The North American carbon budget: Past, present, and future

This chapter nicely summarizes and synthesizes the latest scientific information on the North American carbon budget by incorporating terrestrial, anthropogenic, aquatic, and coastal margin CO₂ and CH₄dynamics. Starting with a historical context, the chapter summarizes current understanding of the magnitudes and trends of carbon stocks and fluxes at the continental scale. It also provides a regional context by stratifying the continent to countries and climate assessment regions and discusses the societal drivers, impacts, and carbon management decisions. Knowledge gaps and research needs are also identified. This chapter is well-written and clearly organized, and provides a broad context beyond individual chapters. Some of the main ways the chapter can be improved include the following:

• some work is needed on the Key Findings (discussed below);
• the goals and objectives should be explicitly described;
• critical content areas missing from the chapter are interannual variability of carbon fluxes and impacts of severe and extended droughts;
• indicators and feedbacks are missing from Section 2.4;
• consistent use of units is recommended;
• numbers with 3-4 significant digits over-state the confidence the reader should have, and all numbers should include uncertainties.

And one broader concern to note: This chapter follows the global overview in Chapter 1, where “sinks” are sinks in the cycle perturbed by anthropogenic CO₂ and CH₄, and the assumption is that globally, the net unperturbed background sinks are zero summed across all reservoirs. Yet in this chapter, “sinks” are net fluxes out of the atmosphere, background + perturbation. For the coastal ocean, inland waters, etc.—where lateral transport is significant—these sources and sinks include background/pre-industrial fluxes that are balanced by fluxes elsewhere. These distinctions must be made clear so that the reader is not given an impression of a greater or lesser sink for anthropogenic CO₂ than is there (e.g., P74, lines 4-6).