Common Misconceptions about Abrupt Climate Change
June 1, 2004 Several decades of scientific research have yielded significant advances in understanding the ocean's role in regulating Earth's climate. Recently, increased media coverage of climate science has also highlighted some common misunderstandings about abrupt climate change, its underlying mechanisms, and possible consequences for society. This summary covers some of the major points about abrupt climate change that are often misunderstood. We hope this digest provides a better understanding of the state of scientific knowledge so far.
» Is the planet warming?
» Have humans contributed to the warming?
» What is the ocean's role in climate?
» How can global warming and sudden cooling happen at the same time?
» What is the North Atlantic heat pump?
» Is this North Atlantic heat pump constant?
» Some reports talk about a "shut down" of the Gulf Stream. What does this mean?
» Can global warming cause an Ice Age?
» What's the difference between an "Ice Age" and "The Little Ice Age"?
» What about Earth's hydrologic cycle?
» Will global warming affect the hydrologic cycle?
» What happens if the hydrologic cycle accelerates?
» When will regional cooling happen?
» If cooling does happen, how cold will it get?
» How long will it stay cold?
» Is there anything we can do about it?
Q. Is the planet warming?
A. Yes. Since records began around 1860, globally-averaged surface temperatures have been rising (see figure "Variations of the Earth's Surface Temperature"). Eleven of the warmest years on record have occurred since 1990, and the five warmest of all have occurred in the last seven years (in descending order: 2002, 1998, 2003, 2001, 1997). Because of these recent extremes, the pace at which average global temperatures have been rising, which amounted to about +0.6°C over the past century, accelerated in the past two decades to an equivalent rate of +1.0°C per century.
|Variations of the Earth's Surface Temperature|
|(From the Intergovernmental Panel on Climate Change)|
Q. Have humans contributed to the warming?
A. Yes, but there is debate over how much. Natural variability - such as that arising from changes in the sun's energy input to Earth, volcanic activity, and regional climate phenomena like El Niño-Southern Oscillation (ENSO) - does play a role in adjusting the global thermometer. But the observed temperature record cannot be wholly accounted for by natural causes. As the American Geophysical Union recently concluded: "It is scientifically inconceivable that - after changing forest into cities, putting dust and soot into the atmosphere, putting millions of acres of desert into irrigated agriculture, and putting greenhouse gases into the atmosphere - humans have not altered the natural course of the climate system." Greenhouse gases such as carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), ozone (O3) and chlorofluorocarbons (CFC) are being added to the atmosphere largely as a result of burning fossil fuels, tropical deforestation and other human activities. These gases trap energy that would normally be radiated into space, and raise Earth's surface temperatures.
Q. What is the ocean's role in climate?
A. The ocean stores heat, freshwater, salt and carbon dioxide, and transports these components around the planet's surface. Because seawater can hold heat much more efficiently than air, the ocean stores about a thousand times more heat than the atmosphere. But the atmosphere moves heat around much more quickly. The result is that the ocean and atmosphere each transport approximately equal amounts of heat. Certain parts of the planet - such as northern Europe - are warmed especially by ocean currents. The temperate climate of the British Isles, for example, is made possible by warm ocean currents transferring heat to the air.
Q. How can global warming and sudden cooling happen at the same time?
A. Confusion arises because a cooling can be a regional event, superimposed on top of continuously warming earth. Global warming is driven by the increased capture of solar energy due to the increasing concentration of greenhouse gases (such as carbon dioxide and methane) in the atmosphere, caused mainly by human activities. The warming has global consequences. The energy gained from higher greenhouse gas concentrations is distributed around the globe and affects many systems - warming the atmosphere, warming the oceans, increasing evaporation in some regions and precipitation in others, and melting glaciers.
Complications arise when you consider how heat and water are moved around the planet. Warming is causing more water to evaporate from the tropics, more rainfall in subpolar and polar regions, and more ice to melt at high latitudes. As a result, fresh water is being lost from the tropics and added to the ocean at higher latitudes. In the North Atlantic Ocean, the additional fresh water can change ocean circulation patterns, disrupting or redirecting currents that now carry warm water to the north. Redirecting or slowing this "Atlantic heat pump" would mean colder winters in the northeast U.S. and Western Europe. But the heat gained from higher greenhouse gas concentrations is still in the climate system, just elsewhere. The result: a warmer earth, a colder North Atlantic.
|Great Ocean Conveyor Belt
A.The North Atlantic heat pump refers to the fact that the Atlantic Ocean transports heat much farther north than, for example, the Pacific Ocean. This is because a part of its circulation is unique. Ocean currents are mainly driven by two forces: winds and ocean density differences. The portion of ocean flow that is driven by density is called the thermohaline circulation (temperature and salinity together determine the density of ocean water). Global thermohaline circulation is sometimes described as a great ocean conveyor belt (see "Great Ocean Conveyor Belt") - with warm, less dense waters flowing in one direction at the sea surface and cold, dense waters flowing in the opposite direction in the deep ocean.
The critical points of this "conveyor belt" are where surface waters sink into the deep ocean. This happens only in a few places - along the Antarctic shelf and at two sites in the northern North Atlantic (the Labrador Sea and the Nordic Seas). In these two North Atlantic sites, as the ocean loses its heat to the atmosphere, the surface waters become so cold - and so dense - they sink downward into the deep ocean and then flow downhill over the seafloor toward the equator. This sinking and southward flow help drive the ocean conveyor. The dense waters that are exiting to the south in the deep ocean have to be replaced. This draws warm, surface currents farther north than they would normally flow and pumps additional heat to high northern latitudes.
Q. Is this North Atlantic heat pump constant?
A. No. If conditions change in the North Atlantic Ocean such that surface waters can no longer become dense enough to sink, then the "conveyor belt" would slow or possibly stop altogether. The most likely agent of change is extra freshwater added to the ocean's sinking sites. If too much freshwater is added - from melting ice and/or increased precipitation - then no matter how cold the surface waters become, they cannot become dense enough to sink. They may turn to ice, but still will not sink.
Although large changes in the circulation of the North Atlantic have not been observed in the last century, ice cores and deep sea sediments provide abundant evidence that the circulation has changed in the geologic past. The best records of reduced Atlantic circulation come from long ago when conditions on earth were quite different. The most prominent event occurred about 12,000 years ago, when the Atlantic heat pump ceased for a period of about 1000 years. Scientists have speculated that changes in North Atlantic circulation may have contributed to a widespread cooling from 1300 AD to 1800 AD called The Little Ice Age (see below).
Q. Some reports talk about a "shut down" of the Gulf Stream. What does this mean?
A. Under no conditions will the Gulf Stream shut off entirely! This strong ocean current is driven by winds as well as ocean density differences. It is the latter portion of the flow -the thermohaline circulation-that brings ocean heat to the high northern latitudes and could be affected by salinity changes that are now taking place in the North Atlantic. The winds will continue to blow over the ocean and the Gulf Stream will continue to flow even if the thermohaline circulation slows or shuts down. Its flow may be reduced, or its route slightly redirected, but it will continue to flow.
Q. Can global warming cause an Ice Age?
A. No. Properly used, the term "Ice Age" refers to a major glacial epoch, in which glaciers cover large portions of continents. The last Ice Age ended about 12,000 years ago, and we have been in an interglacial period since then. No one is predicting that global warming will cause an Ice Age. That is far too extreme a term to describe the regional cooling effects of a slowing (or shutdown) of the North Atlantic heat pump (thermohaline circulation). Slowing of the thermohaline circulation may result in a cooling from eastern North America to Europe, but we will still be in an interglacial period. Unfortunately, "Ice Age" is a great sound bite and too often ends up in headlines, movies and magazine covers.
Several conditions now are far different than those during the last ice advance. The earth receives a different pattern of solar energy now because the shape of the earth's orbit is not the same as it was 20,000 years ago. The tilt of the earth's axis toward the sun and the position of northern hemisphere summer on the orbit are both different now than during the last glacial advance. There is also significantly more CO2 in the atmosphere now than during the Ice Age and atmospheric carbon dioxide continues to rise, making the earth's average temperature much warmer. These conditions combine to make it unlikely for near future changes in ocean circulation to cause the large scale cooling seen during an Ice Age.
Q. What's the difference between an "Ice Age" and "The Little Ice Age"?
A. The "Little Ice Age" refers to a historical period of colder climate that occurred from about 1300 AD to about 1800 AD, well within the present interglacial period. During this Little Ice Age, widespread cooling was observed throughout the North Atlantic region, winters were more severe in Europe and eastern North America and mountain glaciers advanced throughout Europe. The changes in climate at this time caused much hardship and famine. The cold winters associated with The Little Ice Age drove Viking settlements out of Greenland and North America and affected historical events like the American Revolution (remember Washington's troops attacking the Hessians at Trenton, in 1775 and the cold winter endured by his troops at Valley Forge in 1777-78).
The causes of the Little Ice Age are still unclear, but may have been triggered by changes in the amount of solar energy received by the earth from the Sun. The coldest interval of The Little Ice Age occurred during a period of reduced solar activity call the Maunder Minimum, when the Sun was observed to have fewer sunspots. Climate models suggest that changes in the Sun's energy output may have caused a small cooling at that time, but it is still unclear how these small changes in solar activity may have triggered such a widespread cooling.
Q. What about Earth's hydrologic cycle?
A. The ocean contains 97% of the fresh water on Earth, experiences 86% of evaporation, and directly receives 78% of precipitation. It is, therefore, a key element in the planetary hydrologic cycle, which is itself fundamental to the climate system. Together, the atmosphere and ocean maintain a global balance in the distribution of fresh water. Fresh water is evaporated mainly from the tropical and subtropical ocean. The atmosphere then transports that water vapor to other locations (especially to the high latitudes) where it falls as precipitation. The ocean completes the cycle by transporting that fresh water back to the low latitudes.
Q. Will global warming affect the hydrologic cycle?
A. Yes. Evaporation rates should increase as the surface ocean heats up. Also, because water vapor pressure rises exponentially as temperature increases, a warmer atmosphere will hold more water vapor. Since water vapor is itself a potent greenhouse gas (much more so than carbon dioxide), increased water vapor concentrations will trap additional heat and cause the surface temperatures to rise even faster. So the expectation is that the hydrologic cycle should accelerate as a result of global warming, and that this will in turn accelerate global warming.
But the climate system is not quite that simple. The atmosphere is divided into distinct layers, and while the lower layers have become warmer and wetter, the upper layers have cooled slightly. Researchers are not in agreement about whether forces associated with greenhouse warming will be enough to warm those upper layers. As a result of this uncertainty, climate models give widely different predictions of the effects of greenhouse warming.
Q. What happens if the hydrologic cycle accelerates?
A.In polar regions, surface and deep waters have been gaining freshwater for the last forty years. In the tropics, surface waters have been losing freshwater because of increased evaporation during the same period. These two changes point toward an enhanced hydrological cycle: greater evaporation in the tropics as the earth and its oceans warm, and greater precipitation at higher latitudes in the regions where the atmosphere gives up its water vapor. This provides another source of freshwater - in addition to melting ice - that can affect the North Atlantic heat pump.
Q. When will regional cooling happen?
A. There is no certainty that ocean circulation will change or that a regional cooling will occur. Computer models of future climate on earth predict a variety of outcomes, ranging from no change in the Atlantic circulation to a near complete shutdown of its thermohaline component in 50 to 75 years. Part of the uncertainty is in the models themselves: the ocean components of climate models tend to be less advanced than the atmospheric components, the ice components even less well developed. Accurate predictions will require significant improvements in all parts of climate models.
Q. If cooling does happen, how cold will it get?
A. Much depends on the timing and pace of changes in ocean circulation. A reduction in the Atlantic heat pump in the next several decades would cause winters to be colder and more severe than today in the regions around the North Atlantic. If changes in the heat pump occur instead a century from now, then the effects would be different. Since greenhouse gases are projected to rise over the next 100 years, the global average temperature will also continue to rise. Cooling in the Atlantic region might actually mitigate that warming such that winters then would not be colder than today. As greenhouse gas concentrations continue to rise, the planet's surface will become increasingly warm on average. The distribution of that extra heat will largely determine how the planetary climate system responds to global warming.
Q. How long will it stay cold?
A. The duration of any change in ocean circulation depends on the magnitude and duration of the disruption. For instance, a rapid, decades-long increase in freshwater may perturb the circulation for decades and more. Compared with the atmosphere, the ocean responds slowly to changes and ocean perturbations tend to persist longer. The climate system is complex. With present knowledge, it is difficult to say exactly how long any changes might last.
Q. Is there anything we can do about it?
A. The major stress on the climate system now is rapidly rising greenhouse gas concentrations in the atmosphere, and a significant portion of that is from human activity. Take steps to reduce the rate of increase (and eventually decrease) the greenhouse gas concentration, and future changes in climate may be reduced or delayed.
Compiled by William Curry, Director, Ocean and Climate Change Institute.