The term "nuclear winter" was commonly used in the 1980s and represented a scenario where a nuclear war could profoundly alter the earth's climate through injection of aerosols into the stratosphere, which could be considered a highly extreme example of human activity altering the earth system. Paul Crutzen, in his Nobel Prize speech, said that this concept is politically important as it "magnifies and highlights the dangers of a nuclear war and convinces me that in the long run mankind can only escape such horrific consequences if nuclear weapons are totally abolished by international agreement". While nuclear war is hopefully something that we will never experience, it is highly interesting to return to this topic four decades later, after the major advancements in the science of aerosol-climate interactions that have occurred in the meantime (partly motivated by an exploration of geoengineering concepts), and improvements in our abilities to model the agricultural and economic impacts of climate change. The conclusions of the paper by Robock et al. offer a sobering reminder of the manifold dangers of this type of warfare to the earth system and society.

Polar Stratospheric Clouds (PSCs) catalyze ozone depletion at high latitudes with important impacts on energetic, biologically effective UV radiation. The present study is a unique experiment into PSC particle optical properties and formation performed in a large, coolable cloud chamber. Robert Wagner and his colleagues find evidence that a nitric acid dihydrate can be efficiently crystallized in PSCs through heterogeneous nucleation, for example by mineral dust or micrometeorites. It has a "beta-polymorph" crystalline structure - with distinct optical properties - clearly separable from an alpha-polymorph that forms at low temperatures through homogeneous nucleation. Interestingly, electron microscopy, infrared spectroscopy, X-ray diffraction and calculations all point to the same conclusion. The results have important implications for our understanding and the detection of PSCs.

Large volcanic eruptions can have significant effects on the atmosphere and on climate. Comparing the observed effects against model predictions is also a very valuable test of our scientific understanding of how the atmosphere works. This paper describes a model investigation of a hypothetical eruption, about twice the size of that of Pinatubo in 1991, and is unusual that it focuses on the response in the mesosphere -- the part of the atmosphere in the 50-80km altitude range. The eruption deposits aerosol in the tropics in the 20-25km altitude range, in the lower stratosphere and well below the mesosphere. The effect is felt in the mesosphere through dynamical coupling communicated through small-scale gravity waves, which transport momentum and as a result can drive changes in winds and temperatures. The predictions made in the paper will be helpful in future in interpreting changes in the mesosphere observed following volcanic eruptions.