Climatic Data - Climate Change: Identification and Projections - page 12

2.2. Global temperature
2.2.1. Modern temperatures
Global temperatures are collected, processed and updated by the large
organizations mentioned above. We chose the data set HadCRUT4 of the
Hadley-Center-CRU [MOR 12] for the simple reason that it starts in 1850,
whereas those of GISS and NCDC don't start until 1880. In any case, the
databases overlap well with each other. The average annual global
temperatures of Earth surfaces and oceans, over a given year t, are noted as
T G ( t ) 1 .
Up until the last few years, when satellites have partially taken over,
global temperatures were measured under a shelter, at 1.50 m above the
ground, and ocean temperatures were taken from the surface of the water, at
a depth which varies depending on the depth of the cooling water intake on
ships' engines (a few meters). The global average may not be foolproof, but
it is nevertheless an essential indicator.
Since the end of the last great glacial period 8,000 years ago, global
temperatures have not varied by more than one or two degrees.
Climatologists measure variations with respect to a reference temperature,
which is more practical than using 0°C. By convention, global temperature
T G is defined as an anomaly 2 of global temperature, assessed in relation to its
average over the period 1850-1900.
There are other reference periods, depending on organizations and
requirements: 1961-1990 (Hadley) and 1951-1980 (GISS, NCDC). When the
IPCC presents its future temperature predictions, they are sometimes in
relation to a “current temperature” based on the average of the period 1986-
2005 (see Figure 9.4).
When processing data, we will do it from raw values of annual
temperatures (shown by thin lines on Figure 2.1). For visualization purposes,
they can be smoothed (bold lines), either by moving averages, or by
Gaussian or binomial filters. It is preferable to use spline functions with
penalties on derivatives (Matlab®'s csasp tool), which work without data
1 In this year t = 2014 at the time of writing, T G (2014) is as yet unknown.
2 Anomaly, without the connotation of abnormality.