Abrupt climate change
Abrupt climate change refers to significant and rapid shifts in the Earth's climate system, occurring over a relatively short time frame, often within decades or even years. Historical evidence from ice cores, tree rings, and ocean sediments reveals that past climate transitions, such as the Younger Dryas, have manifested dramatic changes in temperature and conditions, often linked to alterations in ocean salinity or atmospheric components. The concept emerged from early theories about gradual climate change, evolving as scientists gathered data indicating that abrupt changes can happen when climate systems cross certain thresholds, leading to feedback loops that exacerbate the initial changes.
Recent concerns focus on the implications of anthropogenic global warming due to increasing greenhouse gas emissions, primarily from fossil fuel combustion. This warming could trigger abrupt climate shifts, such as rapid ice melt in polar regions, which may result in rising sea levels and altered weather patterns. The potential for severe weather events and environmental disruptions raises alarms about the resilience of human societies in the face of such changes. As scientists warn of these risks, the possibility of entering a new geological age, termed the Anthropocene, highlights humanity's role in shaping the climate and environment. Understanding these dynamics is crucial, as historical examples, like the collapse of the Maya civilization, illustrate the profound impacts abrupt climate change can have on societies.
Abrupt climate change
Abrupt climate change entails drastic warming or cooling, regionally or globally, that takes place within a few years or decades and that persists for at least a few decades. Such a transformation would have a lasting effect on human institutions and infrastructure.
Background
In 1840, Louis Agassiz published his theory that the Earth had passed through an ice age. As a result of Agassiz’s work, the corollary idea that the globe’s climate could change dramatically for extended periods entered scientific thinking. Scientists assumed, however, that such change occurred very slowly and smoothly over many millennia. When, in 1922, meteorologist C. E. P. Brooks first proposed that climate can change swiftly, he was largely ignored. During the early 1990s, however, a steady accumulation of data from four main sources strongly supported Brooks’s hypothesis. These data suggested that in past epochs the atmosphere went from warm to cool or from cool to warm within decades, perhaps even within a few years.
Proxy Evidence for Abrupt Change
The theory of abrupt change rests on proxy data from ice cores taken from the ice sheets covering Greenland and Antarctica, as well as from tree rings, sediments in oceans and lakes, and coral. In each of these proxies, layers of material are laid down annually and vary in thickness in accordance with annual atmospheric conditions. In tree rings, for instance, wet years foster greater growth in trees, which is reflected in wider rings than those produced during dry years. In addition to such evidence, gases in bubbles trapped in ice reveal the relative abundance of elements at the time they were trapped, which in turn provides clues to atmospheric temperatures at that time.
Taken together, proxy evidence demonstrates not only sudden climate change in past epochs but also frequent change. The most recent of four ice ages lasted from 120,000 to 14,500 years ago. Even during that frigid period, there were twenty-five periods of abrupt warming, called Dansgaard-Oeschger events, and six extended plunges in temperature, called Heinrich events; in all of them, change took place within decades.
The most studied example of abrupt change is the period known as the Younger Dryas, which began about 12,800 years ago. As the Northern Hemisphere was warming from the ice age, it suddenly relapsed into ice-age temperatures and stayed cold until 11,500 years ago, when temperatures over Greenland rose by 10° Celsius within a decade.
Mechanisms for Abrupt Change
The United States National Research Council defines abrupt climate change as occurring when the climate system is forced to cross some threshold, triggering a transition to a new state at a rate determined by the climate system itself and faster than the cause.
Mechanisms for such change are poorly understood. It appears that some physical process forces an aspect of the climate system to pass a tipping point—for instance, in the albedo, average cloud cover, or salinity of ocean water. After the tipping point, positive feedback in the system accelerates the warming or cooling trend.
In the case of the Younger Dryas, scientists know that the water of the North Atlantic suddenly became less salty, which slowed or altered the course of the thermohaline cycle. The warm waters of the Gulf Stream no longer flowed north of Iceland and back down along the European coast, causing the continent to relapse to ice-age temperatures. The freshening probably resulted from a sudden outflow of water from a freshwater inland sea, Lake Agassiz, in north-central North America. The physical event that led to this forcing is a point of controversy. Scientists have proposed the breaking of an ice dam after gradual warming or possibly a meteor impact. Forcings for other abrupt changes in past climates include alterations in the salinity of the tropical Atlantic Ocean, evaporation and cloud cover in the South Pacific Ocean, melting of methane clathrates (frozen methane in the ocean beds), and the periodic warming of the South Pacific known as the El Niño-Southern Oscillation (ENSO).
Anthropogenic Global Warming
Scientists worry that increasing levels of greenhouse gases (GHGs) in the atmosphere, much of them released by the burning of fossil fuels, have trapped radiant energy from the Sun in the atmosphere and increased average global temperatures in both the atmosphere and the oceans. This greenhouse effect could lead to abrupt climate change in several ways.
The vast ice sheets in the Arctic and Antarctica have the highest regional albedo on Earth, but they are shrinking rapidly, especially in the Arctic. There, the ice rests primarily on water, which is darker than ice and absorbs more heat. As the ice disappears, there is more exposed ocean surface to absorb solar energy, and the warmed water in turn helps melt the ice faster, creating a positive feedback loop. This melted ice will not affect ocean levels or salinity, but if ice sheets melt off the land of Antarctica or Greenland, ocean levels could rise by dozens of meters within a century, lowering ocean salinity enough to stall the thermohaline cycle, which could cool Europe rapidly and drastically even while the rest of the world warmed. Should ocean water heat up too much, clathrates could melt and send billions of metric tons of methane into the atmosphere, accelerating global warming further. The augmented thermal energy in the atmosphere is likely to redistribute wind and rainfall patterns, plunging some regions into drought while making others wetter; catastrophic storms, such as hurricanes and tornados, could become more frequent and severe.
Context
Some scientists argue than the Earth is entering a new geological age, the Anthropocene, because humanity itself now takes part in shaping Earth’s overall surface conditions, climate in particular. Particulate pollution (especially soot), waste heat, release of Greenhouse gases, water consumption, and alteration of soil and plant cover affect not only the land, water bodies, and atmosphere but also modern civilization. If human effects on the environment trigger abrupt climate change, the onset of icy conditions in the Northern Hemisphere, droughts, superstorms, or rising sea level—all of which are possible according to computer models of climate change—would require radical, swift, and comprehensive measures to adapt or relocate much of Earth’s human population. Not only would that be an expensive undertaking, but it would also mark a shift in the course of human history as profound as the Industrial Revolution. In terms of disruptive effects on institutions and infrastructures, previous episodes where abrupt climate change may have occurred can illustrate its harmful impacts on human populations. The Maya were a civilization that, between 600 and 800 AD, had achieved significant advancements in astronomy and mathematics. By 950 AD, this society had largely collapsed. Scientists have hypothesized a climatic change involving drought may have been a significant contributor to the demise of the Maya.
Key Concepts
albedo: the fraction of radiation reflected by a surfacefeedback: a process in which any change accelerates further changes of the same type (positive feedback) or counteracts itself (negative feedback)greenhouse gases (GHGs): atmospheric gases, such as carbon dioxide, water vapor, and methane, that trap heat radiation from Earth’s surface by absorbing it and reemitting itproxy: remnant physical evidence from which past climatic conditions can be inferredthermohaline cycle: the “great conveyor belt” of ocean currents powered by density gradients created by heat and relative salt contenttipping point: the point at which the transition from one state in a system to another becomes inevitable
Bibliography
"Abrupt Climate Change." Woods Hole Oceanographic Institution, 2017, http://www.whoi.edu/main/topic/abrupt-climate-change. Accessed 31 Jan. 2017.
Cox, John D. Climate Crash: Abrupt Climate Change and What It Means for Our Future. Washington, D.C.: Joseph Henry Press, 2005.
Cronin, Thomas M. and Ingrid H.H. Zabel. "What is Abrupt Climate Change?" Paleontological Research Institution, . 20 Oct. 2021, www.priweb.org/blog-post/what-is-abrupt-climate-change. Accessed 16 Jan. 2021.
"Drought and the Ancient Maya Civilization." National Oceanic and Atmospheric Administration, Oct. 2021, www.ncei.noaa.gov/products/paleoclimatology/paleo-perspectives/abrupt-climate-change. Accessed 26 Jan. 2023.
Flannery, Tim. We Are the Weather Makers: The Story of Global Warming. Rev. ed. London: Penguin, 2007.
Fountain, Henry. “Failure to Slow Warming Will Set Off Climate ‘Tipping Points,’ Scientists Say.” The New York Times, 8 Sept. 2022, www.nytimes.com/2022/09/08/climate/global-warming-climate-tipping-point.html. Accessed 16 Jan. 2023.
Lynas, Mark. Six Degrees: Our Future on a Hotter Planet. Washington, D.C.: National Geographic, 2008.
Pearce, Fred. With Speed and Violence: Why Scientists Fear Tipping Points in Climate Change. Boston: Beacon Press, 2007.