Evolution of global temperature over the past two million years

Journal name:
Nature
Volume:
538,
Pages:
226–228
Date published:
DOI:
doi:10.1038/nature19798
Received
Accepted
Published online

Reconstructions of Earth’s past climate strongly influence our understanding of the dynamics and sensitivity of the climate system. Yet global temperature has been reconstructed for only a few isolated windows of time1, 2, and continuous reconstructions across glacial cycles remain elusive. Here I present a spatially weighted proxy reconstruction of global temperature over the past 2 million years estimated from a multi-proxy database of over 20,000 sea surface temperature point reconstructions. Global temperature gradually cooled until roughly 1.2 million years ago and cooling then stalled until the present. The cooling trend probably stalled before the beginning of the mid-Pleistocene transition3, and pre-dated the increase in the maximum size of ice sheets around 0.9 million years ago4, 5, 6. Thus, global cooling may have been a pre-condition for, but probably is not the sole causal mechanism of, the shift to quasi-100,000-year glacial cycles at the mid-Pleistocene transition. Over the past 800,000 years, polar amplification (the amplification of temperature change at the poles relative to global temperature change) has been stable over time, and global temperature and atmospheric greenhouse gas concentrations have been closely coupled across glacial cycles. A comparison of the new temperature reconstruction with radiative forcing from greenhouse gases estimates an Earth system sensitivity of 9 degrees Celsius (range 7 to 13 degrees Celsius, 95 per cent credible interval) change in global average surface temperature per doubling of atmospheric carbon dioxide over millennium timescales. This result suggests that stabilization at today’s greenhouse gas levels may already commit Earth to an eventual total warming of 5 degrees Celsius (range 3 to 7 degrees Celsius, 95 per cent credible interval) over the next few millennia as ice sheets, vegetation and atmospheric dust continue to respond to global warming.

At a glance

Figures

  1. Reconstruction of global average surface temperature (GAST) over the past 2 million years compared to other key palaeoclimate variables.
    Figure 1: Reconstruction of global average surface temperature (GAST) over the past 2 million years compared to other key palaeoclimate variables.

    a, GAST as temperature deviation (in °C) from present (average over 0–5 ka) in blue. b, Stacked reconstruction of change in Antarctic temperature14 (°C) in cyan. c, Stacked reconstruction of atmospheric CO2 concentrations18 (p.p.m.) in red. d, Stack of deep-sea oxygen isotopes30, δ18O (‰), in grey. In all panels, the solid black lines show the median estimate and the colour shaded areas show the 95% interval.

  2. Relationship of changes in GAST to changes in Antarctic temperature and GHG radiative forcing over the past 800 kyr.
    Figure 2: Relationship of changes in GAST to changes in Antarctic temperature and GHG radiative forcing over the past 800 kyr.

    a, b, Each point represents randomly sampled estimates from simulations of GAST plotted against Antarctic temperature14 (a) and GHG radiative forcing14, 17, 18 (b) over the past 800 kyr. The dashed black line shows the median estimated relationship in °C per °C in a and in °C per W m−2 in b. The red dashed line shows the median estimated quadratic relationship in b.

  3. Probabilistic breakpoint analysis of global temperature trends over the past 2 million years.
    Figure 3: Probabilistic breakpoint analysis of global temperature trends over the past 2 million years.

    Shown are empirically fitted frequency distributions for the timing of when the trend in global temperature changes (a; in black), the global temperature trend before the breakpoint (b; in blue), and the global temperature trend after the breakpoint (c; in green).

  4. Spatial distribution of the SST proxy reconstructions used in this analysis.
    Extended Data Fig. 1: Spatial distribution of the SST proxy reconstructions used in this analysis.

    a, All 61 SST records, with methods as follows: from alkenone indices, blue circles; from Mg/Ca ratios, red triangles; and from species assemblage methods, brown squares. b, Repeated after clustering records within 5° latitude/longitude of each other, with the 11 clusters in cyan diamonds and the remaining 18 records as in a.

  5. Temporal distribution of the 61 SST proxy reconstructions used in this analysis.
    Extended Data Fig. 2: Temporal distribution of the 61 SST proxy reconstructions used in this analysis.

    a, Reconstruction length versus latitude, colours as in Extended Data Fig. 1. b, Empirical cumulative distribution function for lengths of the SST proxy reconstructions. c, Empirical cumulative distribution function for lengths of GAST time series in the final simulation ensemble of potential GAST time series.

  6. Comparison of different methods used to estimate GAST.
    Extended Data Fig. 3: Comparison of different methods used to estimate GAST.

    a, The primary GAST estimate (using 61 proxy reconstructions) is plotted as a function of time, with the median in black and the 95% interval in grey. The GAST estimation method is repeated for a clustering of the data (11 clusters and 18 individual reconstructions), with the median shown in cyan, and for only the 5 proxy reconstructions that cover the past 2 Myr, with the median shown in orange. b, The median time series from each alternative method are plotted against the primary median GAST estimate, with the clustered version in cyan circles and the 5-record version in orange squares. c, The primary GAST estimate is plotted as a function of time, with the median in black and the 95% interval in grey. An alternative GAST estimation method using a time-varying scalar based on the deep-sea oxygen isotopes median estimate is shown in green, and another estimation method based on the relative sea level median estimate is shown in purple. d, The median time series from each alternative method is plotted against the primary median GAST estimate, with the reconstruction scaled using deep sea oxygen isotopes shown in green circles and the reconstruction scaled using relative sea level shown in purple squares.

  7. Estimates of the ratio of change in GAST to change in average SST.
    Extended Data Fig. 4: Estimates of the ratio of change in GAST to change in average SST.

    a, b, Scatter plots show the dependency of the ratio of change in GAST to change in average SST over the latitudinal zone 60° N to 60° S from PMIP2 and PMIP3 climate model simulations31, 32 as a function of change in GAST at the LGM (a) and of model climate sensitivity (b). The climate sensitivity estimates (in °C per W m−2) are from ref. 33. Dashed lines show the scalar range used in this analysis.

  8. Estimating change in GAST at the LGM using simulations drawn from PMIP model outputs.
    Extended Data Fig. 5: Estimating change in GAST at the LGM using simulations drawn from PMIP model outputs.

    The solid, purple line is the empirically fitted frequency distribution (shown in density on the y axis) of GAST estimated from the full air surface temperature outputs from the 9 PMIP models. The dashed, black line is the distribution of GAST estimated using the method in the present paper and the PMIP SST outputs drawn from only the locations of the 61 proxy reconstructions. The short-dashed, orange line is the same analysis completed for only the 5 proxy reconstructions that cover the past 2 Myr. The thin vertical lines are the medians of each distribution.

  9. The dependence of coupling relationships over time for GAST on changes in Antarctic temperature and GHG radiative forcing.
    Extended Data Fig. 6: The dependence of coupling relationships over time for GAST on changes in Antarctic temperature and GHG radiative forcing.

    a, b, Regression results of change in GAST as a function of change in Antarctic temperature14 (a) and of change in GHG radiative forcing17, 18, 54 (b) are calculated for moving 200-kyr-long time windows every 5 kyr. The solid line shows the median estimates, with the coloured and grey-shaded areas showing the 50% and 95% intervals, respectively. The dashed lines show the 95% intervals calculated from the entire time series.

  10. Comparison of changes in GAST to changes in CO2 radiative forcing.
    Extended Data Fig. 7: Comparison of changes in GAST to changes in CO2 radiative forcing.

    Boron-isotope-based proxy reconstruction of CO2 from refs 17, 56. Blue points are from 0–1 Ma, red points are from 1–2 Ma, and error bars show 95% intervals.

Tables

  1. Database of SST proxy reconstructions based on Mg/Ca ratio and species assemblages used in estimating GAST
    Extended Data Table 1: Database of SST proxy reconstructions based on Mg/Ca ratio and species assemblages used in estimating GAST
  2. Database of SST proxy reconstructions based on alkenone indices used in estimating GAST
    Extended Data Table 2: Database of SST proxy reconstructions based on alkenone indices used in estimating GAST
  3. Comparisons of GAST with other important palaeoclimate reconstructions
    Extended Data Table 3: Comparisons of GAST with other important palaeoclimate reconstructions

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Affiliations

  1. 1Interdisciplinary Program in Environment and Resources, Stanford University, Stanford, California 94305, USA

    • Carolyn W. Snyder

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The author declares no competing financial interests.

Corresponding author

Correspondence to:

Reviewer Information Nature thanks E. J. Rohling and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figures

  1. Extended Data Figure 1: Spatial distribution of the SST proxy reconstructions used in this analysis. (418 KB)

    a, All 61 SST records, with methods as follows: from alkenone indices, blue circles; from Mg/Ca ratios, red triangles; and from species assemblage methods, brown squares. b, Repeated after clustering records within 5° latitude/longitude of each other, with the 11 clusters in cyan diamonds and the remaining 18 records as in a.

  2. Extended Data Figure 2: Temporal distribution of the 61 SST proxy reconstructions used in this analysis. (153 KB)

    a, Reconstruction length versus latitude, colours as in Extended Data Fig. 1. b, Empirical cumulative distribution function for lengths of the SST proxy reconstructions. c, Empirical cumulative distribution function for lengths of GAST time series in the final simulation ensemble of potential GAST time series.

  3. Extended Data Figure 3: Comparison of different methods used to estimate GAST. (362 KB)

    a, The primary GAST estimate (using 61 proxy reconstructions) is plotted as a function of time, with the median in black and the 95% interval in grey. The GAST estimation method is repeated for a clustering of the data (11 clusters and 18 individual reconstructions), with the median shown in cyan, and for only the 5 proxy reconstructions that cover the past 2 Myr, with the median shown in orange. b, The median time series from each alternative method are plotted against the primary median GAST estimate, with the clustered version in cyan circles and the 5-record version in orange squares. c, The primary GAST estimate is plotted as a function of time, with the median in black and the 95% interval in grey. An alternative GAST estimation method using a time-varying scalar based on the deep-sea oxygen isotopes median estimate is shown in green, and another estimation method based on the relative sea level median estimate is shown in purple. d, The median time series from each alternative method is plotted against the primary median GAST estimate, with the reconstruction scaled using deep sea oxygen isotopes shown in green circles and the reconstruction scaled using relative sea level shown in purple squares.

  4. Extended Data Figure 4: Estimates of the ratio of change in GAST to change in average SST. (112 KB)

    a, b, Scatter plots show the dependency of the ratio of change in GAST to change in average SST over the latitudinal zone 60° N to 60° S from PMIP2 and PMIP3 climate model simulations31, 32 as a function of change in GAST at the LGM (a) and of model climate sensitivity (b). The climate sensitivity estimates (in °C per W m−2) are from ref. 33. Dashed lines show the scalar range used in this analysis.

  5. Extended Data Figure 5: Estimating change in GAST at the LGM using simulations drawn from PMIP model outputs. (127 KB)

    The solid, purple line is the empirically fitted frequency distribution (shown in density on the y axis) of GAST estimated from the full air surface temperature outputs from the 9 PMIP models. The dashed, black line is the distribution of GAST estimated using the method in the present paper and the PMIP SST outputs drawn from only the locations of the 61 proxy reconstructions. The short-dashed, orange line is the same analysis completed for only the 5 proxy reconstructions that cover the past 2 Myr. The thin vertical lines are the medians of each distribution.

  6. Extended Data Figure 6: The dependence of coupling relationships over time for GAST on changes in Antarctic temperature and GHG radiative forcing. (184 KB)

    a, b, Regression results of change in GAST as a function of change in Antarctic temperature14 (a) and of change in GHG radiative forcing17, 18, 54 (b) are calculated for moving 200-kyr-long time windows every 5 kyr. The solid line shows the median estimates, with the coloured and grey-shaded areas showing the 50% and 95% intervals, respectively. The dashed lines show the 95% intervals calculated from the entire time series.

  7. Extended Data Figure 7: Comparison of changes in GAST to changes in CO2 radiative forcing. (332 KB)

    Boron-isotope-based proxy reconstruction of CO2 from refs 17, 56. Blue points are from 0–1 Ma, red points are from 1–2 Ma, and error bars show 95% intervals.

Extended Data Tables

  1. Extended Data Table 1: Database of SST proxy reconstructions based on Mg/Ca ratio and species assemblages used in estimating GAST (219 KB)
  2. Extended Data Table 2: Database of SST proxy reconstructions based on alkenone indices used in estimating GAST (193 KB)
  3. Extended Data Table 3: Comparisons of GAST with other important palaeoclimate reconstructions (256 KB)

Supplementary information

PDF files

  1. Supplementary Methods (187 KB)

    This file contains the R code for key methods described in the paper.

Excel files

  1. Supplementary Data (898 KB)

    This file shows the new global average surface temperature (GAST) reconstruction at 2.5%, 5%, 25%, 50%, 75%, 95%, and 97.5% likelihood values and the 61 sea-surface temperature reconstructions used to create the GAST reconstruction, including a detailed summary table.

Additional data