Claims of global warming are often accompanied by graphs showing
the difference (i.e. anomaly) between global average temperatures
and the long-term average during 1961-90 inclusive.
global averages might imply that warming is uniform around the
world but this is far from the truth. A comparison of the
temperature anomalies in the northern and southern hemispheres
shows some significant differences and perhaps the differences
between the two hemispheres provides some interesting clues about
This article has 11 sections...
- sections 2-4 - Land-based temperature
- section 5 - Land-based anomalies for winter and summer
- sections 6-8 -
Temperature anomalies for land and sea (and combined) in both hemispheres
- section 9 - Land
and sea temperature anomalies for the northern hemisphere
- section 10 - Some
differences between the hemispheres
- section 11 - Observations and conclusions
>>SPECIAL NOTE: The hemispheric temperature anomalies
are corrected for the geometry which sees lines of longitude being closer as one moves
further away from the equator. (Data from the UK's Climate
Below is a graph based on data from the Climate Research Unit at the University of
East Anglia, in the UK. (The year is noted beneath the data point for January.)
Three points stand out
- the anomalies in the northern hemisphere are
usually much higher than the anomalies for the southern hemisphere
- the temperature anomaly
in the northern hemisphere is generally increasing but the anomaly for the southern
hemisphere changes little
- the greatest anomalies for the northern hemisphere often
occur during the northern winter.
This graph is the five-month average of the above anomalies and it produces a smoother
graph in which to observe the above points.
This graph is the five-month average of the monthly anomalies between January 1980 and December 2004.
It shows the variation that has occurred compared to the recent trend of the anomaly for the
northern hemisphere being greater.
temperature anomaly in the northern hemisphere has increased more rapidly than that for the
southern hemisphere since 1985. Typically the January anomaly is an annual peak for the northern
hemisphere although at times December or February are higher.
This graph is the annual anomalies for the two hemispheres with June, July and August in one
season and December (previous year), January and February in the other. Below it is a graph of the
winter and summer anomalies averaged over 5 years (from -2 to +2 from the current year).
the period of warmer NH winters from 1957 to 1964 - slightly
above zero anomaly when before and after were below zero - and
then the rapid decline into colder winters until about 1975.
Also notice the different starting times for the increasing
trend that might already have peaked in the SH. In the NH the
winter temperature anomaly started increasing about 1974 but the
summer temperature anomaly a year or two later. By contrast SH
temperature anomalies started increasing about 1955 according to
this graph but perhaps even earlier.
|The image below shows temperature anomalies for the two hemispheres
over the last 10 years.
The differences between the two hemispheres are obvious and these are
despite the relatively uniform levels of carbon dioxide in both hemispheres.
also that the global average temperature has been on a downward trend since the start of 2002!
Temperature anomalies as running 5 month averages (i.e. current month -2 to current month + 2)
for period of 25 years. The difference between northern and
southern hemispheres is clear. Note that in 1998 both
hemispheres had temperatures well above the long term average
for an extended period and this meant that global temperatures
were higher than normal.
Sea surface temperature anomalies in both hemispheres. These anomalies are generally lower
in the southern hemisphere but not always.
|Sea and land temperature anomalies from
1951 to 1980. The difference between the anomalies at any month
is relatively small. Note also that the six coldest periods (inc
start of 1951) all occurred during the northern winter...
...but between Jan 1980
and December 2004 the picture is quite different. Land
temperature anomalies often exceed sea temperature anomalies,
both data sets are showing an increase since about 1985 and now
the northern winter is the time of the peaks in temperature
That latter point is interesting but it may just be a consequence
of lower average winter temperatures during 1961-90 making the more recent
temperatures higher compared to that average.
There are many differences between the northern and southern hemispheres that might have an
influence on weather and climate.
land area (and less sea area) in the NH (and probably more
forests, agriculture, lakes etc.)
(b) ocean currents circle the earth
at high latitudes in the SH but not in the NH
(c) the polar regions of the SH are
colder than the polar regions in the NH
(d) the NH has land areas which are
further from the oceans along the same line of latitude - i.e. east-west distance to the
sea is greater.
(e) NH has a greater population and this means more
large cities (which may be distorting the observed temperatures)
(f) the use
of energy (for heating, transport, power etc.) is greater in the NH and that means more
"man-made" heat is released to the atmosphere
(g) winter peak
values for the anomaly may be a reflection of improved heating methods releasing
(h) carbon dioxide
emissions in the NH are greater (although as we saw above, the increases in the
NH and SH were similar in 1995-2004)
atmospheric pollution occurs in the NH
(j) Countries which are rapidly
becoming industrialised - and increasing atmospheric emissions - such as China
and India are located in the NH.
(a) Severe El Nino
conditions occurred in the Pacific during 1998-99. It was recently claimed by
NASA's James Hansen (as if it was the first time it had been thought of) that the 1998
El Nino had an influence on global temperatures. (Hansen also claimed that weak El Nino
conditions in 2002 and 2003 had an influence on temperatures in those
(d) Carbon dioxide levels continued to increase across the
10 years of the above graphs. Carbon dioxide levels measured at Mauna Loa
(Hawaii) increased by 16.3 ppmv, at the South Pole by 16 ppmv and at Alert (northern
Canada) by 16.7ppmv in the period from December 1994 to December 2003. Despite the
relative consistency in the increase of carbon dioxide,
temperature variations have been irregular in the two hemispheres.
(c) Carbon dioxide levels are cyclic in both locations due
largely to photosynthesis and seasonal temperature with the northern
hemisphere cycle showing maximum levels in March or April and minimum levels in
September or October. This does not correspond to changes
in the above temperature anomalies.
(d) Patterns of temperature anomalies
are quite different. In the
southern hemisphere these have been relatively stable for the last 3
to 4 years but much larger variations have occurred in the
northern hemisphere where the pattern of monthly values is less regular.
There is some correlation in 1983, 88, 93 and 98 and for a
longer period in the 1960s but generally the lack of good correlation between the
temperature anomalies in the two hemispheres suggests that the principal driver probably
operates on a multi-decadal (or longer) timescale. Sustained increases in carbon dioxide
appear to have no consistent impact.