More than 40 model groups worldwide are participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6), providing a new and rich source of information to better understand past, present, and future climate change. Here, we use the Earth System Model Evaluation Tool (ESMValTool) to assess the performance of the CMIP6 ensemble compared to the previous generations CMIP3 and CMIP5. While CMIP5 models did not capture the observed pause in the increase in global mean surface temperature between 1998 and 2013, the historical CMIP6 simulations agree well with the observed recent temperature increase, but some models have difficulties in reproducing the observed global mean surface temperature record of the second half of the 20^th century. While systematic biases in annual mean surface temperature and precipitation remain in the CMIP6 multi‐model mean, individual models and high‐resolution versions of the models show significant reductions in many long‐standing biases. Some improvements are also found in the vertical temperature, water vapor and zonal wind speed distributions, and root mean square errors for selected fields are generally smaller with reduced inter‐model spread and higher average skill in the correlation patterns relative to observations. An emerging property of the CMIP6 ensemble is a higher effective climate sensitivity with an increased range between 2.3 and 5.6 K. A possible reason for this increase in some models is improvements in cloud representation resulting in stronger shortwave cloud feedbacks than in their predecessor versions.

Contrail cirrus are a major component of the climate forcing due to air traffic. For a given contrail cirrus cover, ice water content and ice crystal shape, their impact on radiation is dependent on the number and size of ice crystals. Here we use a global climate model to study the impact of a reduction in initially formed ice crystal numbers, as may be caused by reduced soot number emissions. We find that for reduced initial ice crystal numbers the ice water content is decreased and ice crystal sizes increased, leading to a reduction in contrail cirrus optical depth and doubling the fraction of contrail cirrus that cannot be detected by satellite remote sensing. Contrail cirrus lifetimes and coverage are strongly reduced leading to significant reductions in contrail cirrus radiative forcing. The global climate impact of contrail cirrus is nonlinearly dependent on the reduction in initial ice crystal numbers. A reduction in the initial ice crystal number of 80% leads to a decrease in contrail cirrus radiative forcing by 50%, whereas a twofold reduction leads to a decrease in radiative forcing by approximately 20%. Only a few contrail cirrus outbreaks explain a large percentage of the climate impact. The contrail cirrus climate impact can be effectively mitigated by reducing initial ice crystal concentrations in such outbreak situations. Our results are important for assessments dealing with mitigating the climate impact of aviation and discussions about the use of alternative fuels or lean combustion in aviation.

This paper describes the second major release of the Earth System Model Evaluation Tool (ESMValTool), a community diagnostic and performance metrics tool for the evaluation of Earth system models (ESMs) participating in the Coupled Model Intercomparison Project (CMIP). Compared to version 1.0, released in 2016, ESMValTool version 2.0 (v2.0) features a brand new design, with an improved interface and a revised preprocessor. It also features a significantly enhanced diagnostic part that is described in three companion papers. The new version of ESMValTool has been specifically developed to target the increased data volume of CMIP Phase 6 (CMIP6) and the related challenges posed by the analysis and the evaluation of output from multiple high-resolution or complex ESMs. The new version takes advantage of state-of-the-art computational libraries and methods to deploy an efficient and user-friendly data processing. Common operations on the input data (such as regridding or computation of multi-model statistics) are centralized in a highly optimized preprocessor, which allows applying a series of preprocessing functions before diagnostics scripts are applied for in-depth scientific analysis of the model output. Performance tests conducted on a set of standard diagnostics show that the new version is faster than its predecessor by about a factor of 3. The performance can be further improved, up to a factor of more than 30, when the newly introduced task-based parallelization options are used, which enable the efficient exploitation of much larger computing infrastructures. ESMValTool v2.0 also includes a revised and simplified installation procedure, the setting of user-configurable options based on modern language formats, and high code quality standards following the best practices for software development.

The Earth System Model Evaluation Tool (ESMValTool) is a community diagnostics and performance metrics tool designed to improve comprehensive and routine evaluation of Earth system models (ESMs) participating in the Coupled Model Intercomparison Project (CMIP). It has undergone rapid development since the first release in 2016 and is now a well-tested tool that provides end-to-end provenance tracking to ensure reproducibility. It consists of (1) an easy-to-install, well-documented Python package providing the core functionalities (ESMValCore) that performs common preprocessing operations and (2) a diagnostic part that includes tailored diagnostics and performance metrics for specific scientific applications. Here we describe large-scale diagnostics of the second major release of the tool that supports the evaluation of ESMs participating in CMIP Phase 6 (CMIP6). ESMValTool v2.0 includes a large collection of diagnostics and performance metrics for atmospheric, oceanic, and terrestrial variables for the mean state, trends, and variability. ESMValTool v2.0 also successfully reproduces figures from the evaluation and projections chapters of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) and incorporates updates from targeted analysis packages, such as the NCAR Climate Variability Diagnostics Package for the evaluation of modes of variability, the Thermodynamic Diagnostic Tool (TheDiaTo) to evaluate the energetics of the climate system, as well as parts of AutoAssess that contains a mix of top–down performance metrics. The tool has been fully integrated into the Earth System Grid Federation (ESGF) infrastructure at the Deutsches Klimarechenzentrum (DKRZ) to provide evaluation results from CMIP6 model simulations shortly after the output is published to the CMIP archive. A result browser has been implemented that enables advanced monitoring of the evaluation results by a broad user community at much faster timescales than what was possible in CMIP5.

A conceptual framework to predict microphysical and optical properties of contrail particles within a wingspan behind the source aircraft is developed. Results from two decades of contrail observations and numerical simulations are reviewed forming the basis of theoretical model development. The model utilizes cloud theory applied to the dynamics and thermodynamics of jet aircraft exhaust plumes in upper tropospheric conditions. Droplet nuclei include soot particles emitted from aircraft engines and atmospheric particles entrained into the plume. These precursor particles activate into copious homogeneously freezing water droplets as the plume relative humidity rises beyond liquid water saturation. A unimodal size spectrum of ice particles develops wherein ice particles grow to micrometer mean sizes. Contrail particle formation is analyzed over a wide range of soot emissions relating to conventional jet fuels as well as to alternative aviation fuels producing much less soot and volatile particle emissions. For current aviation fuels and propulsion technology, the number of contrail ice particles scales roughly in proportion to the number of emitted soot particles that act as water condensation nuclei despite their poor hygroscopicity. Close to the contrail formation threshold, only few plume particles can be water activated and freeze. Implications for effects of alternative fuels on contrails, an arena for future scientific exploration, are outlined.

The climate impact of air traffic is to a large degree caused by changes in cirrus cloudiness resulting from the formation of contrails. Contrail cirrus radiative forcing is expected to increase significantly over time due to the large projected increases in air traffic. We use ECHAM5-CCMod, an atmospheric climate model with an online contrail cirrus parameterization including a microphysical two-moment scheme, to investigate the climate impact of contrail cirrus for the year 2050. We take into account the predicted increase in air traffic volume, changes in propulsion efficiency and emissions, in particular soot emissions, and the modification of the contrail cirrus climate impact due to anthropogenic climate change. Global contrail cirrus radiative forcing increases by a factor of 3 from 2006 to 2050, reaching 160 or even 180 mW m⁻², which is the result of the increase in air traffic volume and a slight shift in air traffic towards higher altitudes. Large increases in contrail cirrus radiative forcing are expected over all of the main air traffic areas, but relative increases are largest over main air traffic areas over eastern Asia. The projected upward shift in air traffic attenuates contrail cirrus radiative forcing increases in the midlatitudes but reinforces it in the tropical areas. Climate change has an insignificant impact on global contrail cirrus radiative forcing, while regional changes are significant. Of the emission reductions it is the soot number emission reductions by 50 % that lead to a significant decrease in contrail cirrus optical depth and coverage, leading to a decrease in radiative forcing by approximately 15 %. The strong increase in contrail cirrus radiative forcing due to the projected increase in air traffic volume cannot be compensated for by the decrease in initial ice crystal numbers due to reduced soot emissions and improvements in propulsion efficiency.