Long-term prediction is particularly relevant for changes in sea level. The changes in relative sea and land levels measured at the coast by tide gauges can be usefully divided four ways, into the local and the global changes of the land and the sea level. These distinctions are important for understanding the time scales and causes of ongoing changes, and predicting their future evolution. Land levels are significantly influenced by global-scale tectonic effectsthe adjustment of the Earth's mantle to the removal of the glacial-era icesheets, for example. Local changes in land level can result from altered sedimentation rates, or from subsidence due to extraction of groundwater or oil. Sea level is also subject to local changes forced by local winds, river runoff, and the passage of oceanic waves of various frequencies. The global sea level is determined primarily by the mass of water in the ocean and its temperature structure.
Particular effort has gone into understanding the global sea-level component of the tide-gauge measurement, because it is expected to change with climate. Tide-gauge records longer than 50 years are needed to eliminate spurious trends due to low-frequency variability. Once records have been corrected for post-glacial rebound, a trend over the past 100 years of about 1.8 mm per year emerges (Douglas, 1991). There is no firm evidence of an increase in the rise, nor would it be expected from the change in climate that has occurred over that period. Archeological and geological studies indicate that the variation of sea level over the previous two millennia was no more than a few tens of centimeters. The time of onset of the current rise is not known.
While uncertainty about the measured rise remains, because of the lack of global coverage and the possible influence of coastal subsidence, uncertainties about the components of the rise are far larger. Two factors contribute significantly to the change of global sea level with climate: thermal expansion of the ocean, and redistribution of water between land and sea. Surface thermal anomalies penetrate down into the ocean's interior via the wind and thermohaline-driven overturning. These circulations have a range of time scales from decadal to millennial. Existing direct observations of ocean temperature are insufficient to reveal the past global warming of the ocean, although significant local changes have been observed. Models of ocean circulation have therefore been used to calculate the thermal-expansion part of the observed sea-level rise. These models yield estimates ranging from 0.2 to 0.7 mm per year (IPCC, 1996a). This calculation is inherently uncertain, however, since the boundary conditionswind stress, surface temperature, and salinityare not well known. The calculation becomes even more tenuous when made for future climate scenarios with additional greenhouse gases.
Ninety-nine percent of the world's land ice is contained in the Greenland and Antarctic ice sheets. The response of these ice sheets to climate change is difficult to predict (see, e.g., Oppenheimer, 1998). Since the mass balance of these ice sheets reflects long time scales, they are likely still adjusting to past climate changes. In general, the increased supply of moisture in a warmer climate is expected to dominate the increased melting for the Antarctic ice sheet, while the reverse is expected for the Greenland ice sheet. Current observations are insufficient to detect a mass imbalance in either. Here, a climate prediction might attempt at least to determine the relative change in the mass balance of the ice sheets, when models can determine temperature and precipitation in the high latitudes.
To interpret observations of sea-level and ice-volume changes, they must be placed in the context of the past and compared with projections of the future. It is clearly of interest to know when the current rise began and whether there were past rises of comparable magnitude and duration. Paleo-studies and data "archeology" (recovery of unpublished records) can help address these issues. Most projections of sea-level response to anthropogenic forcing have been based on simple models (e.g., one-dimensional up-welling-diffusion ocean models). Sea level is fully embedded in the climate system, however, and a coupled ocean-atmosphere-ice model must be used to maintain consistency in all the elements. Furthermore, the dynamic response of the ocean to climate change gives rise to regional changes in sea level that may be of a magnitude comparable to that of the global mean change. Continued improvement of these sophisticated models will be necessary if useful projections are to be made. Such projections will prove invaluable, though, because sea-level rise can have such a large and devastating impact on the vastly developed and densely occupied coastal regions of the world.
Parameterizations of climate-induced ecosystem changes are rapidly improving. To predict ecosystem changes under scenarios of elevated greenhouse gases, earlier models simply mapped the recently observed biomes to the GCM-predicted locations with similar climatic conditions. Some of the latest models include vegetation interactions with nutrients, CO2 fertilization, and fire (VEMAP, 1995; IPCC, 1998). Recent integrated-assessment models of climate change even include climate-vegetation and carbon cycle-vegetation feedbacks, as well as the effects of changing land use (see, e.g., CIESIN, 1995). While the climate scenarios that have been explored with these models are often derived from transient coupled ocean-atmosphere GCMs, the ecosystem models themselves tend to be designed to simulate an equilibrium land-surface biosphere, rather than the transient ecosystem compositions that will precede the equilibrium state.
The veracity of potential (i.e., omitting land-use changes) vegetation-distribution predictions made from uninitialized climate forecasts is as yet unknown. Because