GULF STREAM NOT TO BLAME FOR PAST CLIMATE CHANGE

Paleo-climate events probably regional rather than global

(New article in press at Quaternary Research)

Abrupt climate change: An alternative view

By Carl Wunsch (Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA)

Abstract

Hypotheses and inferences concerning the nature of abrupt climate change, exemplified by the Dansgaard-Oeschger (D-O) events, are reviewed. There is little concrete evidence that these events are more than a regional Greenland phenomenon. The partial coherence of ice core d18O and CH4 is a possible exception. Claims, however, of D-O presence in most remote locations cannot be distinguished from the hypothesis that many regions are just exhibiting temporal variability in climate proxies with approximately similar frequency content. Further suggestions that D-O events in Greenland are generated by shifts in the North Atlantic ocean circulation seem highly implausible, given the weak contribution of the high latitude ocean to the meridional flux of heat. A more likely scenario is that changes in the ocean circulation are a consequence of wind shifts. The disappearance of D-O events in the Holocene coincides with the disappearance also of the Laurentide and Fennoscandian ice sheets. It is thus suggested that D-O events are a consequence of interactions of the windfield with the continental ice sheets and that better understanding of the wind field in the glacial periods is the highest priority. Wind fields are capable of great volatility and very rapid global-scale teleconnections, and they are efficient generators of oceanic circulation changes and (more speculatively) of multiple states relative to great ice sheets. Connection of D-O events to the possibility of modern abrupt climate change rests on a very weak chain of assumptions.

Introduction

The widely-held view of abrupt climate change during the last glacial period, as manifested, particularly, in the so-called Dansgaard-Oeschger (D-O) events, is that they are at least hemispheric, if not global, in extent, and caused by changes in the ocean circulation. A version of the much disseminated curve that stimulated the discussion is shown in Figure 1. The canonical view that ocean circulation changes were the cause of the abrupt changes seen in Greenland isotope records is widespread (e.g., Schmittner, 2005 and Cruz et al., 2005) and is usually implied even where not explicitly stated. The possibility of abrupt climate change occurring because of the ongoing global warming and its oceanic effects is attracting great attention. For examples of how the hypothesis is influencing the debate about modern global warming, see Broecker, 1997 and Broecker, 2003, or The Guardian, London (2005). Major field programs are underway seeking to see early signs of "collapse" of the North Atlantic circulation, e.g., the UK RAPID Program; see http://www.soc.soton.ac.uk/rapid/Scienceplan.php, with some anticipating the shutoff of the Gulf Stream (Schiermeier, 2004).

Given the implications for modern public policy debate, and the use of this interpretation of D-O events for understanding of past climate change, it is worthwhile to re-examine the elements leading to the major conclusions. Underlying the now very large literature of interpretation are several assumptions, assertions and inferences including:

(1) The d18O variations appearing in the record of Figure 1 are a proxy for local temperature changes.

(2) Fluctuations appearing in Greenland reflect climate changes on a hemispheric, and probably global, basis and of large amplitude.

(3) The cause of the D-O events can be traced back to major changes (extending to "shutdown") of the North Atlantic meridional overturning circulation and perhaps even failure of the Gulf Stream.

(4) Apparent detection of a D-O event signature at a remote location in a proxy implies its local climatic importance.

The purpose of this paper is to briefly re-examine these assumptions and assertions, but with emphasis on lk and lk. A summary of the outcome of the survey is that (1) is in part true; little evidence exists for (2) other than a plausibility argument; and (3) is unlikely to be correct. Inference (4) can only be understood through a quantitative knowledge of controls of local proxies and is briefly likened to the problem of interpreting modern El Ni¤o signals. The paper ends with a discussion of how to move forward.

[...]

North Atlantic circulation control

Our focus now changes to the separate issue of the cause (or "trigger") of the rapid changes seen in central Greenland. Consider the widely accepted scenario that Greenland D-O events are a direct consequence of a major shift in the North Atlantic meridional overturning circulation. Some confusion occurs at the outset because of a failure to specify which elements of that circulation are supposed to generate the climate change (Wunsch, 2002). Often, the focus is on the mass flux associated with the meridional overturning, and the mass flux is indeed central to the dynamics of the ocean. But in terms of the impact on the climate system, it is the oceanic poleward heat flux that has the most immediate consequences for the atmosphere. Alternatively, sea-surface temperatures are most often used to determine how the ocean is affecting the atmospheric state, although these will be in large part a consequence of the heat flux divergence (exchange of enthalpy with the atmosphere).

Figure 10 displays the estimated net meridional transport of heat by the combined ocean-atmosphere system, as well as separate estimates of the oceanic and atmospheric contributions (Wunsch, 2005). A number of features stand out in this figure. First, the oceanic Northern Hemisphere contribution poleward of about 25øN falls very rapidly as heat is transferred to the atmosphere through the intense cyclogenesis in the mid-latitude storm belts, and as the relative oceanic area rapidly diminishes. By 40øN, the oceanic contribution is less than 25% of the atmospheric contribution. Of this 25%, most is in the North Atlantic (e.g., Ganachaud and Wunsch, 2002). The assumption that a fractional change in this comparatively minor contribution to the global heat flux is the prime mover of abrupt climate change is not very appealing, if there is any alternative possibility (one will be proposed below). Furthermore, air mass trajectories circling the globe at high latitudes are in contact with the North Atlantic Ocean for only a very short time compared to the North Pacific, Asia and North America. The oceanic tail may not necessarily be wagging the meteorological dog.

Second, note that within the significant error bars, the "conveyor" (the real "global conveyor" is the combined ocean and atmospheric transport) is nearly indistinguishable from being antisymmetric about the equator. The net oceanic transport is asymmetric about the equator, with the atmospheric contribution compensating within observational error. An overall anti-symmetry, despite the asymmetry of the separate oceanic and atmospheric fluxes, is one of the more remarkable, but rarely noted, elements of the modern climate system. Stone (1978) discusses some of the physical elements controlling the total fluxes.

If this anti-symmetry is maintained as the climate system shifts (assuming the modern antisymmetry is not mere accident), a reduction in the Northern Hemisphere oceanic heat transport would be compensated by a corresponding increase in the atmospheric transport. That is, on a zonally integrated basis, one plausible outcome of a hypothetical "shutdown" of the North Atlantic overturning circulation, with any consequent reduction in oceanic heat transport, is a warmer (and/or wetter) Northern Hemisphere atmosphere rather than a colder one. This argument says nothing at all about a regional atmospheric cooling in the North Atlantic sector, only that should it occur; it would have to be compensated elsewhere so as to maintain the balance of incoming and outgoing radiation. Claims that it is obvious that the North Atlantic sector atmosphere must cool are difficult to sustain (how the tropics, and e.g., its albedo, might shift through all of this, are unspecified in most discussions).

Setting the global problem aside, turn now to the question of how a hypothetical North Atlantic meridional overturning circulation shutdown would occur. The conventional explanation connects it to a strong decrease in surface salinity from melting glacial ice. The hypothesis is that an injection of fresh water would dramatically reduce the meridional overturning circulation (MOC)-that is, the zonally integrated mass flux. An extensive literature (e.g., Manabe and Stouffer, 1999) has collected around this hypothesis and it has been the focus of numerous modeling efforts as well as being presented as "fact" to the public. Despite its intuitive appeal, there are a number of serious difficulties with it. That fresh water injection controls the North Atlantic circulation can be questioned from several points of view. First, existing climate models, which are the main tool that have been used to study the hypothesis, do not have the resolution, either vertical or horizontal, to properly compute the behavior of fresh water and its interaction with the underlying ocean and overlying atmosphere. Models of the modern ocean contain special, high resolution subcomponents designed to calculate mixed layer behavior (e.g., Price et al., 1986 and Large et al., 1994). Despite the great effort that has gone into them, systematic errors in calculating mixed layer properties remain. How these errors would accumulate in climate-scale models, with much less resolution is unknown. Second, some models also use a physically inappropriate surface boundary condition for salinity, leading to serious questions about the physical reality of the resulting flows (Huang, 1993).

Third, the models have almost always been run with fixed diffusion coefficients. A series of papers (Munk and Wunsch, 1998, Huang, 1993, Nilsson et al., 2003 and Wunsch and Ferrari, 2004) have noted that, (a) mixing coefficients have a profound influence on the circulation; (b) fixed mixing coefficients as the climate system shifts and/or as fresh water is added are very unlikely to be correct; (c) depending upon exactly how the mixing coefficients are modified, fresh water additions can actually increase the North Atlantic mass circulation (Nilsson et al., 2003). Finally (d), the prime mover of the ocean circulation, including its mixing coefficients as well as providing the major direct input of energy (Wunsch and Ferrari, 2004), is the wind. If one wishes to change the ocean circulation efficiently and very rapidly, all existing theory points to the wind field as the primary mechanism. Scenarios that postulate shifts in the ocean circulation leading to major climate changes (e.g., D-O events) imply important changes in the overlying wind field in response, with a consequent feedback (positive or negative). It is extremely difficult to evaluate proposals that the climate shifted owing to a "shutdown" of the MOC if no account is taken of how the overlying atmospheric winds would have responded. In any event, much of the temperature flux of the modern North Atlantic is carried in the Gulf Stream; scenarios requiring wind shifts sufficient to shut it down are likely a physical impossibility because of the need to conserve angular momentum in the atmosphere. Apparent correlations of the Greenland ?18O shifts with possible corresponding events in North Atlantic deep-sea cores (e.g., Bond et al., 1993) are rationalized most directly as reflecting oceanic circulation changes induced by moderate, acceptable, changing wind stress fields.

Coupled models that have been claimed to show atmospheric response to oceanic mass flux shifts do not themselves resolve the major property transport pathways of either ocean or atmosphere. Some of these models are of the "box" form, with as few as four parameters. More sophisticated models are commonly described as "intermediate complexity" ones; despite the label, they still lack adequate resolution and dynamical and physical components required for true realism. Little evidence exists that such simplified representations of the climate system can be integrated skillfully over the long periods required to describe true climatic time scales as both systematic and random errors accumulate.1 Whether dynamical thresholds in oversimplified models correspond to those in the enormously higher dimension real system also remains unproven. Combined with the resolution issue, one concludes that as yet, modeling studies neither support nor undermine the canonical scenario. They remain primarily as indicators of processes that can be operating, but with no evidence that they dominate.

Furthermore, there is no known simple relationship between the zonally integrated overturning stream function defining the MOC, and either the heat flux or the sea surface temperatures. Even if the MOC did weaken, there is no logical chain leading to the inference that the sea-surface temperatures must be reduced (although in some models, they are reduced).

Much of the evidence for "shutdown" deals with elements of the circulation that do not directly imply anything about shifts in heat flux or sea-surface temperature. Some of the inference concerns the net export of properties from the North Atlantic (e.g., Yu et al., 1996 for protactinium). But in the modern system (e.g., Ganachaud and Wunsch, 2002), estimation of the mean meridional property transport depends upon the property (temperature, oxygen, etc.) and the ability to calculate the integral of velocity times property concentration as a function of longitude and depth across the entire ocean basin. That is the net meridional property flux is,

Other evidence (e.g., Boyle, 1995 and Curry and Oppo, 2005) strongly supports the inference that the high latitude distribution of water masses has shifted through time-unsurprising in an ocean with a radically different overlying atmospheric state. The simplest, and widely accepted, interpretation of much of the data is, for example, that during the Last Glacial Maximum, the equivalent of North Atlantic Deep Water shifted upward in the water column. Displacement of a water mass and a change in its defining properties carries no information about changes in either its mass flux or temperature transport properties: they could separately or together increase, decrease or remain unchanged. Indeed, the export of some properties could increase and some simultaneously decrease, without any contradiction. If, as is widely believed, the North Atlantic Deep Water migrated upwards, its southward cold temperature transport could actually have increased if its mass flux increased correspondingly (no evidence exists one way or the other). Furthermore, because HC involves an integral over the entire water column, one also needs to understand the changes in properties and mass fluxes of all of the water masses making up the North Atlantic. Specific attention (e.g., Wunsch, 2003b) is called to the question of whether the southern component waters, with their extreme properties, were not more actively present in the North Atlantic during this time, probably because of an intensified, not a weakened, circulation. One cannot calculate oceanic heat fluxes without specifying a complete oceanic cross-section mass flux and temperature distribution.2

Postulate, however, that the scenario of a weakened MOC requires a reduced air/sea transfer of heat in the North Atlantic. What will the atmosphere do? The net outgoing radiation must balance the incoming, and so compensation for a reduced ocean heat transport must occur. It can occur by increasing the ocean heat transport somewhere else (the North Pacific?) with consequent changes in atmospheric circulation, or by having the atmosphere compensate, as it seems to be doing in Figure 10, or by quite different changes elsewhere (increased tropical albedo, for example). Will the Northern Hemisphere atmosphere warm on average? It seems foolhardy to speculate further at this time. What does seem clear is that the hypothesis of a reduced North Atlantic MOC produces at best a regional story, one whose global implications have to be determined using convincing global data and models.

[...]

Discussion

The observational record supports at least one alternate interpretation of the D-O events as being primarily a central Greenland phenomenon, dependent for their existence upon the Laurentide and Fennoscandian ice sheets. A possible exception to this inference lies with the methane record of the Greenland ice cores, but the interpretation rests upon the sketchy knowledge of the behavior of glacial-period methane source and sink distributions. The primarily local interpretation implicates the wind field as the central element by which central Greenland temperatures change abruptly, and the mechanism by which larger-scale signatures would be carried to distant locations, including those induced by ocean circulation shifts under the changing wind system. Given the comparatively small contribution of the ocean to the high-latitude meridional flux of heat, it seems an unlikely primary stimulus of major climate shifts beyond the North Atlantic basin. It can readily operate as an integrator and as a transmitter of signals, but that is a different role.

Like the widely accepted view, this reinterpretation has not been demonstrated. But maintenance of an open mind is important when the data are so ambiguous. Several steps can be taken to reduce the uncertainty about how the D-O events were generated and manifested. The most important parameters, wind-direction and speed through time, may be beyond reach of the proxy record. But perhaps the search for such proxies will be rewarded (dust, sand dunes, pollen distributions). The question of the spatial extent and intensity of D-O events can be addressed by continuing efforts to reduce age model uncertainties. Existing oceanic 14C dates have an typical uncertainty of ñ300 yr, with a multi-valued structure, through much of this period-sufficiently wide that great freedom is available in adjusting records to apparently coincide. Interpretation of the methane signature in ice cores would be greatly strengthened by better understanding of how, and how rapidly, wetlands evolve under glacial conditions, and particularly with an ability to distinguish low and high latitude sources. Other gases detectable in ice cores (e.g., N20; see Fliikiger et al., 2004) might, if there is adequate knowledge of the changing source/sink distributions, shed light on the spatial extent question. The presence in other proxies of signals, perhaps convincingly, corresponding to remote manifestations of D-O events needs, however, to be interpreted very cautiously as they may be no more than measurable signals not corresponding to important local climate shifts: the analogy is drawn with remote detectability of modern Los Ni¤os (one might detect El Ni¤o signals over central Asia; they may not be of any importance).

Modeling efforts, with sufficient spatial resolution to respond to shifts in the ice orography, and to produce realistic feedbacks as begun by Jackson (2000) and Roe and Lindzen (2001), would be very informative. If interactions do not produce atmospheric flows exhibiting sustained multiple state shifts in the hemispheric standing wave patterns, then the hypothesis proposed here would be less attractive. Existing models are unlikely to have adequate skill when integrated over the thousands of years required to understand D-O type events, given the propensity, as the duration of the computation increases, of all models to accumulate random and systematic errors.

Finally, a better understanding of how the global ocean and atmosphere conspire to maintain outgoing radiation sufficient to balance the incoming component would be very helpful in understanding how the coupled system actually behaves.






THE BRITISH GOVERNMENT REHASHES OLD DECEPTIONS WITHOUT REGARD FOR CONTRARY EVIDENCE AND ARGUMENT

Prof. Brignell's comments on the latest pack of official British lies are reproduced below

Some people get upset when you use the L word. They find it unduly provocative and therefore counterproductive. But what other word is there for the creation of an elaborate structure of deliberate falsehoods and exaggerations designed to bamboozle the public into a baseless panic? The British Government has issued a "new" report. It was described across the media as more bad news about global warming: yet it was recycled from something that got full blown coverage last year. Number Watch covered it in a piece entitled A tale of two conferences. Your bending author paid to attend one conference out of his meagre pension, including the rail fare and a modest buffet, but the galling aspect is that he also paid for the other through extortionate taxes, covering elaborate banquets and all the other benefits of a Government sponsored jamboree. Benny Peiser bravely attended both and gave us his depressing account of the second.

This recycling a year after the event is a typical ploy by the Green establishment, but you have to hand it to them. They never miss a trick. We have seen them take over international organisations, media outlets (including the once fiercely independent BBC) and major scientific journals, such as the once great Nature. But who would have given them a chance of taking over the British Conservative Party in a bloodless coup? Yet they have done just that! Any bookie would have given you a million to one against only a year ago. Unfortunately, one of the more tragic errors of Margaret Thatcher was to ignore the party in the country. The arrogance that accompanies a large majority was responsible for this, and we can see the same thing happening on the other side now. The best people left, one by one, and the world's most efficient electoral machine was allowed to wither away. The remaining rump were mostly lifeless and incompetent. In their panic they bought a pig in a poke, with the result that they lost control of their party to a faction (the Green Toffs) that was an offence to everything in which they believed. The new leadership demands absolute loyalty and democracy be damned.

What is not generally known is that an iron curtain has descended around the Conservative Party. All debate is suppressed. Would-be candidates who have given their all to the cause are told "If you are white, male and middle class, forget it!" Not only policy, but party procedures are handed down from on high. MPs, MEPs and ministers stay silent, in fear for their jobs.

All this is, of course, rather unnerving for the Great Leader. He chose to ride the Green tiger and, despite all his prevarication, is having to face up to the consequences, such as the disastrous energy policy that has opened him to blackmail by the ex-KGB man in the Kremlin.

Yet the Government have decided to issue a report on the basis of that year-old conference, whose outcome was decided before it even opened. It recycles many of the ludicrous extrapolations that were already fully covered a year ago. The people who cannot tell us what the weather is going to be next week can predict sea levels in a thousand years time. And not, of course, one mention of water vapour, the only important greenhouse gas.

Why do they do it? The answer was of course given to us by that great seer H L Mencken, quoted here only last month:

The art of practical politics is to keep the public in a state of constant alarm by menacing it with hobgoblins, both real and imagined, - so making people clamorous to be led to safety.




BRITAIN'S CHIEF SCIENCE ADVISOR TURNS REALIST: CURBING CLIMATE CHANGE "UNFEASIBLE"

Below is the BBC promotion for the report discussed above by Brignell. But it sadly notes that the UK government has recently changed course and no longer buys the panic scenario

Rising concentrations of greenhouse gases may have more serious impacts than previously believed, a major new scientific report has said. The report, published by the UK government, says there is only a small chance of greenhouse gas emissions being kept below "dangerous" levels. It fears the Greenland ice sheet is likely to melt, leading sea levels to rise by seven metres over 1,000 years. The poorest countries will be most vulnerable to these effects, it adds. The report, "Avoiding Dangerous Climate Change", collates evidence presented by scientists at a conference hosted by the UK Meteorological Office in February 2005.

The conference set two principal objectives: to ask what level of greenhouse gases in the atmosphere is too much, and what are the options for avoiding such levels? Environment Secretary Margaret Beckett said the report's conclusions would be a shock to many people. "The thing that is perhaps not so familiar to members of the public... is this notion that we could come to a tipping point where change could be irreversible," she told BBC Radio 4's Today programme. "We're not talking about it happening over five minutes, of course, maybe over a thousand years, but it's the irreversibility that I think brings it home to people."

One collection of scientific papers sets out the impacts associated with various levels of temperature increase. "Above a one degree Celsius increase, risks increase significantly, often rapidly for vulnerable ecosystems and species," concludes Bill Hare from the Potsdam Institute of Climate Impact Research in Germany, who produced an overview of more than 70 studies of impacts on water resources, agriculture and wildlife. "In the one to two degree range, risks across the board increase significantly, and at a regional level are often substantial," he writes. "Above two degrees the risks increase very substantially, involving potentially large numbers of extinctions or even ecosystem collapses, major increases in hunger and water shortage risks as well as socio-economic damages, particularly in developing countries."

The European Union has adopted a target of preventing a rise in global average temperature of more than two Celsius. That, according to the report, might be too high, with two degrees perhaps being enough to trigger melting of the Greenland ice sheet. This would have a major impact on sea levels globally, though it would take up to 1,000 years to see the full predicted rise of seven metres. The western half of the much larger Antarctic ice sheet is also causing concern to the British Antarctic Survey, whose head Chris Rapley describes it as a "sleeping giant". Previous assessments had concluded the ice here was unlikely to melt in significant amounts in the foreseeable future; but Professor Rapley says that question needs revisiting in the light of new evidence.

A key task undertaken by some scientists contributing to the report was to calculate which greenhouse gas concentrations in the atmosphere would be enough to cause these "dangerous" temperature increases. Currently, the atmosphere contains about 380 parts per million (ppm) of carbon dioxide, the principal greenhouse gas, compared to levels before the industrial revolution of about 275ppm. "For achieving the two Celsius target with a probability of more than 60%, greenhouse gas concentrations need to be stabilised at 450 ppm CO2-equivalent or below," conclude Michel den Elzen from the Netherlands Environmental Assessment Agency and Malte Meinshausen of the US National Center for Atmospheric Research. "A stabilisation at 450 ppm CO2-equivalent requires global emissions to peak around 2015, followed by substantial overall reductions in the order of 30%-40% compared to 1990 levels in 2050."

But, speaking on Today, the UK government's chief scientific advisor Sir David King said that is unlikely to happen. "We're going to be at 400 parts per million in 10 years time, I predict that without any delight in saying it," he said. "But no country is going to turn off a power station which is providing much-desired energy for its population to tackle this problem - we have to accept that. "To aim for 450 (ppm) would, I am afraid, seem unfeasible."

A rise of two Celsius, researchers conclude, will be enough to cause:

* Decreasing crop yields in the developing and developed world
* Tripling of poor harvests in Europe and Russia
* Large-scale displacement of people in north Africa from desertification
* Up to 2.8bn people at risk of water shortage
* 97% loss of coral reefs
* Total loss of summer Arctic sea ice causing extinction of the polar bear and the walrus
* Spread of malaria in Africa and north America

But Miles Allen, a lecturer on atmospheric physics at Oxford University, said assessing a "safe level" of CO2 in the atmosphere was "a bit like asking a doctor what's a safe number of cigarettes to smoke per day". "There isn't one but at the same time people do smoke and live until they're 90," he told Today. "It's one of those difficult areas where we're talking about changing degrees of risk rather than a very definite number after which we can say with absolute certainty that certain things will happen."

More here





IN CLIMATE POLICY POKER, ALARMISTS PLAY THE DOOMSDAY CARD AGAIN

Including that NASA guy. For someone who was gagged, he is pretty talkative. See below. Extreme assumptions (I was going to highlight all the "ifs" in red but it looked too big a job) all round, of course. Note that their assumptions concerning the Gulf stream are contradicted by the article leading today's posts on this blog.

There is a skeptical Early-English saying still sometimes heard today that is very relevant to the prophecies of doom below: "If ifs and ans were pots and pans, there'd be no room for tinkers". In Middle English and Early New English, "an" was a synonym for "if" (Cognate with the German "wenn"). A tinker, of course, used to repair pots and pans. But you would not find that out from a modern literary education, of course. Only specialist linguists and old guys like me now know that stuff.

"Now that most scientists agree human activity is causing Earth to warm, the central debate has shifted to whether climate change is progressing so rapidly that, within decades, humans may be helpless to slow or reverse the trend. This "tipping point" scenario has begun to consume many prominent researchers in the United States and abroad, because the answer could determine how drastically countries need to reduce their greenhouse gas emissions in the coming years. While scientists remain uncertain when such a point might occur, many say it is urgent that policymakers cut global carbon dioxide emissions in half over the next 50 years or risk the triggering of changes that would be irreversible.

There are three specific events that these scientists describe as especially worrisome and potentially imminent, although the time frames are a matter of dispute: widespread coral bleaching that could damage the world's fisheries within three decades; dramatic sea level rise by the end of the century that would take tens of thousands of years to reverse; and, within 200 years, a shutdown of the ocean current that moderates temperatures in northern Europe.

The debate has been intensifying because Earth is warming much faster than some researchers had predicted. James E. Hansen, who directs NASA's Goddard Institute of Space Studies, last week confirmed that 2005 was the warmest year on record, surpassing 1998. Earth's average temperature has risen nearly 1 degree Fahrenheit over the past 30 years, he noted, and another increase of about 4 degrees over the next century would "imply changes that constitute practically a different planet." "It's not something you can adapt to," Hansen said in an interview. "We can't let it go on another 10 years like this. We've got to do something."

Princeton University geosciences and international affairs professor Michael Oppenheimer, who also advises the advocacy group Environmental Defense, said one of the greatest dangers lies in the disintegration of the Greenland or West Antarctic ice sheets, which together hold about 20 percent of the fresh water on the planet. If either of the two sheets disintegrates, sea level could rise nearly 20 feet in the course of a couple of centuries, swamping the southern third of Florida and Manhattan up to the middle of Greenwich Village.

While both the Greenland and the Antarctic ice sheets as a whole are gaining some mass in their cold interiors because of increasing snowfall, they are losing ice along their peripheries. That indicates that scientists may have underestimated the rate of disintegration they face in the future, Oppenheimer said. Greenland's current net ice loss is equivalent to an annual 0.008 inch sea level rise. The effects of the collapse of either ice sheet would be "huge," Oppenheimer said. "Once you lost one of these ice sheets, there's really no putting it back for thousands of years, if ever."

Last year, the British government sponsored a scientific symposium on "Avoiding Dangerous Climate Change," which examined a number of possible tipping points. A book based on that conference, due to be published Tuesday, suggests that disintegration of the two ice sheets becomes more likely if average temperatures rise by more than 5 degrees Fahrenheit, a prospect "well within the range of climate change projections for this century."

The report concludes that a temperature rise of just 1.8 degrees Fahrenheit "is likely to lead to extensive coral bleaching," destroying critical fish nurseries in the Caribbean and Southeast Asia. Too-warm sea temperatures stress corals, causing them to expel symbiotic micro-algae that live in their tissues and provide them with food, and thus making the reefs appear bleached. Bleaching that lasts longer than a week can kill corals. This fall there was widespread bleaching from Texas to Trinidad that killed broad swaths of corals, in part because ocean temperatures were 2 degrees Fahrenheit above average monthly maximums.

Many scientists are also worried about a possible collapse of the Atlantic thermohaline circulation, a current that brings warm surface water to northern Europe and returns cold, deep-ocean water south. Hans Joachim Schellnhuber, who directs Germany's Potsdam Institute for Climate Impact Research, has run multiple computer models to determine when climate change could disrupt this "conveyor belt," which, according to one study, is already slower than it was 30 years ago. According to these simulations, there is a 50 percent chance the current will collapse within 200 years.

Some scientists, including President Bush's chief science adviser, John H. Marburger III, emphasize there is still much uncertainty about when abrupt global warming might occur. "There's no agreement on what it is that constitutes a dangerous climate change," said Marburger, adding that the U.S. government spends $2 billion a year on researching this and other climate change questions. "We know things like this are possible, but we don't have enough information to quantify the level of risk."

This tipping point debate has stirred controversy within the administration; Hansen said senior political appointees are trying to block him from sharing his views publicly. When Hansen posted data on the Internet in the fall suggesting that 2005 could be the warmest year on record, NASA officials ordered Hansen to withdraw the information because he had not had it screened by the administration in advance, according to a Goddard scientist who spoke on the condition of anonymity. More recently, NASA officials tried to discourage a reporter from interviewing Hansen for this article and later insisted he could speak on the record only if an agency spokeswoman listened in on the conversation. "They're trying to control what's getting out to the public," Hansen said, adding that many of his colleagues are afraid to talk about the issue. "They're not willing to say much, because they've been pressured and they're afraid they'll get into trouble."

But Mary L. Cleave, deputy associate administrator for NASA's Office of Earth Science, said the agency insists on monitoring interviews with scientists to ensure they are not misquoted. "People could see it as a constraint," Cleave said. "As a manager, I might see it as protection."

John R. Christy, director of the Earth Science System Center at the University of Alabama in Huntsville, said it is possible increased warming will be offset by other factors, such as increased cloudiness that would reflect more sunlight. "Whatever happens, we will adapt to it," Christy said.

Scientists who read the history of Earth's climate in ancient sediments, ice cores and fossils find clear signs that it has shifted abruptly in the past on a scale that could prove disastrous for modern society. Peter B. deMenocal, an associate professor at the Lamont-Doherty Earth Observatory of Columbia University, said that about 8,200 years ago, a very sudden cooling shut down the Atlantic conveyor belt. As a result, the land temperature in Greenland dropped more than 9 degrees Fahrenheit within a decade or two. "It's not this abstract notion that happens over millions of years," deMenocal said. "The magnitude of what we're talking about greatly, greatly exceeds anything we've withstood in human history."

These kinds of concerns have spurred some governments to make major cuts in the carbon dioxide emissions linked to global warming. Britain has slashed its emissions by 14 percent, compared with 1990 levels, and aims to reduce them by 60 percent by 2050. Some European countries, however, are lagging well behind their targets under the international Kyoto climate treaty.

David Warrilow, who heads science policy on climate change for Britain's Department of Environment, Food and Rural Affairs, said that while the science remains unsettled, his government has decided to take a precautionary approach. He compared consuming massive amounts of fossil fuels to the strategy of the Titanic's crew, who were unable to avoid an iceberg because they were speeding across the Atlantic in hopes of breaking a record. "We know there are icebergs out there, but at the moment we're accelerating toward the tipping point," Warrilow said in an interview. "This is silly. We should be doing the opposite, slowing down whilst we build up our knowledge base."

The Bush administration espouses a different approach. Marburger said that though everyone agrees carbon dioxide emissions should decline, the United States prefers to promote cleaner technology rather than impose mandatory greenhouse gas limits. "The U.S. is the world leader in doing something on climate change because of its actions on changing technology," he said.

Stanford University climatologist Stephen H. Schneider, who is helping oversee a major international assessment of how climate change could expose humans and the environment to new vulnerabilities, said countries respond differently to the global warming issue in part because they are affected differently by it. The small island nation of Kiribati is made up of 33 small atolls, none of which is more than 6.5 feet above the South Pacific, and it is only a matter of time before the entire country is submerged by the rising sea. "For Kiribati, the tipping point has already occurred," Schneider said. "As far as they're concerned, it's tipped, but they have no economic clout in the world."

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Many people would like to be kind to others so Leftists exploit that with their nonsense about equality. Most people want a clean, green environment so Greenies exploit that by inventing all sorts of far-fetched threats to the environment. But for both, the real motive is to promote themselves as wiser and better than everyone else, truth regardless.

Global warming has taken the place of Communism as an absurdity that "liberals" will defend to the death regardless of the evidence showing its folly. Evidence never has mattered to real Leftists

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