GCOS Implementation Plan - WMO

Implementation Plan for the Global Observing System for Climate in Support of the UNFCCC
(2010 Update)
To consider possible impacts, the effect of isostatic rebound and human action on subsidence needs
to be quantified which locally can be of the same order as global sea-level change. Spatial scales of
drainage-induced subsidence are small requiring a substantial enhancement of the local tide gauge
network or geodetic measurements in critical regions. Jointly with the tide gauge network, continued
operation of high-precision satellite altimetry and sun-synchronous altimeter measurements complete
the sea-level network. Together they represent an integrated strategy for monitoring of sea-level
variability and change globally and on regional scales. The Arctic Ocean is an essential component of
this and needs to be observed as well.
Figure 10: Sea-level gauges in the GCOS subset of the GLOSS Core Network. "Near real-time" and "Fast
Delivery" stations provide high-frequency sea level (hourly or better reports) which are necessary for
understanding of sea-level variability in addition to mean sea-level rise. Near real-time stations (blue)
provide data typically within 1 hour of collection; Fast delivery (green) within one month. Delayed-mode
data within 5 years (orange) or greater (red) include monthly averages provided to the Permanent Service
for Mean Sea Level (PSMSL). White dots are placed on tide gauges that are geo-referenced, measuring
local land movements, in order to measure absolute sea-level change.
Issues related to sea-level observing and integrated global analysis include:
• The spatially complex variability of the vertical motion of the surfaces upon which tide gauges are
located. In addition to the ongoing larger-scale tectonic post-glacial adjustment, there are often
local vertical movements of consequence caused by human activities. These vertical movements
are of the same magnitude as the anticipated sea-level change from global warming, and must be
measured at each tide gauge in order to reveal water level changes relative to the centre of the
Earth.
• The existence of large-scale low frequency variability of the oceanic density field which, if not
appropriately and globally sampled, can introduce uncertainty in estimates of global sea-level
change.
• The lack of detailed knowledge of mass being exchanged between polar ice sheets and the
ocean has the potential to introduce a substantial uncertainty in understanding predictions of sealevel
change which can by far exceed the uncertainties arising from changes in the density field of
the ocean.
• The need to know variability and change in surface pressure, which affects water level.
• The technical challenges of making accurate high-frequency water level measurements over long
periods.
• The present shortcomings of international data exchange.
• The need for significant regional and local network enhancements in order to support impact
assessment and monitoring.
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