Figure 3. Comparison of the brGDGTs-based MAAT record for
Huguangyan Maar Lake with historical documentary evidence, instrumental
data, and possible climate forcings during the last millennium.
(a ) brGDGTs-based MAAT record obtained in this study (blue
line); annual mean air temperature from Zhangjiang Station (1921–1939
and 1951–2010) (yellow line); modern MAAT for the study area (green
dashed line). (b ) Number of counties with cold surges per 30
years in tropical China years from historical documents; (c )
Synthetic temperature reconstruction for China (Ge et al., 2015);
(d ) Record of tropical solar forcing (Mann et al., 2005);
(e ) record of GHG radiative forcing (CO2,
CH4, N2O) (Köhler et al., 2017);
(f ) Record of tropical volcanic forcing (Mann et al., 2005);
Green bars represent the extreme cold winters based on historical
documentary evidence from China (Chu and Ching, 1973).
Extreme cold events are concentrated in the Little Ice Age, especially
between 800 CE and 1900 CE, while the Medieval Warm Period (MWP) had few
fewer cold events (Figure 3b). Historical records are less abundant and
intermittent before 1400 CE but are more continuous after the Ming
Dynasty (the Fang Zhi (local chronic) period of 1400 CE-1900 CE). The
brGDGTs-based MAAT record from Huguangyan Maar Lake shows three cold
intervals: 440 CE-1640 CE, 1800 CE-1890 CE, and during the 1970s (Figure
3a). Within the limits of the dating uncertainties, these intervals are
consistent with the documentary evidence (Figure 3b).
Natural climate forcings such as changes in solar output and volcanic
eruptions are widely recognized as causes of
decadal-to-centennial–scale climatic variations. Figure 3 shows that
most of the cold intervals recorded at Huguangyan Maar Lake correspond
to sunspot minima and episodes of intensified volcanic activity.
However, the decrease in MAAT at ~1740 CE is not linked
to these forcings.
3.2 Holocene brGDGTs-based temperature change
Although the climate of the Holocene, the most recent interval of Earth
history, was more stable than that during the last glacial period, it
was interrupted by several abrupt cooling events on the decadal- to
centennial-scale (Bond et al., 2001). The brGDGTs-based temperature
record from Huguangyan Maar Lake shows an increase in MAAT from 22℃
after the Younger Dryas (11.5 ka BP) to 28℃ by ~7.0 ka
BP, followed by a gradual decrease from 23.5℃ in recent decades. This
general trend is punctuated by eight cooling events on the decadal- to
centennial-scale, centered at 0.4, 2.0, 3.7, 5.3, 5.8, 8.3, 9.0, and 9.6
ka B.P. (Figure 4, 5).