The NASA-GISS ModelE2.1-CC2 ESM: development and evaluation

This paper describes the NASA Goddard Institute for Space Studies (GISS) ModelE2.1-Carbon Cycle version 2 Earth System Model (NASA GISS E2.1-CC2 ESM), assesses its skill against observations and the previous version of the same model (GISS‐E2.1‐G‐CC). NASA GISS E2.1-CC2 ESM includes the same physical climate model E2.1-G as its predecessor, updated ocean and land carbon cycle and longer equilibrium simulations. While the focus here is on the land and ocean carbon components and their interactions with the atmosphere and ice, we also describe in detail the physical coupled model for consistency and ease of reference. We detail parameterizations, tuning, and conservation diagnostics that are relevant to the global (land and ocean) carbon cycle. We also describe the pre-industrial control and the historical simulations with this model while future climate scenarios will be addressed in a companion paper. The model is an improvement to prior releases, has been better tuned and spun up (all reservoirs: the ocean, the ocean carbon but especially the soil carbon reservoir) and exhibits smaller drifts in all carbon components. However, some persistent biases remain, particularly with regards to low ocean productivity and land primary production. Low ocean productivity may be due to (i) low iron concentration in high-nutrient and low chlorophyll regions, and (ii) lack of photoadaptation, which would increase the chl/C ratio in low-light environments, relieving light limitation beneath the surface of the ocean. Low land productivity may result from an insufficient gross primary productivity increase due to lack of prognostic LAI, or drought stress, or an underestimated CO 2 fertilization effect, or an overestimate of soil respiration due to biases in soil moisture, soil temperature, and/or how soil respiration responds to these environmental variables.