Climate Science and Satellite Altimetry Meeting of the Committee on a Strategy to Mitigate the Impact of Sensor De-scopes and De-manifests on the NPOESS and GOES-R Spacecraft December 17, 2007 Boulder, Colorado R. Steven Nerem CCAR & CIRES University of Colorado at Boulder

What Causes Sea Level to Change?

Past Sea Level Change Lambeck & Chappell [2001]

G RAVITY R EBOUND Satellite Measurements of Sea Level Change

TOPEX/Poseidon Jason ? Jason-2 June 2008 Satellite Altimetry Jason-3 ?

TOPEX/Poseidon and Jason 10-day Groundtrack

TOPEX Sea Level: January 1998

Monitoring ENSO Processes Sea level averaged 5S-5N, E

Other Satellite Altimetry Climate Applications ENSO processes (onset, strength, planetary waves, etc.) and improving ENSO forecasts ENSO processes (onset, strength, planetary waves, etc.) and improving ENSO forecasts Other climate oscillations (PDO, NAO, etc.) Other climate oscillations (PDO, NAO, etc.) Long-term changes in ocean circulation Long-term changes in ocean circulation Mesoscale variability, eddy kinetic energy Mesoscale variability, eddy kinetic energy

Instrument Calibration via Tide Gauges

Global Mean Sea Level Variations 60-day smoothing Seasonal variations removed GIA correction applied 3.7 ± 0.4 mm/year

Tide Gauge Observations of Sea Level Change Average Rate ~ 1.8 mm/year 0.8 mm/year 2.0 mm/year 3.2 mm/year [Church and White, 2006]

Observed Sea Level Trends: Total Sea Level (Altimetry) Thermosteric Sea Level (XBTs, Argo, etc.)

Land Ice and Sea Level Change [Mitrovica et al., 2001] Antarctica Greenland Land ice gravitationally attracts the ocean water Land ice gravitationally attracts the ocean water Melting of ice causes regional fall of sea level in the vicinity of the ice Melting of ice causes regional fall of sea level in the vicinity of the ice Patterns fit to tide gauge observations: Patterns fit to tide gauge observations: Mountain glaciers & ice caps

Complementary Measurements of Sea Level Change + = Total Sea Level Rise Argo JasonGRACE (roughly) Addition of Freshwater Addition of Heat

Total vs Thermosteric Sea Level:

Future Sea Level and Temperature Change IPCC SPM, 2007 IPCC 4th Assessment, 2007

Sea Level Observations versus Projections [Rahmstorf et al., 2007] Tide Gauge Observations Satellite Observations Climate Model Predictions (IPCC 3rd Assessment, 2001)

Projecting Future Sea Level Change Future sea level projections are uncertain because of uncertainties in the contributions of Greenland and Antarctica. Future sea level projections are uncertain because of uncertainties in the contributions of Greenland and Antarctica. The IPCC 4th Assessment projects m of sea level rise by 2100, but did not exclude higher rates. The IPCC 4th Assessment projects m of sea level rise by 2100, but did not exclude higher rates. In fact, a recent study suggests higher rates ( m by 2100) using an empirically derived regression coefficient (3.4 mm/yr/°C). In fact, a recent study suggests higher rates ( m by 2100) using an empirically derived regression coefficient (3.4 mm/yr/°C). [Rahmstorf, 2007] 1.4° 5.8°

Earth Science Decadal Survey WatER Mission:

#1 Implications for Climate Data Records Sea level change measurements are critical for monitoring the impact of climate change and for corroborating the predictions of climate models. Sea level change measurements are critical for monitoring the impact of climate change and for corroborating the predictions of climate models. Satellite altimetry is also an indispensable tool for providing a historical record of sea level change - tide gauge and other in situ measurements will not suffice. Satellite altimetry is also an indispensable tool for providing a historical record of sea level change - tide gauge and other in situ measurements will not suffice. Data continuity is very important for detecting climate-related variations in the record - tide gauges and sun-synchronous altimeters are of limited use for patching gaps in the record. Data continuity is very important for detecting climate-related variations in the record - tide gauges and sun-synchronous altimeters are of limited use for patching gaps in the record. Score: 1

#2 Role in Determining Climate Sensitivity Sea level varies in response to temperature changes, which in turn varies in response to changes in GHGs and other factors. Sea level varies in response to temperature changes, which in turn varies in response to changes in GHGs and other factors. Sea level has also been shown to drop in response to an influx of volcanic aerosols (due to thermal and hydrologic changes) Sea level has also been shown to drop in response to an influx of volcanic aerosols (due to thermal and hydrologic changes) Therefore, sea level has an important, though not critical, role to play in climate sensitivity studies. Therefore, sea level has an important, though not critical, role to play in climate sensitivity studies. However, the timing of the ice sheet response to temperature changes is still poorly understood. However, the timing of the ice sheet response to temperature changes is still poorly understood. Score: 2

#3 Role of Sea Level in Climate Prediction Satellite altimeter sea level measurements used extensively as initial conditions in short-term climate predictions. Satellite altimeter sea level measurements used extensively as initial conditions in short-term climate predictions. Sea level measurements currently not used directly in long-term climate model projections. Sea level measurements currently not used directly in long-term climate model projections. However, sea level is used extensively to corroborate the climate model projections. However, sea level is used extensively to corroborate the climate model projections. Projecting future sea level change is difficult because the response of the ice sheets is poorly understood, and thus often omitted from climate models. Projecting future sea level change is difficult because the response of the ice sheets is poorly understood, and thus often omitted from climate models. Score: 1

#4 Sea Level and Reanalysis Products Satellite altimeter sea level measurements are used extensively in ocean reanalysis efforts (ECCO, SODA, etc.). Satellite altimeter sea level measurements are used extensively in ocean reanalysis efforts (ECCO, SODA, etc.). Sea level measurements currently not widely used in global climate models. Sea level measurements currently not widely used in global climate models. Potential for increased use once climate models are improved (freshwater input from the continents) Potential for increased use once climate models are improved (freshwater input from the continents) Score: 1

#5 Satellite Altimetry Measurement Maturity Satellite altimetry measurements started over 30 years ago, with the precision measurements starting with the launch of TOPEX/Poseidon 15 years ago. Satellite altimetry measurements started over 30 years ago, with the precision measurements starting with the launch of TOPEX/Poseidon 15 years ago. Thus, the measurement characteristics are well- understood and the maturity is very high. Thus, the measurement characteristics are well- understood and the maturity is very high. Nevertheless, improvements to the measurement processing are still be developed and the datasets are being incrementally improved over time. Nevertheless, improvements to the measurement processing are still be developed and the datasets are being incrementally improved over time. Swath altimetry promises to provide new challenges, but nadir altimetry will continue for several more decades and may never be supplanted by swath measurements. Swath altimetry promises to provide new challenges, but nadir altimetry will continue for several more decades and may never be supplanted by swath measurements. Score: 1

#6 Satellite Altimetry: Measurement Uniqueness Satellite altimetry is the only technique that provides highly accurate sea level measurements with excellent temporal and spatial resolution - the measurements are important by themselves. Satellite altimetry is the only technique that provides highly accurate sea level measurements with excellent temporal and spatial resolution - the measurements are important by themselves. Complemented by in situ oceanographic measurements (Argo), satellite gravity measurements (GRACE), and laser altimeter measurements (Icesat). Complemented by in situ oceanographic measurements (Argo), satellite gravity measurements (GRACE), and laser altimeter measurements (Icesat). International missions tend to be in sun-synchronous orbits, which is undesirable for detecting climate- related sea level variations. International missions tend to be in sun-synchronous orbits, which is undesirable for detecting climate- related sea level variations. Score: 1

#7 Satellite Altimetry: Use by the IPCC Each assessment report has employed satellite altimetry more prominently as the sea level time series has lengthened. Each assessment report has employed satellite altimetry more prominently as the sea level time series has lengthened. The most recent 4th assessment contained a controversial assessment of future sea level change, largely due to a lack of understanding of how the ice sheets will respond to warming. The most recent 4th assessment contained a controversial assessment of future sea level change, largely due to a lack of understanding of how the ice sheets will respond to warming. Satellite altimetry is likely to play a fundamental role in future assessments to help corroborate the GCMs, monitor the melting of ice, and help improve predictions of future sea level rise. Satellite altimetry is likely to play a fundamental role in future assessments to help corroborate the GCMs, monitor the melting of ice, and help improve predictions of future sea level rise. Score: 1

#8 Role in Understanding the Climate System Sea level measurements are critical for understanding changes in the Earth’s heat budget, because 80% of the excess heat from global warming is being stored in the ocean. Sea level measurements are critical for understanding changes in the Earth’s heat budget, because 80% of the excess heat from global warming is being stored in the ocean. Sea level measurements are also a critical indicator of the melting of mountain glaciers and the polar ice sheets - direct measurements of the ice sheets must eventually close the sea level budget. Sea level measurements are also a critical indicator of the melting of mountain glaciers and the polar ice sheets - direct measurements of the ice sheets must eventually close the sea level budget. Satellite altimeter measurements are a critical tool for studying ENSO processes and other climate oscillations. Satellite altimeter measurements are a critical tool for studying ENSO processes and other climate oscillations. Score: 1

#9 Benefit to Related Disciplines Sea level measurements from satellite altimetry benefits a broad array of climate science displines including ocean science, cryospheric science, hydrology, and climate modeling applications. Sea level measurements from satellite altimetry benefits a broad array of climate science displines including ocean science, cryospheric science, hydrology, and climate modeling applications. Ultimately, long-term changes in the Earth’s water storage reservoirs is the most important aspect of climate change for socio-economic impacts. Ultimately, long-term changes in the Earth’s water storage reservoirs is the most important aspect of climate change for socio-economic impacts. Score: 1

Summary Rankings 1) 1)To what extent are the data used in monitoring and providing a historical record of the global climate? Is there a requirement for data continuity? 2) 2)To what extent is this measurement important in reducing “uncertainty”— e.g. to reduce error bars in climate sensitivity forcing and monitoring. 3) 3)Consider the importance of the measurement’s role in climate prediction and projections (forcing/response/sensitivity) 4) 4)To what extent is the measurement needed for reanalysis? 5) 5)Describe the measurement’s maturity—for example, its readiness to be assimilated into a particular model(s)—and its heritage. 6) 6)Are other sensors and ancillary data required to make the measurement useful? Is this measurement unique? Are there complementary international sensors? 7) 7)To what extent are the data used by, for example, the IPCC and the CCSP? 8) 8)Provide a qualitative assessment of the measurement’s role in contributing to an overall improved understanding of the climate system, climate processes. 9) 9)To what extent does the measurement contributing to improved understanding in related disciplines? Are there other aspects of this measurement that should be considered by the committee? Score: 1 Score: 2 Score: 1