IPCC Climate Change Report 2001 (Bewertung von Partikelausbringungen)

Opti­ons to Enhan­ce, Main­tain, and Mana­ge Bio­lo­gi­cal Car­bon Reser­voirs and Geo-engi­nee­ring                                                                                                         S.   333

This might invol­ve pro­vi­ding nitro­gen or phos­pho­rus in lar­ge quan­ti­ties, but the quan­ti­ties to be sup­plied would be much smal­ler if growth were limi­ted by a micro­nut­ri­ent. In par­ti­cu­lar, the­re is evi­dence that in lar­ge are­as of the Sou­thern Oce­an pro­duc­tivi­ty is limi­ted by avai­la­bi­li­ty of the micro­nut­ri­ent iron. Mar­tin (1990, 1991) sug­gested that the oce­an could be sti­mu­la­ted to take up addi­tio­nal CO2 from the atmo­s­phe­re by pro­vi­ding addi­tio­nal iron, and that 300,000 ton­nes of iron could result in the remo­val of 0.8GtC from the atmo­s­phe­re.  Other ana­ly­ses have sug­gested that the effect may be more limi­ted.  Peng and Bro­ecker (1991) exami­ned the dyna­mic aspec­ts of this pro­po­sal and con­clu­ded that, even if the iron hypo­the­sis was com­ple­te­ly cor­rect, the dyna­mic issu­es of mixing the excess car­bon into the deep oce­an would limit the magnitu­de of the impact on the atmo­s­phe­re. Joos et al. (1991) repor­ted on a simi­lar model expe­ri­ment and found the oce­an dyna­mics to be less important, the time path of anthro­po­ge­nic CO2 emis­si­ons to be very important, and the maxi­mum poten­ti­al effect of iron fer­ti­li­za­ti­on to be some­what grea­ter than repor­ted by Peng and Bro­ecker (1991).

Some of the con­cepts of iron fer­ti­li­za­ti­on have now been tested with 2 small-sca­le expe­ri­ments in the equa­to­ri­al Paci­fic Oce­an. In expe­ri­ment IronEX 1 (Novem­ber, 1993) 480 kg of iron were added over 24 hours to a 64 km2 area of the equa­to­ri­al Paci­fic. In IronEX 2 (May/June, 1995) a simi­lar 450 kg of iron (as aci­dic iron sulp­ha­te) were added over a 72 km2 area, but the addi­ti­on occur­red in 3 doses over a peri­od of one week.  The IronEx 1 expe­ri­ment show­ed une­qui­vo­cal­ly that the­re was a bio­lo­gi­cal respon­se to the addi­ti­on of iron. Howe­ver, alt­hough plant bio­mass dou­bled and phy­to­plank­ton pro­duc­tion increa­sed four­fold, the decrea­se in CO2 fuga­ci­ty (in effect the par­ti­al pres­su­re of CO2 decrea­sed by 10 micro atm) was only about a tenth of that expec­ted (Mar­tin et al., 1994; Wat­son et al., 1994; Wells, 1994). In the IronEX 2 expe­ri­ment the abundance and growth rate of phy­to­plank­ton increa­sed dra­ma­ti­cal­ly (by grea­ter than 20 and twice, respec­tively), nitra­te decrea­sed by half, and CO2 con­cen­tra­ti­ons were signi­fi­cant­ly redu­ced (the fuga­ci­ty of CO2 was down 90ìatm on day 9).  Wit­hin a week of the last fer­ti­li­za­ti­on, howe­ver, the phy­to­plank­ton bloom had waned, the iron con­cen­tra­ti­on had decrea­sed below ambi­ent, and the­re was no sign that the iron was retai­ned and recy­cled in the sur­face waters (Monas­ters­ky, 1995; Coa­le et al., 1996; Coo­per et al., 1996; Frost, 1996).  The­se two expe­ri­ments have demons­tra­ted that week-long, sustai­ned addi­ti­ons of iron to nut­ri­ent-rich, but iron-poor, regi­ons of the oce­an can pro­du­ce mas­si­ve phy­to­plank­ton blooms and lar­ge draw­downs of CO2 and nut­ri­ents. While the results of the­se two expe­ri­ments can­not be uncri­ti­cal­ly extra­po­la­ted, they sug­gest a very important role for iron in the cycling of car­bon (Coo­per et al., 1996). The con­se­quen­ces of lar­ger, lon­ger-term intro­duc­tions of iron remain uncer­tain.  Con­cerns that have been expres­sed rela­te to the dif­fe­ren­ti­al impact on dif­fe­rent algal spe­ci­es, the impact on con­cen­tra­ti­ons of dime­thyl sul­phi­de in sur­face waters, and the poten­ti­al for crea­ting anoxic regi­ons at depth (Coa­le et al., 1996; Frost, 1996; Tur­ner et al., 1996). The­re is much to be lear­ned of the eco­lo­gi­cal con­se­quen­ces of lar­ge-sca­le fer­ti­li­za­ti­on of the oce­an.

Jones and Young (1998) sug­gest that the addi­ti­on of reac­tive nitro­gen in appro­pria­te are­as, perhaps in con­junc­tion with trace nut­ri­ents, would increa­se pro­duc­tion of phy­to­plank­ton and could both increa­se CO2 upt­ake and pro­vi­de a sustainab­le fishe­ry with grea­ter yield than at pre­sent.

Che­mi­cal buf­fe­ring of the oce­ans to decrea­ses in pH asso­cia­ted with upt­ake of CO2 leads to an increa­se in dis­sol­ved inor­ga­nic car­bon that does not rely on alte­ra­ti­on of the bio­lo­gi­cal pump. Buf­fe­ring of the oce­ans is enhan­ced by dis­so­lu­ti­on of alka­li­ne mine­rals. Dis­so­lu­ti­on of alka­li­ne mate­ri­als in oce­an sedi­ments with rising pH occurs in natu­re, but does so on a time-sca­le of thousands of years or more (Archer et al., 1997).  Inten­tio­nal dis­so­lu­ti­on of mined

Opti­ons to Enhan­ce, Main­tain, and Mana­ge Bio­lo­gi­cal Car­bon Reser­voirs and Geo-engi­nee­ring                                                                                                    S.  333334

mine­rals has been con­si­de­red, but the quan­ti­ty (in moles) of dis­sol­ved mine­rals would be com­pa­ra­ble to the quan­ti­ty of addi­tio­nal car­bon taken up by the oce­ans (Khesh­gi, 1995).

Stall­ard (1998) has shown that human modi­fi­ca­ti­ons of the earth’s sur­face may be lea­ding to increa­sed car­bon stocks in lakes, water reser­voirs, pad­dy fields, and flood plains as depo­si­ted sedi­ments. Buri­al of 0.6 to 1.5GtC/yr may be pos­si­ble theo­re­ti­cal­ly. Alt­hough Stall­ard (1998) does not sug­gest inten­tio­nal mani­pu­la­ti­on for the pur­po­se of increa­sing car­bon stocks, it is clear that human activi­ties are likely lea­ding to car­bon sequestra­ti­on in the­se envi­ron­ments alrea­dy, that the­re are oppor­tu­nities to mana­ge car­bon via the­se pro­ces­ses, and that the rate of car­bon sequestra­ti­on could be eit­her increa­sed or decrea­sed as a con­se­quence of human decisi­ons on how to mana­ge the hydro­lo­gi­cal cycle and sedi­men­ta­ti­on pro­ces­ses.  The term “geo-engi­nee­ring” has been used to cha­rac­te­ri­ze lar­ge-sca­le, deli­be­ra­te mani­pu­la­ti­ons of earth envi­ron­ments (NAS, 1992; Mar­land, 1996; Flan­ne­ry et al., 1997). Keith (2001) empha­si­zes that it is the deli­be­ra­teness that dis­tin­guis­hes geo-engi­nee­ring from other lar­ge-sca­le, human impac­ts on the glo­bal envi­ron­ment; impac­ts such as tho­se that result from lar­ge-sca­le agri­cul­tu­re, glo­bal forestry activi­ties, or fos­sil fuel com­bus­ti­on. Manage­ment of the bio­s­phe­re, as dis­cus­sed in this chap­ter, has some­ti­mes been inclu­ded under the hea­ding of geo-engi­nee­ring (e.g., NAS, 1992) alt­hough the ori­gi­nal usa­ge of the term geo-engi­nee­ring was in refe­rence to a pro­po­sal to collect CO2 at power plants and inject it into deep oce­an waters (Mar­chet­ti, 1976). The con­cept of geo-engi­nee­ring also inclu­des the pos­si­bi­li­ty of engi­nee­ring the earth’s cli­ma­te sys­tem by lar­ge-sca­le mani­pu­la­ti­on of the glo­bal ener­gy balan­ce.

It has been esti­ma­ted, for examp­le, that the mean effect on the earth sur­face ener­gy balan­ce from a doub­ling of CO2 could be off­set by an increa­se of 1.5% to 2% in the earth’s albe­do, i.e. by reflec­ting addi­tio­nal inco­m­ing solar radia­ti­on back into space. Becau­se the­se later con­cepts offer a poten­ti­al approach for miti­ga­ting chan­ges in the glo­bal cli­ma­te, and becau­se they Opti­ons to Enhan­ce, Main­tain, and Mana­ge Bio­lo­gi­cal Car­bon Reser­voirs and Geo-engi­nee­ring are trea­ted nowhe­re else in this volu­me, the­se addi­tio­nal geo­en­gi­nee­ring con­cepts are intro­du­ced brief­ly here.

Sum­ma­ries by Ear­ly (1989), NAS (1992), and Flan­ne­ry et al.  (1997) con­si­der a varie­ty of ways by which the albe­do of the earth might be increa­sed to try to com­pen­sa­te for an increa­se in the con­cen­tra­ti­on of infra­red absor­bing gases in the atmo­s­phe­re (see also Dick­in­son, 1996). The pos­si­bi­li­ties inclu­de atmo­s­phe­ric aero­sols, reflec­tive bal­loons, and space mir­rors. Most recent­ly, work by Tel­ler et al. (1997) has re-exami­ned the pos­si­bi­li­ty of opti­cal scat­te­ring, eit­her in space or in the stra­to­s­phe­re, to alter the earth’s albe­do and thus to modu­la­te cli­ma­te.  The lat­ter work cap­tures the essence of the con­cept and is sum­ma­ri­zed brief­ly here to pro­vi­de an examp­le of what is envi­sio­ned.  In agree­ment with the 1992 NAS stu­dy, Tel­ler et al.  (1997) found that ~107 t of dielec­tric aero­sols of ~100 nm dia­me­ter would be suf­fi­ci­ent to increa­se the albe­do of the earth by ~1%. They show­ed that the requi­red mass of a sys­tem based on alu­mi­na par­ti­cles would be simi­lar to that of a sys­tem based on sul­phu­ric acid aero­sol, but the alu­mi­na par­ti­cles offer dif­fe­rent envi­ron­men­tal impact. In addi­ti­on, Tel­ler et al. (1997) demons­tra­te that use of metal­lic or opti­cal­ly reso­nant scat­te­rers can, in princip­le, great­ly redu­ce the requi­red total mass of scat­te­ring par­ti­cles requi­red. Two con­fi­gu­ra­ti­ons of metal scat­te­rers that were ana­ly­zed in detail are mesh micro­st­ruc­tures and micro-bal­loons. Con­duc­tive metal mesh is the most mass-effi­ci­ent con­fi­gu­ra­ti­on. The thic­k­ness of the mesh wires is deter­mi­ned by the skin-depth of opti­cal radia­ti­on in the metal, about 20 nm, and the spa­cing of wires is deter­mi­ned by the wav­elength of scat­te­red light, about 300nm. In princip­le, only ~105t of such mesh struc­tures are requi­red to achie­ve the bench­mark 1% increa­se in albe­do. The pro­po­sed metal bal­loons have dia­me­ters of ~4 mm and a skin

Opti­ons to Enhan­ce, Main­tain, and Mana­ge Bio­lo­gi­cal Car­bon Reser­voirs and Geo-engi­nee­ring                                                                                                                 334

 

thic­k­ness of ~20nm. They are hydro­gen fil­led and are desi­gned to float at alti­tu­des of ~25km.  The total mass of the bal­loon sys­tem would be ~106t. Becau­se of the much lon­ger stra­to­s­phe­ric resi­dence time of the bal­loon sys­tem, the requi­red mass flux (e.g., ton­nes repla­ced per year) to sustain the two sys­tems would be com­pa­ra­ble. Final­ly, Tel­ler et al. (1997) show that eit­her sys­tem, if fabri­ca­ted in alu­mi­ni­um, can be desi­gned to have long stra­to­s­phe­ric life­ti­mes yet oxi­di­ze rapidly in the tro­po­s­phe­re, ensu­ring that few par­ti­cles are depo­si­ted on the sur­face.

One of the peren­ni­al con­cerns about pos­si­bi­li­ties for modi­fy­ing the earth’s radia­ti­on balan­ce has been that even if the­se methods could com­pen­sa­te for increa­sed GHGs in the glo­bal and annu­al mean, they might have very dif­fe­rent spa­ti­al and tem­po­ral effec­ts and impact the regio­nal and sea­so­nal cli­ma­tes in a very dif­fe­rent way than GHGs. Recent ana­ly­ses using the CCM3 cli­ma­te model (Govin­da­sa­my and Caldei­ra, 2000) sug­gest, howe­ver, that a 1.7% decrea­se in solar lumi­no­si­ty would clo­se­ly coun­ter­ba­lan­ce a doub­ling of CO2 at the regio­nal and sea­so­nal sca­le (in addi­ti­on to that at the glo­bal and annu­al sca­le) despi­te dif­fe­ren­ces in radia­ti­ve for­cing pat­terns.  It is unclear whe­ther the cost of the­se novel scat­te­ring sys­tems would be less than that of the older pro­po­sals, as is clai­med by Tel­ler et al. (1997), becau­se alt­hough the sys­tem mass would be less, the scat­te­rers may be much more cost­ly to fabri­ca­te.  Howe­ver, it is unli­kely that cost would play an important role in the decisi­on to deploy such a sys­tem. Even if we accept the hig­her cost esti­ma­tes of the NAS (1992) stu­dy, the cost may be very small com­pa­red to the cost of other miti­ga­ti­on opti­ons (Schel­ling, 1996). It is likely that issu­es of risk, poli­tics (Bodan­sky, 1996), and envi­ron­men­tal ethics (Jamie­son, 1996) will pro­ve to be the decisi­ve fac­tors in real choices about imple­men­ta­ti­on.  The impor­t­an­ce of the novel scat­te­ring sys­tems is not in mini­mi­zing cost, but in their poten­ti­al to mini­mi­ze risk. Two of the key pro­blems with ear­lier pro­po­sals were the poten­ti­al impact on atmo­s­phe­ric che­mi­stry, and the chan­ge in the ratio of direct to dif­fu­se solar radia­ti­on, and the asso­cia­ted whiten­ing of the visu­al appearan­ce of the sky. The pro­po­sals of Tel­ler el al.  (1997) sug­gest that the loca­ti­on, scat­te­ring pro­per­ties, and che­mi­cal reac­tivi­ty of the scat­te­rers could, in princip­le, be tun­ed to mini­mi­ze both of the­se impac­ts. None­theless, most papers on geo-engi­nee­ring con­tain expres­si­ons of con­cern about unex­pec­ted envi­ron­men­tal impac­ts, our lack of com­ple­te under­stan­ding of the sys­tems invol­ved, and con­cerns with the legal and ethi­cal impli­ca­ti­ons (NAS, 1992; Flan­ne­ry et al., 1997; Keith, 2000).  Unli­ke other stra­te­gies, geo-engi­nee­ring addres­ses the sym­ptoms rather than the cau­ses of cli­ma­te chan­ge.

4.8 Future Research Needs

This chap­ter sug­gests a host of future rese­arch needs. A com­bi­na­ti­on of sta­tis­ti­cal, eco­lo­gi­cal, and socio-eco­no­mic rese­arch would be hel­pful to bet­ter under­stand the situa­ti­on of the land, the forces of land-use chan­ge and the dyna­mic of forest car­bon pools in rela­ti­on to human activi­ties and natu­ral dis­tur­ban­ce.  More pre­ci­se infor­ma­ti­on is nee­ded about degra­da­ti­on or impro­ve­ment of secon­da­ry and natu­ral forests throughout the world, but par­ti­cu­lar­ly in deve­lo­ping coun­tries.

Some spe­ci­fic examp­les are:

• assess­ment of land avail­ab­le for miti­ga­ti­on opti­ons based on socio-eco­no­mic pres­su­res and land ten­ure poli­ci­es. Fur­ther­mo­re, it would be bene­fi­ci­al if the impact of mar­ket pri­ce of car­bon miti­ga­ted on land avail­ab­le for miti­ga­ti­on oppor­tu­nities in dif­fe­rent coun­tries was unders­tood;

• impli­ca­ti­ons of finan­ci­al incen­ti­ves and mecha­nisms on LULUCF sec­tor miti­ga­ti­on poten­ti­al in dif­fe­rent coun­tries;

• com­pa­ra­ti­ve advan­ta­ge (miti­ga­ti­on cost, ancil­la­ry bene­fits, etc.) of LULUCF sec­tor miti­ga­ti­on opti­ons over ener­gy sec­tor oppor­tu­nities;

• deve­lop­ment and assess­ment of dif­fe­rent approa­ches to deve­lo­ping base­li­nes for LULUCF activi­ties and com­pa­ri­son with other sec­tors; and • socio-eco­no­mic and envi­ron­men­tal costs and bene­fits of imple­men­ting LULUCF sec­tor miti­ga­ti­on opti­ons in deve­lo­ping coun­tries, inclu­ding issu­es such as pro­per­ty rights and land ten­ure.