Introduction to Abrupt Changes in the Earth’s Climate
Large, abrupt climate changes have affected hemispheric to global regions repeatedly, as shown by numerous paleoclimate records (Broecker, 1995, 1997). Changes of up to 16°C and a factor of 2 in precipitation have occurred in some places in periods as short as decades to years (Alley and Clark, 1999; Lang et al., 1999). However, before the 1990s, the dominant view of past climate change emphasized the slow, gradual swings of the ice ages tied to features of the earth’s orbit over tens of millennia or the 100-million-year changes occurring with continental drift. But unequivocal geologic evidence pieced together over the last few decades shows that climate can change abruptly, and this has forced a reexamination of climate instability and feedback processes (NRC, 1998). Just as occasional floods punctuate the peace of river towns and occasional earthquakes shake usually quiet regions near active faults, abrupt changes punctuate the sweep of climate history.
The climate system in the past has made large jumps between typical patterns of behavior, as in the mechanical analogy presented in Box 1.1. Especially large and abrupt climate changes have occurred repeatedly over the last 100,000 years during the slide into and climb out of the most recent global ice age. Those changes persisted into the current warm period and probably occurred during previous ice ages (Sarnthein et al., 1994; Broecker, 1995, 1997; Alley and Clark, 1999; Stocker, 2000). Our ability to understand the potential for future abrupt changes in climate is limited by our
lack of understanding of the processes that control them. For example, mechanisms proposed to explain abrupt climate shifts do not fully describe the patterns of variability seen in either the paleoclimate or the historical records.
Long-term geological records show that in the past there were different stable states of the climate system from those of today. Differences in these climate states involved the coupled atmosphere, ocean, ice, and biological systems. Some of these old states, such as the proposed “snowball earth” (when most or all of the planet was frozen) (Harland, 1964; Caldeira and Kasting, 1992; Kirschvink, 1992; Hoffman et al., 1998), occurred long ago when geological and astronomic conditions were substantially different from today. However, others, such as the warm-polar “hothouse” pattern, were reached relatively recently, when geological conditions were similar to those of our modern earth (Barron, 1987). Despite the recognition that extreme shifts in the climate system can occur, little information is available on whether transitions between climate states are possible under modern or near-future conditions and whether such transitions would be abrupt.
Understanding how and why climate might change abruptly has important implications. The just-completed US National Assessment (National Assessment Synthesis Team, 2000) emphasizes the possible effects of gradual climate change on societies and ecosystems, and it concludes that effects will probably be larger in the case of faster changes that leave less time for adaptation or that give less warning that might make mitigation possible. Thus, when considering the possible causes and impacts of abrupt climate change, numerous important questions arise, including:
What caused the large, widespread, abrupt climate changes of the past? Could they recur? Might human activities affect the possibility of recurrence?
To what extent is abruptness a fundamental characteristic of regional and global climate changes? Might future regional changes be abrupt?
Could global or regional climates shift into modes different from those observed recently, such as the warm-period modes recorded in geological archives? Might such a shift be abrupt?
How do societies and ecosystems maintain resilience and adaptability? How might these be enhanced in the face of severe tests posed by abrupt climate changes?
Answers to these and related questions are best guided by a focused re-
Box 1.1 Analogy of Abrupt Climate Change
Abrupt climate change is not unusual, and in fact many simple physical systems exhibit abrupt changes. Here, we illustrate a few basic points using a mechanical analogy.
Imagine a balance consisting of a curved track poised on a fulcrum, as shown above. The track is curved so that there are two “cups” where a ball may rest. A ball is placed on the track and is free to roll until it reaches its point of rest. This system has three equilibria denoted (a), (b) and (c) in the top row of the figure. The middle equilibrium (b) is unstable: if the ball is displaced ever so slightly to one side or another, the displacement will
accelerate until the system is in a state far from its original position. In contrast, if the ball in state (a) or (c) is displaced, the balance will merely rock a bit back and forth, and the ball will roll slightly within its cup until friction restores it to its original equilibrium.
Suppose we push down gently on the right arm of the balance causing a slight tilt, as shown in (a1). When we let go, the ball will rattle around for a bit as the balance tilts back and forth. Once things settle down, the system will return to its original state of rest, with the ball in the left cup. As noted above, this position is stable in the face of small perturbations.
If instead we push the right arm somewhat farther down, as shown in (a2), the ball will eventually roll over the fulcrum and slide down into the right cup. This is an example of a system passing a threshold. When the pressure is relieved, the system does not return to its original state. A temporary influence can have permanent effects; this is what is known as hysteresis .
This device illustrates other kinds of behavior that are common in the climate system. The equilibria illustrated in the top row are steady, in that the system sits still without moving. But suppose that the ball in state (a) or (c) is given a gentle push. If the friction is low, the ball in either case will rattle around for a long time, but will remain in its original cup. This illustrates the notion of an unsteady regime—the “left cup” regime and the “right cup” regime. A strong enough push at the right time could cause a transition between one regime and the other.
An unusual application of force could cause unexpected behavior. Hit it hard enough, and the device might do something different from anything seen before. For example, the arm of the balance might bang against the table, and the ball could bounce out of the cup and roll away.
Now imagine that you have never seen the device and that it is hidden in a box in a dark room. You have no knowledge of the hand that occasionally sets things in motion, and you are trying to figure out the system’s behavior on the basis of some old 78-rpm recordings of the muffled sounds made by the device. Plus, the recordings are badly scratched, so some of what was recorded is lost or garbled beyond recognition. If you can imagine this, you have some appreciation of the difficulties of paleoclimate research and of predicting the results of abrupt changes in the climate system.
search strategy, and this report seeks to identify the key supporting research needs.
DEFINITION OF ABRUPT CLIMATE CHANGE
What defines a climate change as abrupt? Technically, an abrupt climate change occurs 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. Chaotic processes in the climate system may allow the cause of such an abrupt climate change to be undetectably small.
To use this definition in a policy setting or public discussion requires some additional context, as is explored at length in Chapter 5, because while many scientists measure time on geological scales, most people are concerned with changes and their potential impacts on societal and ecological time scales. From this point of view, an abrupt change is one that takes place so rapidly and unexpectedly that human or natural systems have difficulty adapting to it. Abrupt changes in climate are most likely to be significant, from a human perspective, if they persist over years or longer, are larger than typical climate variability, and affect sub-continental or larger regions. Change in any measure of climate or its variability can be abrupt, including change in the intensity, duration, or frequency of extreme events. For example, single floods, hurricanes, or volcanic eruptions are important for humans and ecosystems, but their effects generally would not be considered abrupt climate changes unless the climate system is pushed over a threshold into a new state; however, a rapid, persistent change in the number or strength of floods or hurricanes might be an abrupt climate change.
The quintessential abrupt climate change was the end of the Younger Dryas interval about 11,500 years ago, when hemispheric to global climate shifted dramatically, in many regions by about one-third to one-half the difference between ice-age and modern conditions, with much of the change occurring over a few years (Alley, 2000). The changes affected many environmental parameters such as temperature and rainfall (Figure 1.2). Weaker, but still of hemispheric extent, was a short cooling spell 8,200 years ago that lasted for about 200 years (Alley et al., 1997). Although more regionally limited, the apparent change in El Niño behavior toward generally warmer and wetter conditions around 1976 (Nitta and Yamada, 1989; Trenberth, 1990; Graham, 1994) could also be considered an abrupt change. Thus, studies of abrupt climate change overlap with studies of ice
ages and other features of deeper time and with studies of decadal-centennial climate modes and variability. In focusing on the tendency of climate to change in fits and starts rather than smoothly, and thus to surprise humans and ecosystems, the study of abrupt climate change is distinct from related branches of climatology.
SCIENTIFIC EVIDENCE, PROCESSES, AND CONSEQUENCES FOR SOCIETY AND ECOSYSTEMS
Abrupt climate change has affected societies. For example, evidence in geologic records suggests that abrupt but persistent droughts caused so-
cial disruption for Mayan culture (Hodell et al., 1995; Gill 2000) and that abrupt climate shifts played a role in the collapse of Mesopotamian civilization (Weiss et al., 1993). Recognizing the potential for abrupt changes in climate has constituted a paradigm shift for the research community, but many questions concerning the processes that cause and mediate abrupt climate change remain, including the following.
What are the patterns of environmental variability associated with abrupt climate change in the tropics and high latitudes?
What is the role of freshwater cycling in abrupt climate change?
Will warmer climates influence the occurrence of abrupt climate change?
Might climate changes occur that are unassociated with a change in external forcing?
What feedback processes are dominant, and what is their role in causing the persistence of abrupt changes including droughts?
Recent research has shown that human activities are affecting climate, but it is often difficult to separate human-induced changes from those occurring naturally (Intergovernmental Panel on Climate Change, 2001b). The question arises whether anthropogenic influences will trigger abrupt climate change. It is not now possible to answer that question, because the processes that cause abrupt climate change are not sufficiently understood.
There is little doubt that the rate, magnitude, and regional extent of abrupt transitions to different climate states could have far-reaching implications for society and ecosystems. Research has shown that, in response to gradual changes in climate, much of the economic capital stock1 and some plants and animals may adjust without major disruption. But rapid changes in climate could have major effects, disrupting ecological or economic systems in a manner that prevents their timely replacement, repair, or adaptation.
Ecological systems are particularly vulnerable to abrupt climate change because they are long-lived and relatively immobile. In addition, these systems often have low adaptive capability. Their vulnerability is increased by human activities that alter ecosystems, reducing species abundance and composition and blocking migration. One reason for the vulnerability of eco-
logical and economic systems to the effects of abrupt climate change is that these systems are peculiar to particular locations and adapted to particular climates. Effects are likely to increase when abrupt climate change causes ecological systems to cross thresholds. For example, climatic conditions change greatly over short distances in some areas, and cause similarly steep gradients in vegetation types, allowing even a small climate change to cause dramatic change in vegetation type in a given locale. Rapid climate change will probably result in the redistribution—and possibly in the extinction—of terrestrial and marine species and have major effects on ecosystems worldwide.
Water resources might be greatly affected by abrupt changes in climate. Changes in water supplies could result in increased demands for water, affect agricultural production, and potentially trigger adverse health effects. Those consequences and the economic effects resulting from them provide a strong motivation for enhancing the understanding of physical processes that cause abrupt climate change.
There is increased effort to understand the probability, rate, and magnitude of abrupt climate change. But there is virtually no research on the economic and ecological impacts of abrupt climate change; most research has concentrated on gradual change. It is important to improve knowledge of climate processes that lead to abrupt climate change, and knowledge of the impacts of such changes.
THE CHARGE TO THE COMMITTEE
Because there was no comprehensive review of the science and potential impacts of abrupt climate change and no overall plan for improving the understanding of abrupt climate change, the US Global Change Research Program asked the National Research Council for assistance. In response, the NRC formed the Committee on Abrupt Climate Change, which was charged to:
describe what is currently known from paleoclimate proxies, historical observations, and numerical modeling about abrupt climate change, including patterns and magnitudes of possible changes, mechanisms, forcing thresholds, and probability of occurrence (at least qualitatively);
identify the critical knowledge gaps concerning the potential for future abrupt climate changes, including those aspects of change that are of potential importance to society and economies; and
outline a research strategy to fill the critical knowledge gaps.
A committee of 11 members was appointed from the paleoclimate, climate modeling, and observational climate communities, including those investigating the cryosphere, atmosphere, ocean, and terrestrial systems. To help to focus priorities in abrupt climate change research, representatives of the economic and social science research communities were also part of the committee. Oversight for the committee was provided jointly by the Ocean Studies Board, the Polar Research Board, and the Board on Atmospheric Science and Climate.
To conduct its work, the committee met three times and held two workshops to allow for broad community participation. The first workshop, at Lamont-Doherty Earth Observatory, brought together members of the physical science community as well as those from the social science community who specialize in the societal impacts of climate change. From this workshop, it was decided that to properly address the ecological and societal impacts of abrupt climate change, an additional two-day workshop concentrating on these issues would be needed. This meeting, held in Washington, D.C., was primarily funded by the Yale/NBER Program on International Environmental Economics and provided essential information used to develop the climate impacts portion of this report.
This report is based on the knowledge and experience of the committee’s members and input gained from the two workshops. This report discusses the evidence for abrupt climate change (Chapter 2), the processes that can cause these changes (Chapter 3), global warming as a potential trigger for rapid climate change (Chapter 4), and the potential economic and ecological impacts of abrupt climate change (Chapter 5). The final chapter of the report presents the committee’s findings and recommendations.