IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Synthesis Report

III.2 Scientific units

SI (Système Internationale) units 
Physical Quantity Name of Unit Symbol 
lengtht metre 
mass kilogram kg 
time second 
thermodynamic temperature kelvin 
Fractions and multiples 
Fraction Prefix Symbol Multiple Prefix Symbol 
10-1 deci 10 deca da 
10-2 centi 102 hecto 
10-3 milli 103 kilo 
10-6 micro µ 106 mega 
10-9 nano 109 giga 
10-12 pico 1012 tera 
10-15 femto 1015 peta 
Non-SI units, quantities and related abbreviations 
°C degree Celsius (0°C = 273 K approximately); temperature differences are also given in °C (=K) rather than the more correct form of “Celsius degrees” 
ppm mixing ratio (as concentration measure of GHGs): parts per million (106) by volume 
ppb  mixing ratio (as concentration measure of GHGs): parts per billion (109) by volume 
ppt mixing ratio (as concentration measure of GHGs): parts per trillion (1012) by volume 
watt power or radiant flux; 1 watt = 1 Joule / second = 1 kg m2 / s3 
yr year 
ky thousands of years 
bp before present  
GtC gigatonnes (metric) of carbon 
GtCO2 gigatonnes (metric) of carbon dioxide (1 GtC = 3.7 GtCO2
CO2-eq carbon dioxide-equivalent, used as measure for the emission (generally in GtCO2-eq) or concentration (generally in ppm CO2-eq) of GHGs; see Box “Carbon dioxide-equivalent emissions and concentrations” in Topic 2 for details