Introduction: Observing the Continental-Scale Greenhouse Gas Balance

The concentrations of CO₂ and CH₄ in the atmosphere are at the highest level they have been in the past 25 million years. Current levels of CO₂ have increased by 30% from 280 ppm in pre-industrial times to 380 ppm today, and they continue to rise. These changes are caused by human activities. The primary inputs of carbon into the atmosphere arise from fossil fuel combustion and modifications of global vegetation cover through land-use change (e.g. land conversion to agriculture including pasture expansion, biomass burning). For the decade of the 1990s, an average of about 6.3 Pg C year⁻¹ was released to the atmosphere from the burning of fossil fuels. for the recent period of 2000–2005, 7.2 Pg C year⁻¹ was emitted from fossil fuel burning. It was estimated that an average of 1.5–2.5 Pg C year⁻¹ was emitted because of deforestation and land-use change during the same intervals (Solomon et al. 2007).

The increasing CO₂ concentrations in the atmosphere raise concern regarding the heat balance of the atmosphere. In particular, the increasing concentration of these gases leads to an intensification of the Earth’s natural greenhouse effect. This shift in the planetary heat balance may force the global climate system in ways which are currently not well understood. Complex interactions and feedbacks are involved, but there is a general consensus that global patterns of temperature and precipitation will change. The magnitude, distribution and timing of these changes, however, are far from certain. Recent Intergovernmental Panel on Climate Change (IPCC) assessments suggest a wide range of temperature and precipitation changes (Solomon et al. 2007).