Challenger7, 8, 1872–1876, was the first globe-circling study of the oceans, obtaining multidisciplinary data along a 69,000-nautical-mile track. “One of the objects of the Expedition was to collect information as to the distribution of temperature in the waters of the ocean … not only at the surface, but at the bottom, and at intermediate depths”3. The thermal stratification of the oceans was described for the first time from about 300 temperature profiles made using pressure-protected thermometers. The Challenger temperature data set was still prominent in large-scale maps and analyses even into the 1940s (ref. Challenger expedition was undertaken until the 1920s–1950s, when theMeteor10, Discovery11, Discovery II11, and Atlantis12 systematically explored the Atlantic and Southern oceans. The voyage of HMS, 1872–1876, was the first globe-circling study of the oceans, obtaining multidisciplinary data along a 69,000-nautical-mile track. “One of the objects of the Expedition was to collect information as to the distribution of temperature in the waters of the oceannot only at the surface, but at the bottom, and at intermediate depths”. The thermal stratification of the oceans was described for the first time from about 300 temperature profiles made using pressure-protected thermometers. Thetemperature data set was still prominent in large-scale maps and analyses even into the 1940s (ref. 9 ). Nothing remotely comparable to theexpedition was undertaken until the 1920s–1950s, when the, andsystematically explored the Atlantic and Southern oceans.

Although the Challenger temperature profiles were global in scale, as they were made along the vessel’s track they were not global in the sense of areal sampling. The modern-day Argo Programme, by contrast, is the first globally and synoptically sampled data set of temperature and salinity. Argo’s free-drifting profiling floats collect more than 100,000 temperature/salinity profiles per year, nominally every 3° of latitude and longitude, every 10 days and to depths as great as 1,980 m.

Challenger transect from New York to Bermuda to St Thomas with nearby tracks sampled in the 1950s, C. Wunsch observed “One is hard pressed to detect any significant differences on the large scale”13. Now, with an added 50-year interval, a quantitative comparison is made by interpolating Argo data14 to the location and depth of eachChallenger measurement and to the same time of year, to minimize seasonal sampling bias in theChallenger data set ( m. The largest values are in the Gulf Stream (about 38° N), indicating that the current is at a higher latitude in the Argo data than in the Challenger data. Obviously, these local differences may represent any timescale in the 135-year interval—from a transient meander of the Gulf Stream in 1873 to a long-term change in the current’s latitude. Similarly, regional to ocean-scale differences may be affected by interannual to decadal15, 16 variability, including in the deep ocean17, and hence our Challenger-to-Argo difference based on stations along the Challenger track must be viewed with caution. When qualitatively comparing features of thetransect from New York to Bermuda to St Thomas with nearby tracks sampled in the 1950s, C. Wunsch observed “One is hard pressed to detect any significant differences on the large scale”. Now, with an added 50-year interval, a quantitative comparison is made by interpolating Argo datato the location and depth of eachmeasurement and to the same time of year, to minimize seasonal sampling bias in thedata set ( Fig. 1 ). Warming is predominant from the sea surface to below 1,800m. The largest values are in the Gulf Stream (about 38°N), indicating that the current is at a higher latitude in the Argo data than in thedata. Obviously, these local differences may represent any timescale in the 135-year interval—from a transient meander of the Gulf Stream in 1873 to a long-term change in the current’s latitude. Similarly, regional to ocean-scale differences may be affected by interannual to decadalvariability, including in the deep ocean, and hence our-to-Argo difference based on stations along thetrack must be viewed with caution.

Figure 1: New York–St Thomas transect differences.





14 along theChallenger’s New York to Bermuda to St Thomas transect. Colour spots show where Argo values are warmer (red), unchanged (white), or cooler (blue) than Challenger, with magnitudes according to the colour scale. Background contours indicate mean temperature (2004–2010) from Argo dataalong the’s New York to Bermuda to St Thomas transect. Colour spots show where Argo values are warmer (red), unchanged (white), or cooler (blue) than, with magnitudes according to the colour scale. Full size image (95 KB) Figures/tables index

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Challenger differences reveal warming in both the Atlantic and Pacific oceans (Challenger made only a few stations in the Indian Ocean, all at high southern latitudes, so that region is omitted here. Out of 273 Challenger temperature stations analysed, the Argo-era sea surface temperature (SST) is higher at 212. The mean SST difference is 1.0 °C±0.11 for the Atlantic and 0.41 °C±0.09 for the Pacific Ocean. As the Challenger’s sampling was more intensive in the Atlantic and the warming may be greater in that ocean, we estimate the global difference as the area-weighted mean of the Atlantic and Pacific values, 0.59 °C±0.12. There are extensive historical measurements of SST, providing context for the Argo-minus-Challengercomparison. A time series of reconstructed global mean SST from 1856 to the present day5 indicates a cooling of SST from 1880 to 1910, with larger warming since 1910. The overall warming5 between the Challenger and Argo eras of about 0.5 °C is consistent with the Argo-minus-Challenger estimate, given the sampling errors. Seasonally adjusted Argo-minus-differences reveal warming in both the Atlantic and Pacific oceans ( Fig. 2a ). Themade only a few stations in the Indian Ocean, all at high southern latitudes, so that region is omitted here. Out of 273temperature stations analysed, the Argo-era sea surface temperature (SST) is higher at 212. The mean SST difference is 1.0°C±0.11 for the Atlantic and 0.41°C±0.09 for the Pacific Ocean. As the’s sampling was more intensive in the Atlantic and the warming may be greater in that ocean, we estimate the global difference as the area-weighted mean of the Atlantic and Pacific values, 0.59°C±0.12. There are extensive historical measurements of SST, providing context for the Argo-minus-comparison. A time series of reconstructed global mean SST from 1856 to the present dayindicates a cooling of SST from 1880 to 1910, with larger warming since 1910. The overall warmingbetween theand Argo eras of about 0.5°C is consistent with the Argo-minus-estimate, given the sampling errors.

Challenger’s subsurface temperature measurements were made using Six’s (Miller–Casella) thermometers. These are maximum/minimum thermometers, with the mercury column displacing a sliding index to record the maximum or minimum temperature, and are fitted with an external bulb to remove the influence of pressure. These instruments were used in the initial belief that temperature decreased monotonically with increasing depth, an assumption discovered to be incorrect during the voyage. Other types of thermometer were used less often, including reversing thermometers that became commonplace later. The Six’s thermometers were graduated in increments of 1° F (0.56 °C) and “the length occupied by one degree (F) could not easily have been subdivided beyond a quarter”3. Hence the temperature, which was recorded to a precision of 0.1° F (0.06 °C), had a reading accuracy of about 0.14 °C. In the report of results18, the type and serial number of thermometers used on each station are not specified. The sounding line was 8-mm-diameter hemp, with a bottom weight of 25–75 kg (ref. min…” and after recovery the thermometers were “…carefully read and registered…and …corrected for errors of zero point…and a curve of temperatures drawn”. It is noted that if there were outliers “…the temperatures at those depths were taken again”3. TheChallenger also deployed water-filled and mercury-filled piezometers, constructed like unprotected Six’s thermometers with one end open3. Together with data from the protected thermometers, these could be used to estimate depth. However, the ratio of temperature to depth sensitivity of these instruments was 1 °C for 783 m of depth change3, so they were useful for correcting only large errors in near-bottom depths. The Challenger data listings18 do not explicitly state that the fathom (fm) values are uncorrected line-out, but this is evident because the 100-fm and other evenly spaced increments in the data records were obtainable only by measuring and marking the sounding line. Most of’s subsurface temperature measurements were made using Six’s (Miller–Casella) thermometers. These are maximum/minimum thermometers, with the mercury column displacing a sliding index to record the maximum or minimum temperature, and are fitted with an external bulb to remove the influence of pressure. These instruments were used in the initial belief that temperature decreased monotonically with increasing depth, an assumption discovered to be incorrect during the voyage. Other types of thermometer were used less often, including reversing thermometers that became commonplace later. The Six’s thermometers were graduated in increments of 1°F (0.56°C) and “the length occupied by one degree (F) could not easily have been subdivided beyond a quarter”. Hence the temperature, which was recorded to a precision of 0.1°F (0.06°C), had a reading accuracy of about 0.14°C. In the report of results, the type and serial number of thermometers used on each station are not specified. The sounding line was 8-mm-diameter hemp, with a bottom weight of 25–75kg (ref. 3 ). During the measurements the line was “…kept quite perpendicular for 5min…” and after recovery the thermometers were “…carefully read and registered…and …corrected for errors of zero point…and a curve of temperatures drawn”. It is noted that if there were outliers “…the temperatures at those depths were taken again”. Thealso deployed water-filled and mercury-filled piezometers, constructed like unprotected Six’s thermometers with one end open. Together with data from the protected thermometers, these could be used to estimate depth. However, the ratio of temperature to depth sensitivity of these instruments was 1°C for 783m of depth change, so they were useful for correcting only large errors in near-bottom depths. Thedata listingsdo not explicitly state that the fathom (fm) values are uncorrected line-out, but this is evident because the 100-fm and other evenly spaced increments in the data records were obtainable only by measuring and marking the sounding line.

Challenger subsurface data. First, taking depth from line-out overestimates the true depth of the thermometer, resulting in a warm bias in the recorded temperature. “If there be a current of any appreciable force, the sounding line begins to wander about, and has to be followed by the ship…an operation of considerable delicacy, even in good weather”3. Second, before the Challenger’s voyage, laboratory measurements of pressure effects on the Challenger thermometers had been made erroneously19. The post-voyage analysis by P. Tait showed that the actual compression effect on the protected glass thermometers was about 0.04 °C km−1 (0.3° F per 2,500 fm; ref. 18 rather than the overcorrected version listed in otherChallenger reports. Finally, the Challenger thermometers were mounted in their frames using vulcanite, compression warming of which might be transferred to the glass, causing a small warm bias in the reading19. Thus, the errors in depth and temperature all tend to make the Challengertemperatures systematically warm at the recorded depths. A small number of temperature measurements were discarded in our analysis. Stations at high southern latitude were excluded owing to the shallow-temperature minimum found there and at a few other locations where Argo indicates a temperature minimum, making them incompatible with the use of maximum/minimum thermometers. The lack of high-latitude and Indian Ocean stations could produce a sampling error in global averages, as multidecadal ocean warming is known to have been strong in the Southern Ocean since the 1930s (ref. 17. Error bars on our estimates of globally averaged temperature differences are discussed in the Three sources of systematic error are considered in thesubsurface data. First, taking depth from line-out overestimates the true depth of the thermometer, resulting in a warm bias in the recorded temperature. “If there be a current of any appreciable force, the sounding line begins to wander about, and has to be followed by the ship…an operation of considerable delicacy, even in good weather”. Second, before the’s voyage, laboratory measurements of pressure effects on thethermometers had been made erroneously. The post-voyage analysis by P. Tait showed that the actual compression effect on the protected glass thermometers was about 0.04°Ckm(0.3°F per 2,500fm; ref. 19 ), much less than the prevoyage estimates. We therefore used the raw temperature datarather than the overcorrected version listed in otherreports. Finally, thethermometers were mounted in their frames using vulcanite, compression warming of which might be transferred to the glass, causing a small warm bias in the reading. Thus, the errors in depth and temperature all tend to make thetemperatures systematically warm at the recorded depths. A small number of temperature measurements were discarded in our analysis. Stations at high southern latitude were excluded owing to the shallow-temperature minimum found there and at a few other locations where Argo indicates a temperature minimum, making them incompatible with the use of maximum/minimum thermometers. The lack of high-latitude and Indian Ocean stations could produce a sampling error in global averages, as multidecadal ocean warming is known to have been strong in the Southern Ocean since the 1930s (ref. 20 ) as well as having substantial basin-to-basin differences. Error bars on our estimates of globally averaged temperature differences are discussed in the Methods section.

m (200 fm, m (500 fm, °C±0.18 at 366 m and 0.12 °C±0.07 at 914 m, reaching zero at about 1,500 m ( Proceeding downwards to 366m (200fm, Fig. 2b ) and 914m (500fm, Fig. 2c ), the pattern of mostly warm differences persists in both oceans, diminishing in magnitude with depth. The global average temperature difference (ocean area weighted) decreases to 0.39°C±0.18 at 366m and 0.12°C±0.07 at 914m, reaching zero at about 1,500m ( Fig. 3 ).





Figure 3: Globally averaged difference.

Challenger temperature difference ±1 s.e.m. The black line is a simple mean over all stations with data at 183-m (100-fm) intervals. The red line uses values for the Atlantic and Pacific oceans in a weighted mean, with weights proportional to the area of the two oceans. The blue line applies the Tait pressure correction19 (−0.04 °C km−1) to the weighted mean. Error estimates are described in Mean Argo-minus-temperature difference ±1 s.e.m. The black line is a simple mean over all stations with data at 183-m (100-fm) intervals. The red line uses values for the Atlantic and Pacific oceans in a weighted mean, with weights proportional to the area of the two oceans. The blue line applies the Tait pressure correction(−0.04°Ckm) to the weighted mean. Error estimates are described in Methods Full size image (75 KB) Previous

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m, the ocean area-weighted difference, using only those stations with samples every 183 m, is 0.33 °C±0.14, corresponding to a heat gain of 1×109 J m−2. This increases to 1.3×109 J m−2 for 0–1,500 m, or 0.3 W m−2 of ocean surface area, averaged over the 135-year interval. The average differences, 0–700 m, are 0.58 °C±0.12 for the Atlantic and 0.22 °C±0.11 for the Pacific Ocean. The Tait pressure correction19, equivalent to −0.04 °C km−1 would increase these values by only 4 % . The temperature bias caused by depth errors is difficult to assess, but may be significant at locations such as the equatorial Pacific, where the strong subsurface shear of the Equatorial Undercurrent, not known in Challenger’s time, would cause a slant in the line. Evidence of this bias can be seen ( m at near-equatorial Pacific stations. A systematic overestimate of 1 % in the depth of Challenger measurements would result in a warm bias in the 0–700 m average temperature of about 0.05 °C. For the upper 700m, the ocean area-weighted difference, using only those stations with samples every 183m, is 0.33°C±0.14, corresponding to a heat gain of 1×10. This increases to 1.3×10for 0–1,500m, or 0.3of ocean surface area, averaged over the 135-year interval. The average differences, 0–700m, are 0.58°C±0.12 for the Atlantic and 0.22°C±0.11 for the Pacific Ocean. The Tait pressure correction, equivalent to −0.04°Ckmwould increase these values by only 4. The temperature bias caused by depth errors is difficult to assess, but may be significant at locations such as the equatorial Pacific, where the strong subsurface shear of the Equatorial Undercurrent, not known in’s time, would cause a slant in the line. Evidence of this bias can be seen ( Fig. 2a,b ) where temperature differences change from positive to negative between the sea surface and 366m at near-equatorial Pacific stations. A systematic overestimate of 1in the depth ofmeasurements would result in a warm bias in the 0–700m average temperature of about 0.05°C.

Challenger temperature measurements are known to be far from perfect and were the subject of controversy as instanced in the correspondence between J. Murray and W. Leighton Jordan in the late 1880s (ref. Challenger and Argo data sets. Thetemperature measurements are known to be far from perfect and were the subject of controversy as instanced in the correspondence between J. Murray and W. Leighton Jordan in the late 1880s (ref. 21 ). However, the data were collected with great care and attention, and the large temperature changes over the subsequent 135 years are revealed by comparing theand Argo data sets.

Challenger in the 1870s and that the warming signal is global in extent. Challenger obtained enough measurements of temperature for statistical confidence at about the 95 % level in the mean temperature differences and the nature of systematic errors in the Challenger data makes these differences a lower bound on the true values. Moreover, comparisons with other temperature records including global SST (ref. 6, all indicate that the warming has occurred on the centennial timescale rather than being limited to recent decades. From 1969 to 2009, globally distributed temperature measurements, 0–700 m, showed warming of an average of 0.17 °C (ref. °C) than the Pacific (0.12 °C). The larger temperature change observed between the Challenger expedition and Argo Programme, both globally (0.33 °C±0.14, 0–700 m) and separately in the Atlantic (0.58 °C±0.12) and Pacific (0.22 °C±0.11), therefore seems to be associated with the longer timescale of a century or more. We find that the modern upper ocean is substantially warmer than the ocean measured by HMSin the 1870s and that the warming signal is global in extent.obtained enough measurements of temperature for statistical confidence at about the 95level in the mean temperature differences and the nature of systematic errors in thedata makes these differences a lower bound on the true values. Moreover, comparisons with other temperature records including global SST (ref. 5 ), extensive subsurface data in the Atlantic as early as the 1920s (ref. 22 ) and global subsurface data over the past 50 years, all indicate that the warming has occurred on the centennial timescale rather than being limited to recent decades. From 1969 to 2009, globally distributed temperature measurements, 0–700m, showed warming of an average of 0.17°C (ref. 6 ), with the Atlantic Ocean warming more strongly (0.30°C) than the Pacific (0.12°C). The larger temperature change observed between theexpedition and Argo Programme, both globally (0.33°C±0.14, 0–700m) and separately in the Atlantic (0.58°C±0.12) and Pacific (0.22°C±0.11), therefore seems to be associated with the longer timescale of a century or more.

23, 24, 25 and extending the record length of subsurface temperature can help in the understanding of the centennial timescale in sea-level rise26, 27. Furthermore, changes in subsurface temperature and in SST are closely related. SST is important in determining air–sea exchanges of heat and increasing SST is linked to increasing rates of evaporation, and hence precipitation, in the global hydrological cycle28, 29. The long-term increase of SST should be understood in the context of changes in both temperature and salinity extending deep into the water column. The implications of centennial-scale warming of the subsurface oceans extend beyond the climate system’s energy imbalance. Thermal expansion is a substantial contributor to global sea-level riseand extending the record length of subsurface temperature can help in the understanding of the centennial timescale in sea-level rise. Furthermore, changes in subsurface temperature and in SST are closely related. SST is important in determining air–sea exchanges of heat and increasing SST is linked to increasing rates of evaporation, and hence precipitation, in the global hydrological cycle. The long-term increase of SST should be understood in the context of changes in both temperature and salinity extending deep into the water column.

Enormous advances in ocean-observing technology have occurred from the time of the Challenger, when about 300 deep-ocean temperature profiles were acquired over three-and-a-half years by a ship with more than 200 crew on board, to today’s Argo Programme, obtaining more than 100,000 temperature profiles annually by autonomous instrumentation. The Challenger data set was a landmark achievement in many respects. With regard to climate and climate change, Challengernot only described the basic temperature stratification of the oceans, but provided a valuable baseline of nineteenth-century ocean temperature that, along with the modern Argo data set, establishes a lower bound on centennial-scale global ocean warming.