New paper that selects hockey-stick data finds only 0.25C global warming over past 1000 years
Even if one believes this highly-flawed paper that selects only hockey-stick proxies, the proxy reconstruction of Northern Hemisphere temperatures shown in Fig 2a below indicate only a small ~0.25C difference between the Medieval Warm Period temperatures ~1000 years ago and temperatures at the end of the record in 2000. The Southern Hemisphere reconstruction shows almost no difference in temperature between the MWP peak and the year 2000. In addition, Figure 2a shows Northern Hemisphere temperatures not statistically-significantly different between the early 1300's [at the beginning of the Little Ice Age] and the year 2000.
Inter-hemispheric temperature variability over the past millennium
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- Nature Climate Change
- doi:10.1038/nclimate2174
- Received
- Accepted
- Published online
The Earth’s climate system is driven by a complex interplay of internal
chaotic dynamics and natural and anthropogenic external forcing. Recent
instrumental data have shown a remarkable degree of asynchronicity
between Northern Hemisphere and Southern Hemisphere temperature
fluctuations, thereby questioning the relative importance of internal
versus external drivers of past as well as future climate variability1, 2, 3. However, large-scale temperature reconstructions for the past millennium have focused on the Northern Hemisphere4, 5,
limiting empirical assessments of inter-hemispheric variability on
multi-decadal to centennial timescales. Here, we introduce a new
millennial ensemble reconstruction of annually resolved temperature
variations for the Southern Hemisphere based on an unprecedented network
of terrestrial and oceanic palaeoclimate proxy records. In conjunction
with an independent Northern Hemisphere temperature reconstruction
ensemble5,
this record reveals an extended cold period (1594–1677) in both
hemispheres but no globally coherent warm phase during the
pre-industrial (1000–1850) era. The current (post-1974) warm phase is
the only period of the past millennium where both hemispheres are likely
to have experienced contemporaneous warm extremes. Our analysis of
inter-hemispheric temperature variability in an ensemble of climate
model simulations for the past millennium suggests that models tend to
overemphasize Northern Hemisphere–Southern Hemisphere synchronicity by
underestimating the role of internal ocean–atmosphere dynamics,
particularly in the ocean-dominated Southern Hemisphere. Our results
imply that climate system predictability on decadal to century
timescales may be lower than expected based on assessments of external
climate forcing and Northern Hemisphere temperature variations5, 6 alone.
From over 25 hemispheric-scale temperature reconstructions published in
recent decades, only three cover the ocean-dominated Southern Hemisphere7. These Southern Hemisphere temperature reconstructions include only seven8 or fewer9 proxy
datasets for the entire Southern Hemisphere, or were provided as
peripheral components of Northern Hemisphere and global reconstruction
efforts4 with
the caveat that ‘more confident statements about long-term temperature
variations in the Southern Hemisphere and globe on the whole must await
additional proxy data collection’4. Consequently, attribution of temperature changes to external forcings10, 11 and investigations of the coupling between temperature and greenhouse gas concentrations5, 6 have focused on the Northern Hemisphere.
Data spanning inter-annual to multi-millennial timescales suggest
limited temperature coherence between the two hemispheres. The degree of
independence in Northern Hemisphere and Southern Hemisphere temperature
trends over the past 150 years2 indicates
that responses to external forcing may be modulated by ocean–atmosphere
variability, reducing predictability of the climate system in
twenty-first century model projections1, 3.
Patterns of late Quaternary deglaciation have also demonstrated high
inter-hemispheric variability, attributed to a coupling of orbital
forcing, ice-albedo feedbacks and the Atlantic Meridional Overturning
Circulation12, 13.
Finally, a recent evaluation of multi-centennial reconstructions from
seven continents also suggests stronger regional temperature coherence
within the hemispheres than between them14.
Yet, the preliminary nature of existing annually resolved Southern
Hemisphere temperature reconstructions has hindered knowledge of the
existence and driving mechanisms of inter-hemispheric climate
variability on the societally relevant multi-decadal to centennial
timescales.
Here, we introduce a Southern Hemisphere temperature reconstruction
ensemble and assess inter-hemispheric temperature variability over the
past millennium in both empirical reconstructions and state-of-the-art
climate model simulations. We use an extensive Southern Hemisphere
palaeoclimate data network from more than 300 individual sites15 yielding 111 temperature predictors (Supplementary Section 1). This proxy collection nearly doubles the number of records considered in the most advanced previous reconstruction attempt4,
now allowing the development of an annually resolved and well-verified
Southern Hemisphere temperature reconstruction for the past millennium (Fig. 1 and Supplementary Section 2) which is insensitive to moderate changes in reconstruction methodology or proxy network composition (Supplementary Section 3).
a, Southern Hemisphere temperature reconstruction proxies. Shading represents GISS instrumental grid-cell temperature28 correlations
in the period 1911–1990, with the Southern Hemisphere field mean used
as reconstruction target (all data linearly detrended). Cells with less
than 30 years of data are blank. b, Temporal evolution of the
number of proxy time series used in the reconstruction, with colours
indicating the relative contribution of each archive and calibration
(red) and verification (green) RE skill metric for the period
1000–2000. c, Instrumental target temperatures (with respect to
1961–1990) over the 1911–1990 calibration/verification period (black)
and reconstruction ensemble means of the years used for calibration
(red) and verification (green) for the most replicated proxy nest.
Details in Methods and Supplementary Section 3.1.3.
Although our database is weighted towards the Pacific sector of the Southern Hemisphere (Fig. 1a),
the proxy network captures the inter-annual to long-term variability in
Southern Hemisphere mean temperatures recorded by instrumental data
(calibration: r = 0.73–0.95, RE = 0.50–0.91; verification: r = 0.57–0.88, RE = 0.32–0.77; all p 
0.01; Fig. 1b, c).
The Southern Hemisphere reconstruction ensemble shows temperatures in
the period 1000 to 1200 CE (all years hereafter Common Era) that are
close to the long term (1000–2000) average. This is followed by an
approximately 150-year warm phase (1200–1350) containing the warmest
pre-industrial temperatures of the past millenium (Fig. 2a).
The subsequent long-term cooling trend reaches a minimum around 1600,
with negative decadal-scale ensemble-mean temperature anomalies
prevailing until the early twentieth century. 99.7% of the Southern
Hemisphere reconstruction ensemble members indicate that the late
twentieth century contained the warmest decade of the past millennium.
This finding complements well-established evidence for the anomalous
characteristics of Northern Hemisphere industrial-era warming5.
Besides the positive twentieth century temperature anomalies,
simultaneous cold anomalies in both hemispheres are identified between
1571 and 1722 (based on the 1000–2000 long term mean; Fig. 2a).
During the rest of the millennium, the Northern Hemisphere and Southern
Hemisphere are more prominently characterized by differences in the
occurrence, timing and phase of warm and cold episodes. In medieval
times, Southern Hemisphere temperature anomalies are notably colder than
the Northern Hemisphere both before 1100 and around 1400, and warmer
between 1280 and 1350. Expression of the industrial-era warming trend in
the Southern Hemisphere also lags by approximately 25 years behind the
Northern Hemisphere. Moreover, Southern Hemisphere temperatures tend to
show a weaker cooling response to strong volcanic eruptions, for
example, during the early nineteenth century.
0.01; Fig. 1b, c).
The Southern Hemisphere reconstruction ensemble shows temperatures in
the period 1000 to 1200 CE (all years hereafter Common Era) that are
close to the long term (1000–2000) average. This is followed by an
approximately 150-year warm phase (1200–1350) containing the warmest
pre-industrial temperatures of the past millenium (Fig. 2a).
The subsequent long-term cooling trend reaches a minimum around 1600,
with negative decadal-scale ensemble-mean temperature anomalies
prevailing until the early twentieth century. 99.7% of the Southern
Hemisphere reconstruction ensemble members indicate that the late
twentieth century contained the warmest decade of the past millennium.
This finding complements well-established evidence for the anomalous
characteristics of Northern Hemisphere industrial-era warming5.
Besides the positive twentieth century temperature anomalies,
simultaneous cold anomalies in both hemispheres are identified between
1571 and 1722 (based on the 1000–2000 long term mean; Fig. 2a).
During the rest of the millennium, the Northern Hemisphere and Southern
Hemisphere are more prominently characterized by differences in the
occurrence, timing and phase of warm and cold episodes. In medieval
times, Southern Hemisphere temperature anomalies are notably colder than
the Northern Hemisphere both before 1100 and around 1400, and warmer
between 1280 and 1350. Expression of the industrial-era warming trend in
the Southern Hemisphere also lags by approximately 25 years behind the
Northern Hemisphere. Moreover, Southern Hemisphere temperatures tend to
show a weaker cooling response to strong volcanic eruptions, for
example, during the early nineteenth century.
a, 30-year loess filtered ensemble mean temperature
reconstruction for the Southern Hemisphere (SH; blue) and Northern
Hemisphere (NH; red) relative to the millennium mean for the period
1000–2000. Blue shading based on Southern Hemisphere reconstruction
uncertainties (Supplementary Section 2.4).
Thin orange lines represent the ensemble means of the nine individual
temperature reconstructions making up the Northern Hemisphere dataset5. b, as a but
for the 24-member climate model ensemble. Note for consistency with
reconstruction data, simulated temperatures are shown as individual
simulations for the Northern Hemisphere and a probabilistic range based
on ensemble percentiles for the Southern Hemisphere.
To determine the extent to which reconstructed temperature patterns are
independently identified by climate models, we investigate
inter-hemispheric temperature coherence from a 24-member multi-model
ensemble (simulation details in Supplementary Table 9). Very similar temperature evolutions are modelled for the two hemispheres (Fig. 2b).
The majority of model ensemble members show warmest pre-industrial
temperatures sometime between 1050 and 1250 in the Northern Hemisphere
(79% of ensemble members), the Southern Hemisphere (75%), and
simultaneously in both (67%) hemispheres. Interestingly, this simulated
warm period is delayed compared to the reconstructed medieval warmth in
the Northern Hemisphere and precedes the phase of maximum Southern
Hemisphere pre-industrial warmth16.
Between 1300 and 1900, simulated temperatures are close to the
1000–2000 average, periodically interrupted by shorter volcanically
induced cold excursions. In contrast to the delay in industrial Southern
Hemisphere warming in the reconstructions, the climate model
simulations show a mostly synchronous temperature increase after 1850.
Mean correlations between 30-year filtered reconstructions and
simulations for all possible ensemble pairs are r = 0.29 ± 0.22 (2σ ensemble spread) for the Southern Hemisphere and r = 0.47 ± 0.33 for the Northern Hemisphere. These values increase to r= 0.35 and r =
0.77 for the ensemble means. As the model ensemble means are subjected
less to internal variability than each individual simulation and better
represent the temperature response to external forcing, these values
suggest substantially weaker links between external forcing and Southern
Hemisphere temperature variability compared to the Northern Hemisphere.
We quantify coherence between Northern Hemisphere and Southern
Hemisphere temperature extremes by identifying the percentage of
ensemble members showing decadal average temperatures more than one
standard deviation above or below the 1000–2000 baseline (Fig. 3).
Extended periods where at least 33% of the reconstruction ensemble
members in both hemispheres simultaneously show extreme cold or warm
temperatures are identified only between 1594 and 1677 and since 1967,
respectively. Since 1974 more than 66%, and since 1979 more than 90%, of
ensemble members show synchronous positive extremes (corresponding to
‘likely’ and ‘very likely’ categories using IPCC AR5 calibrated
uncertainty classification). This analysis provides evidence for a
global cold phase coinciding with the peak of the Northern Hemisphere
‘Little Ice Age’ (LIA) and a late-twentieth century warm phase of
unprecedented duration and magnitude within the past 1000 years. In
contrast, we find no empirical support for a globally coherent ‘Medieval
Climate Anomaly’ (MCA) at decadal timescales during the past millennium14.
Our new temperature reconstruction from the Southern Hemisphere
suggests that data from the Northern Hemisphere alone are insufficient
to characterize global scale temperature anomalies, trends and extremes.
a,b, Fraction of ensemble members with extreme warm (red) or cold (blue) decadal temperatures in the Northern Hemisphere, (a) and Southern Hemisphere (b), respectively. Dark shading represents the reconstructions, light shading with black border the model simulations. c, Probabilities for simultaneous extreme periods in both hemispheres are calculated by multiplying the fractions in a and b. d, Volcanic29(brown), solar30 (green) and greenhouse gas forcing19 (yellow) relative to 1961–1990. Dotted lines enclose the globally expressed peak Little Ice Age (LIA) 1594–1677.
Simulated extreme conditions shown in Fig. 3 are a direct expression of external forcing (Fig. 3d).
The notable reconstructed seventeenth-century peak LIA is less
prominent in the model ensemble, which shows the clearest global cooling
signal in response to volcanic eruptions around 1815 (Tambora), the
1450s (Kuwae) and 1258 (Samalas). External climate forcing as
incorporated in the current climate model simulations does not account
for key features of reconstructed temperature variation. This suggests
that internal variability was a key driver for hemispheric and global
decadal-scale extreme periods.
To further investigate inter-hemispheric temperature coherence, we
calculate the ten-year running temperature differences between the
standardized Northern Hemisphere and Southern Hemisphere reconstructions
(Fig. 4a).
The variability and amplitude of the Northern Hemisphere–Southern
Hemisphere temperature difference fluctuates considerably over time,
showing periods with large divergence (for example, around 1100 and
1575) and with more in-phase variability (for example, the thirteenth
and eighteenth centuries). The distinct and internally driven drop in
Northern Hemisphere temperatures around 19702 was
preceded by several other analogous periods of contrasting hemispheric
temperature trends. Internal ocean–atmosphere processes appear to be the
main driver of the larger Northern Hemisphere–Southern Hemisphere
differences; only two episodes of contrasting temperature regimes
coincide with strong volcanic eruptions (Kuwae and Tambora). Model
simulations also contain periods of contrasting inter-hemispheric
temperature trends, but with notably smaller differences between the
hemispheres (Supplementary Figs 36–59):
median reconstructed Northern Hemisphere–Southern Hemisphere
differences are outside the 10th–90th percentile range of model
simulations 42% of the time (Fig. 4).
The lower Northern Hemisphere–Southern Hemisphere temperature contrasts
within the simulations are not only evident in the pre-instrumental
period but also during the twentieth century3 (Fig. 4b, c), when both the reconstructions and instrumental data2 show strong inter-hemispheric variability.
a, Difference between 10-year filtered Northern
Hemisphere–Southern Hemisphere temperatures in the ensemble
reconstructions after detrending and standardizing (black). The solid
line represents the ensemble median, grey shading the percentiles. Cyan
shows the instrumental data 1880–201028. Red lines are the 10th and 90th percentiles from the ensemble of model simulations. Volcanic forcing29 shown at the top in brown. b,
Ensemble distribution of the reconstructed absolute Northern
Hemisphere–Southern Hemisphere temperature difference averaged over the
1000–1900 period. Red boxplot represents model simulations. c as b but for the twentieth century, cyan represents instrumental data.
The Southern Hemisphere reconstruction presented here allows new
insights into the characteristics of the global climate system. For
example, it has been proposed that the Southern Hemisphere response to
external forcing may be delayed and buffered by the large heat capacity
of the oceans17, 18. The greater amplitude of pre-industrial temperature variation in the Northern Hemisphere (0.67 °C ± 0.46 °C (2σ ensemble
spread) versus 0.37 °C ± 0.11 °C in the Southern Hemisphere), the
approximately two-century Northern Hemisphere lead during medieval times
and the approximately 25 year lead during the era of industrial warming
are in line with this hypothesis. However, we find no evidence for a
consistent lag between Northern Hemisphere and Southern Hemisphere
temperatures (Supplementary Section 8).
The coherent and extreme cool conditions in both hemispheres around
1600 are unique within the past millennium and now offer perhaps the
most viable explanation for the drop in global CO2 (difference of 8.37 ppm or 0.19 W m−2 between 1540–1580 and 1600–16405, 19, 20; Fig. 3d), which may not be sufficiently explained by land use change21 or Northern Hemisphere-temperature–CO2 feedbacks5.
Our results suggest that large, internally driven temperature contrasts
between the hemispheres, such as identified in the twentieth century2 have
repeatedly occurred on the policy-relevant multi-decadal to centennial
timescales. This finding is strengthened by evidence from annually
resolved regional temperature reconstructions14, 22, 23 and the timing of glacial fluctuations in New Zealand and the Northern Hemisphere24. Our data support hypotheses that global and hemispheric temperature extremes and transitions may be initiated11, 16, 25 and prolonged26 by
internal variability and feedbacks. Analyses targeting periods where
climate models and reconstructions differ will be necessary to identify
weaknesses in both proxy- and model-based representations of the Earth’s
climate system. However, the strong inter-hemispheric coupling in the
simulations assessed herein suggests that models overestimate the
strength of externally forced relative to internal climate system
variability, therefore implying more limited predictability not only on
regional1, 27 but
also hemispheric scales. The stronger coherence between the Northern
Hemisphere temperature reconstructions and external forcings similarly
implies that detection and attribution studies10 and climate sensitivity estimates5, 6 based
on Northern Hemisphere data alone may not be representative of the
global climate system. Future consideration of Southern Hemisphere
temperature evolution should reduce uncertainties in estimating and
attributing natural and anthropogenically forced climate variations.
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