11. Natural climatic variability
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This article is from the Climate Change FAQ, by Jan Schloerer jan.schloerer@medizin.uni-ulm.de with numerous contributions by others.
11. Natural climatic variability
What course would earth's temperature have taken without human
influences ? We don't know. [Burroughs] opens his intriguing book
on weather cycles: "The history of meteorology is littered with
whitened bones of claims to have demonstrated the existence of
reliable cycles in the weather." Too little is known about natural
climatic fluctuations lasting decades to centuries.
Some players that may cause climatic variations on this time scale:
atmospheric variability including shifts of the polar front, varia-
tions in the circulation of the North Atlantic and Pacific Ocean,
solar variability, volcanism. During the Holocene, the past about
10,000 years, these factors, taken together, probably did not cause
global mean surface temperature changes exceeding 1 o C [Rind].
Unraveling climate's natural vagaries may take a long time, because
sufficiently long and detailed climatic records are scarce [IPCC 95,
p 173-4, 180-1, 411, 418-21].
The Little Ice Age, from about 1450 to the 19th century, and the
Medieval Warm Period, from perhaps the 9th to the 14th century, are
cases in point. The data, including historical, tree ring, coral
and ice core records, are gappy, in particular for the tropics and
southern oceans. The global patterns of the climatic changes and
the mechanisms behind these changes are not yet known. Formerly it
was presumed that both the Medieval Warm Period and the Little Ice
Age were globally more or less uniform. Now the available data begin
to suggest that no major globally synchronous cool or warm period
occurred during the past millennium. Instead, asynchronous regional
coolings and warmings appear to have been common. [Bradley] [Crowley
& North, chapter 5] [Hughes] [IPCC 95, p 174-7] [Overpeck] [Rind]
For illustration, summers in northwest Sweden were, by and large,
warmer than their 1860-1959 mean between AD 1000 and 1200 and, again,
between 1400 and 1550. From 1200 to 1400, summers tended to be cooler.
Year-round sea surface temperatures in the Sargasso Sea appear to have
taken a similar course. On the other hand, summer temperatures over
the northern Urals show more or less the opposite pattern with cool
summers around AD 1000 and warm summers between 1200 and 1400 [Briffa]
[Keigwin 96]. Over Northern Hemisphere land areas, summers tended to
be cool during the 16th, 17th and 19th century, though with strong
regional differences. Chinese summers, for instance, were unusually
cool around 1650. This spell was weaker over the northern Urals and
at other Arctic sites, it is absent or barely noticeable in a central
European and in some North American records [Bradley] [Briffa].
There are not yet enough data to tell whether the so-called Medieval
Warm Period, globally averaged, was warmer than the Little Ice Age,
let alone the present century. The Little Ice Age, though not a
globally synchronous cooling spell, was probably, on average, cooler
than the last hundred years [Bradley] [Hughes] [IPCC 95, p 174].
The warming since around 1900 appears to be one of the globally
most uniform temperature shifts during, at least, the past several
centuries [Crowley & North, p 99] [Overpeck].
Several clues suggest a decline of solar activity during the Maunder
Minimum (about 1645-1715), amounting to a radiative forcing of
somewhere between -0.5 and -1.5 W/m**2. Decline and subsequent rise
of solar activity to its present level may have contributed to the
Little Ice Age and to the warming thereafter. Solar forcing since
1850 has been tentatively estimated at between +0.1 and +0.5 W/m**2.
[IPCC 94, p 189-92, 194] [IPCC 95, p 115-8]
Without knowing natural climatic variations reasonably well, elucida-
ting their causes is difficult. Even the causes of wellknown events
can be hard to identify. 1976-77 the behavior of El Nino-Southern
Oscillation appears to have changed. El Nino episodes got more fre-
quent, sea surface temperatures in the tropical Pacific tended to be
high, precipitation over the tropics and subtropics from Africa to
Indonesia declined. While some model results suggest that greenhouse
gas induced climate change may look similar, it is still open whether
this was incipient climate change or a natural fluctuation [IPCC 95,
p 153-55, 165] [Meehl].
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