The possible roles of algae in restricting the increase in atmospheric CO₂ and global temperature

Anthropogenic inputs are increasing the CO2 content of the atmosphere, and the CO2 and total inorganic C in the surface ocean and, to a lesser degree, the deep ocean. The greenhouse effect of the increased CO2 (and, to a lesser extent, other greenhouse gases) is very probably the major cause of present global warming. The warming increases temperature of the atmosphere and the surface ocean to a greater extent than the deep ocean, with shoaling of the thermocline, decreasing nutrient flux to the surface ocean where there is greater mean photosynthetic photon flux density. These global changes influence algae in nature. However, it is clear that algae are important, via the biological pump, in decreasing the steady state atmospheric and ocean surface CO2, and thus decreasing radiative forcing, a reduction enhanced by algal increases in albedo. As well as these natural processes there are possibilities that algae can, with human intervention, partly offset the increase in atmospheric CO2. One possibility is to grow algae as sources of fuel for transport, in principle providing an energy source that is close to CO2-neutral. The other possibility is to increase the role of algae in sequestering CO2 as organic C over periods of hundreds or more years in the deep ocean and marine sediments and/or increasing albedo and decreasing radiative forcing of temperature. There are problems, currently unresolved, in the economically viable production of algal biofuels without carbon trading subsidies. Enhanced algal CO2 sequestration also has costs, both in resource input (phosphorus (P) from high P content rocks, a limited resource with a competing use as an agricultural fertilizer) and adverse environmental effects. For example, ocean anoxic zones producing N2O and increased algal production of short-lived halocarbons by algae that both, through breakdown, destroy O3 and increase UV flux to the Earth’s surface.