LETTERS PUBLISHED ONLINE: XX MONTH XXXX | DOI: 10.1038/NCLIMATE1229 Model-based evidence of deep-ocean heat uptake during surface-temperature hiatus periods Gerald A. Meehl1*, Julie M. Arblaster1,2, John T. Fasullo1, Aixue Hu1 and Kevin E. Trenberth1 1 There have been decades, such as 2000–2009, when the atmosphere and land, to melt ice or snow, or to be deposited in the 50 2 observed globally averaged surface-temperature time series subsurface ocean and manifested as changes in ocean temperatures 51 1 3 shows little positive or even slightly negative trend (a hiatus and thus heat content. Changes to the cryosphere and land 52 4 period). However, the observed energy imbalance at the subsurface play a much smaller role than the atmosphere and oceans 53 5 5 top-of-atmosphere for this recent decade indicates that a in energy flows , and they are not further considered in this paper. 54 −2 6 net energy flux into the climate system of about 1 W m The time series of globally averaged surface temperature from 55 7 (refs2,3) should be producing warming somewhere in the all five climate-model simulations show some decades with little 56 4,5 8 system . Here we analyse twenty-first-century climate-model or no positive trend (Fig.1a), as has occurred in observations 57 9 simulations that maintain a consistent radiative imbalance at (Supplementary Fig. S1 top). Running ten year linear trends of 58 −2 10 the top-of-atmosphere of about 1 W m as observed for the globally averaged surface temperature from the five model ensemble 59 11 past decade. Eight decades with a slightly negative global mean members reveal hiatus periods (Fig.1a) comparable to observations 60 12 surface-temperature trend show that the ocean above 300 m (Supplementary Fig. S1 middle). Using the first ensemble member 61 13 takes up significantly less heat whereas the ocean below 300 m as an example, the overall warming averaged over the century is 62 ◦ 14 takes up significantly more compared with non-hiatus decades. about C0:15 C per decade. However, the decades centred around 63 15 The model provides a plausible depiction of processes in the 2020, 2054, 2065, 2070, and several decades late in the century 64 16 climate system causing the hiatus periods, and indicates that a show either near zero or slightly negative trends in that ensemble 65 17 hiatus period is a relatively common climate phenomenon and member. We choose two ten year periods in this ensemble member 66 18 may be linked to La Niña-like conditions. when the globally averaged surface temperature is negative, that 67 ◦ 19 Observational datasets derived from the Argo float data and is, less than −0:10 C over the decade (Fig.1a), and six similar 68 20 other sources indicate that the ocean heat content above about periods that meet the same criterion from the other four ensemble 69 21 700 m did not increase appreciably during the 2000s, a time when members, to form an eight-member composite of hiatus periods. 70 6,7 22 the rise in surface temperatures also stalled . Hiatus periods The composite average net energy flux at the top of the 71 23 with little or no surface warming trend have occurred before in atmosphere for the eight hiatus periods (left side of Fig.1b) is 72 1,8 −2 24 observations, and are seen as well in climate-model simulations . C1:00 T0:88;1:11U W m (the positive sign convention indicates 73 25 So where does the excess heat in the climate system go if not net energy flux into the system), where the values in square 74 26 to increase surface temperatures or appreciably increase upper- brackets are error bars defined as (± one standard error ×1:86) 75 27 ocean heat content? There are suggestions that recent increases in to be consistent with a 5% significance level from a one- 76 9 10 28 stratospheric water vapour , stratospheric aerosols , tropospheric sided Student t-test (see Methods). This can be compared with 77 11 11 29 aerosols or the record solar minimum could have contributed the net energy flux averaged over all other 10 year running- 78 30 to the most recent hiatus by decreasing the net top-of-atmosphere average trends (numbering 435) in the five ensemble members 79 5 −2 31 (TOA) energy imbalance. However, the observational analyses of C1:02 T1:00;1:04U W m . The larger sample reduces the error 80 32 inherently include these effects, and the model analysed here bars compared with the hiatus decades. The error bars overlap, 81 33 produces close to the observed net energy imbalance during hiatus indicating no significant difference between the eight members in 82 12 34 periods, whereas some modelling studies do not . Alternatively, the composite and all other ten year periods, which is equivalent 83 35 significant heat could be sequestered in the deep ocean below 700 m to a significance calculation from a Student t-test (see Methods). 84 12–15 36 on decadal timescales . Here we examine simulations from a Thus in all decades in all five ensemble members there is a 85 −2 37 global coupled climate model to first show that the net TOA energy radiative imbalance of roughly 1 W m . This is indicative of 86 38 imbalance in this model is close to what has been observed over the ongoing increase in CO2 as well as in positive trends of 87 39 the past decade or so. Then we analyse where the heat could be globally averaged surface temperature that occur in most time 88 40 going in the observed system during hiatus periods, and point to periods of the twenty-first century in the model (Fig.1a). So 89 41 the processes that could be responsible. where is the energy gained by the climate system going during ten 90 42 Five ensemble members from a future-climate-model year time periods when the globally averaged surface-temperature 91 16,17 43 simulation (see Methods) are examined to track the energy trend is slightly negative? 92 44 flow during the simulation where the net energy flux at the TOA As noted above, the most likely candidate is the deep ocean, 93 −2 45 from increasing greenhouse gases is about 1 W m , indicating a and this is indeed the case for the model (Fig.1b for global 94 46 net energy surplus being directed into the climate system, mainly averages, and Supplementary Fig. S2 by basin). For the upper 95 18 47 from decreases in outgoing long-wave radiation . If there are 300 m layer, the composite global average heat-content trend 96 48 time periods when globally averaged surface temperatures are not for the eight decades with negative surface-temperature trend is 97 23 49 increasing, this excess energy must go elsewhere, either to heat the C0:17 T0:07;0:27U×10 J per decade, a reduction of 60% compared 98 1National Center for Atmospheric Research, Boulder, Colorado, USA, 2CAWCR, Bureau of Meteorology, Melbourne, Australia. *e-mail: [email protected]; [email protected]. NATURE CLIMATE CHANGE j ADVANCE ONLINE PUBLICATION j www.nature.com/natureclimatechange 1 LETTERS NATURE CLIMATE CHANGE DOI: 10.1038/NCLIMATE1229 a 290.0 Ensemble member 1 2 3 4 289.5 5 0.6 Ten year cooling Running Ts trends 289.0 intervals over ten years 0.4 Surface temperature (K) 0.2 K per decade 0 288.5 ¬0.2 2020 2040 2060 2080 2020 2040 2060 2080 2100 Year bcOcean HC trend (10 1.5 1.5 90° N ) Hiatus decades 2.00 ¬2 1.75 Surface temperature 1.2 All other decades 1.2 60° N 1.50 ( 1.25 ° 1.00 C per decade) 30° N 0.75 0.9 0.9 0.50 0.25 0° 0 0.6 0.6 23 ¬0.25 Latitude J per decade) ¬0.50 30° S ¬0.75 ¬1.00 0.3 0.3 ¬1.25 60° S ¬1.50 ¬1.75 TOA net radiation (W m 0 0 ¬2.00 90° S TOA 0¬300 m 300¬750 m 750 m¬bottom 120° E180° 120° W 60° W600° ° E Longitude Figure 1 j a, Annual mean globally averaged surface temperature for the five climate-model ensemble members (coloured lines) and ensemble mean (black line), highlighting two ten-year negative-temperature-trend periods and ten-year running trends from this ensemble member (inset). b, Left: composite global linear trends for hiatus decades (red bars) and all other decades (green bars) for TOA net radiation (positive values denote net energy entering the system). Right: global ocean heat-content (HC) decadal trends (1023 J per decade) for the upper ocean (surface to 300 m) and two deeper ocean layers (300–750 m and 750 m–bottom), with error bars defined as ± one standard error ×1:86) to be consistent with a 5% significance level from a one-sided Student t-test. c, Composite average SST linear trends for hiatus decades; stippling denotes 5% significance. 1 with the average trend over all decades for the five ensemble Examination of ocean heat-content trends by basin (Supple- 23 23 2 members of C0:42 T0:41;0:44U × 10 J per decade. Thus this layer mentary Fig. S2) indicates qualitatively similar results to the global 24 3 is still gaining heat, but at a much reduced rate during hiatus ocean, with decreases in heat-content trends above 300 m and 25 4 periods compared with other decades.