The scientific context - Climate Change: The Facts - page 48

authors considered January 1969 temperature data for measurement and sampling error,
temperature bias effects, and the effect of limited observational coverage on largescale
averages. The analysis revealed worldwide errors in the range of 1-5°C for individual
sampled area-boxes, i.e. errors that far exceed the total claimed twentieth century warming
of ~0.7°C. Clearly, errors for records collected earlier in the twentieth century are likely to
be higher still than the already large 1969 errors.
DespitetheclaimotherwisebyBrohanetal.intheir2006paper,'Uncertaintyestimates
in regional and global observed temperature changes', these results indicate that no
statistically significant modern warming will be able to be inferred on the basis of
HadCRUT or similar thermometer-based records until the current temperature rises over
1°C above that computed for 1969. 4
Though global average temperature may have warmed during the twentieth century, no
direct instrumental records exist that demonstrate any such warming within an acceptable
degree of probability.
Context 2—natural temperature variations over geological time
It is a scientific truism that climate persistently changes through time, one manifestation
of which is the changing global average temperature recorded in many geological data
sets. These data sets are collected, for example, from high latitude ice cores or oceanic
seabed cores. Though in the first instance they yield local or regional temperature data,
the strong commonality that exists between different palaeoclimate records from widely
different geographical regions nonetheless often reflects an underlying global signal.
Because short thermometer temperature records such as HadCRUT manifestly do not
comprise an adequate climate record, it is to these geological datasets that we must turn to
provide the proper climatic context against which to assess modern temperature changes.
A case in point is the high-quality inferred air temperature above the Greenland ice
cap for the last 10,000 years (Figure 1). This record shows (i) that temperatures were up
to a full two degrees warmer than today during the Holocene Climatic Optimum, c. 8,000
years before present (BP); (ii) the presence of a persistent millennial cycle of warmings
and coolings, with all pre-modern peaks of this cycle, including the Mediaeval Warm
Period, being warmer than the late twentieth century peak; 5 and (iii) an overall cooling of
temperature since 8,000 years BP which took place against the background of an increase
of atmospheric carbon dioxide (CO 2 ) of 20 ppm (of natural origin, and as recorded in
Antarctic ice cores).