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of the Earth system. To do this, we propose the development of a two -level set of control var- iables and boundaries. The subglobal -level units of analysis for these six boundaries are not identical; they vary according to the role that the processes play in the Earth system: (i) changes in biosphere integrity occur at the level of land - based biomes, large freshwater ecosystems, or major marine ecosystems as the largest sub- global unit; (ii) the role of direct, human - driven land- system change in biophysical climate regu- lation is primarily related to changes in forest biomes; (iii) freshwater flows and use occur at the largest subglobal level in the major river basins around the world; and (iv) changes in biogeochemical flows, exemplified by phospho- rus and nitrogen cycling, aggregate from rela- tively localized but very severe perturbations in intensive agricultural zones to affect global flows of nutrients. We recognize these as crit- ical regions for Earth - system functioning. Where appropriate, the updates of the individual bound- aries (see below) (33) now contain both the glob- ally aggregated boundary value of the control variable and its regional distribution function. Figure 2 shows the distributions and current status of the control variables for three of the boundaries where subglobal dynamics are crit- A Phosphorus ical: biogeochemical cycles, land - system change, and freshwater use. We emphasize that our subglobal -level focus is based on the necessity to consider this level to understand the functioning of the Earth system as a whole. The PB framework is therefore meant to complement, not replace or supersede, efforts to address local and regional environmental issues. Updates of the individual boundaries Brief updates of all nine of the PBs are given in this section, and more detailed descriptions of the updates for three of the PBs that have under- gone more extensive revision can be found in (33). The geographical distribution issues discussed above are particularly important for five of the PBs, and their control variables and boundaries have been revised accordingly (Table 1). Figure 3 shows the current status of the seven bounda- ries that can be quantified at the global level. Climate change We retain the control variables and boundaries originally proposed —i.e., an atmospheric CO2 con- centration of 350 parts per million (ppm) and an increase in top -of- atmosphere radiative forcing of +1.0 W M-2 relative to preindustrial levels (1). The radiative forcing control variable is the more C Land - system change M1 Beyond zone of uncertainty (high risk) B Nitrogen RESEARCH ( RESEARCHARTICLE inclusive and fundamental, although CO2 is im- portant because of its long lifetime in the atmo- sphere and the very large human emissions. Human - driven changes to radiative forcing in- clude all anthropogenic factors: CO2, other green- house gases, aerosols, and other factors that affect the energy balance (IS). Radiative forcing is generally the more stringent of the two bound- aries, although the relationship between it and CO2 can vary through time with changes in the relative importance of the individual radiative forcing factors. Evidence has accumulated to suggest that the zone of uncertainty for the CO2 control variable should be narrowed from 350 to 550 ppm to 350 to 450 ppm CO2 (17, IS), while retaining the cur- rent zone of uncertainty for radiative forcing of +LO to 1.5 W M-2 relative to preindustrial levels. Current values of the control variables are 399 ppm CO2 (annual average concentration for 2014) (34) and +2.3 W M-2 (1.1 to 3.3 W m 2) in 2011 relative to 1750 (IS). Observed changes in climate at cur- rent levels of the control variables confirm the original choice of the boundary values and the narrowing of the zone of uncertainty for CO2. For example, there has already been an increase in the intensity, frequency, and duration of heat waves globally (35); the number of heavy rainfall D Freshwater use In zone of uncertainty (increasing risk) Below boundary (safe) Fig. 2.The subglobal distributions and current status of the control variables for (A) biogeochemical flows of P; (B) biogeochemical flows of N; (C) land - system change; and (D) freshwater use. In each panel, green areas are within the boundary (safe), yellow areas are within the zone of uncertainty (increasing risk), and red areas are beyond the zone of uncertainty (high risk). Gray areas in (A) and (B) are areas where P and N fertilizers are not applied; in (C), they are areas not covered by major forest biomes; and in (D), they are areas where river flow is very low so that environmental flows are not allocated. See Table 1 for values of the boundaries and their zones of uncertainty and (33) for more details on methods and results. SCIENCE sciencemag.org 13 FEBRUARY 2015 • VOL 347 ISSUE 6223 1259555 -3