An Overview of the Observations of Sea Level Change R. Steven Nerem University of Colorado Department of Aerospace Engineering Sciences Colorado Center for Astrodynamics Research Cooperative Institute for Research in Environmental Sciences NSCE 11 th Conference on Science, Policy, and the Environment: Our Changing Oceans

The Earth is Warming

Why does sea level change? The Bathtub Model + - Precipitation over Oceans Runoff from Continents Evaporation from Oceans Precipitation over Continents Thermal Expansion IceMelt

What is Global Mean Sea Level Change? “Global Mean Sea Level Change” is the change in the average height of the oceans over the entire globe at a single point in time. “Global Mean Sea Level Change” is the change in the average height of the oceans over the entire globe at a single point in time. Sea level change at a specific location in the ocean may be higher or lower than the global mean because of differences in ocean temperature and other effects. Sea level change at a specific location in the ocean may be higher or lower than the global mean because of differences in ocean temperature and other effects. Does not include ocean tides, storm surge Does not include ocean tides, storm surge Does not include effects of land subsidence, which locally can exacerbate the effects of sea level rise. Does not include effects of land subsidence, which locally can exacerbate the effects of sea level rise.

G RAVITY How do we measure sea level change? Argo

What do the tide gauges tell us? Average Rate ~ 1.8 mm/year 3.2 mm/year [Church and White, 2006]

What do the altimeter satellites tell us? What do the altimeter satellites tell us? TOPEX/ Poseidon Jason 1 & 2 Trend = 3.3 ± 0.4 mm/year

What do satellite gravity measurements tell us?

Rate of Ice mass change: All Greenland: -239 Gt/yr South Greenland: -162 Gt/yr North Greenland: -77 Gt/yr Total Greenland ice volume April, 2002 – June, Gton/yr = 0.66 mm/yr sea level rise (1 Gton = 1 km 3 of water) Greenland Ice Mass Changes from GRACE [Wahr, 2010]

Greenland Ice Mass Changes from GRACE

All Antarctica: -143 Gton/yr West Antarctica: -155 Gton/yr East Antarctica: +15 Gton/yr -143 Gton/yr = 0.40 mm/yr sea level rise Total Antarctic ice mass April, 2002 – June, 2010 Antarctica Ice Mass Variations from GRACE [Wahr, 2010]

Is Antarctic Ice Mass Loss Holding Steady? [Wahr, 2010]

IceSat Surface Elevation Changes [Pritchard et al., 2009]

Rate of mass change between April, 2002 and May, 2010 Rate of mass change: -55 Gton/yr = 0.15 mm/yr sea level rise. Alaskan Glaciers from GRACE [Wahr, 2010]

The Argo Array

Current Sea Level Budget Thermal Expansion: ~ 1 mm/year Mountain Glaciers: ~ 1.1 mm/year Greenland Ice Melt: ~ 0.6 mm/year Antarctic Ice Melt: ~ 0.4 mm/year Land Water Storage: ? Total: ~ 3.1 mm/year =

Spatial Variations in Sea Level Rise

Western Pacific Sea Level Change Merrifield (2010)

Predicted Regional Patterns of Sea Level Change from Ice Melt [Bamber, 2009]

What Will Happen in the Future? Coupled climate models IPCC, 2007 Empirical projections Rahmstorf, 2007

1 Meter of Sea Level Rise – Gulf Coast

1 Meter of Sea Level Rise - Florida

6 Meters of Sea Level Rise - Florida

It can happen – because it has happened before [Clark, 2009] The last time the Arctic was 3 to 5°C warmer than present, global sea level was ~6 meters higher than present

Summary Observations of sea level change are consistent with how we expect sea level to respond in a warming climate. Observations of sea level change are consistent with how we expect sea level to respond in a warming climate. Sea level rose twice as fast in the last decade than over the last half of the 20 th century. Sea level rose twice as fast in the last decade than over the last half of the 20 th century. Presently, ocean warming, melting of mountain glaciers, and melting of the polar ice caps are contributing in roughly equal amounts to the observed rise. Presently, ocean warming, melting of mountain glaciers, and melting of the polar ice caps are contributing in roughly equal amounts to the observed rise. The large uncertainty in future sea level rise projections is due mainly to the uncertain contributions of Greenland and Antarctica, which appear to be accelerating. The large uncertainty in future sea level rise projections is due mainly to the uncertain contributions of Greenland and Antarctica, which appear to be accelerating. Whether the current rate of rise is accelerating can only be resolved with longer satellite time series. Whether the current rate of rise is accelerating can only be resolved with longer satellite time series. Many of the remaining questions about sea level rise can only be answered with continued satellite measurements, which have an uncertain future. Many of the remaining questions about sea level rise can only be answered with continued satellite measurements, which have an uncertain future.

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