Turn Do wn t he H e at: W h y a 4 ° C War m e r Wor ld Mu st B e Avoided
impacts of climate change, depending upon the rate and ultimate magnitude of the rise. Substantial progress has been made since the IPCC AR4 in the quantitative understanding of sea-level rise, especially closure of the sea-level rise budget. Updated estimates and reconstructions of sea-level rise, based on tidal gauges and more recently, satellite observations, confirm the findings of the AR4 (Figure 5) and indicate a sea-level rise of more than 20 cm since preindustrial times7 to 2009 (Church and White 2011). The rate of sea-level rise was close to 1.7 mm/year (equivalent to 1.7 cm/decade) during the 20th century, accelerating to about 3.2 mm/year (equivalent to 3.2 cm/decade) on average since the beginning of the 1990s (Meyssignac and Cazenave 2012). In the IPCC AR4, there were still large uncertainties regarding the share of the various contributing factors to sea-level rise, with the sum of individually estimated components accounting for less than the total observed sea-level rise. Agreement on the quantitative contribution has improved and extended to the 1972–2008 period using updated observational estimates (Church et al. 2011) (Figure 6): over that period, the largest contributions have come from thermal expansion (0.8 mm/year or 0.8 cm/decade), mountain glaciers, and ice caps (0.7 mm/year or 0.7 cm/decade), followed by the ice sheets (0.4 mm/year or 0.4 cm/decade). The study by Church et al. (2011) concludes that the human influence on the hydrological cycle through dam building (negative contribution as water is retained on land) and groundwater mining (positive contribution because of a transfer from land to ocean) contributed negatively (–0.1 mm/year or –0.1 cm/decade), to sea-level change over this period. The acceleration of sea-level rise over the last two decades is mostly explained by an increasing land-ice contribution from 1.1 cm/decade over 1972–2008 period to 1.7 cm/decade over 1993–2008 (Church et al. 2011), in particular because of the melting of the Greenland and Antarctic ice sheets, as discussed in the next section. The rate of land ice contribution to sea level rise has increased by about a factor of three since the 1972–1992 period. There are significant regional differences in the rates of observed sea-level rise because of a range of factors, including differential heating of the ocean, ocean dynamics (winds and currents), and the sources and geographical location of ice melt, as well as subsidence or uplifting of continental margins. Figure 7 shows reconstructed sea level, indicating that many tropical ocean regions have experienced faster than global average increases in sea-level rise. The regional patterns of sea-level rise will vary according to the different causes contributing to it. This is an issue that is explored in the regional projections of sea-level rise later in this report (see Chapter 4). Longer-term sea-level rise reconstructions help to locate the contemporary rapid rise within the context of the last few thousand years. The record used by Kemp et al. (2011), for example, shows 8
Figure 5:
Global mean sea level (GMSL) reconstructed from tidegauge data (blue, red) and measured from satellite altimetry (black). The blue and red dashed envelopes indicate the uncertainty, which grows as one goes back in time, because of the decreasing number of tide gauges. Blue is the current reconstruction to be compared with one from 2006. Source: Church and White 2011. Note the scale is in mm of sea-level-rise—divide by 10 to convert to cm.
Source: Church and White (2011).
a clear break in the historical record for North Carolina, starting in the late 19th century (Figure 8). This picture is replicated in other locations globally.
Increasing Loss of Ice from Greenland and Antarctica Both the Greenland and Antarctic ice sheets have been losing mass since at least the early 1990s. The IPCC AR4 (Chapter 5.5.6 in working group 1) reported 0.41 ±0.4 mm/year as the rate of sea-level rise from the ice sheets for the period 1993–2003, while a more recent estimate by Church et al. in 2011 gives 1.3 ±0.4 mm/year for the period 2004–08. The rate of mass loss from the ice sheets has thus risen over the last two decades as estimated from a combination of satellite gravity measurements, satellite sensors, and mass balance methods (Velicogna 2009; Rignot et al. 2011). At present, the losses of ice are shared roughly equally between Greenland and Antarctica. In their recent review of observations (Figure 9),
7
While the reference period used for climate projections in this report is the preindustrial period (circa 1850s), we reference sea-level rise changes with respect to contemporary base years (for example, 1980–1999 or 2000), because the attribution of past sea-level rise to different potential causal factors is difficult.