Background and introduction
Sea ice is a key component of the global climate system, and a very visible indicator of climate change. Arctic summer sea ice cover has declined at a rate of over 13% per decade since satellite observation began, and there is much interest in how this decline will continue in the future. Global coupled models are arguably the best tool we have for making future projections of Arctic sea ice, but generate a wide spread of projections of future decline.
Several previous CMIP analyses/studies have attempted to decrease the spread of plausible future projections by sub-selecting models based on their ability to simulate current day sea ice and/or past observed changes. However, such an approach ignores the considerable influence of internal variability in the Arctic climate system as well as the existence of cancelling errors in models and the potential for these to change in climate simulations as the system warms.
Comparing frequently analysed integrated quantities - such as ice extent and volume - alone is not sufficient to understand the reasons for differences in model projections. It is becoming increasingly clear that it is also necessary to consider, compare and evaluate the underlying processes causing ice growth and decline, and how they are likely to change in a warming world.
For CMIP6 models, the extra sea ice budget diagnostics that are included in the SIMIP data request (Notz et al., 2016) will allow much easier inter-model comparison of these underlying processes via an evaluation of the sea ice mass/volume budget. Improved knowledge of the sea ice volume/mass budget in climate models will help us to better understand the spread in climate simulations and the drivers of Arctic sea ice decline.
Here we propose a methodology for inter-comparing the mass budget of the Arctic sea ice, and how it changes during the 21st century, in CMIP6 model projections. This is based on the analysis of Keen and Blockley (2018) (K&B18), who investigated 21st century changes in the volume budget of the Arctic sea ice in the CMIP5 model HadGEM2-ES.