Home > Triggers of Abrubt Climate Change

Abrupt climate change can occur when the Earth system gets pushed across a threshold, whether by some sudden event like a massive volcanic eruption or by the accumulation of more gradual forces, or "forcings" on the system. Much as the slowly increasing pressure of a finger eventually flips a switch and abruptly turns on a light, or as a passenger’s leaning more and more over the side of a canoe will at some point cause the craft to suddenly capsize, the slow effects of drifting continents or wobbling orbits or changing atmospheric composition may "switch" the climate to a new state. The more rapid the forcing, the more likely it is that it will "flip a switch," causing an abrupt change on the time scale of human economies or global ecosystems. Such forcings may occur through perturbations in key components of the Earth system such as:

• Oceans. Because water has enormous heat capacity, oceans typically store 10-100 times more heat than equivalent land surfaces. Thus the oceans exert a profound influence on climate through their ability to transport heat from one location to another. Changes in ocean circulation, and especially the thermohaline circulation in the North Atlantic (see Figure 2) have been implicated in abrupt climate change of the past such as the Younger Dryas. Floods of glacial meltwaters that would have freshened the North Atlantic and reduced the ability of its waters to sink, immediately preceeded the coolings of the Younger Dryas and another short cold event 8,200 years ago, suggesting causation.
• Cryosphere. The portion of the Earth covered in ice and snow, the cryosphere, greatly affects temperature. When sea ice forms, it increases the planetary reflective capacity, thereby enhancing cooling. Sea ice also insulates the atmosphere from the relatively warm ocean, allowing winter air temperatures to steeply decline and reduce the supply of moisture to the atmosphere. Glaciers and snow cover on land can also provide abrupt-change mechanisms. The water frozen in a glacier can melt if warmed sufficiently, leading to possibly rapid discharge, with consequent effects on sea level and atmospheric flow patterns. Meanwhile, snow-covered lands of all types maintain cold conditions because of their high reflectivity and because surface temperatures cannot rise above freezing until the snow melts.
• Atmosphere. The atmosphere is involved in virtually every physical process of potential importance to abrupt climate change, providing a means of rapidly propagating the influence of any climate forcing from one part of the globe to another. Atmospheric temperature, composition, humidity, cloudiness, and wind determine the Earth’s energy fluxes. Wind fields help dictate the ocean's surface circulation and upwelling patterns. Atmospheric-moisture transport - most prominently, through precipitation - helps govern the freshwater balance, overall water circulation, and the dynamics of glaciers.
• Land surface. The reflective capacity of the land can change greatly, with fresh snow or ice sheets reflecting more than 90% of the sunlight striking them while dense forests absorb more than 90%. Changes in surface characteristics can also affect solar heating, cloud formation, rainfall, and surface-water flow to the oceans, thus feeding back strongly on climate.
• External factors. Phenomena external to the climate system can also be agents of abrupt climate change. For example, the orbital parameters of the Earth vary over time, affecting the planet's distribution of solar energy. Fluctuations in solar output - prompted by sunspot activity or the effects of solar wind - may cause major climate fluctuations. The drying of the Sahara in the Holocene is linked to variations in the Earth's orbit around the sun.