Fossil fuel emissions dominate Northern Hemisphere CO₂ seasonal cycle trends under mitigation scenarios

Variations in the atmospheric CO₂ seasonal cycle across the Northern Hemisphere have historically been dominated by terrestrial ecosystems, making ground-based observations a reliable proxy for terrestrial carbon dynamics. However, whether this dominance will persist in the future remains uncertain. Here we combine atmospheric transport modeling with factorial simulations to assess and attribute future changes in the CO₂ seasonal cycle through 2100. We show that the dominant drivers of these changes shift fundamentally across scenarios. Under the high-emission scenario (SSP5-8.5), strengthening land sinks dominate and amplify CO₂ seasonal variability, preserving ground-based observations as a reliable terrestrial proxy. In contrast, under the low-emission scenario (SSP1-2.6), CO₂ seasonal amplitude declines widely, driven primarily by reduced fossil fuel emissions and their dampened seasonality. Consequently, established ground-based CO₂ observations may no longer reliably track terrestrial carbon dynamics under mitigation pathways, underscoring the need for new approaches for monitoring and climate policy verification.