## Abstract

It is generally accepted that the combustion of fossil fuels over the period 1860 to 1954 has produced an amount of carbon dioxide, containing no radiocarbon, that is equal to approximately 13% of the carbon dioxide in the atmosphere. The addition of this 'old' carbon dioxide to the atmosphere has observably disturbed the steady-state distribution of carbon-14 in nature. In the present paper measurements are described of the carbon-14 concentration in sets of wood samples from the northern and southern hemispheres, and these show that the carbon-14 specific activity of atmospheric carbon dioxide has decreased by 2$\cdot $03 $\pm $ 0$\cdot $15% over the period 1860 to 1954, and that the present-day difference between the decrease in the northern and southern hemispheres is less than 0$\cdot $50%. The response of various mathematical models of the carbon cycle in nature to the addition of 'old' carbon dioxide at an exponential rate has been considered. Using the above data in conjunction with these models it is deduced that: (1) The mean life of a carbon dioxide molecule in the atmosphere before it is absorbed into other reservoirs of carbon must be less than 7 yr, and is probably of the order of 2 yr. (2) The exchange time for mixing of the atmospheres of the two hemispheres (i.e. the mean life of a carbon dioxide molecule in the atmosphere of one hemisphere before transferrence to the other hemisphere) is less than 2 yr.