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where it is currently in excess to areas where the soil is naturally P -poor may simultaneously boost global crop production and reduce the transgres- sion of the regional -level P boundary (3, 52, 54). The N boundary has been taken from the com- prehensive analysis of de Vries et al. (5), which proposed a PB for eutrophication of aquatic eco- systems of 62 Tg N year' from industrial and intentional biological N fixation, using the most stringent water quality criterion. As for the P boundary, a few agricultural regions of very high N application rates are the main contributors to the transgression of this boundary (Fig. 2 and fig. S5B). This suggests that a redistribution of N could simultaneously boost global crop produc- tion and reduce the transgression of the regional- level boundary. Because the major anthropogenic perturba- tion of both the N and P cycles arises from fertil- izer application, we can analyze the links between the independently determined N and P bounda- ries in an integrated way based on the N:P ratio in the growing plant tissue of agricultural crops. Applying this ratio, which is on average 11.8 (55), to the P boundary (6.2 Tg P year-) gives an N boundary of 73 Tg N year'. Conversely, applying the ratio to the N boundary (62 Tg N years) gives a P boundary of 5.3 Tg P years. The small dif- ferences between the boundaries derived using the N:P ratio and those calculated independent- ly, which are likely nonsignificant differences given the precision of the data available for the calculations, show the internal consistency in our approach to the biogeochemical boundaries. More detail on the development of the P and N boundaries is given in (33), where we also em- phasize that the proposed P and N boundaries may be larger for an optimal allocation of N (and P) over the globe. Land - system change The updated biosphere integrity boundary pro- vides a considerable constraint on the amount and pattern of land- system change in all ter- restrial biomes: forests, woodlands, savannas, grasslands, shrublands, tundra, and so on. The land- system change boundary is now focused more tightly on a specific constraint: the biogeo- physical processes in land systems that directly regulate climate— exchange of energy, water, and momentum between the land surface and the atmosphere. The control variable has been changed from the amount of cropland to the amount of forest cover remaining, as the three major forest biomes — tropical, temperate and boreal —play a stronger role in land surface- climate coupling than other biomes (56, 57). In particular, we fo- cus on those land - system changes that can in- fluence the climate in regions beyond the region where the land - system change occurred. Of the forest biomes, tropical forests have sub- stantial feedbacks to climate through changes in evapotranspiration when they are converted to nonforested systems, and changes in the distribu- tion of boreal forests affect the albedo of the land surface and hence regional energy exchange. Both have strong regional and global teleconnections. The biome -level boundary for these two types of forest have been set at 85% (Table I and the supplementary materials), and the boundary for temperate forests has been proposed at 50% of potential forest cover, because changes to tem- perate forests are estimated to have weaker in- fluences on the climate system at the global level than changes to the other two major forest biomes (56). These boundaries would almost surely be met if the proposed biosphere integ- rity boundary of 90% BII were respected. Estimates of the current status of the land - system change boundary are given in Figs. 2 and 3 and fig. S6 and in (58). Freshwater use The revised freshwater use boundary has retained consumptive use of blue water [from rivers, lakes, reservoirs, and renewable groundwater stores (59)] as the global -level control variable and 4000 km3 /year as the value of the boundary. This PB may be somewhat higher or lower de- pending on rivers' ecological flow requirements (6). Therefore, we report here a new assessment to complement the PB with a basin -scale bound- ary for the maximum rate of blue water with- drawal along rivers, based on the amount of water required in the river system to avoid regime shifts in the functioning of flow - dependent ecosystems. We base our control variable on the concept of environmental water flows (EWF), which defines the level of river flows for different hydrological characteristics of river basins adequate to main- tain a fair -to -good ecosystem state (60 -62). The variable monthly flow (VMF) method (33, 63) was used to calculate the basin -scale boundary for water. This method takes account of intra - annual variability by classifying flow re- gimes into high -, intermediate -, and low -flow months and allocating EWF as a percentage of the mean monthly flow (MMF). Based on this analysis, the zones of uncertainty for the river - basin scale water boundary were set at 25 to 55% of MMF for the low -flow regime, 40 to 70% for the intermediate -flow regime, and 55 to 85% for the high -flow regime (table S2). The boundaries were set at the lower end of the uncertainty ranges that encompass average monthly EWF. Our new estimates of the current status of the water use boundary— computed based on grid cell- specific estimates of agricultural, industrial, and domestic water withdrawals —are shown in Figs. 2 and 3, with details in figs. S7 and S8. Atmospheric aerosol loading Aerosols have well- known, serious human health effects, leading to about 7.2 million deaths per year (64). They also affect the functioning of the Earth system in many ways (65) (fig. S9). Here, we focus on the effect of aerosols on regional ocean - atmosphere circulation as the rationale for a separate aerosols boundary. We adopt aero- sol optical depth (AOD) (33) as the control var- iable and use the south Asian monsoon as a case study, based on the potential of widespread aero- sol loading over the Indian subcontinent to switch the monsoon system to a drier state. RESEARCH ( RESEARCHARTICLE The background AOD over south Asia is -0.15 and can be as high as 0.4 during volcanic events (66). Emissions of black carbon and organic car- bon from cooking and heating with biofuels and from diesel transportation, and emission of sul- fates and nitrates from fossil fuel combustion, can increase seasonal mean AODs to as high as 0.4 (larger during volcanic periods), leading to decreases of 10 to 15% of incident solar radiation at the surface (fig. S9). A substantial decrease in monsoon activity is likely around an AOD of 0.50, an increase of 0.35 above the background (67). Taking a precautionary approach toward uncer- tainties surrounding the position of the tipping point, we propose a boundary at an AOD of 0.25 (an increase due to human activities of 0.1), with a zone of uncertainty of 0.25 to 0.50. The annual mean AOD is currently about 0.3 (66), within the zone of uncertainty. Introduction of novel entities We define novel entities as new substances, new forms of existing substances, and modified life forms that have the potential for unwanted geo- physical and /or biological effects. Anthropogenic introduction of novel entities to the environment is of concern at the global level when these en- tities exhibit (i) persistence, (ii) mobility across scales with consequent widespread distributions, and (iii) potential impacts on vital Earth- system processes or subsystems. These potentially in- clude chemicals and other new types of engi- neered materials or organisms [e.g., (65 -71)] not previously known to the Earth system, as well as naturally occurring elements (for example, heavy metals) mobilized by anthropogenic activities. The risks associated with the introduction of novel entities into the Earth system are exempli- fied by the release of CFCs (chlorofluorocarbons), which are very useful synthetic chemicals that were thought to be harmless but had unexpected, dramatic impacts on the stratospheric ozone layer. In effect, humanity is repeatedly running such global -scale experiments but not yet applying the insights from previous experience to new appli- cations (72, 73). Today there are more than 100,000 substances in global commerce (74). If nanomaterials and plastic polymers that degrade to microplastics are included, the list is even longer. There is also a "chemical intensification" due to the rapidly increasing global production of chemicals, the expanding worldwide distribution as chemical products or in consumer goods, and the exten- sive global trade in chemical wastes (75). In recent years, there has been a growing de- bate about the global -scale effects of chemical pollution, leading to calls for the definition of criteria to identify the kinds of chemical sub- stances that are likely to be globally problematic (76, 77). Persson et al. (73) proposed that there are three conditions that need to be fulfilled for a chemical to pose a threat to the Earth system: (i) the chemical has an unknown disruptive effect on a vital Earth - system process; (ii) the disruptive effect is not discovered until it is a problem at the global scale; and (iii) the effect is not readily SCIENCE sciencemag.org 13 FEBRUARY 2015 • VOL 347 ISSUE 6223 1259555 -7