Marine heatwave (MHW) characteristics - such as frequency, intensity and duration - are increasing, largely due to global warming. However, while corresponding marine cold spell (MCS) characteristics have decreased over most regions, importantly, concomitant changes in MHW and MCS characteristics remain unclear. Here, we provide a comparative global assessment of these changes based on satellite sea surface temperature (SST) observations over 1982-2020. Across the planet, we find distinct differences in mean MHW and MCS metrics. Furthermore, decreasing trends in MCS characteristics are not necessarily aligned with increasing trends in MHW characteristics. While differences in intensity trends are mainly explained by SST variance trends, differences in exposure trends are less clear. Overall, decreasing MCS exposure and intensities are found to be largely driven by warming SST, rather than changes to SST variance, so it is expected that MCS will continue to diminish in their frequency, intensity and duration under global warming.

Outputs from new state-of-the-art climate models under the Coupled Model Inter-comparison Project phase 6 (CMIP6) promise improvement and enhancement of climate change projections information for Australia. Here we focus on three key aspects of CMIP6: what is new in these models, how the available CMIP6 models evaluate compared to CMIP5, and their projections of the future Australian climate compared to CMIP5 focussing on the highest emissions scenario. The CMIP6 ensemble has several new features of relevance to policy-makers and others, for example the integrated matrix of socio-economic and concentration pathways. The CMIP6 models show incremental improvements in the simulation of the climate in the Australian region, including a reduced equatorial Pacific cold-tongue bias, slightly improved rainfall teleconnections with regional climate drivers, improved representation of atmosphere and ocean extreme heat events, as well as dynamic sea level. However, important regional biases remain, evident in the excessive precipitation over the Maritime Continent and precipitation pattern biases in the nearby tropical convergence zones. Projections of temperature and rainfall from the available CMIP6 ensemble broadly agree with those from CMIP5, except for a group of CMIP6 models with higher climate sensitivity and greater warming and increase in some extremes after 2050. CMIP6 rainfall projections are similar to CMIP5, but the ensemble examined has a narrower range of rainfall change in summer in the north and winter in the south. Overall, future national projections are likely to be similar to previous versions but perhaps with some areas of improved confidence and clarity.

Climate models exhibit a broad range in the simulated properties of the global climate. In the early historical period, the absolute global mean surface air temperature of models contributing to the fifth phase of the Coupled Model Intercomparison Project (CMIP5) spans a range of ~12-15 °C. Other climate parameters are linked to the global mean temperature, such as sea ice area, atmospheric circulation patterns, and by extension cloudiness, precipitation and albedo. Accurate representation of the baseline climate state is crucial for meaningful future climate projections, since the baseline conditions may dictate the capacity for change. For example, a model with initially smaller sea ice area has less potential to lose sea ice as the planet warms. Amongst the CMIP5 models, it is found that in the baseline climate state there are coherences between Southern Ocean temperature, outgoing shortwave radiation, cloudiness, the position of the mid-latitude eddy-driven jet, and Antarctic sea ice area. The baseline temperature relationship extends to projected future changes in the same set of variables. The tendency for models with initially cooler Southern Ocean surface temperature to exhibit more global warming, and vice versa for initially warmer models, can therefore be linked to baseline Southern Ocean climate system biases. A first look at emerging data from CMIP6 reveals a shift of the tendency towards the Antarctic region, potentially linked to a reduction in biases over the Southern Ocean, which prompts an examination of biases in the Antarctic region as more CMIP6 model data becomes available.