Historical temperature record

The Historical Period: from 1850

The phrase historical temperature record is generally used for the time over which reasonably reliable temperature records exist from actual observations from thermometers. This is generally considered to start in about 1850 - earlier records exist, but coverage and instrument standardisation are less.

In the present day most meteorological observations are taken for use in weather forecasts. Centres such as ECMWF show instantaneous map of their coverage (http://www.ecmwf.int/products/forecasts/d/overview/monitoring/coverage/dcover!ssmi!00!pop!od!oper!w_coverage!latest!obs/); or the Hadley Centre show the coverage for the average of the year 2000 (http://www.meto.gov.uk/research/hadleycentre/CR_data/Annual/HadCRUTanm_2000.gif). Coverage for earlier in the 20th and 19th centuries would be significantly less.

Most of the warming occurred during two periods: 1910 to 1945 and 1976 to 2000. See [1] (http://www.grida.no/climate/ipcc_tar/wg1/figspm-1.htm) for a picture of the temperature record.

The IPCC say:

"The best estimate of global surface temperature change is a 0.6°C increase since the late 19th century with a 95% confidence interval of 0.4 to 0.8°C" [2] (http://www.grida.no/climate/ipcc_tar/wg1/049.htm).

The US National Academy of Science, both in its 2002 report to President George W. Bush, and in its latest publications, has strongly endorsed evidence of an average global temperature increase in the 20th century.

There are important concerns about the historical temperature record, which essentially divide into the fraction of the globe covered; and the effect of changing thermometer designs and observing practice and effects of changing land-use around the observing stations.

There exists a parallel record of marine observations from surface ships. These too suffer from changing practices (such as the switch from collecting water in canvas buckets to measuring the temperature from engine intakes) but they are at least immune to the urban heat island effect. The land and marine records can be compared at http://www.grida.no/climate/ipcc_tar/wg1/fig2-6.htm.

The IPCC says that it has corrected the land station data to account for the urban heat island effect. To do: find and summarize their correction technique.

Secondary data

Secondary evidence for temperature changes can be obtained by observing things that are predicted to be affected by temperature changes, such as variations in the snow cover and ice extent, global average sea level, precipitation, cloud cover, El Niño and extreme weather events. For example, satellite data shows a 10% decrease of snow cover since the late 1960s, and the Northern Hemisphere spring and summer sea-ice extent has decreased by about 10% to 15% since the 1950s and there has been a widespread retreat of mountain glaciers in non-polar regions throughout the 20th century. (Source: IPCC).

Upper atmosphere temperature

Satellite measurements of the troposphere from 1979 to 2002, however, show a warming trend of 0.04 oC/decade. When results are extended to 2003, the trend becomes 0.074 oC/decade [3] (http://www.ghcc.msfc.nasa.gov/MSU/msusci) and [4] (http://www.nsstc.uah.edu/data/msu/t2lt/tltglhmam_5.1). This illustrates a severe problem with the satellite record, its shortness - adding a few years on to the record can change the trends considerably. These satellite data are controversial and incorporate a number of corrections for, amongst other effects, orbital drift of the satellites [5] (http://www.ghcc.msfc.nasa.gov/MSU/hl_measuretemp.htm). Other analyses of the same data produce different trends. The same page [6] (http://www.ghcc.msfc.nasa.gov/MSU/msusci) shows a decline in stratospheric temperatures, interspersed by "noise" from volcanic erruptions. This is what is expected from Global Warming theory: the troposphere should warm, whilst the stratosphere should cool. However, this simple picture is complicated by ozone depletion, which also causes a cooling of the stratosphere.

A National Academy of Sciences panel that reviewed upper air cooling concluded that "the warming trend in global-mean surface temperature observations during the past 20 years is undoubtedly real and is substantially greater than the average rate of warming during the twentieth century. The disparity between surface and upper air trends in no way invalidates the conclusion that surface temperature has been rising."[7] (http://books.nap.edu/books/0309068916/html/2#pagetop)[8] (http://www.msnbc.com/news/356875.asp?cp1=1)

The global average temperature for the last 150 years is shown by http://www.grida.no/climate/ipcc_tar/wg1/figspm-1.htm. Further discussion is available at http://www.grida.no/climate/ipcc_tar/wg1/056.htm#fig27.

Proxies: tree rings, ice cores: the last 1000 years

Longer records exist from proxies: quantities such as tree-ring widths, coral growth or isotope varioations in ice cores. From these, proxy temperature reconstruction of the last 1000 years have been made for the northern hemisphere. However, coverage of these proxies is sparse: even the best proxy records contain far fewer observations that the worst periods of the observational record. Also, problems exist in connecting the proxies (e.g. tree ring width) to the variable of interest (e.g. temperature).

A reconstruction of the temperature of the Northern Hemisphere for the past 1000 years is shown by http://www.grida.no/climate/ipcc_tar/wg1/figspm-1.htm. Further discussion is available from http://www.grida.no/climate/ipcc_tar/wg1/069.htm#fig220

One example of temperature fluctuation in this period is the Medieval Warm Period, which allowed the Vikings to colonise Greenland. These colonies were abandoned during the Little Ice Age that followed.

Indirect historical proxies

As well as natural, numerical proxies (tree-ring widths, for example) there exist records from the human historical period which can be used to infer climate variations, often in a less directly numerical way: reports of Frost Fairs[?] on the Thames; records of good and bad harvests; dates of spring blossom or lambing; extraordinary falls of rain and snow, and unusual floods or droughts. These too can be used to infer historical temperatures, but generally in a more qualitative manner than the natural proxies discussed above.

The long term ice core record: the last 400,000 years

Even longer term records exist for few sites: one ice core (from Vostok, Antarctica[?]) stretches back 420,000 years [9] (http://cdiac.esd.ornl.gov/trends/temp/vostok/graphics/tempplot5.gif); details from [10] (http://cdiac.esd.ornl.gov/trends/temp/vostok/jouz_tem.htm); many others reach more than 100,000 years. The Vostok core covers four glacial/interglacial cycles. Two cores GRIP (http://www.ngdc.noaa.gov/paleo/icecore/greenland/summit/document/gripinfo.htm), GISP (http://earth.agu.org/revgeophys/mayews01/mayews01) from Greenland stretch back as far as the previous interglacial. Whilst the large-scale signals from the cores are clear, there are problems interpreting the detail, and connecting the isotopic variation to the temperature signal.

Various other records exist and are even longer (principally ocean sediment cores) but they await a competent describer...