Fig. 3.

Early winter (October through December) land−ocean sector CO₂ signal (dCO₂) measured at the NOAA BRW tower in Barrow, AK, since 1975. The tower is influenced by a large area of the North Slope. The early winter land sector dCO₂ has increased by 73.4% ± 10.8% at BRW over 41 y [1.51 ± 0.64 ppm in 1975–1989 to 2.62 ± 0.85 ppm in 2004–2015, a statistically significant increase with P value 0.018, indicated by red asterisks (*)]. Shown are the yearly averaged data (gray diamonds) and 11-y average (black circles) with 95% confidence intervals (error bars) and dashed lines to indicate the years sampled. Using the fluxes calculated as part of the aircraft optimization, the mean flux of CO₂ on the North Slope has increased by a comparable amount, from 0.24 μmol⋅m⁻²⋅s⁻¹ in 1975–1989 to 0.43 μmol⋅m⁻²⋅s⁻¹ in 2004–2015.