For carbon dioxide, slightly higher levels in the northern hemisphere reflect the greater emissions there. A global increase in concentration over the few decades of available atmospheric measurements shows the imbalance of emissions over sinks, and a clear annual cycle results from the 'pulsing' of the biosphere with the seasons. The carbon dioxide exchanges can be estimated from measurements of surface processes, scaled up to large geographical regions. Uncertainties in these exchanges can be significantly reduced by analysis of the atmospheric concentrations and isotopic ratios of trace species. However, the frequency and coverage of the atmospheric measurements needs to be dramatically increased if scientists are to determine exchanges with sufficient detail and precision for emissions accounting at regional scales.
Atmospheric measurements show how composition has changed over recent decades.
To place these changes in perspective, the air preserved in polar ice is measured. Air in bubbles in ice cores goes back for thousands of years. The recent changes in carbon dioxide and methane, however, are unprecedented in both the rate of increase and the levels reached, for hundreds of thousands of years. Another tool we can apply to understand atmospheric composition changes is the use of isotopes. For example, organic carbon is depleted in the 13 C isotope, compared to carbon in other reservoirs such as the ocean and atmosphere.