This article from Landscape and Urban Planning looks at carbon emissions and carbon sequestration in Beijing:

During the study period, carbon sequestration only offset 2.4% of carbon emission, indicating a serious imbalance of the city’s carbon metabolism. The city’s core built-up area expanded along eight axes, and its form fluctuated between simpler and more complex. From a small-scale perspective, the spatial pattern mainly showed expansion and aggregation of patches with high carbon emission and shrinkage and fragmentation of patches with high carbon sequestration.

I think this sort of study is useful as we think about what it would mean for cities to be truly sustainable either within their own boundaries or in the context of the larger landscape. 2.4% doesn’t sound like much, but if that is the answer with no system-level planning or management, could it be boosted to 5% or 10% with a more systematic approach to green infrastructure? The rest of the landscape (farms, protected forests, grasslands, and wetlands, and bodies of water) would do its share. Finally, technology would have to make up the remaining gap, if we really want to one day get to carbon neutral or even begin to role back the damage we have done to the atmosphere and oceans.