Boundary Conditions

Boundary Conditions

Boundary conditions in climate models all affect the way that energy is absorbed or exchanged in the climate system. Boundary conditions are not predicted by the model and must be specified. Some boundary conditions are natural, and others are influenced by human activities.

What are the boundary conditions in climate models?

Global Volcanic and Solar Forcing 1850-2000 Used in the Third Paleoclimate Model Intercomparison Project (PMIP3)

Natural boundary conditions include solar radiation and volcanic aerosols. Total solar insolation, observed at the top of the atmosphere, has varied by about 2 W/m2 around an average of about 1361 W/m2 over the past 1150 years. Large volcanic eruptions episodically inject large quantities of aerosols into the atmosphere, which reflects incoming solar radiation.

Sidebar: Natural forcing: How do we know?

Natural forcing: How do we know?

Photo of Antarctic Ice Core with Prominent Ash Layer

Records of volcanic activity used to model past climate come from ice cores in the Arctic and Antarctic. For example, Gao et al. (2008) developed an index based on volcanic deposits in 54 ice core records. Based on the spatial distribution of the deposits and knowledge of stratospheric transport, they produced a volcanic forcing dataset as a function of month, latitude, and altitude for the past 1500 years.

The primary source of information on solar activity (before direct measurements were made) comes from concentrations of carbon-14, which is formed in the atmosphere by the collision of nitrogen-14 and cosmic rays from the sun. The carbon-14 is incorporated into plant material, where it slowly decays back to nitrogen-14. By measuring the carbon-14 concentration in trees that are well dated through their tree rings, and then accounting for the radioactive decay of carbon-14, we can determine the past concentration of carbon-14 in the atmosphere. From this, we can deduce the solar irradiance over time.

Global Land Use Forcing 1850-2000 Used in the Third Paleoclimate Model Intercomparison Project (PMIP3)

Human-influenced boundary conditions include changes at the surface and changes in the atmosphere. At the surface, cutting forest for pasture and crops changes surface reflectivity and moisture, heat, and momentum exchanges between land and atmosphere.

Sidebar: Land use: How do we know?

Land use: How do we know?

Global historical cropland area (% of grid cell)

Land use reconstructions for times prior to the 20th century are based on population estimates and historical relationships of land use for different population densities. The calculations are done country-by-country to account for regional differences like crop types, farming technology, and diet. All the reconstruction methods largely rely on the similar historical population estimates. The results are gridded maps of land use through time.

Global Greenhouse Gas Forcing 1850-2000 Used in the Third Paleoclimate Model Intercomparison Project (PMIP3)

In the atmosphere, the most important changes are those that affect greenhouse gases. Greenhouse gases, principally water vapor and carbon dioxide, keep Earth habitable by absorbing enough long-wave radiation to keep surface temperatures tens of degrees Celsius warmer than they would be otherwise. These graphs show a rapid rise in different greenhouse gases over the past 2 centuries, primarily due to burning of fossil fuels.

Human emissions of atmospheric aerosols also alter the Earth's energy balance. Depending on the composition of the aerosols and where they are, they contribute to both warming and cooling of the climate. Overall, aerosols are thought to contribute a cooling effect equal to about half of the warming caused by greenhouse gases when averaged over the globe.

Sidebar: Greenhouse gases: How do we know?

Greenhouse gases: How do we know?

Photo of ice core section with bubbles

The concentration of greenhouse gases for climate reconstructions is based on measurements of the composition of air bubbles preserved in glacial ice in Antarctica. This photograph shows air bubbles trapped from an ice core from Antarctica. The prolonged darkness in winter and prolonged sunlight in summer leads to easily recognized and counted annual layers in the ice, yielding a high-resolution record of changes in atmospheric composition.