through the century, storing all of the carbon emitted during the 21<sup>st</sup> century in the terrestrial reservoir would require storing an amount of carbon roughly equal to all of the carbon now present in the world’s aboveground biomass and short-lived detritus, or roughly doubling the 30% of the Earth’s land area covered by forests (Fischlin and Midgley, 2007). As a result, early ideas about planting more trees along highways or other limited corridors would not accomplish much. Indeed, given the demand for land for food and fiber products and for communities, fully offsetting CO2 emissions by expanding terrestrial carbon uptake is just not possible. While every effort should be made to increase land storage of carbon (IPCC, 2000a; Lovejoy et al., 2008), it is likely to be a challenge just to keep the net amount of terrestrial carbon constant.

Given the difficulty of accumulating and storing sufficient carbon in the terrestrial biosphere, consideration is being given to limiting emissions by capturing the CO2 at power plants and then transporting it to the deep ocean by pipeline. While this would be categorized as mitigation based on the definitions used in this report, gathering the carbon after it has been emitted into the atmosphere and dispersed, and then sequestering it in the ocean is generally categorized as geoengineering. An early approach proposed for capturing dispersed carbon involved growing trees on land, harvesting them, and then sinking them to the bottom of the ocean (Marchetti, 1975). To counterbalance fossil fuel emissions, this would require harvesting and sinking to the ocean bottom roughly 1% of the existing vegetation per year as well as replanting and likely having to fertilize those areas to encourage regrowth of the trees. Assuming the average time to maturity of a tree is 50 years, this approach would require including about 50% of the world’s forested area in a sequential process of harvesting trees, sinking them in the ocean, and then regrowing trees on the cleared land. This level of sequestration seems far more than would be possible, and, in addition, it would be a terrible waste of the solar energy that has been captured in the wood over such an extended time. If indeed the economic costs of climate change impacts justified a harvest/sink/regrow process of this magnitude, it would seem to make far more sense, and likely be far more economical, to be transforming the biomass to biofuels and not mining and combusting the fossil fuels at all. Indeed, pursuing a combined biomass-fossil fuel process that generated energy while also sequestering carbon underground appears to be a quite promising technology (Kreutz et al., 2008).

At the present rate of CO2 emissions, the net uptake of CO2 by the oceans is ~2.3 PgC/yr, split between the extra amount transmitted to the deep ocean by the overturning circulation (actually a reduction in the net upward transport) and by the sinking of biomass (fecal pellets, skeletons, etc.). Warming of the ocean is projected to reduce the overturning component, and warmer ocean waters (like carbonated beverages) hold less CO2, so climate change is likely to affect the natural carbon cycle in a way that makes the situation worse, not better. However, because there are some indications that biological activity could be increased in some regions of the ocean by the addition of apparently missing micro- or macro-nutrients, there is considerable interest in determining if fertilizing the ocean could be used to increase ocean uptake and downward transport of carbon. Basically, powered by solar energy, the notion would be to make an investment of micronutrients to drive additional uptake of carbon by the marine biosphere while perhaps also increasing fish stocks to help feed the world.

A number of small-scale tests of ocean fertilization have been conducted and more have been proposed, some funded by venture capitalists hoping that success might provide carbon credits 13