Forward projections of solar cyclicity imply the next few decades may be marked by global cooling rather than warming, despite continuing CO2 emissions.
Beyond the next few decades, how much the climate changes will depend primarily on the amount of greenhouse gases emitted into the atmosphere; how much of those greenhouse gases are absorbed by the ocean, the biosphere, and other sinks; and how sensitive Earth’s climate is to those emissions.23 Climate sensitivity is typically defined as the long-term change that would result from a doubling of carbon dioxide in the atmosphere relative to preindustrial levels; its exact value is uncertain due to the interconnected nature of the land–atmosphere–ocean system. Changes in one aspect of the system can lead to self-reinforcing cycles that can either amplify or weaken the climate system’s responses to human and natural influences, creating a positive feedback or self-reinforcing cycle in the first case and a negative feedback in the second.18 These feedbacks operate on a range of timescales from very short (essentially instantaneous) to very long (centuries). While there are uncertainties associated with modeling some of these feedbacks,24,25 the most up-to-date scientific assessment shows that the net effect of these feedbacks over the industrial era has been to amplify human-induced warming, and this amplification will continue over coming decades18 (see Box 2.3).