By 1860, enough temperature readings were taken all over planet's land surface and seas to begin a comprehensive global record - only the Antarctic and extreme Arctic were missing by then.
Prior to that, temperatures have to be inferred from a variety of sources which offer much poorer global coverage - tree growth rings, the number of days for which rivers were ice-covered, the length of glaciers and the ratio of isotopes of common gases trapped in bubbles within Arctic and Antarctic ice.
The global warming since 1860 has been uneven in both time and space. It appears to have begun around 1920, then halted for 30 years before resuming at the end of the 1970s.
A steady, even rise is impossible because of the vagaries of the earth's highly complex climate system. Average annual temperatures fluctuate markedly without any human influence; factors such as volcanic eruptions and changes in sea currents exert a strong influence. The British scientists forecast that 1996 will be slightly cooler, globally, than this year.
England has had several periods of record-breaking warmth this year, but the current freeze may stop it beating 1990 into being England's hottest since records began.