information that will be needed to better understand and develop more effective responses to sea level rise and other coastal management challenges.

Sea level has varied dramatically over Earth’s history. For the past two to three million years, the ice age cycles—which are driven by periodic variations in Earth’s orbit (see Chapter 6)—have led to regular fluctuations in sea level of several hundred feet. During an ice age, significant amounts of water are stored in continent-sized glaciers called ice sheets that are up to several miles thick. Much of this ice melts during warm interglacial periods, and the resulting water raises global sea level substantially when it enters the oceans. During the most recent ice age, which peaked about 26,000 years ago, global average sea level was approximately 400 feet (120 meters) lower than it is today.

By carefully analyzing the depths and dates of coral reefs, geologists have reconstructed the temporal history of sea level rise during the recovery from the last ice age (Fairbanks, 1989; Peltier and Fairbanks, 2006). This rise was not steady, but rather punctuated by periods of rapid rise of as much as 2 inches (5 centimeters) per year; it is inferred that these periods of rapid rise were driven by pulses of water from melting ice (Figure 7.1). By approximately 6,000 years ago—or around the time that agriculture expanded and larger-scale civilizations were first established—global average sea level had risen to close to its present-day value, and it subsequently remained relatively steady. Other direct and indirect observations have allowed oceanographers to estimate past sea levels going back a few thousand years. These historical records suggest that there was little net change in sea level from the first century A.D. to 1800 (Church et al., 2008; Sivan et al., 2004).

Instrumental records for sea level date back about 140 years, when systematic measurements by tide gauges became available (NRC, 1990b). During the past few decades, tide gauge records have been augmented by satellite measurements that give precise sea level maps across the entire globe. Together, these modern records indicate that sea level has been rising since the mid-19th century, and that the rate of increase has been accelerating in recent years (Figure 7.2). For example, in the late 19th century, when tide gauge readings begin to provide accurate global sea level estimates, the rate of sea level rise was about 0.02 inches (0.6 millimeters) per year