://www. climatechange2013.org/images/uploads/WGIAR5_WGI-12Doc2 b_FinalDraft_All.pdf Jansen E, Andersson C, Moros M, Nisancioglu KH, Nyland BF, Telford RJ (2008) The early to mid-Holocene thermal optimum in the North Atlantic. In: Battarbee RW, Binney HA (eds) Natural climate variability and global warming: a Holocene perspective. WileyBlackwell Pub, Oxford; Hoboken, NJ, pp. 123–137 Jones PD, Mann ME (2004) Climate over past millennia. Rev Geophys 42. doi:10.1029/2003RG000143 Jones PD, Briffa KR, Barnett TP, Tett SFB (1998) High-resolution palaeoclimatic records for the last millennium: interpretation, integration and comparison with general circulation model control-run temperatures. The Holocene 8:455–471 Kobashi T, Severinghaus JP, Barnola J-M, Kawamura K, Carter T, Nakaegawa T (2010) Persistent multi-decadal Greenland temperature fluctuation through the last millennium. Clim Change 100 Kobashi T et al. (2011) High variability of Greenland surface temperature over the past 4000 years estimated from trapped air in an ice core. Geophys Res Lett 38. doi:10.1029/2011GL049444 Larocque I, Hall RI (2004) Holocene temperature estimates and chironomid community composition in the Abisko Valley, northern Sweden. Quat Sci Rev 23:2453–2465 Larocque-Tobler I, Grosjean M, Heiri O, Trachsel M, Kamenik C (2010) Thousand years of climate change reconstructed from chironomid subfossils preserved in varved lake Silvaplana, Engadine, Switzerland. Quat Sci Rev 29:1940–1949 Lindholm M, Jalkanen R, Salminen H, Aalto T, Ogurtsov M (2010) The height-increment record of summer temperature extended over the last millennium in Fennoscandia. The Holocene 21:319–326 Liu Y et al. (2009) Annual temperatures during the last 2485 years in the mid-eastern Tibetan Plateau inferred from tree rings. Science in China series D: earth. Sciences 52:348–359 Ljungqvist FC (2010) A new reconstruction oftemperature variability in the extra-tropical Northern Hemisphere during the last two millennia. GeografiskaAnnaler. Phys Geogr 92:339–351 Loso MG (2009) Summer temperatures during the Medieval Warm Period and Little Ice Age inferred from varved proglacial lake sediments in southern Alaska. J Paleolimnol 41:117–128 Luckman BH, Wilson RJS (2005) Summer temperatures in the Canadian Rockies during the last millennium: a revised record. Clim Dyn 24: 131–144 MacDonald GM, Porinchu DF, Rolland N, Kremenetsky KV, Kaufman DS (2009) Paleolimnological evidence of the response of the central Canadian treeline zone to radiative forcing and hemispheric patterns of temperature change over the past 2000 years. J Paleolimnol 41: 129–141

Mangini A, Spötl C, Verdes P (2005) Reconstruction of temperature in the Central Alps during the past 2000 yr from a δ18O stalagmite record. Earth Planet Sci Lett 235:741–751 Mann ME, Jones PD (2003) Global surface temperatures over the past two millennia. Geophys Res Lett 30:1820 Mann ME, Bradley RS, Hughes MK (1999) Northern Hemisphere temperatures during the past millennium: inferences, uncertainties, and limitations. Geophys Res Lett 26:759 Mann ME, Rutherford S, Wahl E, Ammann C (2005) Testing the fidelity of methods used in proxy-based reconstructions of past climate. J Clim 18:4097–4107 Mann ME, Zhang Z, Hughes MK, Bradley RS, Miller SK, Rutherford S, Ni F (2008) Proxy-based reconstructions of hemispheric and global surface temperature variations over the past two millennia. Proc Natl Acad Sci 105:13252–13257 Mann ME et al. (2009) Global signatures and dynamical origins of the Little Ice Age and Medieval Climate Anomaly. Science 326:1256– 1260 Marcott SA, Shakun JD, Clark PU, Mix AC (2013) A reconstruction of regional and global temperature for the past 11,300 years. Science 339:1198–1201 Martín-Chivelet J, Muñoz-García MB, Edwards RL, Turrero MJ, Ortega AI (2011) Land surface temperature changes in northern Iberia since 4000 yr BP, based on δ13Cof speleothems. Glob Planet Chang 77: 1–12 Moberg A, Sonechkin DM, Holmgren K, Datsenko NM, Karlén W (2005) Highly variable northern hemisphere temperatures reconstructed from low- and high-resolution proxy data. Nature 433: 613–617 Moore JJ, Hughen KA, Miller GH, Overpeck JT (2001) Little ice age recorded in summer temperatures from varved sediments of Donard Lake, Baffin Island, Canada. J Paleolimnol 25:503–517 Osborn TJ, Briffa KR (2006) The spatial extent of 20th-century warmth in the context of the past 1200 years. Science 311:841–844 PAGES-2k-Consortium (2013) Continental-scale temperature variability during the past two millennia. Nat Geosci 6:339–346 Risebrobakken B, Dokken T, Smedsrud LH, Andersson C, Jansen E, Moros M, Ivanova EV (2011) Early Holocene temperature variability in the Nordic seas: the role of oceanic heat advection versus changes in orbital forcing. Paleocenography 26:PA4206 Rohling E, Mayewski P, Challenor P (2003) On the timing and mechanism of millennial-scale climate variability during the last glacial cycle. Clim Dyn 20:257–267 Salzer MW, Kipfmueller KF (2005) Reconstructed temperature and precipitation on a millennial timescale from tree-rings in the Southern Colorado Plateau, U.S.A. Clim Chang 70:465–487 Shi F, Yang B, Gunten LV (2012) Preliminary multiproxy surface air temperature field reconstruction for China over the past millennium. Science China Earth Sciences 55:2058–2067 Si Yet al. (2013) Climate fluctuation record from China’s Salawusu River valley during the early last glacial. Geochem Int 51:240–248 Stuiver M et al. (1998) INTCAL98 Radiocarbon Age Calibration, 240000 cal BP. Radiocarbon 40:1041–1083 Tan M, Liu T, Hou J, Qin X, Zhang H, Li T (2003) Cyclic rapid warming on centennial-scale revealed by a 2650-year stalagmite record of warm season temperature. Geophys Res Lett 30 Thomas EK, Briner JP (2008) Climate of the past millennium inferred from varved proglacial lake sediments on northeast Baffin Island, Arctic Canada. J Paleolimnol 41:209–224 Thompson DWJ, Wallace JM (2001) Regional climate impacts of the Northern Hemisphere annular mode. Science 293:85–89 Trachsel M et al. (2012) Multi-archive summer temperature reconstruction for the European Alps, AD 1053-1996. Quat Sci Rev 46:66–79 Viau AE, Gajewski K (2009) Reconstructing millennial-scale, regional paleoclimates of boreal Canada during the Holocene. J Clim 22: 316–330

Proxy-based Northern Hemisphere temperature reconstruction Viau AE, Gajewski K, Sawada MC, Fines P (2006) Millennial-scale temperature variations in North America during the Holocene. Journal of Geophysical Research: Atmospheres 111. doi:10.1029 /2005JD006031 Viau AE, Gajewski K, Sawada MC, Bunbury J (2008) Low- and highfrequency climate variability in eastern Beringia during the past 25 000 years. Can J Earth Sci 45:1435–1453 Wang Y et al. (2005) The Holocene Asian monsoon: links to solar changes and North Atlantic climate. Science 308:854–857 Wang Y et al. (2008) Millennial- and orbital-scale changes in the east Asian monsoon over the past 224,000 years. Nature 451:1090–1093 Wang J, Yang B, Ljungqvist FC (2015) A millennial summer temperature reconstruction for the eastern Tibetan Plateau from tree-ring width. J Clim 28:5289–5530

Wanner H et al. (2008) Mid- to late Holocene climate change: an overview. Quat Sci Rev 27:1791–1828 Wheeler T, Braun JV (2013) Climate change impacts on global food security. Science 341:508–513 Wilson R, Wiles G, D’Arrigo R, Zweck C (2007) Cycles and shifts: 1, 300 years of multi-decadal temperature variability in the Gulf of Alaska. Clim Dyn 28:425–440 Yang B, Braeuning A, Johnson KR, Yafeng S (2002) General characteristics of temperature variation in China during the last two millennia. Geophysical Research Letter 29 Zhang P et al. (2008) A test of climate, sun, and culture relationships from an 1810-year Chinese cave record. Science 322:940–942 Zhu C, Chen X, Ma C, Zhu Q, Li Z, Xu W (2008) Pollen based paleoclimate reconsctruction in Da Jiu Lake of Shen Nong Jia in China. Chin Sci Bull 53:38–44