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

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.