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.
- Climate Change 2013: The Physical Science Basis Ch. 5 (eds et al.) 383–464 (Cambridge Univ. Press, 2013) et al. in
- Making sense of palaeoclimate sensitivity. Nature 491, 683–691 (2012) et al.
- The middle Pleistocene transition: characteristics, mechanisms, and implications for long-term changes in atmospheric pCO2. Quat. Sci. Rev. 25, 3150–3184 (2006) et al.
- Evolution of ocean temperature and ice volume through the Mid-Pleistocene climate transition. Science 337, 704–709 (2012) et al.
- Sea-level and deep-sea-temperature variability over the past 5.3 million years. Nature 508, 477–482 (2014) et al.
- Hydrographic variations in deep ocean temperature over the mid-Pleistocene transition. Quat. Sci. Rev. 88, 147–158 (2014) , &
- Sea surface and high-latitude temperature sensitivity to radiative forcing of climate over several glacial cycles. J. Clim. 25, 1635–1656 (2012) , , , &
- Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation. Nature 484, 49–54 (2012) et al.
- EPICA Dome C record of glacial and interglacial intensities. Quat. Sci. Rev. 29, 113–128 (2010) et al.
- Trends, rhythms, and aberrations in global climate 65 Ma to present. Science 292, 686–693 (2001) , , , &
- Climate sensitivity, sea level and atmospheric carbon dioxide. Phil. Trans. R. Soc. Lond. A 371, (2013) , , &
- Aerosol radiative forcing and climate sensitivity deduced from the last glacial maximum to Holocene transition. Geophys. Res. Lett. 35, L04804 (2008) &
- Reconstruction of a continuous high-resolution CO2 record over the past 20 million years. Clim. Past 7, 1459–1469 (2011) , , , &
- Synchronous change of atmospheric CO2 and Antarctic temperature during the last deglacial warming. Science 339, 1060–1063 (2013) et al.
- Target atmospheric CO2: where should humanity aim? The Open Atmos. Sci. J. 2, 217–231 (2008) et al.
- Past and future polar amplification of climate change: climate model intercomparisons and ice-core constraints. Clim. Dyn. 26, 513–529 (2006) et al.
- What caused Earth’s temperature variations during the last 800,000 years? Data-based evidence on radiative forcing and constraints on climate sensitivity. Quat. Sci. Rev. 29, 129–145 (2010) et al.
- Revision of the EPICA Dome C CO2 record from 800 to 600kyr before present. Geophys. Res. Lett. 42, 542–549 (2015) et al.
- High Earth-system climate sensitivity determined from Pliocene carbon dioxide concentrations. Nat. Geosci. 3, 27–30 (2010) , , &
- Earth system sensitivity inferred from Pliocene modelling and data. Nat. Geosci. 3, 60–64 (2010)
- The timing of major climate terminations. Paleoceanography 12, 577–585 (1997)
- Early Pleistocene glacial cycles and the integrated summer insolation forcing. Science 313, 508–511 (2006)
- A depth-derived Pleistocene age model: uncertainty estimates, sedimentation variability, and nonlinear climate change. Paleoceanography 19, PA1028 (2004) &
- A phase-space model for Pleistocene ice volume. Earth Planet. Sci. Lett. 307, 94–102 (2011) , &
- Pleistocene sea-surface temperature evolution: early cooling, delayed glacial intensification, and implications for the mid-Pleistocene climate transition. Earth Sci. Rev. 123, 173–193 (2013) , , &
- On the mid-Pleistocene transition to 100-kyr glacial cycles and the asymmetry between glaciation and deglaciation times. Paleoceanography 18, PA000627 (2003) &
- Insolation-driven 100,000-year glacial cycles and hysteresis of ice-sheet volume. Nature 500, 190–193 (2013) et al.
- The role of orbital forcing, carbon dioxide and regolith in 100 kyr glacial cycles. Clim. Past 7, 1415–1425 (2011) &
- Subpolar link to the emergence of the modern Equatorial Pacific Cold Tongue. Science 328, 1550–1553 (2010) , , , &
- A Pliocene-Pleistocene stack of 57 globally distributed benthic δ18O records. Paleoceanography 20, PA1003 (2005) &
- Results of PMIP2 coupled simulations of the Mid-Holocene and Last Glacial Maximum — Part 1: experiments and large-scale features. Clim. Past 3, 261–277 (2007) et al.
- Climate model benchmarking with glacial and mid-Holocene climates. Clim. Dyn. 43, 671–688 (2014) et al.
- Can the Last Glacial Maximum constrain climate sensitivity? Geophys. Res. Lett. 39, L24702 (2012) , , &
- Environmental processes of the ice age: land, oceans, glaciers (EPILOG). Quat. Sci. Rev. 20, 627–657 (2001) , &
- Calibration of the alkenone paleotemperature index U37K' based on core-tops from the eastern South Atlantic and the global ocean (60°N-60°S). Geochim. Cosmochim. Acta 62, 1757–1772 (1998) , , , &
- Review of alkenone calibrations (culture, water column, and sediments). Geochem. Geophys. Geosyst. 2, http://dx.doi.org/10.1029/2000GC000055 (2001)
- Glacial-interglacial changes in Subantarctic sea surface temperature and δ18O-water using foraminiferal Mg. Earth Planet. Sci. Lett. 170, 417–432 (1999) , &
- Past temperature and δ18O of surface ocean waters inferred from foraminiferal Mg/Ca ratios. Nature 405, 442–445 (2000) &
- Long-term sea surface temperature and climate change in the Australian-New Zealand region. Paleoceanography 22, PA2215 (2007) , , , &
- A test for the presence of covariance between time-uncertain series of data with application to the Dongge Cave speleothem and atmospheric radiocarbon records. Paleoceanography 25, PA001713 (2010) &
- Probabilistic sequence alignment of stratigraphic records. Paleoceanography 29, 976–989 (2014) , , &
- Age dating and the orbital theory of the ice ages: development of a high-resolution 0 to 300000-year chronostratigraphy. Quat. Res. 27, 1–29 (1987) et al.
- Glacial variability over the last two million years: an extended depth-derived age model, continuous obliquity pacing, and the Pleistocene progression. Quat. Sci. Rev. 26, 37–55 (2007)
- Large-scale features of Pliocene climate: results from the Pliocene Model Intercomparison Project. Clim. Past 9, 191–209 (2013)
- The Plio-Pleistocene development of Atlantic deep-water circulation and its influence on climate trends. Quat. Sci. Rev. 123, 265–282 (2015) , &
- The Last Glacial Maximum climate over Europe and western Siberia: a PMIP comparison between models and data. Clim. Dyn. 17, 23–43 (2001) et al.
- Meta-analysis of tropical surface temperatures during the Last Glacial Maximum. Geophys. Res. Lett. 32, L05712 (2005) , , , &
- How cold was the Last Glacial Maximum? Geophys. Res. Lett. 33, L14709 (2006) , , &
- Probabilistic assessment of sea level during the last interglacial stage. Nature 462, 863–867 (2009) , , , &
- Evidence for warmer interglacials in East Antarctic ice cores. Nature 462, 342–345 (2009) , , &
- Does the Agulhas Current amplify global temperatures during super-interglacials? J. Quat. Sci. 25, 839–843 (2010) &
- How warm was the last interglacial? New model–data comparisons. Phil. Trans. R. Soc. 371, http://dx.doi.org/10.1098/rsta.2013.0097 (2013) et al.
- Modern Applied Statistics with S 4th edn (Springer, 2002) &
- Orbital and millennial-scale features of atmospheric CH4 over the past 800,000 years. Nature 453, 383–386 (2008) et al.
- Introduction to Regression Modeling (Duxbury Press, 2006) &
- Atmospheric carbon dioxide concentration across the Mid-Pleistocene Transition. Science 324, 1551–1554 (2009) , , , &
- Detecting trend and seasonal changes in satellite image time series. Remote Sens. Environ. 114, 106–115 (2010) , , &
- Stable sea surface temperatures in the western Pacific warm pool over the past 1.75 million years. Nature 433, 294–298 (2005) , , &
- The 100 000-yr cycle in tropical SST, greenhouse forcing, and climate sensitivity. J. Clim. 17, 2170–2179 (2004)
- Paleoclimate history of Galapagos surface waters over the last 135,000yr. Quat. Sci. Rev. 25, 1152–1167 (2006) et al.
- The mid-Pleistocene transition in the tropical Pacific. Science 310, 1009–1012 (2005) &
- Late Pleistocene surface and thermocline conditions of the eastern tropical Indian Ocean. Quat. Sci. Rev. 29, 887–896 (2010) et al.
- Paleo-sea surface temperature calculations in the equatorial east Atlantic from Mg/Ca ratios in planktic foraminifera: A comparison to sea surface temperature estimates from U37K', oxygen isotopes, and foraminiferal transfer function. Paleoceanography 15, 124–134 (2000) , &
- Amplitude and timing of sea-surface temperature change in the northern South China Sea: dynamic link to the East Asian monsoon. Geology 33, 785–788 (2005) &
- 340,000-year centennial-scale marine record of southern hemisphere climatic oscillation. Science 301, 948–952 (2003) , , &
- Planktonic foraminiferal Cd/Ca: paleonutrients or paleotemperature? Paleoceanography 14, 293–303 (1999) &
- Inter-hemispheric asymmetry in the early Pleistocene Pacific warm pool. Geophys. Res. Lett. 37, L11601 (2010) et al.
- A first look at past sea surface temperatures in the equatorial Indian Ocean from Mg/Ca in foraminifera. Geophys. Res. Lett. 32, L24605 (2005) , , , &
- High-resolution sea surface temperature records derived from foraminiferal Mg/Ca ratios during the last 260 ka in the northern South China Sea. Palaeogeogr. Palaeoclimatol. Palaeoecol. 250, 126–138 (2007) , , &
- 155,000 years of West African monsoon and ocean thermal evolution. Science 316, 1303–1307 (2007) , , &
- Late Quaternary variations in sea surface temperatures and their relationship to orbital forcing recorded in the Southern Ocean (Atlantic sector). Paleoceanography 14, 135–148 (1999) &
- Sea surface temperature variability in the North Atlantic during the last two glacial-interglacial cycles: comparison of faunal, oxygen isotopic, and Mg/Ca-derived records. Palaeogeogr. Palaeoclimatol. Palaeoecol. 204, 145–164 (2004) , &
- Hydrographic changes of the Southern Ocean (southeast Indian sector) over the last 230 kyr. Paleoceanography 11, 57–76 (1996) et al.
- Spatial and temporal oceanographic variability of the eastern equatorial Pacific during the late Pleistocene: evidence from Radiolaria microfossils. Paleoceanography 12, 381–393 (1997) &
- Response of surface water masses and circulation to Late Quaternary climate change east of New Zealand. Paleoceanography 13, 70–83 (1998) , &
- Combined coccolith, foraminiferal, and biomarker reconstruction of paleoceanographic conditions over the past 120 kyr in the northern North Atlantic (59°N, 23°W). Paleoceanography 14, 336–349 (1999) et al.
- Climate shock — Abrupt changes over millennial time scales. Phys. Today 55, 32–38 (2002)
- Interhemispheric synchrony of the last deglaciation inferred from alkenone palaeothermometry. Nature 385, 707–710 (1997) , &
- Southern Hemisphere forcing of Pliocene δ18O and the evolution of Indo-Asian monsoons. Paleoceanography 23, PA4210 (2008) , , , &
- Tropical ocean temperatures over the past 3.5 million years. Science 328, 1530–1534 (2010) , , &
- Millennial-scale variations in hydrography and biogeochemistry in the Eastern Equatorial Pacific over the last 100 kyr. Quat. Sci. Rev. 30, 210–223 (2011) et al.
- Molecular record of secular sea surface temperature changes on 100-year timescales for glacial terminations I, II and IV. Nature 356, 423–426 (1992) et al.
- Spatial and temporal sea-surface temperatures in the eastern equatorial Pacific over the past 150 kyr. Geophys. Res. Lett. 33, L13605 (2006) , , , &
- Evolution of the eastern tropical Pacific through Plio-Pleistocene glaciation. Science 312, 79–83 (2006) , &
- High-amplitude variations in North Atlantic sea surface temperature during the early Pliocene warm period. Paleoceanography 24, PA2218 (2009) , , , &
- Alkenone and coccolithophorid species changes in late Quaternary sediments from the Walvis Ridge: implications for the alkenone paleotemperature method. Palaeogeogr. Palaeoclimatol. Palaeoecol. 135, 71–96 (1997) , , &
- Sea surface temperatures of southern midlatitudes 0-160 kyr BP. Paleoceanography 21, PA2003 (2006) &
- High-resolution UK37 temperature reconstructions in the South China Sea over the past 220 kyr. Paleoceanography 14, 224–231 (1999) , , , &
- South Tasman Sea alkenone palaeothermometry over the last four glacial/interglacial cycles. Mar. Geol. 230, 73–86 (2006) , , , &
- Sea surface temperature and productivity records for the past 240 kyr in the Arabian Sea. Deep-Sea Res. 44, 1461–1480 (1997) , , , &
- Fidelity of alkenone paleotemperatures in southern Cape Basin sediment drifts. Paleoceanography 18, 1082 (2003) &
- Late Quaternary surface circulation in the east equatorial South Atlantic: evidence from alkenone sea surface temperatures. Paleoceanography 10, 197–219 (1995) , &
- Biomarker stratigraphic records over the last 150 kyears off the NW African coast at 25°N. Org. Geochem. 31, 577–588 (2000) et al.
- Assessment of sea surface temperature variations in the central North Atlantic using the alkenone unsaturation index U37k'. Geochim. Cosmochim. Acta 62, 2421–2427 (1998) , , , &
- Orbital-scale anti-phase variation of sea surface temperature in mid-latitude North Pacific margins during the last 145,000 years. Geophys. Res. Lett. 31, L16311 (2004) , , &
- The California current system during the last 136,000 years: response of the North Pacific High to precessional forcing. Quat. Sci. Rev. 26, 405–414 (2007) , &
- A millennial-scale U37K' sea-surface temperature record from the South China Sea (8°N) over the last 150 kyr: monsoon and sea-level influence. Palaeogeogr. Palaeoclimatol. Palaeoecol. 236, 39–55 (2006) , , &
- Atmospheric science: GRIP deuterium excess reveals rapid and orbital-scale changes in Greenland moisture origin. Science 309, 118–121 (2005) et al.
- Past temperature reconstructions from deep ice cores: relevance for future climate change. Clim. Past 2, 145–165 (2006) et al.
Extended data figures and tables
Extended Data Figures
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Extended Data Figure 7: Comparison of changes in GAST to changes in CO2 radiative forcing. (332 KB)
Extended Data Tables
- Supplementary Methods (187 KB)
This file contains the R code for key methods described in the paper.
- 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.