Table 3.

Impacts of Control, SRM, and CDR scenarios on ecosystem components

	Control	CDR examples			SRM examples
	Doubled atmospheric CO2	Afforestation (land)	Engineered carbon capture and storage (underground)	Ocean fertilization (ocean)	Stratospheric aerosol injection (land/ocean)	Cloud albedo enhancement (ocean)
Community structure and taxonomic diversity	Changes in biome distributions in land (tundra, Amazon, boreal, desertification) and ocean (Boyd and Doney 2002; Boyd and Ellwood 2010; Boyd et al. 2010) ecosystems; increased weediness; reduced biodiversity; reduced sea ice causing polar bear extinction (Durner 2009); ocean acidification; large effects on community structure in coral reefs; uncertain effects on calcifying plankton; effects will be different for different species (Fabry et al. 2008)	Reduced changes in biome distribution, more forests retained; reallocation of land use (e.g., from grassland to forest)	Changes for the “Control” case would be mitigated to some extent: reduced changes in biomes, extinctions	Purposeful redistribution of phytoplankton species (Boyd et al. 2007). Short-term (multi-week) changes in phytoplankton, heterotrophic bacteria and higher trophic levels during bloom (permanent changes are possible, with increased toxic diatoms in some regions) (Trick et al. 2010; Silver et al. 2010)	Changes for the “Control” case would be mitigated to some extent: reduced changes in biomes, extinctions. Change in seasonality will impact phenology, especially for near-freezing ecosystems	Changes for the “Control” case would be mitigated to some extent: more biodiversity than base case; unknown effects on surface-ocean species distributions (e.g., reduced light could favor phytoplankton that are adapted to lower light), but likely smaller than the “Control” case
Biomass and productivity	Benefits from warming: increased biological productivity because of CO2 fertilization and water efficiency, including increases in oceanic net primary productivity (Saba et al. 2010); losses from warming: sensitivity to drought, possible transition of Amazon to tundra, decreased productivity from coastal changes in hydrology, increased vulnerability to wild fires	Increased forest biomass and productivity	Changes for the “Control” case would be mitigated to some extent, but localized changes in land use are likely in deserts	Purposeful increase in net primary productivity and phytoplankton biomass in surface waters (Boyd et al. 2004; Boyd et al. 2007); increases could be either sustained or transient, depending on region and amount of unused nutrients; changes on land for “Control” case would be mitigated	Biomass and productivity will be stimulated by increased diffuse radiation and synergy with high CO2 (reduction in PAR will limit this effect); in some places, productivity could be reduced by different or exacerbated regional drought	May be more biomass than base case if global temperature is reduced, but intense cooling over small ocean regions could change ocean productivity and circulation (e.g., El Nino and monsoon cycles), which could have detrimental effects on land; changes in stratification, nutrient supply, sunlight
Biogeochemical cycling	Increased nitrogen deposition from continued fossil fuel combustion (e.g., in Arctic); changing nutrient loads in coastal and to some extent open oceans due to eutrophication and atmospheric deposition (Duce et al. 2008); overall decreased particulate export flux in open ocean (Bopp et al. 2002)	Increased nitrogen deposition	Changes for the “Control” case would be mitigated to some extent, including possible restoration of nutrient imbalances	Increased biogeochemical cycling in surface layers (including CO2 uptake and trace gases, DMS, N2O); unknown extent of CO2 drawdown; expected acceleration and enhanced re-mineralization of sinking particles (Boyd et al. 2004)	Cooler soil temperatures could reduce nutrient turnover in soils; reduced carbon loss; small change in sulfur deposition in rain; changes in atmospheric circulation and precipitation could have large scale impacts on terrestrial biogeochemical cycling	Potentially high regional changes in ocean cycling; changes in atmospheric circulation and precipitation could have large scale impacts on terrestrial biogeochemical cycling; possible localized changes in ocean chemical cycling