Principal indicators of past climates - Climate Change: Biological and Human Aspects - page 41

Huaynaputina Ta m b o r a
Kuwae

Cometary debris(?)/volcano

Laki Krakatau

1.5

Medieval
optimum

Little Ice Age

1

0.5

0

-0.5

-1

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Ye a r

Fig. 2.1 Approximate,smoothed-average(withan 70-yearilter)northernhigh-latitudetemperaturechangesover

thepast2000years,basedoriginallyonseveraldendrochronologicalseriesanddeviatingfromthe1601-1974
average.Possiblecometarydebrisinoneinstance,togetherwithivevolcaniceruptions,coincidewith
subsequentlowerannualtree-ringgrowth.SeeChapter3:before1000thedendrochronologysuggestsitwas
warmerratherthancooler;alsoshownarethemedievalwarmperiod,900-1200,theLittleIceAge,
1550-1750,andthe20th-centurywarming.Thedeclineinsomeborealdendrochronologicalseriesattheend
ofthe20thcentury(dashedline)isportrayedseparatelyandisthoughttoperhapsbeduetoincreasedsnow
falldelayingspringgreeningorsomeotherenvironmentalfactorimpedinggrowthmostlikelycausedbya
warmingclimate.SeethesectioninChapter5onthecurrentbiologicalsymptomsofwarming.Datainpart
fromBrifa(1998).

≈

It is important to note these underpinning tenets (as it is for all assumptions made
in science), for it is easy to assume that they will always hold. This is not so and
invariably there are exceptions. Tree-ring series derived from high-latitude growth
have provided a solid record for nearly all of the past two millennia. However,
ring growth in recent decades, principally since the 1960s, has deviated from the
pronounced warming experienced (and instrumentally measured). Growth is less
than is expected from temperature alone (see Figure 2.1). The reason for this is
the failure of the above principles. There is some evidence, for example (Vag-
anov et al., 1999), to suggest that what has happened is that our globally warmed
world of recent decades has seen increased winter precipitation (snow) in these
high-latitude regions. (One would expect more ocean evaporation in a warmer
world, and hence, overall, more precipitation. More evidence from high latitudes
is covered in Chapter 6.) In these permafrost environments this extra precipita-
tion has delayed snow melt: a greater depth of snow takes longer to melt. As
a result the initiation of cambial activity necessary for the formation of wood
cells is delayed. Fortunately, dendrochronological problems of such significance are
rare.