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Closure of the Earth’s energy budget relies on strong aerosol cooling since 1998, if the same feedbacks apply for anthropogenic and natural variability. An analysis of climate model simulations suggests that these feedbacks are instead distinct.

Arctic warming affects weather and climate thousands of miles to the south. Scientists are split on how large this effect is.

Some aspects of the Earth system—such as global mean temperatures, and sea-level rise due to thermal expansion or melting of large ice sheets—continue to respond to climate change long after the stabilization of radiative forcing. Simulations with a coupled climate–vegetation model show that similarly ecosystems may be committed to significant change after climate stabilization. Targets for stabilizing climate change are often based on considerations of the impacts of different levels of global warming, usually assessing the time of reaching a particular level of warming. However, some aspects of the Earth system, such as global mean temperatures1 and sea level rise due to thermal expansion2 or the melting of large ice sheets3, continue to respond long after the stabilization of radiative forcing. Here we use a coupled climate–vegetation model to show that in turn the terrestrial biosphere shows significant inertia in its response to climate change. We demonstrate that the global terrestrial biosphere can continue to change for decades after climate stabilization. We suggest that ecosystems can be committed to long-term change long before any response is observable: for example, we find that the risk of significant loss of forest cover in Amazonia rises rapidly for a global mean temperature rise above 2 ∘C. We conclude that such committed ecosystem changes must be considered in the definition of dangerous climate change, and subsequent policy development to avoid it.

The quest to determine climate sensitivity has been going on for decades, with disturbingly little progress in narrowing the large uncertainty range. But fascinating new insights have been gained that will provide useful information for policy makers, even though the upper limit of climate sensitivity will probably remain uncertain for the near future.

Human activity alters the atmospheric composition, which leads to global warming. Model simulations suggest that reductions in emission of sulfur dioxide from Europe since the 1970s could have unveiled rapid Arctic greenhouse gas warming.

Permafrost soils contain almost twice as much carbon as the current atmospheric carbon pool. Climate model simulations suggest that the feedback generated by future permafrost carbon release could lead to a further warming of 0.13–1.69 °C by 2300. Permafrost soils contain an estimated 1,700 Pg of carbon, almost twice the present atmospheric carbon pool1. As permafrost soils thaw owing to climate warming, respiration of organic matter within these soils will transfer carbon to the atmosphere, potentially leading to a positive feedback2. Models in which the carbon cycle is uncoupled from the atmosphere, together with one-dimensional models, suggest that permafrost soils could release 7–138 Pg carbon by 2100 (refs 3, 4). Here, we use a coupled global climate model to quantify the magnitude of the warming generated by the feedback between permafrost carbon release and climate. According to our simulations, permafrost soils will release between 68 and 508 Pg carbon by 2100. We show that the additional surface warming generated by the feedback between permafrost carbon and climate is independent of the pathway of anthropogenic emissions followed in the twenty-first century. We estimate that this feedback could result in an additional warming of 0.13–1.69 °C by 2300. We further show that the upper bound for the strength of the feedback is reached under the less intensive emissions pathways. We suggest that permafrost carbon release could lead to significant warming, even under less intensive emissions trajectories.

Tue perception that future climate warming is inevitable stands at the centre of current climate-policy discussions. W e argue that the notion of unavoidable warming owing to inertia in the climate system is based on an incorrect interpretation of climate science. Stable atmospheric concentrations of greenhouse gases would lead to continued warming, but if carbon dioxide emissions could be eliminated entirely, temperatures would quickly stabilize or even decrease over time. Future warming is therefore driven by socio-economic inertia, and is only as inevitable as future emissions. As a consequence, mitigation efforts to minimize future greenhouse-gas emissions can successfully restrict future warming to a level that may avoid dangerous anthropogenic interference with the climate system. Tue challenge of climate mitigation, although daunting, is fully within the scope of human control.

Natural climate variability and limited observational records have made identifying human-influenced climate change at the poles difficult. But a human signature is now emerging in rising Arctic and Antarctic temperatures.

The influence of global warming on temperature trends at higher altitudes has been hotly debated. Stratospheric ozone depletion is another piece in the remaining tropical climate puzzle.