Abrupt Climate Change

Two high-profile events in 2004 put the issue of "abrupt climate change" squarely in the public eye. The first was a February 2004 Fortune Magazine article that broke the news of a report prepared for the Pentagon on abrupt climate change and its implications for U.S. national security. The Pentagon report describes a scenario in which human-caused global warming leads to a near-term collapse of the ocean's thermohaline circulation, which brings warm surface waters from the tropics to the North Atlantic, warming parts of Western Europe. The authors propose dramatic impacts, including rapid cooling in Europe, greatly diminished rainfall in many important agricultural and urban centers and consequent disruptions in food supply and water supply with enormous geopolitical and security implications.

The second was the May 2004 release of The Day After Tomorrow a 20th Century Fox blockbuster disaster movie with a similar premise. With a dashing paleoclimatologist as the action hero, The Day After Tomorrow depicts a world careening toward an ice age over a few weeks' time. Here too, the culprit is the warming-induced shutdown of the thermohaline circulation.

The authors of the Pentagon report and the producers of The Day After Tomorrow caution readers and viewers against treating these extreme scenarios as serious possibilities. The Pentagon report intentionally considers the worst possible scenario, one that stretches the boundary of scientific plausibility. The Day After Tomorrow leaps beyond that boundary to unleash a collection of climate catastrophes intended to thrill audiences and showcase the latest special effects. Yet underlying even these extreme scenarios are the sober facts of human-caused global warming and the real opportunities to minimize climate change by reducing emissions of heat-trapping gases.

UCS views the publicity generated by these events as an opportunity to help the public and decision makers better understand what we know about the causes, consequences and solutions to climate change. Toward that end, we provide the following answers to some frequently asked questions.

Can what happens in The Day After Tomorrow happen in real life?

No. The dramatic, virtually instantaneous and widespread cooling envisioned in the film is fiction. But like all good science fiction, the film is premised on several important scientific facts. We know with great certainty that Earth is already warming, largely because as we burn fossil fuels and clear forests we are releasing carbon dioxide and other heat-trapping gases in the atmosphere. This warming is expected to continue in the coming decades, accompanied by changes in rainfall patterns and rising sea levels. The possibility of an abrupt shift in the climate system is only one feature of a changing climate that is expected to become more erratic, with extreme weather events like droughts, torrential rainfall, and extreme heat becoming more common. We can slow down global warming and reduce the likelihood of future abrupt climate changes by reducing our emissions of heat-trapping gases.

What is "abrupt climate change?"

The term "abrupt climate change" describes changes in climate that occur over the span of years to decades, compared to the human-caused changes in climate that are occurring over the time span of decades to centuries. From ice cores, ocean sediments, tree rings, and other records of Earth's past climate, scientists have found that changes in climate have occurred quickly in the past—over the course of a decade. An example of an abrupt climate change event is the Younger Dryas (~12,000 years ago), a period of abrupt cooling that interrupted a general warming trend as Earth emerged from the last Ice Age. During the Younger Dryas period, average summertime temperatures in New England cooled by about 5-7°F (3-4°C). This and other abrupt events have been linked to changes in an ocean circulation pattern known as thermohaline circulation.

How might abrupt climate change affect people?

While the scenarios depicted in the Day After Tomorrow and the Pentagon report are extreme, changes in climate, including possible abrupt climate changes, will have serious consequences for people's lives and livelihoods. As Earth warms, higher temperatures and more common extreme heat conditions will affect human health, energy demand, water supply and demand, and agriculture. Rising sea levels will impact coastal communities as flooding happens more often and damage from coastal storms becomes more severe. Some regions will become much drier, while others will become much wetter, affecting agriculture, water supply, and the spread of diseases. Many of these impacts will be most severe in developing countries, where scarce resources and limited technological capacity will limit options for coping with the consequences of climate change.

Can we avoid abrupt climate change?

Yes. While abrupt climate change is not a certainty, human-caused climate change makes abrupt events more likely. What is certain is that human-caused climate change is already under way, and is expected to continue over the next century as a result of our emissions of carbon dioxide and other heat-trapping gases to the atmosphere. Levels of carbon dioxide in the atmosphere are higher today than they have been for more than 400,000 years. Earth's surface temperature has increased measurably over the past 100 years, and 10 of the warmest years on record have occurred since 1990. This warming has caused changes in rainfall—some regions have become wetter while others have become drier—and droughts and severe rainfall events have become more common. By making choices now to reduce our emissions of heat-trapping gases, we can slow the rate of global warming and reduce the likelihood of unexpected climate changes.

How can global warming make things cooler?

As we rapidly increase Earth's average temperature, some regions, such as high latitudes, will experience greater warming than others, such as the tropics. As warming alters ocean and atmosphere circulation patterns, some regions could even experience cooling. Much of Western Europe is now warmed by ocean circulation as well as the atmosphere. Heat is transported to the region by a global ocean circulation pattern variably known as thermohaline circulation, the North Atlantic heat pump, or the "Great Ocean Conveyor Belt." This "heat pump" pulls warm salty water northward from the tropics into the North Atlantic, where heat is released, warming air temperatures over Europe.

As Earth warms, melting of ice caps and glaciers, increased precipitation and other inflows of fresh water to the North Atlantic Ocean may weaken or shut down thermohaline circulation. This change in ocean circulation could disrupt the transfer of heat northward from the tropics, resulting in cooling in the North Atlantic region. Regional cooling of as much as 14-29°F (8-16°C) has been seen in the past climate record. However, any regional cooling would be superimposed on the global warming that is already underway. Contrary to the "Day After Tomorrow" dramatization, abrupt climate change will not result in an ice age, because the cooling effects are regional and Earth is currently in an interglacial, or warm, period.

What is thermohaline circulation?

Thermohaline circulation is a global ocean circulation pattern that distributes water and heat both vertically, through the water column, and horizontally across the globe. As cold, salty water sinks at high latitudes, it pulls warmer water from lower latitudes to replace it. Water that sinks in the North Atlantic flows down to the southern hemisphere, skirts the Antarctic continent, where it is joined by more sinking water, and then crosses south of the Indian Ocean to enter the Pacific Ocean basin. There, the cold deep water rises to the surface, where heat from the tropical sun warms the water at the ocean's surface and drives evaporation, leaving behind saltier water. This warm, salty water flows northward to join the Gulf Stream, traveling up the Eastern coast of the United States and across the Atlantic Ocean into the North Atlantic region. There, heat is released to the atmosphere, warming parts of Western Europe. Once this warm, salty water reaches the North Atlantic and releases its heat, it again becomes very cold and dense, and sinks to the deep ocean.

IPCC figure as corrected by Woods Hole Oceanographic Institution

What is the difference between thermohaline circulation and the Gulf Stream?

Media coverage has tended to cite a shutdown of the Gulf Stream as the mechanism for abrupt climate change, but this is not fully accurate. The Gulf Stream is a strong ocean current that flows north along the coast of the U.S. then crosses the North Atlantic to Western Europe, forming part of a larger clockwise circulation of surface waters in the North Atlantic Ocean. The Gulf Stream is part of the global thermohaline circulation, but it is also driven by winds (resulting form uneven heating of Earth's surface)contacting the ocean surface. Both the winds and the resulting currents are greatly influenced by Earth's rotation. If the deep-ocean thermohaline circulation were shut off, these winds would still drive the Gulf Stream and the clockwise currents in the Atlantic. However, the influx of warm, salty water from the tropics would decrease and the Gulf Stream might become fresher and cooler, and might not extend as far north.

How might thermohaline circulation be turned off?

Thermohaline circulation is driven by the sinking of cold, salty water at high latitudes. Fresh water flowing into the North Atlantic Ocean from rainfall or the melting of ice and permafrost can make the ocean water less salty, and therefore less dense. If it becomes "light" enough, it will not sink any more, possibly slowing or shutting down global thermohaline circulation. Indeed, during some of the abrupt events in Earth's past climate, scientists find evidence of large catastrophic flows of fresh water into the North Atlantic from the melting of glaciers and ice caps, and due to flooding from glacier-dammed lakes. Without the large-scale sinking of salty water in the North Atlantic the influx of warm water to replace it from the tropics would not occur, effectively switching off the thermohaline circulation.

Past changes in thermohaline circulation have occurred during periods of relatively rapid climate change, such as transitions in and out of glaciations. Similarly, the rapid warming we are currently experiencing could trigger an abrupt thermohaline shutdown and subsequent regional cooling. While a shutdown of thermohaline circulation is unlikely to occur in the next century, scientists have recently found that freshwater inputs have already caused measurable "freshening" of North Atlantic surface waters over the past 40 years. Human activities may be driving the climate system toward a threshold and thus increasing the chance of abrupt climate changes occurring.

Will abrupt climate change cause another Ice Age?

No. Human-caused climate change, including possible abrupt climate changes, will not lead to another Ice Age. Earth is warming, and will continue to warm as a result of our emissions of heat-trapping gases, even if an abrupt change in climate occurred. The cooling that might take place if thermohaline circulation switched off would only occur in regions that are currently warmed by the current.