The COVID‐19 pandemic changed emissions of gases and particulates. These gases and particulates affect climate. In general, human emissions of particles cool the planet by scattering away sunlight in the clear sky and by making clouds brighter to reflect sunlight away from the earth. This paper focuses on understanding how changes to emissions of particulates (aerosols) affect climate. We use estimates of emissions changes for 2020 in two climate models to simulate the impacts of the COVID‐19 induced emission changes. We tightly constrain the models by forcing the winds to match observed winds for 2020. COVID‐19 induced lockdowns led to reductions in aerosol and precursor emissions, chiefly soot or black carbon and sulfate (SO₄). This is found to reduce the human caused aerosol cooling: creating a small net warming effect on the earth in spring 2020. Changes in cloud properties are smaller than observed changes during 2020. The impact of these changes on regional land surface temperature is small (maximum +0.3 K). The impact of aerosol changes on global surface temperature is very small and lasts over several years. However, the aerosol changes are the largest contribution to COVID‐19 affected emissions induced radiative forcing and temperature changes, larger than ozone, CO₂ and contrail effects.

The severity of climate change is closely related to how much the Earth warms in response to greenhouse gas increases. Here we find that the temperature response to an abrupt quadrupling of atmospheric carbon dioxide has increased substantially in the latest generation of global climate models. This is primarily because low cloud water content and coverage decrease more strongly with global warming, causing enhanced planetary absorption of sunlight—an amplifying feedback that ultimately results in more warming. Differences in the physical representation of clouds in models drive this enhanced sensitivity relative to the previous generation of models. It is crucial to establish whether the latest models, which presumably represent the climate system better than their predecessors, are also providing a more realistic picture of future climate warming.

We examine simulations of Arctic sea ice from the latest generation of global climate models. We find that the observed evolution of Arctic sea‐ice area lies within the spread of model simulations. In particular, the latest generation of models performs better than models from previous generations at simulating the sea‐ice loss for a given amount of CO₂ emissions and for a given amount of global warming. In most simulations, the Arctic Ocean becomes practically sea‐ice free (sea‐ice area <1 million km²) in September for the first time before the Year 2050.

Climate models provide an important way to understand future changes in the Earth's climate. In this paper we undertake a thorough evaluation of the performance of various climate models published between the early 1970s and the late 2000s. Specifically, we look at how well models project global warming in the years after they were published by comparing them to observed temperature changes. Model projections rely on two things to accurately match observations: accurate modeling of climate physics and accurate assumptions around future emissions of CO₂ and other factors affecting the climate. The best physics‐based model will still be inaccurate if it is driven by future changes in emissions that differ from reality. To account for this, we look at how the relationship between temperature and atmospheric CO₂ (and other climate drivers) differs between models and observations. We find that climate models published over the past five decades were generally quite accurate in predicting global warming in the years after publication, particularly when accounting for differences between modeled and actual changes in atmospheric CO₂ and other climate drivers. This research should help resolve public confusion around the performance of past climate modeling efforts and increases our confidence that models are accurately projecting global warming.