Biodiversity
A property of ecosystems related to the number of different plant and animal species they contain.

Biogenic gas
Any gaseous substance that is produced by biological activity.

Biomass burning
A term often used to describe the way by which unwanted plant life is disposed in "slash and burn" operations designed to prepare pristine lands for agriculture.

Blizzard
A storm of at least three hours' duration with winds over 16 m s^-1 accompanied by falling or blowing snow that causes visibility to drop below 400 m.

Boreal forest
Areas mostly adjacent to and south of the arctic tundra in North America and Eurasia with vegetation that is relatively uniform and dominated by coniferous trees, including spruce, fir, and pine.  Soils are typically acidic and nutrient-poor, usually containing permafrost less than a meter below the surface.

Boreholes
Holes drilled in ice, soil or rock to very great depths for the purpose of obtaining temperature profiles that can be used to reconstruct the temperature history at the surface of the hole.

Branch bag
A transparent plastic bag in which branches are enclosed to study gas exchange characteristics of leaves or needles.  The validity of this technique is sometimes questioned, however, as only portions of a tree are exposed to applied treatments, so observations may not be representative of whole trees.

Bundle sheath cells
These are specialized photosynthetic cells, found within C4 plant species, where the initial products of C4 photosynthesis (four-carbon sugars) are imported and decarboxylated.  Since bundle sheath cell walls are highly impervious to the diffusion of CO2, the CO2 concentration within such cells increases dramatically following decarboxylation events.  Consequently, the released CO2 is converted back into sugars, for the second time, by the enzyme rubisco, which operates much more efficiently due to the locally high CO2 concentration present in bundle sheath cells.