Relative Homogeneity of the Medieval Warm Period (MWP) and The Little Ice Age (LIA) | Watts Up With That?
Relative Homogeneity of the Medieval Warm Period
(MWP) and The Little Ice Age (LIA)
Guest Opinion: Dr. Tim Ball
The 2001 Intergovernmental Panel on Climate Change (IPCC) “Hockey
Stick” graph produced solutions for three facts challenging the
claims of key IPCC climate scientists at the Climatic Research Unit
(CRU). The problems appeared in
Figure
7c in the 1990 First Assessment Report (FAR) (Figure 1).
Figure 1: Source: Figure 7c in the IPCC FAR
The three problems it created were
The Medieval Warm Period (MWP)
contradicted their claim that the 20th century was the
warmest ever.
The Little Ice Age (LIA) showed
that the present warming trend began prior to the major impact of
the Industrial Revolution. Significant production of CO2 by humans
began only after World War II.
- The trend of the warming since circa 1650 A.D., the nadir of
the LIA, was well within natural variability.
The hockey stick graph dealt with all those by eliminating the MWP
and the LIA. It inappropriately tacked on, as the blade of the stick,
an upturn in temperature in the 20
th century. Phil Jones
produced the upturn that claimed a 0.6°C ±0.2°C increase in 120
years. They claimed this rate of increase was beyond any natural
increase, conveniently ignoring the ±33% error factor.
A second part of their problem involved a paper by Soon and
Baliunas titled “
Proxy climatic and environmental changes of the
past 1,000 years” (Climate Research, 23, 89–110) that
detailed worldwide evidence of existence of the MWP. The personal
attacks on Soon and Baliunas are now exposed, particularly the
role
of John Holdren when at Harvard.
A third part involved the claim that
Figure 7c was the temperature for the Northern Hemisphere and neither
the LIA nor the MWP was a global
event.
The argument that a climatic period was regional or for a given
portion of a record, was used to counter the problem that the
Holocene Optimum was warmer than the 20
th century.
Steve
McIntyre examined the response of AGW supporters, including a
quotation from
NOAA that says,
In summary,
the mid-Holocene, roughly 6,000 years ago, was generally warmer than
today, but only in summer and only in the northern hemisphere.
Timing of Events
The issue involved here is classic and essential to proper
understanding of climate. It involved a standard practice in
climatology called Relative Homogeneity. If you are going to
establish a pattern of climate change, you must determine if it is
local, regional, hemispheric or global. It addresses the very
definition of climatology, which is the study of climate in a region
or the change over time.
I dealt with the issue while preparing for my doctoral thesis that
involved reconstructing climate change at Churchill, Manitoba, on
Hudson Bay for the period from 1714 – 1850. My supervisor, Dr.
Bruce Atkinson, directed me to reconstruct climate changes at York
Factory, also on Hudson Bay, but 200 km away. The comparison
identified local changes at Churchill while examination of regional,
hemispheric and global changes provided a wider context.
Two major assumptions complicate determination of the length and
extent of the change. First is that an event, such as the MWP, begins
and ends at the same time. Second is that changes created by an
external forcing is evident in all
records.
I discussed both issues at length with Hubert Lamb because my thesis
period covered the transition from the LIA to the modern warm period.
Lamb did not use the term Medieval Warm Period (MWP) and Jean Grove
attributes the Little Ice Age (LIA) name to Mathes (1939), but Lamb
gave much thought to the
dates
of onset and termination of both. In Volume 2 of his
Climatic
History
and the Future: Climate Present, Past and Future he used the
transitional terms of “
the medieval warm epoch or Little
Climatic Optimum.” This was the distinction between epochs and
events, like that used with magnetic reversals. At that time the
Holocene Optimum was called the Climatic Optimum, so the MWP was the
Little Climatic Optimum.
Lamb pointed out that, even though an event was global, the dates
of onset, peak, and termination varied considerably depending on
different conditions at all levels from local to regional to
hemispheric. He also indicated that the difference in dates could
vary by decades. You can look at a single station or even a few in a
region for a particular time and not find evidence of a trend like
the MWP or the LIA. As Lamb wrote
“Evidence already cited at various places in this
volume suggests that for a few centuries in the Middle Ages the
climate in most parts of the world regained something approaching the
warmth of the warmest postglacial times. The climax of the warm epoch
was not quite contemporaneous everywhere …” “in the heartland
of North America, as in European Russia and Greenland, the warmest
times may be placed between A.D. 950 and 1200. In most of Europe, the
warmest period seems to have been between 1150 and about 1300 though
with notable warmth also in the later 900s.”
The original IPCC FAR Figure 7c (Figure 1) indicates in the legend
that
“The dotted line nominally represents conditions near
the beginning of the twentieth century.”
Presumably somebody added the line and in doing so provided a
general range for the MWP and the LIA. By eyeball the MWP covers A.D.
950 to 1350, and the LIA from A.D. 1350 to 1900. This does not match
with the numbers in the text, particularly for the LIA with numbers
attributed to Grove of 150 to 450 years ago or A.D. 1540 to 1840.
There are a few interesting comments that needed correction for the
politically motivated 2001 IPCC Report. In referring to the MWP, they
note,
This period of widespread warmth is notable in that there
is no evidence that it was accompanied by an increase of greenhouse
gases.
Discussing the various explanations for the LIA cooling they
wrote;
Some have argued that an increase in explosive volcanism
was responsible for the coolness (for example Hammer, 1977, Porter,
1986), others claim a connection between glacier advances and
reductions in solar activity (Wigley and Kelly, 1989) such as the
Maunder and Sporer solar activity minima (Eddy, 1976), but see also
Pittock (1983).
Note that these are the same arguments made for the current
“hiatus” in temperatures. Also, notice the name of Tom Wigley,
former Director of the CRU and a central figure in the 2001 IPCC,
attributing the cooling to solar activity. The actual reference given
is:
Wigley, T M L , and PM Kelly, 1989 Holocene climatic change,
14C wiggles and variations in solar irradiance Phil. Trans. Royal
Society London, (in press).
Apparently, the article listed as “(in press)” never appeared
because
Wigley’s
publication list at the CRU doesn’t list it.
On page 203 of the IPCC Report they observe,
Thus some of the global
warming since 1850 could be a recovery from the Little Ice Age rather
than a direct result of human activities So it is important to
recognise that natural variations of climate are appreciable and will
modulate any future
changes induced by man.
All these points confronted and contradicted the political agenda
of blaming human CO2 for global warming and latterly climate change.
Refutation began in the 1995 Second Assessment Report (SAR) and hit
full stride with the 2001 Third Assessment Report (TAR) and its
central feature, the “Hockey Stick”. Accurate determination of
the onset and termination dates for the MWP and LIA, the relative
homogeneity, was essential to identifying the underlying mechanisms.
The Hockey Stick ‘solved’ the problem by eliminating the events
completely and tacking on a modern blade with an error factor that
made the numbers meaningless.
Welcome to IPCC climate science.
