Admiral MacDonald RN
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Apparently cable (pay TV) is running a "documentary" on Global Cooling. See other thread elsewhere. Here are some facts.
RealClimate RealClimate is a commentary site on climate science by working climate scientists for the interested public and journalists. We aim to provide a quick response to developing stories and provide the context sometimes missing in mainstream commentary.
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14 Jan 2005 The global cooling myth Filed under: Climate Science Paleoclimate Greenhouse gases Instrumental Record— william @ 5:31 am - () Every now and again, the myth that "we shouldn't believe global warming predictions now, because in the 1970's they were predicting an ice age and/or cooling" surfaces. Recently, George Will mentioned it in his column (see Will-full ignorance) and the egregious Crichton manages to say "in the 1970's all the climate scientists believed an ice age was coming" (see Michael Crichton’s State of Confusion ). You can find it in various other places too [here, mildly here, etc]. But its not an argument used by respectable and knowledgeable skeptics, because it crumbles under analysis. That doesn't stop it repeatedly cropping up in newsgroups though. I should clarify that I'm talking about predictions in the scientific press. There were some regrettable things published in the popular press (e.g. Newsweek; though National Geographic did better). But we're only responsible for the scienti press. If you want to look at an analysis of various papers that mention the subject, then try http://www.wmconnolley.org.uk/sci/iceage/. Where does the myth come from? Naturally enough, there is a kernel of truth behind it all. Firstly, there was a trend of cooling from the 40's to the 70's (although that needs to be qualified, as hemispheric or global temperature datasets were only just beginning to be assembled then). But people were well aware that extrapolating such a short trend was a mistake (Mason, 1976) . Secondly, it was becoming clear that ice ages followed a regular pattern and that interglacials (such as we are now in) were much shorter that the full glacial periods in between. Somehow this seems to have morphed (perhaps more in the popular mind than elsewhere) into the idea that the next ice age was predicatable and imminent. Thirdly, there were concerns about the relative magnitudes of aerosol forcing (cooling) and CO2 forcing (warming), although this latter strand seems to have been short lived. The state of the science at the time (say, the mid 1970's), based on reading the papers is, in summary: "...we do not have a good quantitative understanding of our climate machine and what determines its course. Without the fundamental understanding, it does not seem possible to predict climate..." (which is taken directly from NAS, 1975). In a bit more detail, people were aware of various forcing mechanisms - the ice age cycle; CO2 warming; aerosol cooling - but didn't know which would be dominant in the near future. By the end of the 1970's, though, it had become clear that CO2 warming would probably be dominant; that conclusion has subsequently strengthened. George Will asserts that Science magazine (Dec. 10, 1976) warned about "extensive Northern Hemisphere glaciation.". The quote is from Hays et al. But the quote is taken grossly out of context. Here, in full, is the small section dealing with prediction: Future climate. Having presented evidence that major changes in past climate were associated with variations in the geometry of the earth's orbit, we should be able to predict the trend of future climate. Such forecasts must be qualified in two ways. First, they apply only to the natural component of future climatic trends - and not to anthropogenic effects such as those due to the burning of fossil fuels. Second, they describe only the long-term trends, because they are linked to orbital variations with periods of 20,000 years and longer. Climatic oscillations at higher frequencies are not predicted. One approach to forecasting the natural long-term climate trend is to estimate the time constants of response necessary to explain the observed phase relationships between orbital variation and climatic change, and then to use those time constants in the exponential-response model. When such a model is applied to Vernekar's (39) astronomical projections, the results indicate that the long-term trend over the next 20,000 years is towards extensive Northern Hemisphere glaciation and cooler climate (80). The point about timescales is worth noticing: predicting an ice age (even in the absence of human forcing) is almost impossible within a timescale that you could call "imminent" (perhaps a century: comparable to the scales typically used in global warming projections) because ice ages are slow, when caused by orbital forcing type mechanisms. Will also quotes "a full-blown 10,000-year ice age" (Science, March 1, 1975). The quote is accurate, but the source isn't. The piece isn't from "Science"; its from "Science News". There is a major difference: Science is (jointly with Nature) the most prestigous journal for natural science; Science News is not a peer-reviewed journal at all, though it is still respectable. In this case, its process went a bit wrong: the desire for a good story overwhelmed its reading of the NAS report which was presumably too boring to present directly. The Hays paper above is the most notable example of the "ice age" strand. Indeed, its a very important paper in the history of climate, linking observed cycles in ocean sediment cores to orbital forcing periodicities. Of the other strand, aerosol cooling, Rasool and Schneider, Science, July 1971, p 138, "Atmospheric Carbon Dioxide and Aerosols: Effects of Large Increases on Global Climate" is the best exemplar. This contains the quote that quadrupling aerosols could decrease the mean surface temperature (of Earth) by as much as 3.5 degrees K. If sustained over a period of several years, such a temperature decrease could be sufficient to trigger an ice age!. But even this paper qualifies its predictions (whether or not aerosols would so increase was unknown) and speculates that nuclear power may have largely replaced fossil fuels as a means of energy production (thereby, presumably, removing the aerosol problem). There are, incidentally, other scientific problems with the paper: notably that the model used was only suitable for small perturbations but the results are for rather large perturbations; and that the estimate of CO2 sensitivity was too low by a factor of about 3. Probably the best summary of the time was the 1975 NAS/NRC report. This is a serious sober assessment of what was known at the time, and their conclusion was that they didn't know enough to make predictions. From the "Summary of principal conclusions and recommendations", we find that they said we should: Establish National climatic research program Establish Climatic data analysis program, and new facilities, and studies of impact of climate on man Develope Climatic index monitoring program Establish Climatic modelling and applications program, and exploration of possible future climates using coupled GCMs Adoption and development of International climatic research program Development of International Palaeoclimatic data network Which is to say, they recommended more research, not action. Which was entirely appropriate to the state of the science at the time. In the last 30 years, of course, enormous progress has been made in the field of climate science. Most of this post has been about the science of 30 years ago. From the point of view of todays science, and with extra data available: The cooling trend from the 40's to the 70's now looks more like a slight interruption of an upward trend (e.g. here). It turns out that the northern hemisphere cooling was larger than the southern (consistent with the nowadays accepted interpreation that the cooling was largely caused by sulphate aerosols); at first, only NH records were available. Sulphate aerosols have not increased as much as once feared (partly through efforts to combat acid rain); CO2 forcing is greater. Indeed IPCC projections of future temperature inceases went up from the 1995 SAR to the 2001 TAR because estimates of future sulphate aerosol levels were lowered (SPM). Interpretations of future changes in the earths orbit have changed somewhat. It now seems likely (Loutre and Berger, Climatic Change, 46: (1-2) 61-90 2000) that the current interglacial, based purely on natural forcing, would last for an exceptionally long time: perhaps 50,000 years. Finally, its clear that there were concerns, perhaps quite strong, in the minds of a number of scientists of the time. And yet, the papers of the time present a clear consensus that future climate change could not be predicted with the knowledge then available. Apparently, the peer review and editing process involved in scientific publication was sufficient to provide a sober view. This episode shows the scientific press in a very good light; and a clear contrast to the lack of any such process in the popular press, then and now. Further Reading: Imbrie & Imbrie "Ice Ages: solving the mystery" (1979) is an interesting general book on the discovery of the ice ages and their mechanisms; chapter 16 deals with "The coming ice age". Spencer Weart's History of Global Warming has a chapter on Past Cycles: Ice Age Speculations. An analysis of various papers that mention the subject is at www.wmconnolley.org.uk/sci/iceage/.
Global Glacial Retreat
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18 Mar 2005 Worldwide glacier retreat Filed under: Climate Science Paleoclimate Instrumental Record— eric @ 1:09 pm - () One of the most visually compelling examples of recent climate change is the retreat of glaciers in mountain regions. In the U.S. this is perhaps most famously observed in Glacier National Park, where the terminus of glaciers have retreated by several kilometers in the past century, and could be gone before the next century (see e.g. the USGS web site, here, and here). In Europe, where there is abundant historical information (in the form of paintings, photographs, as well as more formal record-keeping), retreat has been virtually monotonic since the mid 19th century (see e.g. images of the glaciers at Chamonix). These changes are extremely well documented, and no serious person questions that they demonstrate long term warming of climate in these regions. New work published in Science ("Extracting a Climate Signal from 169 Glacier Records") highlights these results, and uses them to make a new estimate of global temperature history since about 1600 A.D., which agrees rather well with previous, independent temperature reconstructions. Of course, as we frequently remind readers on this site, changes in one particular region do not necessarily translate to worldwide trends. That is why the work of such groups of scientists as the World Glacier Monitoring Service, which compiles observations on changes in mass, volume, area and length of glaciers, is important. From the compilations of WGMS (and many other groups and individuals), we know that glacier retreat is in fact an essentially global phenomenon, with only a few isolated (and well understood) counter-examples, such as western Norway. The figure at right shows an example from WGMS, as published in the 2001 IPCC report. (Click on the figure for details).The photos at left show South Cascade Glacier in Washington State in 1928 and 2000. What causes glaciers to retreat like this? With the exception of glaciers that terminate in the ocean, and glaciers in the polar regions or at extreme high altitudes where the temperature is always below freezing, essentially just two things determine whether a glacier is advancing or retreating: how much snow falls in the winter, and how warm it is during the summer.  For typical glaciers in mid latitudes, the role of temperature is generally more important than winter precipitation. This is because a bit of extra heat in summer is a very efficient way to get rid of ice. A 1°C increase in temperature, applied uniformly across a glacier, is enough to melt a vertical meter of ice each year. For typical mid-latitude glaciers, winter snow accumulation is on the order of 1 m/year (ice equivalent -- or about 3 m of snow). On balance then, a 1°C rise in summer temperature has roughly the same effect as a year in which no snow accumulates. Put another way, for every degree rise in summer temperature, an extra meter of ice-equivalent would be required to offset the extra loss. (This makes it clear why glaciers in coastal Norway are not as strongly influenced by temperature – at these locations, winter precipitation typically exceeds several ice-equivalent meters per year). To give another, more specific example, at a typical glacier on Mt. Baker, in Washington State, a summer temperature increase of 1°C translates to a ~150 m increase in the altitude of the equilibrium line (the point where annual ice accumulation = annual loss), and a resulting ~2 km retreat of the glacier terminus. The same change, if driven by winter precipitation, would require about a 25% decrease in local precipitation at this site. What all this means is that glaciers comprise a rather nice “proxy” for climate change in general, and for temperature change in particular. Glaciologists have for many years used this fact to make estimates of temperature change from records of glacier change. This work received an important update in the journal Science, with the publication of a paper by J. Oerlemans, of Utrecht University. Oerlemans's paper does three useful things. First, it provides a compilation of global trends in glacier terminus positions since 1600 A.D. Second, it uses this compilation to create a new estimate of global temperature change. Third it provides an estimate of uncertainties on the temperature estimates, taking into account plausible changes in winter precipitation. Oerlemans’s reconstruction of global temperatures (largely from mid latitude glaciers) is entirely independent of the much talked about temperature records from other paleoclimate proxy data (e.g. Moberg and others, Mann and others, Crowley and others). Yet Oerlemans’s findings basically agree with the earlier results, as shown in the figure, below. Indeed, the reconstruction of temperature from glacier data is notable for having a rather distinctive "hockey stick" shape, the aspect of the original Mann, Bradley & Hughes reconstruction that seems to attract the most attention and criticism. This poses a substantial challenge to those who have dismissed the “hockey stick” as due to biases or errors. Some will of course quibble with this perspective, because the data prior to the 19th century are rather sparse. (Only a few records go back to the 17th and 18th centuries). However, the "hockey stick" shape is clearly in the data, from both the Northern and Southern hemispheres (see for example the data for Grindelwald, d'Argentière, and Franz Joseph in the figure at right).  Figure shows comparison of the Oerlemans reconstruction with those of Mann et al. 1999, and Moberg et al., 2005. Click on the figure for comparisons with other temperature reconstrutions. A few comments: First, the exact relationship between a glacier and temperature is a bit more complex than implied above, and also depends on the glacier geometry and aspect (which direction it is facing), and on radiative as well as sensible heat fluxes. (The difference between radiative and sensible heat fluxes may be thought of as the difference between the ambient temperature is, and how intense the sun is. We all have had the experience of feeling warmth when sitting in the sun on a day when the air temperature is quite cold. Glaciers experience the same thing.) Oerlemans addresses this by using a simple linear model that relates the glacier length to temperature, with adjustments for the glacier geometry and the local annual precipitation for each glacier. It should be noted that a lot of work was required to do these calculations, much of it presumably by Oerlemans’s student L. Klok. Many of the details are not given in the paper due to the short space provided by Science, but all the information most will want is in the online Supplemental Data on the Science website. (If you want more, see the paper by Klok and Oerlemans in The Holocene.) Second, Oerlemans’s reconstruction doesn’t say anything about the ongoing debate of whether the “Medieval warm period” was as warm as today. Certainly there is evidence that some glaciers were as small or smaller than they are today at some locations, around 1000 years ago. However, the extent to which the “Medieval warm period” was a pervasive, essentially synchronous retreat of glaciers worldwide (as is happening now) is still open to question (see e.g. Bradley et al., 2003). Finally, Oerlemans’s work doesn’t address whether or not the worldwide glacier retreat is part of a “natural” phenomenon. Indeed, the fact that glaciers were generally more advanced in the 19th century than they are today is exactly what gave rise to the term Little Ice Age (coined by a newspaper reporter in California, writing about F.E. Matthes work on glaciers in the Sierra Nevada). Again though, the evidence that the Little Ice Age advances were as synchronous worldwide as the current glacier retreats are today is sketchy. In any case, what Oerlemans’s paper does very well is to demonstrate (one more time) what we already knew: global temperatures have risen more than 0.5 degrees C in the last century (up to 1990 -- we don't yet have a compilation of the latest data). As Oerlemans points out, the only way for this to be substantially in error is if there has been worldwide decreases in summertime cloudiness (by 30% or so!), or in winter precipitation (by 25%!). There is no evidence for either of these changes occurring, and if there were, it would be a remarkable discovery in and of itself.
If the we are experiencing Global Cooling - how can there be simultaneous Global Glacial Retreat ?
Anthropomorphic Global Cimate Change Connections
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26 Apr 2005 Pollution-Climate Connections Filed under: Climate Science Greenhouse gases Climate modelling Aerosols— group @ 10:16 am - () Guest commentary by Loretta Mickley, Harvard University
Every summer over much of the United States, we brace ourselves for heat waves. During these periods, the air turns muggy and usually smoggy. After a few days, a cold front moves in, sweeping away the pollution and ending the heat. Given that we are on a path towards global warming, atmospheric chemists are asking how climate change could affect air quality. Will warmer temperatures mean more pollution during these episodes? Will episodes last longer? Most importantly, what effect will changes in air quality have on human health?
Recently the National Resource Defense Council (NRDC) released Heat Advisory, a report warning that surface air quality could suffer greatly as a result of climate change. In response, a group called the Pacific Research Institute (PRI), together with another group called United for Jobs, published Air Quality False Alarm, a detailed criticism of the NRDC forecast. PRI argues, among other things, that anthropogenic emissions in the U.S. will drop sharply in coming decades. In their view, air pollution will become a thing of the past, no matter what happens to climate.
What’s the story here? First, a little background on ozone and particulate matter (PM), two major components of smog. Surface ozone is formed from a mix of natural and anthropogenic precursors like nitrogen oxides and volatile organic carbon. We have measurements of surface ozone dating back to the late 1800s which imply that ozone in some regions has increased 2-5 times due to emissions of ozone precursors from cars, industry, and power plants. As for PM, there are many different kinds – e.g., organic carbon, soot, and sulfate-ammonium-nitrate. Some kinds of PM, like soot, are directly emitted into the air, but other kinds condense from gas-phase molecules. Like ozone, PM has both natural and anthropogenic ingredients.
Many factors govern the severity and timing of pollution episodes. An obvious factor is the magnitude of precursor emissions. But there are meteorological factors, like how stagnant the surface air is and whether it’s clear or cloudy, warm or cool. The summer of 1998, for example, saw a record number of ozone exceedances averaged over New England. That summer was also the warmest on record for that region. The hot summer that Europe endured in 2003 was also a summer of high pollution levels for that continent. But the cool summer in the U.S. that same year meant that the we saw low levels of pollution.
So how will pollution evolve over the coming decades as climate changes? The easy answer is: oh, the warmer temperatures mean greater pollution! But it’s more complicated than that. Then there are other meteorological factors to consider. As the surface temperatures rise, will the depth of the boundary layer increase, diluting the pollutants within it? Maybe stronger surface winds will carry all the pollution away. What about changes in cloud cover or rainfall?
To tackle issues of this complexity, modelers often turn to sensitivity studies. A sensitivity study is one in which you change just one or two variables, and keep everything else constant. By taking the problem apart in this way, you can isolate the effect of one or two factors at a time.
In one sensitivity study, Aw and Kleeman [2003] imposed a 5ºC increase in temperature over the Los Angeles basin, but kept all other meteorological variables (like windspeed) constant in their model. Ozone in the region increased by 10-15%, but concentrations of sulfate-ammonia-nitrate PM decreased by 10-15%. That’s because ammonia condenses less readily at high temperatures. This is an interesting result. But in the real world, stalled high pressure systems, like the one over the Midwest and Northeast last week (April 18-20), can lead to both warm temperatures and high PM. With clear skies and weak winds, PM can accumulate over the source regions. As the climate changes, not only could temperatures change, but also the behavior of these high pressure systems.
In my research group, we tried a different sensitivity study [Mickley et al., 2004]. We devised our model experiment to test just the effect of changing wind patterns on pollutant concentrations. What we found was that the severity summertime regional pollution episodes in the Midwest and Northeast U.S. increased significantly by 2050, relative to present. Also, the average length of an episode increased from 2 to 3-4 days. Why did this happen? Our model forecast a 20% decline in the frequency of cold fronts sweeping into the U.S., so stagnation events in the model persisted longer. That allowed both gas-phase and PM pollution to build to higher concentrations.
Another model study [Hogrefe et al., 2004] focused on the effect of climate change on just surface ozone. The authors found that even with emissions of ozone precursors in the model held at 1990s levels, the total number of “exceedance days” increased by about 60% over the eastern U.S. (An exceedance day is a day in which ozone averaged over 8 hours exceeds the EPA threshold of 84 ppb.) Because of the complexity of the study, Hogrefe et al. [2004] could not diagnose precisely all the meteorological changes (temperature? circulation patterns?) contributing to the increased surface ozone in their model. But they did find that one factor accounting for about half the increase was enhanced emissions of natural ozone precursors, which are temperature-sensitive.
One of the biggest unknowns, of course, is how anthropogenic emissions will evolve in the future. The Clean Air Act has led to tremendous improvements in air quality since the 1970s. But even if our emissions do decline, the consequences for air pollution are uncertain. Fiore et al. [2002] have shown that decreases in U.S. emissions may be offset by increases elsewhere in the world. Specifically, rising methane emissions elsewhere in the world could significantly enhance background levels of ozone over the U.S., leading to as much pollution in 2030 as we saw in the mid-1990s.
So there’s a lot more to be learned about the links between climate and pollution. Since both surface ozone and PM have adverse effects on human health, understanding the link is important.
References:
Aw, J., and M.J. Kleeman, Evaluating the first-order effect of intraannual air pollution on urban air pollution, J. Geophys. Res., 108, 4365, 10.1029/2002JD002688, 2003.
Fiore, A.M., D.J. Jacob, B.D. Field, D.G. Streets, S.D. Fernandes, and C. Jang, Linking ozone pollution and climate change: The case for controlling methane, Geophys. Res. Lett., 29, 1919, doi:10.1029/2002GL015601, 2002.
Hogrefe, C., B. Lynn, K. Civerolo, J.-Y. Ku, J. Rosenthal, C. Rosenzweig, S. Gaffin, K. Knowlton, and P.L. Kinney, Simulating changes in regional air pollution over the eastern United States due to changes in global and regional climate and emissions. J. Geophys. Res., 109, D22301, doi:10.1029/2004JD004690, 2004.
Mickley, L. J., D. J. Jacob, B. D. Field, and D. Rind, Effects of future climate change on regional air pollution episodes in the United States, Geophys. Res. Let., 30, L24103, doi:10.1029/2004GL021216, 2004.
What about the Gulf Stream ?
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26 May 2005 Gulf Stream slowdown? Filed under: Climate Science Paleoclimate Oceans— gavin @ 3:21 am There has been an overwhelming popular demand for us to weigh in on recent reports in the Times Britain faces big chill as ocean current slows and CNN Changes in Gulf Stream could chill Europe (note the interesting shift in geographical perspective!). At the heart of the story was a statement at the recent EGU meeting by Peter Wadhams from Cambridge University, that convection in a normally active area of the Greenland Sea was much reduced last winter. Specifically, in an area where a dozen or so convective 'chimneys' form, only two small chimneys were seen. (Unfortunately, I can't seem to be able to find a relevant abstract of Dr. Wadhams talk, and so I have to rely on the Times' news reports for the specifics).  Convective chimneys in the seas bounded by Greenland, Iceland and Norway occur when intense cooling of the ocean, usually associated with a low-pressure system passing through, breaks down the normally stable ocean layers and causes the now colder, denser water to convect and mix down to a relatively deep layer. This area of the world is one of only a few places where the underlying ocean column is marginally stable enough that this process can occur in the open ocean and lead to convective chimneys going down 2000 to 3000 meters. The deep water masses formed in this way are then exported out of the area in deep currents that eventually make up "North Atlantic Deep Water" (which also contains contributions from the Labrador Sea and entrainment of other water masses). This process is part of what is called the 'thermohaline' or 'overturning' circulation and is associated with a significant amount of heat transport into the North Atlantic, which indeed keeps Britain and the rest of the North Atlantic region 3 to 6 degrees C warmer than they otherwise would be. The figure gives two model estimates for the impact of this circulation (Stocker, 2002). This heat transport is often associated with the Gulf Stream in the media and among the public. However, my pedantic side obliges me to point out that the Gulf Stream is a predominantly wind-driven western boundary current that moves up from the Gulf of Mexico along the US coast to Cape Hatteras, at which point it heads off into the central Atlantic (see also this letter by Carl Wunsch). It then turns into the North Atlantic Drift which is really the flow of water responsible for the anomalous northward heat transport in the Atlantic. There is good evidence from past climates, theoretical studies and climate models that large changes, a slowing down or even a complete collapse, in the North Atlantic Drift and the thermohaline circulation can happen. Indeed climate models generally (though not exclusively) forecast a slowdown in this circulation by 2100. This occurs mainly as a function of increased rainfall in the region which strengthens the ocean layering and reduces the amount of convection in the region. It is probably futile to insist on it at this point, but a collapse of the overturning circulation is not the same as a collapse or reversal of the Gulf Stream (which as I mentioned above is predominantly wind-driven). Getting back to the statement by Peter Wadhams though, how does this relatively small-scale observation get translated into headlines forecasting changes in the Gulf Stream and chilly times ahead for Europe? The major problem is that the background story and the climate model results are now very well known, and any scientific result that appears to project onto this storyline therefore gets a lot of attention. However, it is a long way from the Greenland Sea to the Gulf Stream and some important points did not get a mention in the news stories. Firstly, we know that there is a great deal of decadal variability in how much and where deep convection takes place. Indeed, it was reported by Schlosser et al (1991), that based on CFC measurements, very little convection had occured in the Greenland Sea over the previous 7 years. Subsequently, convection was renewed. Similarly, convection in the Labrador Sea (the other main component) has also oscillated, possibly out of phase with the convection in Greenland. Studies by Dickson et al (1999, 2002) showed that properties of the deep water overflowing the Denmark Strait (between the Greenland Sea and rest of the Atlantic) appear to be related to patterns of variability like the North Atlantic Osillation, and this may help explain some of the variabilty. To be sure, there are some long term trends that are becoming discernable. There is a freshening of the North Atlantic visible since the 1950s. Long continuous records of temperature and salinity at Ocean Weather Station M in the Norwegian Sea indicate that the deep water has also warmed noticeably. However, monitoring networks are now starting to be put in place (Osterhus et al, 2005) and better integrated data will be available in the future. It is important to bear in mind that while the changes being seen are indeed significant given the accuracy of modern oceanography, the magnitude of the changes (a few hundredths of a salinity unit) are very much smaller (maybe two orders of magnitude) than the kinds of changes inferred from the paleo data or seen in climate models. Thus while continued monitoring of this key climatic area is clearly warranted, the imminent chilling of the Europe is a ways off yet.
Yes , Global Warming can lead to regional cooling perhaps most the northern hemisphere might cool dramatically if the Gulf Stream slows or shuts down, but the world is more than just the northern hemisphere.
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