From measurements of air trapped in ice cores and from direct measurements of the atmosphere, we know that in the past 200 years the abundance of CO₂ in the atmosphere has increased by over 30% (i.e., from a concentration of 280 ppm by volume (pppmv) in 1700 to nearly 370 ppmv in 2000). We also know that the concentration was relatively constant (roughly within ±10 ppmv of 275) for more than 1,000 years prior to the human-induced rapid increase in atmospheric CO₂ (see Chapter 3, Figures 3.2a and 3.2b).

Looking further back in time, we find an extraordinarily regular record of change. The Vostok core (Figure 3.2d) captures a remarkable and intriguing signal of the periodicity of inter-glacial and glacial climate periods in step with the transfer of significant pools of carbon from the land (most likely through the atmosphere) to the ocean and then the recovery of terrestrial carbon back from the ocean. The repeated pattern of a 100 to 120 ppmv decline in atmospheric CO₂ from an inter-glacial value of 280 to 300 ppmv to a 180 ppmv floor and then the rapid recovery as the planet exits glaciation suggests a tightly governed control system. There is a similar methane (CH₄) cycle between 320 to 350 ppbv (parts per billion by volume) and 650 to 770 ppbv. What begs explanation is not just the linked periodicity of carbon and glaciation, but also the apparent consistent limits on the cycles over the period. See Chapter 3, Box 3.4.

Today�s atmosphere, imprinted with the fossil fuel CO₂ signal, stands at nearly 90 to 70 ppmv above the previous inter-glacial maximum of 280 to 300 ppmv. The current methane value is even further (percentage-wise) from its previous inter-glacial high values. In essence, carbon has been moved from a relatively immobile pool (in fossil fuel reserves) in the slow carbon cycle to the relatively mobile pool (the atmosphere) in the fast carbon cycle, and the ocean, terrestrial vegetation and soils have yet to equilibrate with this �rapidly� changing concentration of CO₂ in the atmosphere.

Given this remarkable and unprecedented history one cannot help but wonder about the characteristics of the carbon cycle in the future (Chapter 3). To understand better the global carbon cycle, two themes are clear: (1) there is a need for global observations that can contribute significantly to determining the sources and sinks of carbon and (2) there is a need for fundamental work on critical biological processes and their interaction with the physical system. Two observational needs must be highlighted:

• Observations that would decisively improve our ability to model the carbon cycle. For example, a dense and well-calibrated network for monitoring CO₂ and O₂ concentrations that will also be required for international verification of carbon sources and sinks is central.
  
• �Benchmarks� data sets that allow model intercomparison activities to move in the direction of becoming data-model comparisons and not just model-model comparisons.
  

We note that the Subsidiary Body for Scientific and Technological Advice (SBSTA) of the United Nations Framework Convention on Climate Change (UNFCCC) recognised the importance of an Integrated Global Observing Strategy Partnership in developing observing systems for the oceans and terrestrial carbon sources and sinks in the global carbon cycle and in promoting systematic observations.

There is also a range of areas where present day biogeochemistry modelling is not only in need of additional data, but is also crucially limited by insufficient understanding at the level of physical or biological processes. Clarifying these processes and their controls is central to a better understanding of the global carbon cycle.

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