Intergovernmental Panel on Climate Change:
Fifth Assesment Report; Working Group One:
Chapter 13: Sea Level Change
Chapter Summary and Implications
13.1: Components and Models of Sea Level Change
Introduction
This section provides an overview of the chapter and a review of the different components and processes affecting sea level rise and how models are used to represent and predict future change. Previous IPCC reports laid the groundwork for our understanding of global sea level change. In the First and Second Assessment Reports (FAR 1990, SAR 1995), it was recognized that global sea level had risen during the 20th century and that it would continue to rise and even accelerate into the 21st century. It was also understood that sea level would not rise uniformly around the globe and sea level would continue to rise even after Greenhouse Gases (GHGs) were reduced. Current evidence at that point also suggested that large ice sheets would not play a significant role in the 21st century.
However, even by the time of the Fourth Assessment Report (AR4 2007) significant issues remained. First, recent observed sea level rise was larger than the sum of the individual contributors predicted through models and observations. Second, confident regional predictions of sea level rise were not possible. Third, the contribution of ice sheets to sea level rise was still poorly understood.
Processes affecting Sea Level
The land, ocean, atmosphere, ice and hydrological cycle all contain climate sensitive processes that have some capacity to alter sea level at regional to global scale (Fig. 13.1). Expansion of seawater due to lowered density (either cause by increased temperatures and/or decreasing salinity) provides a significant increase in global sea level. Due to the importance of temperature and salinity, ocean currents play a large role on the location and severity of sea level change due to advection of these ocean properties.
Since the AR4, observations have increased and improved, models have been improved and we have a better understanding of the role of ice sheet dynamics. This chapter provides a synthesis of past, current and future sea level change at regional to global scales. It provides the mechanisms and reasons for sea level change (Fig. 13.1) as well as predictions of likely sea level change around the world.
Changes in the flux and storage of ice between the land and ocean have a direct impact on Global Mean Sea Level (GMSL). An introduction of land ice melt water into the ocean can propagate sea level change around the globe in a matter of days. Additionally, the influx of freshwater changes ocean temperatures and salinities, affecting sea level. However, these changes take decades to influence GMSL.
The visco-elastic deformation of the earths crust in response to mass movement on the surface also changes GMSL. It should be noted here that the movement of the Earth's continental crust in response to loading or unloading (such as the growth and demise of ice sheets) causes the land to rise or fall relative to sea level and does not affect the volume of water present in the oceans. Since climate change is responsible for mass fluxes of ice on the continents, it is important that this process be taken into account for future predictions.
Humans have a dramatic effect on the water flux over the land surface (agriculture, dams, land surface change). These changes affect the hydrological cycle and alter how much water moves between the ocean-land reservoirs, therefore altering sea level.
The combination of these mechanism and processes (Fig 13.2) are the driving forces behind GMSL now and into the future.
Models Used to Interpret Past and Project Future Changes in Sea Level
Several different types of models are used to model and predict sea level change. Each of them is designed differently and useful in different scenarios. Atmosphere-Ocean Global Circulation Models (AOGCMs) are fully coupled land-ocean models that represent the sea surface as a result of natural and anthropogenic forcings placed on the different components of the model. In these complex models, all the parameters (such as ocean temperature and density) are dynamically linked to all the other processes involved and evolve together over the life of the model.
Geodynamic surface loading models simulate the sea surface as a result of mass changes between the land and ocean and changes to gravity around the globe. Semi-empirical models model and predict sea level using the statistical relationships between observed GMSL and global mean temperature. In this way, these systems do not model the processes affecting sea level but rather relationships of the cause and effect parameters. Unfortunately these types of models use characteristically long times scale largely inappropriate for modeling sea level in the short term.
Models Used to Project Changes in Ice Sheets and Glaciers
The contribution of an ice mass to sea level is characterized by changes in its mass balance or changes in its dynamics which affects outflow. Although observational data has greatly improved in the last decade alone, the contribution of ice masses to sea level is still difficult to model or predict. Complete, up-to-date observations of most glaciers do not exist, and so one must rely more on remotely sensed data and models of glacier change. Even more difficult is the modeling of internal ice dynamics, which makes predicting large ice sheet movement and contribution to sea level hard to predict and uncertain.
Figure 13.1: A schematic of various cliamte-sensitive processes and cycles that affect both global and regional sea level
Figure 13.2: The key components and their relationships within the Ocean-Land-Atmosphere and how they are all intertwined with sea level change. Numbers in figure refer to the IPCC AR5 WG1 chapter and section that discusses that particular component.