Past changes in sea level
One hallmark of global temperature change is a commensurate change in sea level. There are two main processes at work: (1) during colder times, more of the Earth’s water is tied up in ice sheets, leading to a drop in sea level (and, conversely, a rise in sea level during warm, ice-deficient times); and (2) warming of the ocean causes thermal expansion, which increases the ocean’s volume and thus sea level (and vice-versa for cooling). Projected future sea level rise has very serious implications for coastal areas occupied by humans, so studying sea level during past climate extremes is a useful exercise. The IPCC focuses on the Mid-Pliocene Warm Period, the Last Interglacial, and the Last Glacial Termination/Holocene for sea level change analyses.
One complication of reconstructing global mean sea level is that relative sea level varies widely region-to-region. This is due to two primary processes: (1) large masses of continental ice exert a gravitational force that essentially pulls ocean toward the ice bodies, causing a change in relative sea level without a change in ocean volume; and (2) the presence of a large mass of ice results in a long-term vertical adjustment of continents in terms of how far they sink into the mantle due to the weight of the ice sheet (see schematic figure to the right). Despite these complications, multiple lines of proxy evidence from different parts of the Earth can help paleoclimatologists to constrain changes in global ocean volume.
Schematic figure illustrating isostatic adjustment, the process of a large mass (like ice) depressing a continent and changing relative sea level. From ocean.tamu.edu.
The Last Interglacial
During the warmth of the Last Interglacial (LIG; 129,000 to 116,000 years ago), sea level was likely between 5 and 10 m higher than present, with 6 m being the best estimate from multiple studies. Moreover, sea level during this time likely fluctuated by several meters, and rates of sea level rise may have been up to 1.1 to 2.6 m per century. Figure 5.15 above highlights proxy estimates of LIG sea level change for the global average (a) and Western Australia (b). Western Australia is tectonically stable and does not hold ice during glacial periods, so the magnitude of sea level change is dampened there relative to more isostatically active regions. High sea level during the LIG was likely a result of ice melt from the Greenland Ice Sheet, the West Antarctic Ice Sheet, certain margins of the East Antarctic Ice Sheet, and mountain glaciers. Evidence from ice cores and other observational data indicate that the Greenland Ice Sheet did not completely disappear during the LIG, and quantitative estimates of the relative contributions to sea level rise are not currently possible.
Last Glacial Termination and Holocene
Following the Last Glacial Maximum (LGM; ~20,000 years ago), ice sheets melted and sea level rose ~130 m over 13,000 years. During this time, sea level rise was not steady—a gradual rise was punctuated by rapid increases during warm times. One of these rapid increases was during Meltwater Pulse 1A, during which sea level rose ~20 m in only ~340 years. This rate is significantly higher than modern rates of sea level rise.
During the Holocene, sea level gradually rose, including a rise of ~2 to 3 m between 7,000 and 3,000 years ago. Sea level change then leveled out until the industrial era, when anthropogenic warming began causing sea level rise. Figure 5.17 on the right highlights these Holocene changes and the transition to anthropogenic sea level rise.
Mid-Pliocene Warm Period
The Mid-Pliocene Warm Period (MPWP; 3.3 to 3.0 million years ago) was marked by warmer temperatures and higher atmospheric CO2 (350–450 ppm), and, correspondingly, higher sea level. Estimates of peak sea level based on proxy data and model output vary widely—from 10 to 30 meters above present—but the balance of evidence suggests that sea level was not higher than 20 m above present. The main reason for this high sea level was drastically reduced ice volume in the Greenland Ice Sheet and West Antarctic Ice Sheet (and some areas of the East Antarctic Ice Sheet). Large ice sheets did not grow in North America or Eurasia until ~2.7 million years ago.
Figure 5.17: Reconstructed global mean sea level for the last 300 years (e) and the last 6,000 years (f). Sea level rose ~2-3 meters between 7,000 and 3,000 years ago, and then was relatively stable until the industrial era (post-1850).
Figure 5.15: Proxy-generated sea level reconstructions for the Last Interglacial for the global average (a) and Western Australia (b). Note that regional sea level can differ significantly from the global average. Values are changes in sea level (in meters) relative to present; note the different y-axis scales in the two panels.