The Earth Institute at Columbia University, by Andrea Thompson and Ker Than

Our planet and its inhabitants – including we humans – are in a precarious position as we celebrate Earth Day, April 22.

While global warming is widely accepted as a reality by scientists and many governments and industrial leaders, progress to curb greenhouse gases and other forms of pollution remains limited. The current economic climate will likely make pollution control efforts more difficult, analysts say.

Recent studies, as well as the 2007 report by the Intergovernmental Panel on Climate Change (IPCC) have pointed to some of the likely effects of uncurbed greenhouse gas emissions: rising global temperatures, rising sea levels, Arctic sea ice melt, the disappear of glaciers, epic floods in some areas and intense drought in others.

These effects are intensified when combined with other forms of pollution the world’s rising population.

Humans will face widespread water shortages. Famine and disease will increase. Earth’s landscape will transform radically, with a quarter of plants and animals at risk of extinction.

While putting specific dates on these traumatic potential events is challenging, this timeline paints the big picture and details Earth’s future based on several recent studies and the longer scientific version of the IPCC report. This timeline is an updated version of one first published by LiveScience in 2007.


Arctic sea ice hits its second lowest summer ice extent on record (the lowest extent was in 2007). A massive chunk of ice breaks away from Greenland’s Petermann Glacier. Several breakups of ice shelves in Antarctica are observed. (NSIDC; Jason Box, Ohio State University; ESA, NSIDC)

Global oil production peaks sometime between 2008 and 2018, according to a model by a Swedish physicist. Others say this turning point, known as “Hubbert’s Peak,” won’t occur until after 2020. Once Hubbert’s Peak is reached, global oil production will begin an irreversible decline, possibly triggering a global recession, food shortages and conflict between nations over dwindling oil supplies. (Doctoral dissertation of Frederik Robelius, University of Uppsala, Sweden; report by Robert Hirsch of the Science Applications International Corporation)

The Bush Administration enacts changes to the Endangered Species Act that affect reviews of government projects.

Polar bears and beluga whales are placed on the Endangered Species List. Gray wolves are delisted in certain areas.


The U.S. Environmental Protection Agency declares carbon dioxide and other heat-trapping gases to be pollutants under the Clean Air Act.

An ice bridge connected to the Wilkins Ice Sheet of Antarctica breaks apart.

Many of the world’s major rivers are found to be losing water. (Aiguo Dai, NCAR, Journal of Climate)


The first phase of the Kyoto Protocol, an international environmental treaty created to limit the production of greenhouse gases, expires. Nations will have to draw up and enact a succesor treaty to further limit emissions, should they choose to do so.


Flash floods will very likely increase across all parts of Europe. (IPCC)

Less rainfall could reduce agriculture yields by up to 50 percent in some parts of the world. (IPCC)

World population will reach 7.6 billion people. (U.S. Census Bureau)


Diarrhea-related diseases will likely increase by up to 5 percent in low-income parts of the world. (IPCC)

Up to 18 percent of the world’s coral reefs will likely be lost as a result of climate change and other environmental stresses. In Asian coastal waters, the coral loss could reach 30 percent. (IPCC)

World population will reach 8.3 billion people. (U.S. Census Bureau)

Warming temperatures will cause temperate glaciers on equatorial mountains in Africa to disappear. (Richard Taylor, University College London, Geophysical Research Letters:)

In developing countries, the urban population will more than double to about 4 billion people, packing more people onto a given city’s land area. The urban populations of developed countries may also increase by as much as 20 percent. (World Bank: The Dynamics of Global Urban Expansion)

The Arctic Sea could be ice-free in the summer. (James Overland, NOAA, Muyin Wang, University of Washington, Geophysical Research Letters)


The Group of Eight top industrial nations has endorsed cutting greenhouse gas emission in half by this year.

Small alpine glaciers will very likely disappear completely, and large glaciers will shrink by 30 to 70 percent. Austrian scientist Roland Psenner of the University of Innsbruck says this is a conservative estimate, and the small alpine glaciers could be gone as soon as 2037. (IPCC)

Ocean acidification could kill off most coral reefs. (Ken Caldeira, Carnegie Institution of Washington, Science)

At least 400 bird species could become endangered or extinct due to deforestation and climate change. (Walter Jetz, University of California, Davis, PLoS Biology)

In Australia, there will likely be an additional 3,200 to 5,200 heat-related deaths per year. The hardest hit will be people over the age of 65. An extra 500 to 1,000 people will die of heat-related deaths in New York City per year. In the United Kingdom, the opposite will occur, and cold-related deaths will outpace heat-related ones. (IPCC)

World population reaches 9.4 billion people. (U.S. Census Bureau)

Crop yields could increase by up to 20 percent in East and Southeast Asia, while decreasing by up to 30 percent in Central and South Asia. Similar shifts in crop yields could occur on other continents. (IPCC)

As biodiversity hotspots are more threatened, a quarter of the world’s plant and vertebrate animal species could face extinction. (Jay Malcolm, University of Toronto, Conservation Biology)


As glaciers disappear and areas affected by drought increase, electricity production for the world’s existing hydropower stations will decrease. Hardest hit will be Europe, where hydropower potential is expected to decline on average by 6 percent; around the Mediterranean, the decrease could be up to 50 percent. (IPCC)

Warmer, drier conditions will lead to more frequent and longer droughts, as well as longer fire-seasons, increased fire risks, and more frequent heat waves, especially in Mediterranean regions. (IPCC)


While some parts of the world dry out, others will be inundated. Scientists predict up to 20 percent of the world’s populations live in river basins likely to be affected by increased flood hazards. Up to 100 million people could experience coastal flooding each year. Most at risk are densely populated and low-lying areas that are less able to adapt to rising sea levels and areas which already face other challenges such as tropical storms. (IPCC)

Coastal population could balloon to 5 billion people, up from 1.2 billion in 1990. (IPCC)

Between 1.1 and 3.2 billion people will experience water shortages and up to 600 million will go hungry. (IPCC)

Sea levels could rise around New York City by more than three feet, potentially flooding the Rockaways, Coney Island, much of southern Brooklyn and Queens, portions of Long Island City, Astoria, Flushing Meadows-Corona Park, Queens, lower Manhattan and eastern Staten Island from Great Kills Harbor north to the Verrazano-Narrows Bridge. (NASA GISS)


The risk of dengue fever from climate change is estimated to increase to 3.5 billion people. (IPCC)


A combination of global warming and other factors will push many ecosystems to the limit, forcing them to exceed their natural ability to adapt to climate change. (IPCC)

Atmospheric carbon dioxide levels will be much higher than anytime during the past 650,000 years. (IPCC)

Ocean pH levels will very likely decrease by as much as 0.5 pH units, the lowest it’s been in the last 20 million years. The ability of marine organisms such as corals, crabs and oysters to form shells or exoskeletons could be impaired. (IPCC)

Thawing permafrost and other factors will make Earth’s land a net source of carbon emissions, meaning it will emit more carbon dioxide into the atmosphere than it absorbs. (IPCC)

Roughly 20 to 30 percent of species assessed as of 2007 could be extinct by 2100 if global mean temperatures exceed 2 to 3 degrees of pre-industrial levels. (IPCC)

New climate zones appear on up to 39 percent of the world’s land surface, radically transforming the planet. (Jack Williams, University of Wisconsin-Madison, Proceedings of the National Academy of Sciences)

A quarter of all species of plants and land animals—more than a million total—could be driven to extinction. The IPCC reports warn that current “conservation practices are generally ill-prepared for climate change and effective adaptation responses are likely to be costly to implement.” (IPCC)

Increased droughts could significantly reduce moisture levels in the American Southwest, northern Mexico and possibly parts of Europe, Africa and the Middle East, effectively recreating the “Dust Bowl” environments of the 1930s in the United States. (Richard Seager, Lamont Doherty Earth Observatory, Science)


An Earth day will be 0.12 milliseconds shorter, as rising temperatures cause oceans to expand away from the equator and toward the poles, one model predicts. One reason water will be shifted toward the poles is most of the expansion will take place in the North Atlantic Ocean, near the North Pole. The poles are closer to the Earth’s axis of rotation, so having more mass there should speed up the planet’s rotation. (Felix Landerer, Max Planck Institute for Meteorology, Geophysical Research Letters)


Lamont Doherty Earth Observatory at Columbia University — Researchers have developed the first year-by-year record of rainfall in sub-Saharan West Africa for the past 3,000 years, and identified a daunting pattern: a 30-to-60-year cycle of serious droughts that last a decade or more, punctuated by killer megadroughts that last for centuries. The last great dry periods, from about AD 1100 to 1300, and 1400 to 1750, dwarfed the recent notorious Sahel drought, which killed some 100,000 people and displaced millions in the 1970s and 1980s. The region is now home to tens of millions of the world’s poorest people. Global warming could worsen this natural cycle, say the researchers. Their study appears in the April 17 issue of the journal Science.

“It suggests that the most recent drought was relatively minor,” said lead author Timothy Shanahan, a geoscientist at Jackson School of Geosciences at the University of Texas, Austin. Coauthor Kevin Anchukaitis, a paleoclimatologist at Columbia University’s Lamont-Doherty Earth Observatory, added, “What’s scary is that we don’t know precisely what causes the megadroughts. That is cause for great concern.”

The researchers drew their climate portrait from Ghana’s Lake Bosumtwi, where they took cores of yearly sediment layers holding isotopes and elements that reflect lake levels. They combined this data with outlines of old beaches both above and below the present water line, which indicate dramatic rises and falls in water levels. They also dated dead trees that sprouted when lake levels fell but were later drowned as levels rose again. During the 20th-century Sahel drought, the lake fell 5 meters (15 feet); but during the megadroughts, it fell as much as 31 meters (95 feet).


The study shows that the shorter-period droughts have been a regular occurrence for many centuries. It also tends to confirm many scientists’ belief that these droughts come during natural cycles when temperatures decrease over the northern Atlantic Ocean—one phase of the so-called Atlantic Multidecadal Oscillation. Instrumental records of sea-surface temperatures go back only 80 or 100 years, but the new study matches well with these, and other evidence, including tree rings from widely scattered locations. Instrumental records suggest also that the patterns shown in Lake Bosumtwi match the weather across much of West Africa.

The scientists say that both the shorter cycles and the larger droughts could be exacerbated by global warming. “Dry periods will occur against hotter background temperatures, and so they will be that much worse,” said Anchukaitis. In the paper, the scientists raise the possibility that warming could also slow down the overturning circulation of the Atlantic itself—an event thought to have occurred during warm periods of the past–which could create conditions for the centuries-long droughts. But “that’s a fairly unknown aspect,” cautioned Anchukaitis. “The timing and magnitude of past and future circulation changes at these time scales, and what they produce, are still pretty uncertain.”

In a New York Times article about the study, Richard Seager, a climate modeler at Lamont-Doherty who was not involved in the research, described the work as “startling.” Seager, who has studied droughts in the American Southwest, said the new study showed the need to refine computer models so that shifts can be predicted in specific time frames. “The most pressing problem we now face is to predict climate in the near-term future—years to decades,” he said.
Some climate models predict a wetter climate for west Africa, while others predict drying. In either case, the study shows that periodic changes can be far more severe than suggested by historical records alone, said Anchukaitis.

Related links:
University of Texas press release on the study
University of Arizona press release on the study

Related Projects:
North American Drought Research at Lamont-Doherty Earth Observatory
Abrupt Climate Change


Paleogeography 13,400 years ago. Glacial Lake Iroquois is held back by an ice dam in northern New York. When that dam collapsed it drained (red arrows) into the lakes within the Champlain and Hudson Valleys, breaching the Narrows Dam (near New York City). It cascaded across the then exposed continental shelf to the North Atlantic Ocean. This release of meltwater reduced the flow of the Gulf Stream and caused an abrupt climate cooling in the Northern Hemisphere that lasted several hundred years. (Illustration by Jack Cook ©WHOI)


bull, March/April 2009, by Marlowe Hood – A predicted slowdown in Atlantic Ocean currents will cause sea levels along the U.S. northeast coast to rise twice as fast as the global average, exposing New York and other big cities to violent and frequent storm surges, according to a new study.

Manhattan’s Wall Street, barely a meter (three feet) above sea level, for example, will find itself underwater more often as the 21st century unfolds, said the study, published online Sunday in Nature Geoscience.

Sea levels vary across regions by up to 24 centimeters (9.5 inches), influenced in part by powerful currents that coarse around the globe in a pattern called the thermohaline circulation.

In the Atlantic, warm water moving north along the surface from the Gulf of Mexico helps temper cold winters in western Europe and along the US east coast, while frigid Arctic waters run south along the bottom of the sea.

The UN’s Intergovernmental Panel on Climate Change (IPCC) concluded in early 2007 that expanding ocean water driven by climate change will drive up sea levels, on average, anywhere from 18 to 59 centimeters (seven to 23 inches) by 2100, depending on how successful we are at slashing greenhouse gas emissions.

This rising water mark will erase several island nations from the map, and is likely to cause devastation in Asian and African deltas home to tens of millions of people.

More recent studies, taking the impact of melting ice sheets in Greenland and the Western Antarctic into account, forecast an even higher increase of at least one meter (39 inches) over the same period.

Jianjun Yin of Florida State University and two colleagues wanted to find out what impact these sea level rises would have at a regional level, especially along the American eastern seaboard.

The researchers analyzed the projections of nearly a dozen state-of-the art climate change models, under three different greenhouse gas scenarios.

They found that sea levels in the North Atlantic adjusted in all cases to the projected slowing of the Gulf Stream and its northward extension, the North Atlantic Current.

The weakened currents account for nearly half of a predicted sea rise — from thermal expansion alone — of 36 to 51 centimeters for the US northeastern coast, especially near New York, they found.

“This will lead to the rapid sea level rise on the Northeast coast of the United States,” Yin told AFP by phone.
And if, under the influence of melting ice sheets, “the global sea level rise is higher, the relative sea level rise will be superimposed. Proportionally it would be the same,” he added.

Rapid sea level increases would put cities such as New York, Boston, Baltimore and Washington D.C. at significantly greater risk of coastal hazards such as hurricanes and intense winter storm surges.

A study released last year by the Cambridge, Massachusetts-based Union of Concerned Scientists showed that, due to rising sea levels, once-in-a-century storms would occur on average every 10 years by 2100.

A large belt of around the tip of Manhattan — included Wall Street — would have a 10 per cent chance of flooding in any given year, it concluded.


COPENHAGEN – Being a climate scientist these days is not for the faint of heart, as arguably no other area of research yields a sharper contrast between “eureka!” moments, and the sometimes terrifying implications of those discoveries for the future of the planet.

“Science is exciting when you make such findings,” said Konrad Steffen, who heads the Cooperative Institute for Research in Environmental Sciences (CIRES) in Boulder, Colorado.

“But if you stop and look at the implications of what is coming down the road for humanity, it is rather scary. I have kids in college — what do they have to look forward to in 50 years?”

And that’s not the worst of it, said top researchers gathered here last week for a climate change conference which heard, among other bits of bad news, that global sea levels are set to rise at least twice as fast over the next century as previously thought, putting hundreds of millions of people at risk.

What haunts scientists most, many said, is the feeling that — despite an overwhelming consensus on the science — they are not able to convey to a wider public just how close Earth is to climate catastrophe.

That audience includes world leaders who have pledged to craft, by year’s end, a global climate treaty to slash the world’s output of dangerous greenhouse gases.

It’s as if scientists know a bomb will go off, but can’t find the right words to warn the people who might be able to defuse it.
French glaciologist Claude Lorius, one of the first scientists to publish, in 1987, evidence that global warming was real, has despaired of getting the message across.

“At first, I thought that we could convince people. But there is a terrible inertia,” he told AFP. “I fear that society is not up to the challenge of a crisis like this. Today, as a human being I am pessimistic.”

John Church, an expert on sea levels at the Antarctic Climate and Ecosystem Cooperative Research Centre in Hobart, Tasmania, takes an equally dim view of our collective capacity for denial.

“Perhaps society has realized the seriousness, but it certainly hasn’t realized the urgency,” he said.

“But even if you are pessimistic — and sometimes I am — it does not help. What are you going to do? Chop off your hands and give up? That’s not a solution either,” he said.

Most scientists, while no less alarmed by snowballing evidence of a planet out of kilter, still think there is time to act.

“We are actually going to have to decrease the amount of carbon dioxide in the atmosphere if we want to stabilize climate and avoid some highly undesirable effects,” said James Hansen, director since 1981 of NASA’s Goddard Institute for Space Studies. “It is still possible to do that.”

Some of those undesirable effects include massive droughts, more intense hurricanes and a panoply of human misery including expanded disease and tens of millions of climate refugees.

Even gloomier scenarios see a world map redrawn by sea levels rising tens of meters and a planet able to sustain only a fraction of the nine billion people projected to become, as of 2050, Earth’s stable population.

But even if it is urgent to let the world know just how bad it could be, there is also a danger of frightening people into inaction, said other scientists.

“I do worry that people just can’t deal, psychologically, with the enormity of the problem, and that they may revert to doing nothing,” said William Howard, a researcher at the University of Tasmania.

“As a scientist, I deal with climate change on a time scale of hundreds of thousands of years, and even I have a hard time dealing with it,” added Howard, who reported last week that tiny marine animals called forams are losing their capacity to absorb huge amounts of carbon pollution from the atmosphere.

“The risk is that when science pumps out more and more evidence that we are facing dangerous tipping points” — triggers that would make climate change irreversible — “that you put your head in the sand and move from denial to despair,” said Johan Rockstrom, director of the Stockholm Environment Institute.

Hanging over the conference proceedings like an invisible cloud were the apocalyptic predictions of the monstre sacre of Earth sciences, 90-year-old British scientist James Lovelock.

A true iconoclast, Lovelock commands respect because he understood decades before his peers that Earth behaves as a single, self-regulating system composed of physical, chemical and biological components, a concept he dubbed the Gaia principle.

In his just-released book “The Vanishing Face of Gaia”, he basically says we have already passed a point of no return, and that it is now impossible “to save the planet as we know it.”

“Efforts to stabilize carbon dioxide and temperature are no better than planetary alternative medicine,” he wrote.

It is perhaps telling that more than a dozen scientists interviewed could only say that they hoped Lovelock was wrong.

None could say — based on the science — that they knew he was wrong.


Geophysical Research Abstracts, Vol. 5, 07512,
European Geophysical Society

Alexey V. Fedorov, R. Pacanowski, S. G. H. Philander and G. Boccaletti. Princeton University (

The two main components of the oceanic circulation, the wind-driven circulation of
the ventilated thermocline in the upper ocean, and the deep thermohaline circulation,
both involve meridional over-turning and poleward heat transport. The sinking of surface
waters depends critically on salinity in the second component, but in the first
sinking is induced by the winds over subduction zones. Salinity nonetheless has a
strong influence on the wind-driven circulation and hence on its transport of heat.
Today that transport is less than it would be in the absence of salinity variations because the northward surface density gradient created by the decrease in surface temperature with latitude is countered by the decrease in salinity. (Surface salinities are
high in low latitudes regions of strong evaporation, and are low in the rain belts of
higher latitudes.) To explore these matters we use an idealized ocean general circulation model that captures the main aspects of the thermocline ventilation and employs simple parameterizations of surface heat and salt fluxes. Our calculations show that a freshening of the surface waters in the extra-tropics, and the associated reduction in the meridional density gradient at the surface, can reduce the poleward heat transport and deepen the equatorial thermocline. As the freshwater forcing at the surface in high latitudes approaches a critical value, certain aspects of the wind-driven circulation change radically: the horizontal heat transport in the ocean approaches zero,
the heat budget becomes balanced locally everywhere, and the equatorial thermocline
becomes practically horizontal so that permanent El Niño conditions prevail in the tropics.
At that stage the wind-stress at the equator is balanced by salinity gradients in the
upper ocean. Earlier than three million years ago oceanic conditions were apparently
similar to these.


Woods Hole Oceanographic Institute — 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.



Q. Have humans contributed to the warming?
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?
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. Howcan global warming and sudden cooling happen at the same time?
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.


Q. What is the North Atlantic heat pump?
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?
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. Will global warming affect the 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. What happens if the hydrologic cycle accelerates??
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. When will regional cooling happen?
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. If cooling does happen, how cold will it get?
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. How long will it stay cold?
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. Is there anything we can do about it?
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. What is the ocean’s role in climate?
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.


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