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Several records agreeing with the latter view has been quantitatively reconstructed from tree ring and other proxy data by scientists, principally Mann, Jones and Briffa; and used by the IPCC. More recently, the record has been extended to the last 2000 years (Mann and Jones, GRL, 2003 [1]).
However, in the September 30th, 2004 issue of Science, Hans von Storch and his colleagues showed that the particular method of Mann et al. probably underestimates the temperature fluctuations in the past by a factor of two or more.
The graphs of these reconstructions show a separation into two trends. From 1000 C.E. to 1880 the temperature graphs show a slow, irregular steady decline. From 1880 to present temperatures increase about 0.6 °C.
This temperature record has an unofficial name, the "Hockey Stick" graph, first coined by Jerry Malhman , a colleague of Mann's.
right Northern Hemisphere temperature graph showing the temperature for the last 1000 years. 50 year averaged data are shown for 1000-1950, in blue, 10 year averaged data for 1950-2000 are shown, in red
The work of Mann et al. and others [2] [3] forms a major part of the IPCC's conclusion that "the rate and magnitude of global or hemispheric surface 20th century warming is likely to have been the largest of the millennium, with the 1990s and 1998 likely to have been the warmest decade and year" [4].
The reconstructions mentioned above are quantitative and tend to have many data points for each source: numerical temperature time series, either from observations or a variety of proxies, are merged and averaged to produce an average for the northern hemisphere. In the process, it is possible to produce error estimates that generally get larger further back in time.
It is also possible to use historical data - times of grape harvests; ice-free periods in harbours; diary entries of frost or heat waves - to produce indications of when it was warm or cold in particular regions. These records are harder to calibrate, are often only available sparsely through time, may only be available from regions with written records, and are unlikely to come with good error estimates. Calibration of individual items is often needed, such as temperature tolerance of plants being compared to records of types of plants which were grown, or comparing recent temperature and freezing records of a specific canal to historical descriptions and paintings of the canal.
These historical observations of the same time period show periods of both warming and cooling. Scientists such as astrophysicist Sallie Balunias note that these ups and downs correlate with solar activity and assert that the number of observed sunspots give us a rough measure of how bright the sun is.
Balunias and others believe that periods of decreased solar radiation are responsible for historically recorded periods of cooling such as the Maunder Minimum and the Little Ice Age. Similarly, they say, periods of increase solar radiation contributed to the Medieval Warm Period, when the Greenland's icy coastal areas thawed enough to permit farming and colonization.