Towards monitoring climate change on a yearly basis using observational constraints.

Climate change monitoring and adaptation policies require reliable and updated indicators about global warming. Reports of the Intergovernmental Panel on Climate Change (IPCC) provide updated indicators every 5 to 7 years, based in particular on the latest available data and evidence. In the last report, projections were derived from climate model simulations constrained by observations. Current warming was derived from observations averaged over the last 10 years, or again a combination of model and observations in attribution statements. Here we explore the possibility of updating present and future warming estimates on a yearly basis, by incorporating new observations to observational constraints. First, we show that adding the latest temperature observations each year leads to a continuous improvement in estimating warming projections. In particular, warming estimates are not affected by year-to-year internal variability. Second, regarding current warming, we show that observational constraints can be used to derive an estimate of the forced warming for the current year, without having to average over the last 10 years - as the IPCC does in its latest report. This provides an unbiased estimate of current warming, without adding further variance. Third, we show that updating model data can lead to a gap in the estimate of current and future warming, but this remains within the uncertainties we estimate. Finally, we argue that annual updates of current and future warming estimates provide accurate, robust, and reliable information about climate change, while remaining consistent with previous years' estimates.