On the Present Halting of Global Warming

Abstract

The rise in global average temperature over the last century has halted since roughly the year 2000, despite the fact that the release of CO2 into the atmosphere is still increasing. It is suggested here that this interruption has been caused by the suspension of the near linear (+ 0.5 °C/100 years or 0.05 °C/10 years) temperature increase over the last two centuries, due to recovery from the Little Ice Age, by a superposed multi-decadal oscillation of a 0.2 °C amplitude and a 50~60 year period, which reached its positive peak in about the year 2000—a halting similar to those that occurred around 1880 and 1940. Because both the near linear change and the multi-decadal oscillation are likely to be natural changes (the recovery from the Little Ice Age (LIA) and an oscillation related to the Pacific Decadal Oscillation (PDO), respectively), they must be carefully subtracted from temperature data before estimating the effects of CO2.

Full text

Climate 2013, 1, 4-11; doi:10.3390/cli1010004
climate
ISSN 2225-1154
www.mdpi.com/journal/climate
Article
On the Present Halting of Global Warming
Syun-Ichi Akasofu
International Arctic Research Center, University of Alaska Fairbanks, USA;
E-Mail: sakasofu@iarc.uaf.edu; Tel.: 1-907-474-6012; Fax: 1-907-474-5662
Received: 28 January 2012; in revised form: 15 April 2013/ Accepted: 15 April 2013/
Published: 3 May 2013
Abstract: The rise in global average temperature over the last century has halted since
roughly the year 2000, despite the fact that the release of CO
2
into the atmosphere is still
increasing. It is suggested here that this interruption has been caused by the suspension of
the near linear (+ 0.5 C/100 years or 0.05 C/10 years) temperature increase over the last
two centuries, due to recovery from the Little Ice Age, by a superposed multi-decadal
oscillation of a 0.2 C amplitude and a 5060 year period, which reached its positive peak
in about the year 2000—a halting similar to those that occurred around 1880 and 1940.
Because both the near linear change and the multi-decadal oscillation are likely to be
natural changes (the recovery from the Little Ice Age (LIA) and an oscillation related to the
Pacific Decadal Oscillation (PDO), respectively), they must be carefully subtracted from
temperature data before estimating the effects of CO
2
.
Keywords: global warming; Little Ice Age; multi-decadal oscillation
1. Introduction
One of the standard processes in climatology is to construct a spectral analysis of past changes in
global temperature and to try to interpret component changes. This procedure has been applied
extensively to changes of the last 400 kys [1]. In this paper, we follow this process of studying changes
in global average temperature over the period from 18001850 to 2010 to find the general trend.
Figure 1 shows both temperature changes from 1860 to 2000 [2] and the rate of increase in
temperature estimated for several different intervals. The most consistent trend is a gradual increase in
temperature from 1860 to 2000, indicated here by a straight red line, with a rate of increase of 0.045 
 0.012 /10 years. Superposed on it is the most prominent change, an oscillatory change in amplitude
of about 0.2 C with a period of about 5060 years.
OPEN ACCESS

Climate 2013, 1 5
Figure 1. Global average temperature changes [2]. The rate of increase in temperature is
estimated for several different time intervals.
2. Spectral Analysis
2.1. The Near Linear Change
Fortunately, an excellent spectral analysis of global temperature variations from 1850 to 2000 has
been constructed by Wu et al. [3]. Their results further indicate that the most prominent change over
this period is a near linear change represented by a rate of increase of about 0.5 C/100 years or 0.05
C/10 years. This near linear trend has also been noted by (Bryant, 2001) [4], who showed that there
are only a few points outside the 95 % confidence limits of linear approximation.
Based on various climate change data, such as those of tree rings [5], glacial retreat [6] and river ice
breakup [7], Akasofu [8] showed that global warming began as early as 18001850 and not after CO
2
began to increase very rapidly around 1946. Further, among these factors, the sea level increase from
about 1850 to 2000 was also almost linear, though with a slightly decreasing rate [9].
Figure 2 shows temperature variations from about the year 800 to 2000, deduced from tree ring
changes; it shows the Little Ice Age (LIA) after the Medieval Warm Period circa 1000 and the near
linear increase after 1800 or shortly after [5]. Figure 3 shows basically similar trends in the
temperature changes obtained by several researchers (including Esper et al. [5]), indicating a near
linear rise in temperature from about 1850 to 2000 [10]. All these results clearly show near linear
increases temperature from about 18001850 to 2000, in addition to those results obtained by the
IPCC (Figure 1).
One possibility, then, is that this near linear component is due to a gradual recovery from the Little
Ice Age (LIA) of 18001850 [8], as the LIA did not end abruptly. It is generally perceived that the
temperature during the LIA was about 1 C lower than in the present (Figures 2 and 3). Thus, the rate

Climate 2013, 1 6
of this gradual temperature increase since 1800 would be roughly 1 C /200 years (= 0.5 C/100 years
or 0.05 C/10 years), similar to the rate of the near linear increase of about 0.5 C that we see over the
course of the 20
th
century.
Figure 2. Temperature variations deduced from tree ring records [5].
Figure 3. Temperature changes from 900 to 2000, compiled by the National Research
Council [10].
As the increase in temperature since 18001850 is nearly linear, the trend is quite different from the
increase in CO
2
, which has shown a near quadratic increase over the same period—rapidly increasing
after 1946, after a gradual increase that began around 1900. It is at least problematic, therefore, to
consider this near linear increase in temperature during the 19
th
and 20
th
centuries as mainly due
to CO
2
.
It may also be noted that the solar modulation function derived from C
14
and Be
10
[11]; Muscheler
et al. [12] displays a trend inversely proportional to the temperature trend shown in Figures 2 and 3,
and it may be speculated that the LIA and its recovery is perhaps related to changes in solar activity,
even though changes in solar output during a sunspot cycle (11 years) are known to be small, at  0.1
%.

Climate 2013, 1 7
2.2. The Multi-Decadal Oscillation
Superposed upon the near linear increase in temperature are various changes. The most predominant
of these is the multi-decadal oscillation, with an amplitude of about 0.2 C and a period of about 50–60
years; from Figure 1, we can see three major increases beginning in 1860, 1910 and 1970. These
increases peaked in 1880, 1940 and 2000, respectively. Jevrejeva et al. [9] has shown that the sea level
also superposed similar changes onto the otherwise near linear increase.
Further, the increases in 1860 and 1910 were each followed by a significant decrease lasting about
30 years. A halt and even a slight decrease in the rising trend after 2000 can therefore be expected, on
the basis of this spectral analysis.
This fluctuating change is likely the result of multi-decadal oscillation [13, 14]. Since the Pacific
Decadal Oscillation (PDO) has a similar phase to that shown in Figure 4 [15]; earlier data back to 1900
show the same phase changes), it is reasonable to consider that temperature’s multi-decadal change is
closely related to the PDO, a natural phenomenon. Consistent with this expectation, the PDO shows a
clearly decreasing trend since 2000.
Figure 4. The Pacific Decadal Oscillation (PDO) [15].
http://www.ncdc.noaa.gov/teleconnections/pdo/

Climate 2013, 1 8
3. Synthesis
Figure 5 shows the above findings in graphic form and represents an improved version of Figure 9
of Akasofu [8]. The large rectangular box shaded in yellow shows temperature changes from 1860 to
2010 (standard data, similar to Figure 1), together with a linear black line showing the 0.5 C/100 year
rate of increase and the multi-decadal oscillation shown in red and blue, above and below the line,
respectively. Figure 5 shows a detailed version of data shown in the yellow box. The dotted line before
1860 indicates that the linear line may be extended back to about 1800, assuming that the LIA indeed
began to recover from about 1800. Figure 6 shows the HADCRUT4 data [16], together with its five-
point smoothing data. It is clear from the above data set that the warming trend is halted and that there
is an indication of even slight cooling after 2000.
Figure 5. An interpretation of changes in global average temperature from 1800 to 2012.
The temperature in the vertical axis is for reference scale; for detail, see the text. An insert
above the yellow box is a detailed version of data shown in the yellow. The HadCRUT4
data are discussed by Morice et al. [16].
In the yellow box, the change from 2000 to June of 2012 is emphasized by the thick blue line to
indicate the halting trend as an effect of the multi-decadal oscillation. Above the yellow box is shown a
detailed version of this data. Based on the above synthesis, it may be suggested that the present halt to
global warming is due to the fact that multi-decadal oscillation has overwhelmed the prior, near linear
(LIA recovery) increase. Indeed, such a trend is similar to those after 1880 and 1940, when
temperature actually decreased toward 1910 and 1970, respectively (particularly in light of the fact that
CO
2
had begun to increase rapidly after 1946). It may be noted, however, that Levitus et al. [17]
showed a continuous increase of the world ocean heat content after 2000, although the rate of increase
seems to decrease after 2004; on the other hand, the result by Pielke [18] does not seem to show such
an increase after 2000.

Climate 2013, 1 9
Figure 6. The HADCRUT4 data as illustrated in Figure 5, together with the five-point
smoothing, but shown for the interval 1980–2012 only.
The temperature increase from 1975 to 2000, shown by the thick red line (essentially the same line
as shown in yellow in Figure 1), is likely composed primarily of recovery from the LIA, combined
with the positive phase of the multi-decadal oscillation [8]. In contrast, the IPCC considers the
temperature rise from 1975 to 2000 as “very likely due to the observed increase in anthropogenic
greenhouse gas concentrations [2].
Based on this assumption, the IPCC has predicted a +2  4 C temperature increase by 2100 [2], as
shown in Figure 5 by the dotted extension of the red thick line, to have resulted more immediately in a
+0.2 C or greater temperature increase by 2012. However, the halted increase (or even slight
decrease) in temperature since 2000 indicates a situation more similar to those after 1880 and 1940.
It is quite likely, therefore, that the near linear increase due to LIA recovery has been temporarily
overwhelmed by the multi-decadal oscillation, which had reached a positive peak in about the
year 2000.
Assuming these results obtained by statistical analysis will continue throughout the 21
th
century, we
may observe the dashed line from 2012 to 2100 as the linear extension, in conjunction with
multi-decadal oscillation. The expected rise in temperature due to recovery from the LIA remains
about 0.5 C, though the contribution from multi-decadal oscillation will depend on its phase
(0.2 C).

Climate 2013, 1 10
4. Conclusion
It is likely that both the near linear increase and multi-decadal oscillation are primarily natural
changes. Thus, in order to estimate the effects caused by CO
2
over the last two centuries, it is
important to isolate these natural components of climate change from real temperature data.
Acknowledgements
The author would like to thank Dr. Gerhard Kramm for his discussion and also for improving
Figure 5 and providing Figure 6.
Conflict of Interest
The author declares no conflict of interest.
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