-5ln whlch 6u@ers are coore! and vlnters are lrar'er than no'-al. Thus ,
thls produces uore preclpltation and faster glacier grolrth durr'g the
wlnter and less Eelliog duling the srr@e!,
Flnally, a serlous questlon has been ralsed as to lrhether
c1fuate 1s really predlctable. Thls posslblllty nas ralsed by LorenzS
1n 1970. He drew an aaalogy to eathenatlcal oodellng. Many BatheDatl_
cal oodels of coropllcated phenonena are based upon a large nruber of
noa-llaear equatlons .olth a varlety of cmplex feedback interactlons,
If the l[ath$atlcLan ls fortunate, when a model of thls type ls run on
the couputet, {t rr111 converge and glve hfu a deflalt,e .rri".r. Such a
Dodel ls called transltlve. On the other hand, when a couplicated
nodel 1s tested, 1t ls not at aIl unusual to flnd that the solutlon
v111 trot converge but w111 oscillate back and forth vlthout produclng
a stable aasner. Such a nodel is ca11ed 1ntlansltlve. There 1s alio
an lDterredlate co[dltlon. occasloually, a nodel Ls found to converge
lnltla11y upon a deflnlte a!.s!rer but after a sholt perlod to 1,,,t ofi
thls solutlon aad settle down upon anocher one. Aflet a E.coid io_
definite perLod, lt will J rnp up and couverge agaln upon a thlrd solu_
tlon and eo on produclng a nr:mber of appalent solutlons 1n a randou
DaDner. Such a mdel 1s ca11ed alnost transitlve (or alDost lntran_
sltlve). Lorenz polnted out that the cll.sate ls a systeE whLch is the
result of a 1a!ge number of non-lLnear energy lnputs betlreea whlch there
are l0any corDpllcated feedback lnteractlons. He therefole Euggested that
the cllnate nay be a natula1 exauple of an alEoat tlansltlve ;ysten
whlch does not heve a stable solut1on. It !1111 settle dorrn lnto an
apparently stable condltion but then after a randou perlod w111 J uEp over
to a[other apparent stabillty, etc.
It ls not certain, hovever, that such a pesslmlgtlc outlook
Justlfied and lt has not stopped the development of uany roodels of
the Creenhouse Effect and other cllnate phenoEena. The sluplest of
these are the one-diDenslonal oodels in whlch the Ltrput at the earthrs
16
ls averaged over the globe and detalled caleulatloas are earrled
out to predlct veltlcal verlatlons. such nodels.do not requile uuch conputer t lae and can lnclude detailed treatDent of vertlcal jheumena such
as radlatlve transfer. They suffer, however, flou the fact that the
lnfluence of latltudlnal varlatlons 1s corpletely lgnored.
gurface
The next roore coupllcated oodele are so-called zonally
avereged uodele Ln shlch varlous latltude reglon€ are treated separately
ln e trro-dlDensional oanirer. These take nore co!0puter tlae but are stil1
Bhort enough to per:E1t conslderable sophlstlcatLon ln the calculatLoos.
They Eti1l suffer, however, from an lacomplete treatl0ent of letltudlnal
lntelactlons. In 6plte of thls, oany nodelers feel that they are the
nost valuable type of aodel upon which to work,
The
nost c@plicated models are the so-called general circu-
latlon nodels rhlch are three-dlaenslonal 1n character. These take
very long tlnes to c@pute snd the ratlo of real to mschlne tlme can