Carbon cycle feedbacks represent large uncertainties in climate change projections, and the response of soil carbon to climate change contributes the greatest uncertainty to this. Future changes in soil carbon depend on changes in litter and root inputs from plants and especially on reductions in the turnover time of soil carbon (tau(s)) with warming. An approximation to the latter term for the top one metre of soil (Delta C-s,C-tau) can be diagnosed from projections made with the CMIP6 and CMIP5 Earth System Models (ESMs), and is found to span a large range even at 2 degrees C of global warming (-196 +/- 117 PgC). Here, we present a constraint on Delta C-s,C-tau, which makes use of current heterotrophic respiration and the spatial variability of tau(s) inferred from observations. This spatial emergent constraint allows us to halve the uncertainty in Delta C-s,C-tau at 2 degrees C to -232 +/- 52 PgC.