We report measurements and analysis of a boreal forest fire, integrating the effects of greenhouse gases, aerosols, black carbon deposition on snow and sea ice, and postfire changes in surface albedo. The net effect of all agents was to increase radiative forcing during the first year (34 ± 31 Watts per square meter of burned area), but to decrease radiative forcing when averaged over an 80-year fire cycle (–2.3 ± 2.2 Watts per square meter) because multidecadal increases in surface albedo had a larger impact than fire-emitted greenhouse gases. This result implies that future increases in boreal fire may not accelerate climate warming.

Downs and Ups: Every spring, the concentration of CO ₂ in the atmosphere of the Northern Hemisphere decreases as terrestrial vegetation grows, and every fall, CO ₂ rises as vegetation dies and rots. Climate change has destabilized the seasonal cycle of atmospheric CO ₂ such that Graven et al. (p. 1085 , published online 8 August; see the Perspective by Fung ) have found that the amplitude of the seasonal cycle has exceeded 50% at some latitudes. The only way to explain this increase is if extratropical land ecosystems are growing and shrinking more than they did half a century ago, as a result of changes in the structure and composition of northern ecosystems.

Significance: Climate change and rising CO₂are altering the behavior of land plants in ways that influence how much biomass they produce relative to how much water they need for growth. This study shows that it is possible to detect changes occurring in plants using long-term measurements of the isotopic composition of atmospheric CO₂. These measurements imply that plants have globally increased their water use efficiency at the leaf level in proportion to the rise in atmospheric CO₂over the past few decades. While the full implications remain to be explored, the results help to quantify the extent to which the biosphere has become less constrained by water stress globally.

Stable isotopes in water have the potential to diagnose changes in the earth's hydrological budget in response to climate change and land use change. However, there have been few measurements in the vapour phase. Here, we present high‐frequency measurements of oxygen isotopic compositions of water vapour (δ_v) and evapotranspiration (δ_ET) above a soybean canopy using the tunable diode laser (TDL) technique for the entire 2006 growing season in Minnesota, USA. We observed a large variability in surface δ_v from the daily to the seasonal timescales, largely explained by Rayleigh processes, but also influenced by vertical atmospheric mixing, local evapotranspiration (ET) and dew formation. We used δ_ET measurements to calculate the isotopic composition at the sites of evaporative enrichment in leaves (δ_L,e) and compared that with the commonly used steady‐state prediction (δ_L,s). There was generally a good agreement averaged over the season, but larger differences on individual days. We also found that vertical variability in relative humidity and temperature associated with canopy structure must be addressed in canopy‐scale leaf water models. Finally, we explored this data set for direct evidence of the Péclet effect.