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OCR for page 85
l
10
-Global Temperature Trends
KEVIN E. TRErIBERTH
Notional -Center for Atmospheric Research
This talk focuses mairdy on stratospheric temperature trends but
covers some a~p£C{B of Propose temp~at;ures ~ -weD. The I
part of the talk 6tiSCUS$~S what is h~-~i-ng in the arc region.
The principal Issue to consider ~ the ¢onsmte-~ -of coons am other
trace gas changes, and of the expected temperature Mange best
ing hum such trace gas changes, with actual temperature Amps.
In the Iowe:r stratosphere, espe~c~aIly Dublin the Hider night, !tem-
perat;ure am ozone are simita~y at so :from that -$taMpomrt
temperature an] sconce changes sh-4 -recur in-tandem. But o:zorre
;ch~ges also imply Ganges in Bow he~g rates and th~u~ch-anges
in temperature. Are changes in ozone an;~peratwe`.~sm~tent
that sense?
With regard tc, greenhouse gayest, merry indicates that a warm-
~ng should occur at the surface, but since the oceans add the
cryosphere are involved' cih~ges a" Alley to occur on time scales
of a-~;ecade or ITIQ1~, whereas flee reuse of the ~tr~at~.ihere to
greenhouse gases Ill c~cciur ran a mther shout tinge sold. For the
stratosphere, `ozone decrease and g~eenho; - e .~" morale b`atth Icy
GO:Oli ng; therefore the Ounce ~ d*~g an gun Change is
greater than in the ~tr~.~e. Jerry mamas rheas mead Ridded
out that c~omp-ara~y Platte, ;; Ding; ia-ave Occurred in
the upper str;atos~ere, but e~ewhe;re temperature trencis care much
harder to find.
>85
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l
86
KEVIN E. TRENBERTH
One of the problems with temperature trends is the difficulty
of detecting them. Temperature records are clearly functions of the
measurement systems used. The issue of temperature measurements
needs to be addressed in the future, especially with regard to any
climate change program. The main device for taking temperature
measurements in the atmosphere is the radiosonde, which was not,
however, designed to detect long-term temperature trends. There
have been changes in type and design (especially with regard to
radiation shields), and significant day-night temperature differences
that have varied with design have resulted. A recent international
intercalibration found that instruments of different nations generally
reproduce temperatures within about 0.2°C and pressures within
2 mb. This amount of error is not serious in the troposphere but
becomes significant in the stratosphere, where 2 mb is a sizeable
fraction of the pressure being measured. Hence, radiosondes are not
useful above about 20 mb; ~ fact, few of them ever rise higher than
20 mb.
The other principal source of temperature data in the free atmo-
sphere is satellite radiance data that exist in useful form (as TOVS
data) from about 1979 to present. There are again problems with cal-
ibration, changes to new satellites, and changes in the time of day of
satellite overhead passage. These problems have led to temperature
differences of 2 to 4°C, due to changes of satellite in the stratosphere
especially. Therefore, it has been necessary to adjust and merge
segments of the satellite record in order to get a continuous record
of temperature trends. Some of the analyses have in fact removed
apparent (but spurious) trends by invoking this kind of step-function
adjustment. So it is very difficult to get an absolute calibration and
to use satellite radiance data to get reliable trends.
Rocketsondes are too few and far between to be useful for estate
fishing trends alone, but they may be useful for calibrating satellite
data in the region above the useful radiosonde ascents. But rocket-
sondes have also changed, in type and instrumentation, with time.
The products that exist for determining temperature trends are
station or instrument records and analyses based in some way on
these records. Many station records suffer from missing data, a major
problem. Additionally, the areal coverage is insufficient in many
regions. Various kinds of analyses have been done to fill the record
gaps and to merge different data types (e.g., Labitzke et al. (1986)
have compiled temperature analyses for the Northern Hemisphere
that start in 1965~.
OCR for page 87
GLOBAL TEMPERATURE l:RENDS
87
When one examines a temperature record for a linear trend, one
needs to be concerned with rend effects" and with major temporary
perturbations, such as those attributed to E} Chichon. The seasonal
cycle, which has very large amplitude in the stratosphere, is impor-
tant in that incomplete data during a period of rapid seasonal change
can give misleading results, as explained below. The missing data
problem arises because of balloon loss or data transmission problems.
Because of differing data amounts at different levels, the thickness of
the monthly mean geopotential heights may not equal the monthly
mean of the daily thickness values.
The effects on temperature trends that we need to consider in-
clude the solar cycle with its variation in ultraviolet flux, leading to
stratospheric cooling at the solar minimum; aerosols from volcanic
eruptions, resulting in heating within the stratospheric aerosol layer;
ozone change, with a decrease implying stratospheric cooling; carbon
dioxide, with an increase implying stratospheric cooling; and natu-
ral variability due to dynamical processes and internally generated
variations such as the quasi-biennial oscillation, southern oscillation,
and sudden stratospheric warmings, affecting both temperatures and
the ozone distribution.
The surface temperature record (Figure 10-1) as compiled by
Hansen and Lebedeff (1988) shows that, in terms of global averages,
1987 was the second warmest year on record, being exceeded only by
1981. The 1980s thus far average distinctly warmer than any previous
decade since the record began in the 1880s. It is noteworthy that
global temperatures have been highest during a period of decrease in
insolation as measured by satellite. The magnitude of the insolation
decrease since 1979 is estimated to have offset almost exactly the
expected increase since 1979 in temperature due to greenhouse gages.
The trend of the 108-year record is not linear but has variations,
including a relatively warm period in the 1930s, which has been
attributed to a dearth of volcanic aerosols but could also be due to
natural variability. There is a problem in interpreting this record
because of such anthropogenic effects as development of urban heat
islands (Kukla et al., 1986), deforestation, building of roads, and
relocation of observing stations. Although an attempt has been
made to estimate such effects, we cannot be sure that the record is
indicating changes due to carbon dioxide buildup and other external
factors.
As ~ mentioned earlier, problems can arise in fitting linear trends
to data. One can use least squares regression to fit a linear trend to
OCR for page 88
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Representative terms from entire chapter:
monthly mean
%8
0.4
o.~ _
ala
--~.~2 _
I..]
A.
,,, .
.' At, . ~
lo.- ~ l
, .+
, 'i,
.
-~.6` _4 .
.
r;
~ .
80
!
O. ~ E
KEVIN E. TRENBERTH
Annual Meon
5 Year :*~nning :Nlea2n
. a. 1 1
. , 1
!
-+ - - - - - - - - - - ~i
F I al
[rfOr Estimates (95~0 Confidence]
_ - i ~ :i 1 ! I
. !
~ I 1: :
.1920,1~930 1~~O ;~-~50 .~1~960 '1970 ~1980 1-990
-''E
FIG; 10-:1 D¢p-atture of -mean g1~1 ~-temperat~res from their i951 to I-98Q
period mean -~iiue for ~ndi:Vidua1 yews boots con~ect~1 With dashed Elmer and
for 5-year running ~mea-n valu-es .~anlid ~unre3. ~rar~estim-~es for both in~ndll-al
~anii1 !tu~ining mean val;ues a-re s;}~o~tn i-= sil~.~d yeiars. ~lod -~t recc,rd ~s 1880
through ^~. `~pr~i~ed ~tro-m Ha-n~n ta~ ~efE, 1988. t~opy-r~-ght :~) 198-8
;b.y the ;Pt-meric-an Oe'opb - ~1 C.ni-o-n-.~)
o4ie cyche ~om pi 't0 thr-ce :pi) ~ -a sine wave ~;d get a lar-ge :pos~tive
-trend w~kh a t~a~- on ~ 0.7-8, Wh~Gh ~ ~y ~s~:gn~fica~t. :If hadf a
tyole is added ;~43 th-e tegmn~ng <3f-this ~record and a~her -half ;to $he
en6, a ]=ge 8:oWnwat~ iindar -tPe~n:6 -~re~s ~hai h"as --a correiiatio~ ~f
--~.~3!~. The tm~t ~of tllis is th~at-trui~ds ~re wry sensiti~re to ~-the be-
-gt0tngand~nil-po~n-ts`-ofilietacotd. Co-ncli~sions~abo~;i
GLOBAL TEMPERATURE TRENDS
89
value since the 1920s. Any trend starting with 19" is likely to be
totally unrepresentative of longer-period trends.
Karoly (1987) has looked at temperature trends for 19 Southern
Hemisphere stations from 1964 to 1985. He found slight upward
trends, ranging from 0.1 to 1.2°C, at 700 mb at all of these stations
and slight downward trends, ranging from-0.1 to -1.3, at 50 mb at
all but one of them. The Mt. Agung volcanic eruption that occurred
in 1963 may have affected the trends to some extent; hence the trends
may reflect more than just the carbon dioxide increase. AISQ in the
Southern Hemisphere, Salinger (1979) has composited New Zealand
mean annual and seasonal surface temperatures from 1853 through
1975. A minimum in the rn~-1960s is even more pronounced in this
record than in the global record of Hansen and Lebedeff (1988), and
a marked upward trend since about 1964 is not representative of the
overall long-term trend of this record.
Labitzke et al. (1986) have published time-series of monthly
mean Comb temperatures for the Northern Hemisphere from 1965
through 1983. The E! Chichon eruption of 1982, and perhaps the
Mt. St. Helens eruption of 1980, contarn~nated the record in the
last few years. Therefore Labitzke computed linear trends from
1966 through l9X0. These trends are slightly downward for this
period at latitudes poleward of 20°N, but the time series show much
interannual variability, even though the quasi-biennial oscillation
has been removed from these series. Only the trend at 70°N latitude
appears large enough to be meaningful.
Using a relatively sparse network of stations, Angell (1986a) has
looked at global temperature trends from 1960 to 1985. HE results
show a slight warming in the Southern Hemisphere midtroposphere
and a slight cooling above about 10 km, in general agreement with
Karoly's results. These trends are somewhat stronger for the decade
1975 to 1985 alone. More recent results of Angell (unpublished) show
layer mean temperature time-series for the world through 1987. The
records for the surface, the 850~ to 300-mb layer, and the 300 to
100-mb layer start in 1957, and the 10~ to Comb record starts in
1969. The surface and tropospheric layer both show cooling un-
til about 1964 to 1965 and a warming trend since. The 300 to
100 mb layer does not show a recognizable trend. At 100 to 50 mb,
Angell's data show a pronounced downward trend after a warm peak
in 1982 that was probably associated with the eruption of E! Chichon.
Angell (1986b) did a latitudinal breakdown of the 10() to Comb data,
which shows that a recent rapid decline of 7°C in the South Polar
90
KEVIN E. TRENBERTH
region accounts for much of the globally averaged decline, with the
equatorial zone also making a significant contribution.
The Ozone Trends Pane! (Watson et al., 1988; Trenberth, 1988)
did a comparison of temperature time-series by Angell (1986b; 100
to 50 mb), Labitzke et al. (1986; 100 to 50 mb, for the Northern
Hemisphere only), the National Meteorological Center (NMC) (70
to 50 mb, based on NMC analyses), and the microwave sounder unit
(approximately 90 mb). There is good agreement for the Northern
Hemisphere from 1979 through 1986, showing a warm peak in 1982
to 1983 that is probably due to E! Nino or E! Chichon, followed
by a slight cooling trend. In the Southern Hemisphere, Angell's
values indicate much colder temperatures than those for the other
two series in 1985 to 1986. Angell's series also indicates the coldest
temperatures in recent years in the 30°N to 30°S latitude region.
A similar comparison for the 5- to 1-mb layer, based on the NMC
analysis and two SSU channels, shows a clear downward trend in
temperatures from 1979 to 1986 for all major regions of the world.
For the antarctic region, time-series from 1979 to 1986 of
200-mb heights generated in analyses by the NMC and the European
Centre for Medium-Range Weather Forecasting (ECMWF) analyses
disagree by 50 to 400 m (Trenberth and Olson, 1988~. The obser-
vational input to these analyses has an accuracy of about 20 m, for
comparison. The ECMWF analysis has been getting progressively
better with time. The NMC values were highly erroneous between
1983 and 1986 but improved in May 1986, when a correction to the
analysis procedure was made. Clearly, the operational analyses in
the antarctic region need further improvement.
The record of radiosonde ascents at the Amundsen-Scott South
Polar station has been examined and reveals that for the period
1961 through 1976, about 80 to 85 percent of ascents reached the
100-mb level during the winter (June to August) months, with about
40 percent surviving to 50 mb. For the period 1976 to 1986, on the
other hand, only about 15 percent of these soundings reached 100 mb
or higher, and only about 10 percent managed to reach 50 mb. The
reason for this loss is the bursting of balloons in cold temperatures, so
that a warm bias is likely to result from the remaining observations
that are actually made. The situation is even a little worse at the
McMurdo station. The worst month, in terms of the number of
observations made, is August, but by October the amount of high-
altitude radiosonde data obtained is much greater. This problem has
a marked impact on the temperature records.
GLOBAL TEMPERATURE TRENDS
91
If we look at the thumb, thumb, 5() mb, and 30-mb annual cy-
cles of temperature at Amundsen-Scott (Figure 10-2), we note that
there is a very steep increase in temperature from August to Octo-
ber. At 50 mb, the average temperature is -80°C at the beginning
of October and -55°C at the end of the month. If there is a pre-
ponderance of missing data at the beginning of the month, then the
monthly mean temperature will have a warm bias. To date, analysis
of the records of antarctic stations has not taken this problem into
account. As a result, trends in temperatures deduced from antarctic
station data are of questionable value.
We have computed the departure from the mean annual cy-
cle (Figure 10-2) of every individual day and then compiled these
anomalies into monthly means for the antarctic stations. For 50 mb
at McMurdo Sound, this compilation, when compared with straight
averages of the available observations for each month, gives monthly
means that differ by up to 12°C for some Octobers. For recent years,
our method gives mean October temperatures that are consistently
up to 8°C colder than those obtained by the straight averaging
method. The adjusted October record since 1956 shows a slight
downward trend that is not apparent in the unadjusted record.
The above procedure was used for 3~, 5~, and 10(Ymb data
at both Amundsen-Scott and McMurdo to correct for the annual
cycle-rn~ssing data effect (see Figures 1~3 and 1~4~. For the month
of September, there is little apparent trend from 1961 to 1987 at
Amundsen-Scott or from 1956 to 1986 (with some years missing)
at McMurdo. For October (Figure 10-3), a downward trend, more
pronounced at Amundsen-Scott, is apparent in the last 8 to 10 years
of these records, with the coldest years occurring near the end of
the record. Even so, the recent downturn is modest and would
probably not be considered especially noteworthy were it not for
the known springtime ozone depletion in the antarctic region. For
November (Figure 1~4), the recent downward trend ~ again appar-
ent at Amundsen-Scott for 50 and 100 mb and at McMurdo for 100
mb only. It Is noteworthy, however, that at McMurdo the coldest
November temperatures at all levels occurred in 1959 and 1961 (1960
data are maskings. For 1987, we have recently estimated the anoma-
lies, shown by dashes in Figures 1~3 and 1~4, at the South Pole,
and the lowest values on record occur in October and especially in
November, consistent with the absence of heating due to the ozone
hole.
92
KEVIN E. TRENBERTH
-20
-22
-24
-26
-28
-30
-32
o -34
-36
-38
-40
-42
-44
-46
-48
-50
-20
-30
-35
-40
-45
-
o -50
AL 55
-60
-65
-75
-80
-85
-90
RAW ANNUAL CYCLE
WAVES 0-4
500 MB
92 183 274 365
RAW ANNUAL CYCl
WAVES 0-4
\
1 00 MB
92 183 274 365
DAY OF YEAR
FIGURE 10-2 Mean annual cycle of temperatures at four pressure levels as
observed at the Amundsen-Scott antarctic station. Heavy curve shows the daily
raw means; thin curve is the best fit of combined temporal waves 0 to 4 to the
daily means. Period of record is 1961 through 1986.
GLOBAL TEMPERATURE TRENDS
-20
-28
-36
-44
-52
o -60
Q
~ -68
LD
-76
-84
-92
-100
-20
-28
-36
-44
-52
o
-
-60
-68
-76
-84
-92
-100
RAW ANNUAL CYCLE
WAVES 0-4
50 MB
J
f
,.
'. _ I
92 183 274 365
l
-RAW ANNUAL CYCLE 30 MB
WAVES 0-4
per,
i. ~
FIGURE 10-2 (continued).
93
92 183 274 365
DAY OF YEAR
94
KEVIN E. TRENBERTH
MONTHLY MEAN T-ANOM
20
AMUNDSEN-SCOTT
..... , , .
15
10
o
llJ
-10
-15
-20
-25
/VAVAV \~1 LI V\\.
HA
HA . ,
v ~
.
~ v X ~4
30 MB; +150FFSET CJM = 6.612
- 50 MB GM = 4.574
100 MB: -15 OFFSET OM = 2.832
-30 . , . I
56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88
YEARS
OCT
MONTHLY MEAN T-ANOM
~MCMURDO ~
_
15
10
- 5
o
- O
,_ -5
-10
-15
-20
-25
\ /\ V 1 1?
(JM = 8.646
50 MB (IM = 7.286
_ 100 MB; -15 OFFSET HIM = 4.784
30 MB; + 15 OFFSET
\
~ \
Al \ A
MA
~ ~J:
. ... . . .
-30 - . , . , . , . , .
5 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88
YEARS
OCT
- 30
- 50
100
-30
- 50
100
FIGURE 10-3 Monthly mean departure of daily temperature from the period
normal for three pressure levels at Amundsen-Scott (top) and McMurdo (bot-
tom) for October. Note that curves for the three different levels are offset from
one another by 15°C intervals. Data are missing for some years at McMurdo.
The 1987 values for Amundsen-Scott are preliminary. See text for further
explanation.
l
GLOBAL TEMPERATURE TRENDS
MONTHLY MEAN T-ANOM
AMUNDSEN-SCOTT
30
30 MB; +15 OFFSET OM = 3797
25 - 50 MB cam = 5.066
~ 00 MB; -15 OFFSET AM = 6 339
20 A
15
10
5
o
-
~ O
LL
-5
-10
-15
-20
-25
1
- i~V'
~J~ it
1 1~i ~ 1','.
1
1
At\
-30- ., ., · . , · I
5 ~ 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88
YEARS
NOV
MONTHLY MEAN T-ANOM
30
25
20
15
10
-10
MCMURDO
-
30 MB; + 15 OFFSET aM = 4 915
50 M8 Cam = 4.851
100 MB;-15OFFSET (7M = 5994
~ ~ \1:
AN 1:
-20 \ I
-30 - ........
5 6 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88
YEARS
NOV
MA I,,/\
- 30
~50
100
. . . .
A
so
- 100
95
FIGURE 10-4 Monthly mean departure of daily temperature from the pe-
riod normal for three pressure levels at Amundsen-Scott (top) and McMurdo
(bottom) for November. Note that curves for the three different levels are
offset from one another by 15°C intervals. Data are missing for some years at
McMurdo. The 1987 values for Amundsen-Scott are preliminary. See text for
further explanation.
96
KEVIN E. TRENBERTH
Finally, the Japanese antarctic station Syowa has measured both
Womb temperatures and total column ozone from 1966 through 1985
(Chubachi, 1986~. For October and November, a distinct downward
trend in both temperature and ozone is apparent after about 1978.
December data show a slight downward trend in recent years. The
year-to-year variations in temperature and ozone are approximately
parallel to each other, supporting the argument that ozone is the
key factor affecting temperatures at this level through solar heating.
Syowa 50-mb November temperatures also show a downward trend
from about 1978 on. Syowa is located close to the region where the
largest trends are supposed to be occurring.
(In answer to a question): The global surface temperature trends
that have been computed are weighted toward the Northern Hemi-
sphere, especially for the earlier years. Surface data coverage in the
Southern Hemisphere before World War I! was only about half the
present coverage, and there were no useful antarctic data prior to
about 1954.
REP1:RENCES
Angell, J.K. 1986a. Annual and seasonal global temperature changes in the
troposphere and low stratosphere, 1960-1985. Mon. Weal Rev. 114:1922-
1930.
Angell, J.K. 1986b. The close relation between Antarctic total-ozone depletion
and cooling of the Antarctic low stratosphere. Geophys. Res. Lett. 13:1240-
1243.
Chubachi, S. 1986. On the cooling of stratospheric temperature at Syowa,
Antarctica. Geophys. Res. Lett. 13:1221-1223.
Hansen, J., and S. Lebedeff. 1987. Global trends of measured surface air
temperature. J. Geophys. Res. 92:13345-13372.
Hansen, J., and S. Lebedeff. 1988. Global surface temperatures: Update
through 1987. Geophys. Res. Lett. 15:323-326.
Karoly, D.J. 1987. Southern Hemisphere temperature trends: A possible
greenhouse gas effect? Geophys. Res. Lett. 14:1139-1141.
Kukla, G., J. Gavin, and T.R. Karl. 1986. Urban warming. J. Climate Appl.
Meteorol. 25:1265-1270.
Labitzke, K., G. Brasseur, B. Naujokat, and A. De Rudder. 1986. Long-term
temperature trends in the stratosphere: Possible influence of anthropogenic
gases. Geophys. Res. Lett. 13:52-55.
Salinger, M.J. 1979. New Zealand climate: The temperature record, historical
data and some agricultural implications. Climatic Change 2:109-126.
Trenberth, K.E. 1988. Review of "Executive summary of the Ozone Trends
Panel Report." Environment 30:25-26.
GLOBAL TEMPERATURE TRENDS
Trenberth, K.E., and J.G. Olson. 1988. An evaluation and intercomparison of
global analyses from the National Meteorological Center and the European
Centre for Medium Range Weather Forecasts. Bull. Am. Meteorol. Soc.
69:1047-1057.
Watson, R.T., M.J. Prather, and M.J. Kurylo. 1988. Present State of Knowl-
edge of the Upper Atmosphere 1988: An Assessment Report. NASA
Reference Publication No. 1208. National Aeronautics and Space Admin-
i~tration, Washington, D.C.
