Abstract It is estimated that global anthropogenic carbon dioxide (CO₂) emissions reduced by up to 12% at the start of 2020 compared to recent years due to the COVID-19 related downturn in economic activity. Despite the large decrease in CO₂ emissions, no reduction in the trend in background atmospheric CO₂ concentrations has been detected. So, how long would it take for sustained COVID-19 CO₂ emission reductions to be detected in daily and monthly averaged local CO₂ concentration measurements? CO₂ concentration measurements for five measurement sites in the UK and Ireland are combined with meteorological numerical weather prediction data to build statistical models that can predict future CO₂ concentrations. It is found that 75 % of the observed daily variability can be explained by these simple models. Emission reduction scenario experiments using these simple models illustrate that large daily and seasonal variability in local CO₂ concentrations precludes the rapid emergence of a detectable signal. COVID-19 magnitude emissions reductions would only be detectable in the daily CO₂ concentrations after at least 38 months and in monthly CO₂ concentrations after 11 months of sustained reductions. For monthly CO₂ concentrations the time of emergence is similar for all sites since the seasonal variability is largely driven by non-local fluxes of CO₂ between the terrestrial biosphere and the atmosphere. The COVID-19 CO₂ anthropogenic emissions reductions are similar in magnitude to those that are required to meet the Paris Agreement target of keeping global temperatures below 2 ° C. This study demonstrates that, using measurements alone, there will be a considerable lag between changes in global anthropogenic emissions and a detected signal in local CO₂ concentration trends. Thus, there is likely to be a delay of several years between changes in policy designed to meet CO₂ anthropogenic emissions targets and our ability to detect the impact of these policies on CO₂ concentrations using atmospheric measurements alone.