A number of common metrics to describe ecological consequences are included in most LCAs. The most common example is global warming potential (GWP) which normalizes greenhouse gas emissions into one number with units of mass emissions in carbon dioxide equivalents. Since several chemicals typically contribute to specific ecological impacts, metrics are very useful for consolidating data. Other examples of common metrics in this class are ozone depleting potential, eutrophica­tion potential, and acidification potential.

The most obvious ecological consequence to include in algae-to-energy studies is GWP since many algae-based energy systems are designed to produce intrinsi­cally low carbon neutral fuels. Because of this desire to produce low carbon fuels, many algae projects have used “sequestration” to describe their activities. In reality, algae-to-energy systems are not a sequestration technology. Sequestration implies that there is long-term storage of CO2 either as a solid carbonate mineral or in the subsurface under high pressure. In theory, algae could be grown and the biomass buried to sequester carbon, but it would be necessary to carefully control the condi­tions under which the carbon was buried such that the biomass was not simply digested by bacteria that could generate methane, effectively compounding the problem. What algae-to-energy systems can offer is a fuel that is closer to carbon neutral than conventional fossil fuels. That is, most of the carbon that will be emit­ted from the combustion of the fuel is not new carbon removed from the ground as in the case of coal or petroleum. This won’t help mitigate the impacts of climate change by reducing atmospheric concentrations, but it will reduce the increase in this concentration by not contributing new carbon. How much carbon these pro­cesses can keep out of the atmosphere is a current topic of investigation. It is impor­tant for the industry to adapt norms with regard to the way it treats carbon dioxide for full transparency.