Solar geoengineering is receiving increased policy attention as a potential tool to offset climate warming. While climate responses to geoengineering have been studied in detail, the potential biodiversity consequences are largely unknown. To avoid extinction, species must either adapt or move to track shifting climates. Here, we assess the effects of the rapid implementation, continuation and sudden termination of geoengineering on climate velocities-the speeds and directions that species would need to move to track changes in climate. Compared to a moderate climate change scenario (RCP4.5), rapid geoengineering implementation reduces temperature velocities towards zero in terrestrial biodiversity hotspots. In contrast, sudden termination increases both ocean and land temperature velocities to unprecedented speeds (global medians >10 km yr) that are more than double the temperature velocities for recent and future climate change in global biodiversity hotspots. Furthermore, as climate velocities more than double in speed, rapid climate fragmentation occurs in biomes such as temperate grasslands and forests where temperature and precipitation velocity vectors diverge spatially by >90°. Rapid geoengineering termination would significantly increase the threats to biodiversity from climate change.

Though previous studies have shown that state‐of‐the‐art climate models are rather imperfect in their simulations of the climate response to large volcanic eruptions, the results depend on how the analyses were done. Observations show that all recent large tropical eruptions were followed by winter warming in the first Northern Hemisphere (NH) winter after the eruption, with little such response in the second winter, yet a number of the evaluations have combined the first and second winters. We have looked at just the first winter after large eruptions since 1850 in the Coupled Model Intercomparison Project 5 historical simulations and find that most models do produce a winter warming signal, with warmer temperatures over NH continents and a stronger polar vortex in the lower stratosphere. We also examined NH summer precipitation responses in the first year after these large volcanic eruptions and find clear reductions of summer monsoon rainfall.

Aerosol influences on clouds are a major source of uncertainty to our understanding of forced climate change. Increased aerosol can enhance solar reflection from clouds countering greenhouse gas warming. Recently, this indirect effect has been extended from water droplet clouds to other types including mixed-phase clouds. Aerosol effects on mixed-phase clouds are important because of their fundamental role on sea ice loss and polar climate change, but very little is known about aerosol effects on these clouds. Here we provide the first analysis of the effects of aerosol emitted from ship stacks into mixed-phase clouds. Satellite observations of solar reflection in numerous ship tracks reveal that cloud albedo increases 5 times more in liquid clouds when polluted and persist 2 h longer than in mixed-phase clouds. These results suggest that seeding mixed-phase clouds via shipping aerosol is unlikely to provide any significant counterbalancing solar radiative cooling effects in warming polar regions.

The Laki eruption in Iceland, which began in June 1783, was followed by many of the typical climate responses to volcanic eruptions: suppressed precipitation and drought, crop failure, and surface cooling. In contrast to the observed cooling in 1784–1786, the summer of 1783 was anomalously warm in Western Europe, with July temperatures reaching more than 3 K above the mean. However, the winter of 1783–1784 in Europe was as cold as 3 K below the mean. While climate models generally reproduce the surface cooling and decreased rainfall associated with volcanic eruptions, model studies have failed to reproduce the extreme warming in western Europe that followed the Laki eruption. As a result of the inability to reproduce the anomalous warming, the question remains as to whether this phenomenon was a response to the eruption or merely an example of internal climate variability. Using the Community Earth System Model from the National Center for Atmospheric Research, we investigate the “Laki haze” and its effect on Northern Hemisphere climate in the 12 months following the eruption onset. We find that the warm summer of 1783 was a result of atmospheric blocking over Northern Europe, which in our model cannot be attributed to the eruption. In addition, the extremely cold winter of 1783–1784 was aided by an increased likelihood of an El Niño after the eruption. Understanding the causes of these anomalies is important not only for historical purposes but also for understanding and predicting possible climate responses to future high‐latitude volcanic eruptions.