Re : je kan beter de WG1 AR4 citeren

At least as early as the beginning of the 20th century, diff erent authors were already examining the evidence for climate changes
during the last two millennia, particularly in relation to North America, Scandinavia and Eastern Europe (Brooks, 1922). With regard to
Iceland and Greenland, Pettersson (1914) cited evidence for considerable areas of Iceland being cultivated in the 10th century. At the
same time, Norse settlers colonised areas of Greenland, while a general absence of sea ice allowed regular voyages at latitudes far to
the north of what was possible in the colder 14th century. Brooks (1922) described how, after some amelioration in the 15th and 16th
centuries, conditions worsened considerably in the 17th century; in Iceland, previously cultivated land was covered by ice. Hence, at
least for the area of the northern North Atlantic, a picture was already emerging of generally warmer conditions around the centuries
leading up to the end of the fi rst millennium, but framed largely by comparison with strong evidence of much cooler conditions in
later centuries, particularly the 17th century.
Lamb (1965) seems to have been the fi rst to coin the phrase ‘Medieval Warm Epoch’ or ‘Little Optimum’ to describe the totality of
multiple strands of evidence principally drawn from western Europe, for a period of widespread and generally warmer temperatures
which he put at between AD 1000 and 1200 (Lamb, 1982). It is important to note that Lamb also considered the warmest conditions
to have occurred at diff erent times in diff erent areas: between 950 and 1200 in European Russia and Greenland, but somewhat later,
between 1150 and 1300 (though with notable warmth also in the later 900s) in most of Europe (Lamb, 1977).
Much of the evidence used by Lamb was drawn from a very diverse mixture of sources such as historical information, evidence
of treeline and vegetation changes, or records of the cultivation of cereals and vines. He also drew inferences from very preliminary
analyses of some Greenland ice core data and European tree ring records. Much of this evidence was diffi cult to interpret in terms of
accurate quantitative temperature infl uences. Much was not precisely dated, representing physical or biological systems that involve
complex lags between forcing and response, as is the case for vegetation and glacier changes. Lamb’s analyses also predate any formal
statistical calibration of much of the evidence he considered. He concluded that ‘High Medieval’ temperatures were probably 1.0°C to
2.0°C above early 20th-century levels at various European locations (Lamb, 1977; Bradley et al., 2003a).
A later study, based on examination of more quantitative evidence, in which eff orts were made to control for accurate dating and
specifi c temperature response, concluded that it was not possible to say anything other than ‘… in some areas of the Globe, for some
part of the year, relatively warm conditions may have prevailed’ (Hughes and Diaz, 1994).
In medieval times, as now, climate was unlikely to have changed in the same direction, or by the same magnitude, everywhere (Box
6.4, Figure 1). At some times, some regions may have experienced even warmer conditions than those that prevailed throughout the
20th century (e.g., see Bradley et al., 2003a). Regionally restricted evidence by itself, especially when the dating is imprecise, is of little
practical relevance to the question of whether climate in medieval times was globally as warm or warmer than today. Local climate
variations can be dominated by internal climate variability, often the result of the redistribution of heat by regional climate processes.
Only very large-scale climate averages can be expected to refl ect global forcings over recent millennia (Mann and Jones, 2003; Goosse
et al., 2005a). To define medieval warmth in a way that has more relevance for exploring the magnitude and causes of recent largescale
warming, widespread and continuous palaeoclimatic evidence must be assimilated in a homogeneous way and scaled against
recent measured temperatures to allow a meaningful quantitative comparison against 20th-century warmth (Figure 6.10).
A number of studies that have attempted to produce very large spatial-scale reconstructions have come to the same conclusion:
that medieval warmth was heterogeneous in terms of its precise timing and regional expression (Crowley and Lowery, 2000; Folland
et al., 2001; Esper et al., 2002; Bradley et al., 2003a; Jones and Mann, 2004; D’Arrigo et al., 2006).
The uncertainty associated with present palaeoclimate estimates of NH mean temperatures is signifi cant, especially for the period
prior to 1600 when data are scarce (Mann et al., 1999; Briff a and Osborn, 2002; Cook et al., 2004a). However, Figure 6.10 shows that the
warmest period prior to the 20th century very likely occurred between 950 and 1100, but temperatures were probably between 0.1°C
and 0.2°C below the 1961 to 1990 mean and signifi cantly below the level shown by instrumental data after 1980.
In order to reduce the uncertainty, further work is necessary to update existing records, many of which were assembled up to 20
years ago, and to produce many more, especially early, palaeoclimate series with much wider geographic coverage. There are far from
suffi cient data to make any meaningful estimates of global medieval warmth (Figure 6.11). There are very few long records with high
temporal resolution data from the oceans, the tropics or the SH.
The evidence currently available indicates that NH mean temperatures during medieval times (950–1100) were indeed warm
in a 2-kyr context and even warmer in relation to the less sparse but still limited evidence of widespread average cool conditions in
the 17th century (Osborn and Briff a, 2006). However, the evidence is not suffi cient to support a conclusion that hemispheric mean
temperatures were as warm, or the extent of warm regions as expansive, as those in the 20th century as a whole, during any period in
medieval times (Jones et al., 2001; Bradley et al., 2003a,b; Osborn and Briff a, 2006).

The weight of current multi-proxy evidence, therefore,
suggests greater 20th-century warmth, in comparison with
temperature levels of the previous 400 years, than was shown
in the TAR. On the evidence of the previous and four new
reconstructions that reach back more than 1 kyr, it is likely that
the 20th century was the warmest in at least the past 1.3 kyr.
Considering the recent instrumental and longer proxy evidence
together, it is very likely that average NH temperatures during
the second half of the 20th century were higher than for any
other 50-year period in the last 500 years. Greater uncertainty
associated with proxy-based temperature estimates for
individual years means that it is more diffi cult to gauge the
signifi cance, or precedence, of the extreme warm years observed
in the recent instrumental record, such as 1998 and 2005, in the
context of the last millennium.