| 英文摘要: | In recent decades, over nine-tenths of Earth's top-of-the-atmosphere energy imbalance has been stored in the ocean, which is rising as it warms. Combining satellite sea-level data with ocean mass data or model results allows insights into ocean warming. One could say that global warming is ocean warming. Quantifying how fast, and where, the ocean is warming is vital to understanding how much and how fast the atmosphere will warm, and seas will rise1, 2, 3, 4. Historically, sparse and sometimes uncertain in situ ocean temperature data have made that quantification challenging5. Writing in Nature Climate Change, Paul Durack and colleagues6 argue that previously published 0–700 m ocean warming rates based on in situ ocean temperature data are underestimated in the more sparsely sampled Southern Hemisphere, at least from 1970 until the advent of the Argo array of profiling floats7 in the early 2000s. They reach this conclusion by analysing the in situ data together with satellite sea-level data and climate model results. Another study8 published in Nature Climate Change concludes that the deeper half of the ocean (below 2 km depth) has, on average, not warmed from 2005 to 2013. William Llovel et al.8 arrive at this result by combining satellite sea-level and ocean-mass-change data with unprecedentedly dense and high-quality Argo in situ temperature data from the upper half of the ocean volume (to depths of 2 km). Changes in sea level and ocean temperature are closely linked by the fact that seawater expands as it warms. However, at least three factors complicate inferring one from the other directly. First, the transfer of water from land to ocean also contributes to sea-level rise, at a comparable magnitude to ocean warming1. This transfer has mostly been owing to melting of glaciers and ice sheets over recent decades. Second, the amount of sea-level rise depends on where the ocean warming occurs, because colder seawater can absorb several times more thermal energy than warmer water while contributing to the same amount of sea-level rise. Finally, regional redistributions of ocean salinity9 and mass10 also affect sea level. Durack et al.6 exploit the close relationship between upper-ocean warming and sea-level rise, the latter being well measured by satellites since 1992, to refine estimates of global ocean warming rates. They find good agreement between modelled and observed sea-level rise and poorer agreement between modelled and reported upper-ocean warming rates from 1970 to 2004, especially in the Southern Hemisphere. They argue that observational estimates of ocean warming rates in the Southern Hemisphere are likely biased low owing to the sparse historical temperature data there, and suggest that these estimates should be increased by 48–152% based on the ratio of observed sea-level rise and upper-ocean warming in the better-observed Northern Hemisphere. Their results6 are of importance because they imply an increase of 0.04–0.13 W m−2 in the global energy imbalance from 1970 to 2004, important for evaluating the performance of climate models. Also, quantifying how much of sea-level rise is owing to expansion of warming seawater versus transfer of water from land to ocean is important for understanding how much and how fast sea level will rise. However, as noted above, there are other, neglected, contributions to variations in sea level that could affect the results. Llovel et al.8 report that sea level has been rising at a global average rate of 2.78 mm yr−1 from 2005 to 2013 as estimated from satellite altimetry data. Over that period, ocean expansion from warming in the upper 2 km has accounted for 0.9 mm yr−1 of that rise, according to in situ ocean measurements, and transfer of freshwater into the ocean 2.0 mm yr−1, according to satellite measurements of Earth's changing gravity field. The residual of those numbers implies a deep ocean cooling equivalent to a decrease in sea level of −0.13 mm yr−1. This residual calculation was not possible before the advent of satellite gravity measurements in 2003 and the achievement of near-global measurements of ocean temperature and salinity in the upper 2 km by Argo starting around 2005. Their result8 is interesting because first, in contrast, analyses of sparse ship-based measurements below 2 km depth reveal that the deep ocean warmed from the 1990s to about 2005, contributing about 0.11 mm yr−1 of sea-level rise11. Second, the uncertainty for their residual estimate of the rate of deep ocean temperature change, equivalent to 0.44 W m−2 distributed over the surface area of Earth, is two-thirds the size of their central estimate, 0.64 W m−2, of the net ocean warming from 2005 to 2013. Although their residual calculation is suggestive, its uncertainties are large. Eliminating the deep-ocean measurement gap would reduce uncertainties and increase confidence in the robustness of sea-level and planetary energy-imbalance budgets by allowing complete cross checks among ocean mass, ocean warming and sea-level measurements. Argo has revolutionized large-scale observational physical oceanography for the upper 2 km of the ocean7, but more temperature data and salinity data are now required in the half of the ocean volume below 2 km depth. Global ship-based oceanographic surveys, while invaluable in the accuracy and variety of data they collect, are too expensive to gather the quantity of deep ocean temperature and salinity data needed. To close this deep-ocean measurement gap, work is proceeding on 'Deep Argo', on which one of us, G.C.J., is a principal investigator. This project is a proposed global array of profiling floats capable of measuring temperature and salinity to high accuracy and to depths of 6 km. In June 2014, scientists deployed two Deep Argo floats (Fig. 1) east of New Zealand. Scientists of various nations are developing regional Deep Argo pilot arrays in other areas that have exhibited large abyssal and deep-ocean temperature and salinity variability, including the Southern and North Atlantic oceans. With hard work and commitment, a decade from now, the fledgling regional Deep Argo arrays could grow and merge to become global. If they do, ocean heat content and sea-level budgets will no longer be hampered by a residual term.
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