A tipping point, in ecological or geological terms, is a threshold at which a small “push” (such as additional greenhouse gas emissions) can lead to a runaway feedback loop, resulting in the sudden shift of a local ecosystem or the entire planet to a new state (say, a much hotter climate). These shifts may follow a pattern known as hysteresis, where is it much harder to reverse the shift than it is to cause it. For example, an ice sheet that melts rapidly due to climate change might take a long time to regrow even if climate change were to be reversed. Hence, tipping points could potentially result in environmental changes that are irreversible on human timescales.