| 英文摘要: | Present-day sea-level rise is a major indicator of climate change1. Since the early 1990s, sea level rose at a mean rate of ~3.1 mm yr−1 (refs 2, 3). However, over the last decade a slowdown of this rate, of about 30%, has been recorded4, 5, 6, 7, 8. It coincides with a plateau in Earth’s mean surface temperature evolution, known as the recent pause in warming1, 9, 10, 11, 12. Here we present an analysis based on sea-level data from the altimetry record of the past ~20 years that separates interannual natural variability in sea level from the longer-term change probably related to anthropogenic global warming. The most prominent signature in the global mean sea level interannual variability is caused by El Niño–Southern Oscillation, through its impact on the global water cycle13, 14, 15, 16. We find that when correcting for interannual variability, the past decade’s slowdown of the global mean sea level disappears, leading to a similar rate of sea-level rise (of 3.3 ± 0.4 mm yr−1) during the first and second decade of the altimetry era. Our results confirm the need for quantifying and further removing from the climate records the short-term natural climate variability if one wants to extract the global warming signal10. Precisely estimating present-day sea-level rise caused by anthropogenic global warming is a major issue that allows assessment of the process-based models developed for projecting future sea level1. Sea-level rise is indeed one of the most threatening consequences of ongoing global warming, in particular for low-lying coastal areas that are expected to become more vulnerable to flooding and land loss. As these areas often have dense populations, important infrastructures and high-value agricultural and bio-diverse land, significant impacts such as increasingly costly flooding or loss of freshwater supply are expected, posing a risk to stability and security17, 18. However, sea level also responds to natural climate variability, producing noise in the record that hampers detection of the global warming signal. Trends of the satellite altimetry-based global mean sea level (GMSL) are computed over two periods: the period 1994–2002 and the period 2003–2011 of the observed slowdown (Fig. 1a). GMSL time series from five prominent groups processing satellite altimetry data for the global ocean are considered (Methods). During recent years (2003–2011), the GMSL rate was significantly lower than during the 1990s (average of 2.4 mm yr−1 versus 3.5 mm yr−1). This is observed by all processing groups (Fig. 1a). The temporal evolution of the GMSL rate (computed over five-year-long moving windows, starting in 1994 and shifted by one year) was nearly constant during the 1990s, whereas the rate clearly decreased by ~30% after ~2003 (Fig. 2a). This decreasing GMSL rate coincides with the pause observed over the last decade in the rate of Earth’s global mean surface temperature increase9, 10, an observation exploited by climate sceptics to refute global warming and its attribution to a steadily rising rate of greenhouse gases in the atmosphere. It has been suggested that this so-called global warming hiatus11 results from El Niño–Southern Oscillation- (ENSO-) related natural variability of the climate system10 and is tied to La Niña-related cooling of the equatorial Pacific surface11, 12. In effect, following the major El Niño of 1997/1998, the past decade has favoured La Niña episodes (that is, ENSO cold phases, reported as sometimes more frequent and more intensive than the warm El Niño events, a sign of ENSO asymmetry19). The interannual (that is, detrended) GMSL record of the altimetry era seems to be closely related to ENSO, with positive/negative sea-level anomalies observed during El Niño/La Niña events2. Recent studies have shown that the short-term fluctuations in the altimetry-based GMSL are mainly due to variations in global land water storage (mostly in the tropics), with a tendency for land water deficit (and temporary increase of the GMSL) during El Niño events13, 14 and the opposite during La Niña15, 16. This directly results from rainfall excess over tropical oceans (mostly the Pacific Ocean) and rainfall deficit over land (mostly the tropics) during an El Niño20 event. The opposite situation prevails during La Niña. The succession of La Niña episodes during recent years has led to temporary negative anomalies of several millimetres in the GMSL (ref. 15), possibly causing the apparent reduction of the GMSL rate of the past decade. This reduction has motivated the present study. From seasonal to centennial time scales, the two main contributions to GMSL variability and change come from ocean thermal expansion and ocean mass. Owing to water mass conservation in the climate system, sources of global ocean mass variations are land ice masses, land water storage and atmospheric water vapour content. Studies have shown that ENSO-driven interannual variability in the global water cycle strongly impacts land water storage12, 13, 14, 15 and atmospheric water vapour21, hence ocean mass and GMSL.
| http://www.nature.com/nclimate/journal/v4/n5/full/nclimate2159.html
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