REPRESENTATIVENESS OF ZONALLY INTEGRATED TRENDS

e

1 :

.a E

. .

-.2 I .6 1.4 1 .2 . .

1840 1060 1880 19W 1920 1940 1960

FIG.

2. Trends of mean temperature in Northern and Southern Hemispheres, by pentads, within indicated latitude limits.

Annual average data above and winter-season data below. All curves are area-weighted averages of 100-latitude band data in Table 5.

to the early

1940s,

and a marked tendency for cooling since the early

1940s.

Average temperatures

in

recent years have apparently returned to about the levels of the

1920s.

Turning next to a comparison of the zonally integrated trends for the Northern and Southern Hemispheres, shown in

Fig. 2,

w e

find

that the trends have been

FIG. 3. Trend of annual mean temperature in tropics (30°

N.-

300

S.)

by pentads, based on numbers of stations indicated.

162

qualitatively similar

in

both. Inasmuch as the mag&

tudes of temperature changes are

known

to be a strong function of latitude (at least in the Northern Hemis- phere), the trend for the Northern Hemisphere has been shown in Fig.

2

for

00-600 N.

as well as for

00-800 N.,

to compare with the trend for

Oo-600 S.

(Climatological data for higher southern latitudes are virtually lacking.) Inclusion of data for

600-800 N. in

the Northern Hemis- pheric averages are seen

in

Fig.

2

(dotted curves) to increase the magnitude of the net secular trend of that hemisphere

by

as m u c h as half although the added data represent only a one-seventh increase of surface area.

Of

special interest for this symposium, perhaps, is the fact that the tropics have shared

in

the secular warming of the past century, as well as

in

a tendency for cooling since the

1940s.

Fig.

3

shows the pentadal variation of annual mean temperature since

1880 in

the zone between

300N.

and

300s.

The net warming

in

this zone had reached nearly

lo F.

from about

1880

1940,

and the cooling since about

1940

has thus far totalled about

-0.30F.

Callendar

(1961)

has also published zonally integrated values of m e a n annual temperature change

in

different increments of latitude. For the six decades ending with

1950,

when Callendar's data terminated, these changes have been found to agree closely with the changes derived from

my

o w n and Willett's data (see Landsberg and Mitchell,

1961).

This is reassuring in view of dissimilar data selection and analysis procedures in the two sludies. In terms of Merence between consecutive 30-year periods, this comparison is shown in Table 1.l

REPRESENTATIVENESS OF ZONALLY INTEGRATED TRENDS

Fig. 4

indicates the geographical locations of all stations that contributed at least

20

years of data to the trends illustrated

in

the previous figures. The most casual glance at Fig.

4

serves to remind us that

in

speaking of average trends over large parts of the world w e are really speaking primarily of trends over the continents and a few relatively well populated archipelagoa. Large expanses of ocean, especially

in

the Southern Hemis- phere, are completely devoid of historical data (areas outlined

in Fig. 4). With

this fact in mind, let us next consider the representativeness of the available data from t w o different points of view. First, to what extent are the temperature trends shown in Figs.

1-3

representative of the principal areas of civilization-i.e., primarily the continental land masses and archipe- lagos-for which data are comparatively abundant

?

Second, to what extent are the calculated trends representative of the earth as a whole, and therefore indicative 1. Inasmuch aa C d e n d a r did not treat global data available since 1950, it iß impossible to compare notes with him concerning evidence of the recent culmination of thr warming.

World-wide pattern of secular temperature change

T A B L E

1. Thirty-year change of annual mean temperature

(OF.),

1890-1920 to 1920-501 -

After Callendor After Willett and Mitchell

Zone

Inclusive latitudes A Inclusive latitudes A

World 60° N.-50”

S.

+0.41 600 N.-500

S.

$0.37

North temperate Tropical South temperate

600-250

N.

250 N.-250

S.

250-500

S.

+0.70

$0.31 t0.25

600-30°

N.

60°-200N.

300 N.-300 S.

1

200 N.-200

S.

1

^300-50oS.^200-50’^S.

1. Callendar’s data are 1891-1920 to 1921-50; Willett and Mitchell’s data are 1890-1919 to 1920-49.

$0.64 +O37 +0.35 +0.39 + O J O +O.OS

of variations of the net planetary atmospheric heat budget

?

T R E N D S AS R E P R E S E N T A T I V E O F T H E D A T A A R E A S

( R E G I O N S O F A B U N D A N T D A T A )

Let us postulate that the stations whose data have been used in this trend study are randomly distributed over the region of available data outlined

in

Fig.

4

(hereafter denoted for simplicity as

the

“data areas”, which roughly coincide

with

the continents).

By

the methods given

in my

research paper (Etchell,

1961~)

w e can then obtain an estimate of

the

standard error of the zonally averaged trends as representative of the data areas.

An

outline of these methods and some statistics necessary for their application here are given

in

a n appendix at the end of the paper.

Trends of mean annual and winter temperature since

1890

are summarized along

with

their associated standard errors for various latitude zones of the world

in

Tables

2

and

3. The trends

are shown as 30-year changes from

1890-1919

1920-49

(the same intervals as

in

Table

1) in

Table

2

and as 10-year changes from

1940-49

1950-59 in

Table

3.

T h e ñrst period coincides

with

the great “world-wide” warming, and

the

second coincides with the cooling evident

in

Figs

1-3

which followed. It is clear from these tables that both

the

warming and subsequent cooling were shared by (the data areas of) the Northern and Southern Hemispheres and the tropics.

T h e statistical significance of the warming w a s moder- ately

high in the

Southern Hemisphere (trend equal to

2-3

standard errors) and extremely

high

elsewhere as well as for the world as a whole (trend equal to

4-7

standard errors).

T h e significance of the cooling has also been moder- ately high, but has varied substantially

with

latitude and season. (The trend for winter

in

the Northern Hemis- phere was comparable to its standard error; all other

trends shown in Table

3

are about two or three times

their

standard errors.) T h e recent cooling

in

the tropics, as well as the annual average cooling for the world as a whole, has been

highly

significant (exceeding about

3

standard errors, corresponding to significance levels of

99

per cent and higher).

T R E N D S A S R E P R E S E N T A T I V E O F T H E W O R L D AS A W H O L E

In view of the sizeable oceanic regions of the world totally devoid of data, it is only

with

certain reservations that the trends noted above can be taken as representative of planetary average conditions. Until such time as these data gaps have been ñlled, either

by

conven- tional ocean weather stations,

by

automatic weather buoys, or possibly

by

indirect sensing techniques involving artificial earth satellites, secular fluctuations of temperatures

in

these areas can never be determined

with

m u c h c0nfidence.l

In

order to evaluate the available trend data as a measure of planetary average conditions, w e

will

adopt approaches described below.

In the ñrst approach, w e assume that the u n k n o w n trends averaged for the no-data areas are identically zero. That is to say, the no-data areas (open oceans) are presumed not to have participated to an appreciable extent

in

any of the net temperature changes measured elsewhere (the data regions). Inasmuch as there is little justification-either empirical or theoretical-for sup- posing that average trends over the oceans are normally opposed in sign to those over the continents, this assumption provides the basis for a conservative test of the statistical significance of the observed trends as

1. Climatological analyses of synoptic ship reports of air and sen temper- ature have been m a d e which have Borne value in this regard. Except for certain well-travelled shipping lanes in the northern oceans, however, irrebdar timing and spacing of the observation8 and various sources of inhomogeneity greatly complicate the interpretation of long-term trends evident in 8uch data.

163

_.--

..e-.- ....--.

___

164

wor&wide pattern of secutar:temperature change

TABLE

2. Thirty-year temperature change (OF.), 1890-1919 to 1920-49,

A,

selected latitude belts; standard error of estimate

SE(A)

for data areas; significance ratio

IA]

SE(A)

applicable to areas of good data coverage (t) and whole earth (to), assuming zero net temperature change in areas of no data

Annual Winter

Latitude zone

r 1

A f SE(A) x' A 1 S E M tx

World

800 N.-600

S.

1-0.49 f0.07 6.7 4.3 3-0.71 10.11 6.2 4.5

600 N.-600

S.

1-0.39 rfr0.07 5.9 3.4 +0.44 fO.10 4.6 3.0

800

N.-Oo

+0.64 fO.09 6.8 5.1 +1.14 10.18 6.5 5.0

600

N.-Oo

+0.43 f0.07 6.3 4.4 1-0.61 10.13 4.9 3.4

300 N.-300

S.

+0.35 kO.09 3.8 2.0 1-0.49 rfr0.12 4.0 2.2

00-600

S.

+0.34 f0.11 3.0 1.3 +0.28 10.15 1.9 1.0

Northen Hemisphere Tropics1

Southern Hemisphere

1. Winter value for tropics is average in Jnnuary, February, and December north of Equntor and in June, July, nnd August south of Equator.

TABLE

3. Ten-year temperature change (OF.) 1940-49 to 1950-59

;

standard error of estimate for data areas

;

and significance ratios for data areas for whole earth (see heading to Table 2)

Annual Winter

Latitude zone

t 1

f

S E W tZ A f W A ) rg

World

800 N.-600

S.

-0.19 f0.06 3.3 1.8 -0.23 hO.09 2.4 1.1

600 N.-60O

S.

-0.17 f0.05 3.3 1.7 -0.17 f0.08 2.1 0.6

800

N.-Oo

-0.26 kO.09 2.9 1.7 -0.22 10.17 1.3 0.7

600 N.-OO -0.23 h0.08 3.0 1.6 -0.10 10.14 0.8 0.1

300 N.-300

S.

-0.24 f0.06 3.6 1.9 -0.26 hO.09 2.9 1.5

00-600

S.

-0.11 f0.07 1.6 0.8 -0.23 fO.09 2.6 0.9

Northern Hemisphere Tropics1

Southern Hemisphere

1. Winter vnlue for tropics is average in January, Febrnav, and December north of Equator and in June, Suly, and August south of Equntor.

representative of planetary conditions. Accordingly, let the change of planetary m e a n temperature

in

a given latitudinal band i, between any two time intervals, be given

by

(1)

-

8. ₁

- - c. Jr

+

-

Ci) Fi,

where

&

is the average trend for the data areas,

8'i

is that for the no-data areas (here assumed identically equal to zero), and Ci is the percentage total area of the band at least

1,000

miles distant from any historical- record station within the z0ne.l

In

this w a y w e m a y calculate globally representative trends corresponding to the data-area trends

in

Tables

2

and

3 by

the formula

A, =

ZCiwiSi, (2)

where

the

zui are proportional to the areas of the

100

bands involved

(Cwi = 1).

These trends, in turn, m a y

-

be expressed as ratios to the same standard errors as before,

by

which w e prescribe that the error of estima- tion of the (zero) trends

in

the no-data areas is

hypo-

thetically equal to that of the changes in the data areas.

T h e resulting ratios (i.e., values of tg, also shown

in

Tables

2

and

3)

are suitable for estimating the statistical significance of the trends from the planetary point of view. It

w i l l

be seen in Table

2

that the observed warming trends between

1890

and

19-49,

with the exception

of

those

in

the Southern Hemisphere, remain

highly

significant

(2-5

standard errors). According to Table

3,

however, the cooling since

1940

loses much of its significance w h e n viewed

in

this manner.

In

a second approach to this problem, let us consider 1. This fiyre represents the shortest geographical distance nt which the

spatial eorrelntion coefficient of mean iemperature changes typically diminishes to zero. It is found from cmpiricd evidence that this distance is nearly independent of latitude, at least in the Northern Hemisphere.

Values of C for each 100-latitude band are s h o w n in Tnble 7.

165

Changes of climate Les changements

de

climat

the

€allowing

question: W h a t range of hypothetical average temperature change

in

the no-data areas (i.e., what values of

Sr,)

can apply such that, w h e n these are averaged

with

the measured trends for the data areas, the resulting trends are not significantly different from zero?

If

such values of

8'z,

denoted as

Arc,

can be declared unreasonable, then w e are justified in accepting the measured trends as evidence not only of redistri- butions of heat

within

the atmosphere, but also of changes of the heat budget of the whole planetary atmosphere.

For this purpose, w e m a y set the global trend equal to its expected value

Ãc = Ão +

[Z(l- Ci)wi]Arc, (3) with

Á o

given

by

(2), and define to such that to

>

= Ã/SE(Ã). If

for convenience the sign of

ho

is always taken as positive, and that of

Aro

adjusted to agree, w e are

then led

to the inequality

(4)

For to

=2.58

(corresponding to the

99

per cent significance level), and for the stipulation that

SE@) = SE(A),

ranges of

A',

are shown in Table

4

for the s a m e periods of record and latitude zones as

in

Tables

2

and

3. With

the help of the maps of secular change to be presented in the next section, these values for the no-data areas have been tentatively evaluated as definitely reasonable, marginally reasonable, or unreasonable. Although such an evaluation must at this stage be taken as provisional, it has been extremely

SE@) - ^Ão

"c< ~ ( 1 -

cihi

difñcult

by

this means to avoid the conclusion that the warming trends for

the

world as a whole, and for the Northern Hemisphere in particular, are

truly

planetary

in

scope.

On

the other hand, it cannot

yet he

demons- trated in this w a y beyond a reasonable doubt that the net cooling since the

194~0s

has likewise

been

planetary in scope. That this cooling is of such nature, however, seems reasonable and this should be verifiable

if

the cooling

in the

data areas were to continue for another decade or two in the future.

Dans le document CHANGEMENTS DE CLIMAT OF CLIMATE (Page 164-168)