Abrupt Impacts of Climate Change: Anticipating Surprises

Climate is changing, forced out of the range of the last million years by levels of carbon dioxide and other greenhouse gases not seen in Earth’s atmosphere for a very long time. Lacking action by the world’s nations, it is clear that the planet will be warmer, sea level will rise, and patterns of rainfall will change. But the future is also partly uncertain—there is considerable uncertainty about how we will arrive at that different climate. Will the changes be gradual, allowing natural systems and soci- etal infrastructure to adjust in a timely fashion? Or will some of the changes be more abrupt, crossing some threshold or “tipping point” to change so fast that the time be- tween when a problem is recognized and when action is required shrinks to the point where orderly adaptation is not possible? A study of Earth’s climate history suggests the inevitability of “tipping points”— thresholds beyond which major and rapid changes occur when crossed—that lead to abrupt changes in the climate system. The history of climate on the planet—as read in archives such as tree rings, ocean sediments, and ice cores—is punctuated with large changes that occurred rapidly, over the course of decades to as little as a few years. There are many potential tipping points in nature, as described in this report, and many more that we humans create in our own systems. The current rate of carbon emissions is changing the climate system at an accelerating pace, making the chances of crossing tipping points all the more likely. The seminal 2002 National Academy Re- port, Abrupt Climate Changes: Inevitable Surprises (still required reading for anyone with a serious interest in our future climate) was aptly named: surprises are indeed inevi- table. The question is now whether the surprises can be anticipated, and the element of surprise reduced. That issue is addressed in this report. Scientific research has already helped us reduce this uncertainty in two important cases; potential abrupt changes in ocean deep water formation and the release of carbon from frozen soils and ices in the polar regions were once of serious near-term concern are now understood to be less imminent, although still worrisome as slow changes over longer time horizons. In contrast, the potential for abrupt changes in ecosystems, weather and climate extremes, and groundwater supplies critical for agri- culture now seem more likely, severe, and imminent. And the recognition that a gradu- ally changing climate can push both natural systems, as well as human systems, across tipping points has grown over the past decade. This report addresses both abrupt Copyright © National Academy of Sciences. All rights reserved. Abrupt Impacts of Climate Change:  Anticipating Surprises viii PREFACE climate changes in the physical climate system, and abrupt climate impacts that occur in human and natural systems from a steadily changing climate. In addition to a changing climate, multiple other stressors are pushing natural and human systems toward their limits, and thus become more sensitive to small perturba- tions that can trigger large responses. Groundwater aquifers, for example, are being depleted in many parts of the world, including the southeast of the United States. Groundwater is critical for farmers to ride out droughts, and if that safety net reaches an abrupt end, the impact of droughts on the food supply will be even larger. Must abrupt changes always be surprises? Certainly not. As knowledge of the tipping points in natural and human systems improves, an early warning system can be devel- oped. Careful and vigilant monitoring, combined with a constantly improving scientific understanding of the climate system, can help society to anticipate major changes before they occur. But it is also important to carefully and vigilantly catalog the assets at risk—societies cannot protect everything and will need to prioritize, and without an understanding of what could be lost, such as coastal infrastructure to rising seas, for example, intelligent decisions about what to protect first cannot be made. Can all tipping points be foreseen? Probably not. Some will have no precursors, or may be triggered by naturally occurring variability in the climate system. Some will be dif- ficult to detect, clearly visible only after they have been crossed and an abrupt change becomes inevitable. Imagine an early European explorer in North America, paddling a canoe on the swift river. This river happens to be named Niagara, but the paddler does not know that. As the paddler approaches the Falls, the roar of the water goes from faint to alarming, and the paddler desperately tries to make for shore. But the water is too swift, the tipping point has already been crossed, and the canoe—with the paddler—goes over the Falls. This tipping point is certainly hard to anticipate, but is it inevitable? No. The tipping point in this case could have been detected by an early warning system (listening for the roar of a waterfall), but importantly, prudence was re- quired. Sticking closer to shore, in other words taking some prudent precautions, could have saved the paddler. Precaution will help us today as well, as we face a changing climate, if we are prudent enough to exercise it. Key to this is the need to be watching and listening for the early warning signals. I would like to commend the committee for their hard work, stimulating conversa- tions, scientific expertise, and most importantly, willingness to think outside of the box and take a fresh look at this issue. To Richard Alley (he of the 2002 Report), David Archer, Tony Barnosky, Jon Foley, Rong Fu, Marika Holland, Susan Lozier, Annie Schmitt, Larry Smith, George Sugihara, David Thompson, The Honorable Andrew Weaver and Steve Wofsy, I owe great thanks and heaps of praise. This report was an adventure, Copyright © National Academy of Sciences. All rights reserved. Abrupt Impacts of Climate Change:  Anticipating Surprises ix Preface and I could not have asked for better travelling companions. The staff of the National Research Council, those heroes behind the scenes, worked tirelessly to make this study a success. Rob Greenway and Amanda Purcell spent countless hours pulling together text and organizing meetings. Claudia Mengelt contributed valuable discussions and tight editing. And Edward Dunlea simply did it all, keeping me and the rest of the crew on track, ensuring that we could indeed get this done in the time allotted (never enough, it seems) and making sure it all came together. To the committee and staff, my deep, heartfelt thanks; intelligent, hard-working and industrious folks all. In fact, we have a planet full of intelligent, hard-working and industrious folks. Humans are capable of solving whatever problems nature throws at us, or that we create. But first we have to arm ourselves with information and then commit to using that information intelligently and wisely, and that, in a nutshell, is the message of this report.

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