tipping element
n. An ecosystem or region that undergoes an abrupt and significant change in climate, particularly due to human activity.
Many people think of climate change as a gradual process that occurs over decades and centuries, but environment experts believe that the Earth's climate system can cross tipping points at which a small change can have a big and sudden impact on the fate of large areas. A research team at East Anglia has identified a shortlist of nine potential climate-system tipping elements so that scientists can monitor them with early-warning systems and help policymakers and the public avoid dangerous thresholds.
—Ann Mroz, “THE Awards 2008,” The Times Higher Education Supplement, September 25, 2008
Global change may appear to be a slow and gradual process on human scales. However, in some regions anthropogenic forcing on the climate system could kick start abrupt and potentially irreversible changes. For these sub-systems of the Earth system the researchers introduce the term "tipping element".

Drawing on a workshop of 36 leading climate scientists in October 2005 at the British Embassy, Berlin, Germany, a further elicitation of 52 experts in the field, and a review of the pertinent literature, the authors compiled a short-list of nine potential tipping elements. These tipping elements are ranked as the most policy-relevant and require consideration in international climate politics.

Arctic sea-ice and the Greenland Ice Sheet are regarded as the most sensitive tipping elements with the smallest uncertainty. Scientists expect ice cover to dwindle due to global warming. The West Antarctic Ice Sheet is probably less sensitive as a tipping element, but projections of its future behavior have large uncertainty. This also applies to the Amazon rainforest and Boreal forests, the El Nino phenomenon, and the West African monsoon. "These tipping elements are candidates for surprising society by exhibiting a nearby tipping point," the authors state in the article that is published in PNAS Online Early Edition. The archetypal example of a tipping element, the Atlantic thermohaline circulation, could undergo a large abrupt transition with up to ten percent probability within this century, according to the UN climate report from 2007.
—“Tipping elements in the Earth's climate system,” Science Letter, February 19, 2008
2005 (earliest)
A substantial part of the discussions in this group focused on the confirmed and suspected "Achilles' heels," or tipping elements, in the planetary system. The crucial question was whether those critical elements most susceptible to triggering by human actions can be identified in good time to avoid abrupt—and most likely devastating—global change.
—Hans Joachim Schellnhuber, “Earth System analysis for sustainability,” Environment, October 01, 2005