cogitator
Since Oct 19, 2000
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cogitator
Since Oct 19, 2000
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New in November '07: Climate scepticism: The Top 10 (with short refutations)
So, here we go.
#1. Volcanoes emit more CO2 in one eruption than all of mankind ever did, from the beginning of history. (There are various flavors of this one.)
NOT TRUE. CO2 is not one of the main gaseous components of volcanic emissions. The main gas coming out of a volcano is water vapor, followed by sulfur dioxide (SO2) and some acidic species, foremost being HCl. In an average year, volcanic SO2 emissions can be significant compared to human emissions (generally about 13%), and probably higher in a year with a big eruption. Intriguingly, however, the variable rate of plate tectonics apparently determined CO2 levels in the atmosphere, and modified climate, on geologic time-scales (100,000 to millions of years) because if the rate of oceanic crustal plate subduction speeds up, coastal carbonate sediments get "processed" in the subduction zone and there will be more CO2 in volcanic emissions. But due to the natural fluxes into and out of the atmosphere, it takes a long time for an appreciable shift -- shifts much larger than the current changes or even glacial-interglacial variability in atmospheric CO2 concentration.
Supporting links:
Gases: Man versus the Volcanoes
The Relationship Between Plate Tectonics and the Carbon Cycle
Natural Impacts on the Climate System (this one will be referenced again)
#2. Mars (and Jupiter, Saturn, Enceladus, Triton, and Pluto) is (are) experiencing global warming, and this shows either a) that humans don't have anything to do with global warming because its happening there, or b) that solar variability is causing global warming on Earth and other Solar System bodies.
NOT TRUE. None of the observations of other bodies in the Solar System impply or demonstrate a linkage to a significant recent increase in solar variability that could have created a climate change. Several of the observations cited by skeptics, in fact, have no suggested linkage to solar variability whatsoever.
First: The Role of the Sun in 20th Century Climate Change
Regarding Mars: "Thus inferring global warming from a 3 Martian year regional trend is unwarranted. The observed regional changes in south polar ice cover are almost certainly due to a regional climate transition, not a global phenomenon, and are demonstrably unrelated to external forcing." Global warming on Mars?
UPDATE: Turns out that there may be a global warming trend on Mars now, and this is caused by dust storms, influenced by changes in Mars' albedo. The RealClimate article notes that Mars apparently cooled off due to a reduction in dust storm activity from the 1970s to 2000; the warming trend appears to be due to increased dust storm activity since 2000.
Red planet heating up
Global warming and climate forcing by recent albedo changes on Mars.
Regarding Jupiter: "The latest images could provide evidence that Jupiter is in the midst of a global change that can modify temperatures by as much as 10 degrees Fahrenheit on different parts of the globe. ... The global change cycle began when the last of the white oval-shaped storms formed south of the Great Red Spot in 1939. As the storms started to merge between 1998 and 2000, the mixing of heat began to slow down at that latitude and has continued slowing ever since." [No linkage to solar variability suggested]
New Storm on Jupiter Hints at Climate Change
Jupiter's Winds Come From Inside
Research Predicts Global Climate Change on Jupiter
Regarding Pluto: "The change is likely a seasonal event, much as seasons on Earth change as the hemispheres alter their inclination to the Sun during the planet's annual orbit. ... Though Pluto was closest to the Sun in 1989, a warming trend 13 years later does not surprise David Tholen, a University of Hawaii astronomer involved in the discovery. "It takes time for materials to warm up and cool off, which is why the hottest part of the day on Earth is usually around 2 or 3 p.m. rather than local noon," Tholen said. "This warming trend on Pluto could easily last for another 13 years." [No link to solar variability suggested, though there is a link to solar insolation, similar to Milankovitch forcing of Earth's climate] Pluto is undergoing global warming, researchers find (also discusses Triton)
Regarding Triton: " There are two possible explanations for the moon's warmer weather. One is that the frost pattern on Triton's surface may have changed over the years, absorbing more and more of the sun's warmth. The other is that changes in reflectivity of Triton's ice may have caused it to absorb more heat." [No link to solar variability suggested] Global warming detected on Triton
Regarding Enceladus, warming is not even suggested. The question is about how Enceladus has an internal heat source allowing the generation of water-ice jets.
Enceladus (up-to-date with recent references)
Regarding Saturn: The observation is that Saturn's south pole is "warm", with a polar vortex due to atmospheric circulation. No global warming is suggested and no linkage to solar activity or variability is suggested. Saturn's Bulls-Eye Marks Its Hot Spot
APPARENTLY other like-minded individuals are also perceiving the widespread erroneous attribution of solar variability to changes observed on other Solar System bodies. Here is a prime example: Is global warming solar-induced? from "Bad Astronomy" (The parallelism of incorrect arguments on this topic here on FR with those described at this link is remarkable.)
#3. In the 1970s, scientists were predicting global cooling. They were wrong then so they must be wrong now.
INACCURATE. There are also various flavors of this one around. Rather than devote much space to addressing it, the page cited below on "A Few Things Ill-Considered" provides discussion and links. Briefly, a very mild cooling trend from the late 1940s to the late 1970s was observed. The likely causes were a combination of natural variability and massive increases in SO2 emissions from post-war reindustrialization. It was suggested (and hyped in the media and a particular book, "The Cooling" by non-scientist Lowell Ponte) that if the trends continued that they could cause a significant cooling climate change. Most scientists actually indicated that the climate models of the time were too crude to make any definitive projections. They Predicted Cooling in the 1970s
#4. Actually, global temperatures have been cooling down since 1998.
INCORRECT. This claim was apparently first made by Bob Carter of the University of Queensland in Australia. It is based on the observation that 1998 is the warmest year in the historical record (by a whisker over 2005 for NOAA and second to 2005 for the NASA Goddard Institute of Space Studies [GISS] analysis). As most know, 1998 was the year of what was likely the largest El Nino event ever observed. El Nino years are always climatically warm years; demonstrate this by looking at the two images below (the bottom image can be clicked for a larger version). The top image shows El Nino conditions (red) and La Nina conditions (blue) -- anything between 1 and -1 is "normal" (not in an El Nino or La Nina condition).
Immediately after 1998, the short-term "trend" was negative, as would be expected after a record warm year followed by a La Nina year. But temperatures rebounded, such that there is no perceptible cooling trend from 1998 to present now, and as noted, 2005, a normal year in terms of El Nino, was either just barely less warm than 1998 or just a bit warmer than 1998. It's hard to claim a cooling trend when climatic factors indicate a continuing warming trend. See the links below for more information.
Yet more betting on climate with World Climate Report
NOAA 2005 global temperature summary
NASA GISS 2005 global temperature summary
Quote from the above: "Record warmth in 2005 is notable, because global temperature has not received any boost from a tropical El Niño this year. The prior record year, 1998, on the contrary, was lifted 0.2°C above the trend line by the strongest El Niño of the past century." [I have done a quick back-of-the-envelope calculation for a FR post, which is reproduced here. "OK, it's warmed about 0.4 C since 1975. Call it +0.13 C per century (which actually agrees with Spencer and Christy's MSU satellite data analysis, other groups put it higher). So if 1975 is the "0" point, then 1998 should have been 0.3 C warmer than 1985. Instead, it was 0.2 C above the trend line, i.e. 0.5 C. According to the current decadal trend (barring accelerations), it should take 38.5 years for the global temperature to rise 0.5 C. That would be -- 2013. So it's basically unsurprising (and expected) that there hasn't been a year warmer than 1998 yet, even though 2005 was a virtual tie. So we either have to wait until 2013 (or a year with the next large El Nino) to expect a new global temperature "record". "]
#5. During glacial-interglacial cycles, the initial temperature rise preceded the increase in atmospheric CO2. This proves that increasing atmospheric CO2 doesn't cause increasing global temperatures; in fact, warming temperatures cause CO2 to increase.
NOT TRUE. This is probably the least understood and most misconstrued issue with regard to current global warming. Below is my attempt to do a definitive treatment of this topic. [Completed, though some revision may still be necessary, 04/11/2007.]
First, the basics. Milankovitch cycle forcing is strongly correlated with most (not all) of the glacial/interglacial period variability. The figure below shows why. There is more significant climate change when the Milankovitch cycles superpositions are maxima (high variability) rather than minima (low variability). Note that the most distinct transitions from "hot" conditions to "cold" conditions tend to occur where the cycles shift from hight amplitude to low amplitude. Perhaps the easiest example to perceive occurs after 300K years ago.
The linked figure here is higher resolution, too big to fit in this profile. It nicely shows the main glacial and interglacial terminations.
Climate cycles
The Milankovitch cycle correlations might make it tempting to ascribe the global temperature changes for glacial/interglacial cycles to the variability of solar insolation they induce. But the change in solar insolation is insufficient, by far, to explain the full temperature range. The transitions between Milankovitch superposition maxima and minima only initiate glacial or interglacial transitions.
Support: The CO2-temperature correlation
What controls ice sheet growth? (A large Powerpoint presentation, explains how insolation affects ice sheet growth). Slides 19-22 are most relevant; the glaciation threshold is temperature determined. This helps to explain why not all of the Milankovitch cycle minima and maxima result in glacial or interglacial period terminations. Note that continental glaciation is an important feedback process due to a significant effect on Earth's albedo.
Solar Influences on Global Change, pages 46-47 The key section terminates this chapter.
Some of the discrepancies are described here: a brief introduction to ice age theories
Now that this has been discussed, the next question addresses the role of CO2. The solar insolation changes due to Milankovitch cycles initiate the process, and then climate system feedbacks get going. The big question about the lag time is that the temperature increase leads the CO2 increase. Before getting into how that works, a simple thought experiment.
Imagine a bowling ball sitting on a hillside. The hillside is sufficiently grassy to keep the ball in place, but not sufficient to stop a rolling bowling ball. A person approaches the ball and pushes it with a single finger. The ball starts to roll down the hill, picking up speed all the way down.
Question: was the force imparted by the push from the single finger responsible for the full speed and momentum achieved by the bowling ball?
Clearly (I hope) not. The initiation factor (the push) does not impart nearly enough force to create the bowling ball's top speed. The top speed is a result of feedback in all of the components of the system (positive, the slope of the hill, gravity; negative, the drag of the grass and soil). Until the slope lessens and the drag overcomes the acceleration due to gravity, the ball will pick up speed.
Basically (the details are below) in the transition from a cold glacial period to a warm interglacial period, rapidly changing insolation initiates the climate response by an increase in temperature. When positive feedbacks are stronger than negative feedbacks, warming will ensue. The main positive feedback is increasing atmospheric CO2 concentrations (the cause of that is apparently multifold, see below). An increase in atmospheric CO2 concentration causes a temperature increase, which causes another increase in atmospheric CO2*, and so on, and so on. Because the temperature increase started before the increase in CO2, it "leads" the increase in CO2. But increasing atmospheric CO2 is the positive feedback factor that causes the temperatures to increase.
* It should be noted that increasing temperature caused by CO2 forces another positive feedback effect, increased relative humidity (higher atmospheric water vapor concentration). Water vapor changes very rapidly in the atmosphere, so the increased water vapor concentration caused by global temperature increase is considered a feedback, not a forcing, of global climate. CO2's much longer atmosopheric lifetime makes it a climate forcing factor -- even though alteration of the climate system affects feedback mechanisms that influence atmospheric CO2 concentration, as discussed below. Water vapor changes in a matter of days; CO2 changes over decades, centuries, and millenia.
[For an interglacial to glacial transition, substitute "decrease" for "increase" and "negative" for "positive", and "cooling" for "warming" in the paragaph above.]
Just as the push of the finger was not responsible for most of the speed and momentum of the bowling ball rolling down the hillside, the initial Milankovitch -forced change in solar insolation (and corresponding change in summer temperatures, affecting ice sheet melting and volume) is not responsible for most of the warming (or cooling) observed in Pleistocene climate transitions. For the bowling ball, the main operative factor is gravity; for Pleistocene climate, the main operative factor is the atmospheric concentration of CO2. The next most important factor is the change in planetary albedo caused by continental ice sheets.
Now for the fun part. Why does atmospheric CO2 go up or down in glacial-interglacial cycles??
Paleoclimate scientists will be the first to admit that they haven't fully figured this out yet -- because that means more research is needed!. The best that the models have accomplished is about 2/3 of the 80-100 ppm change observed between the atmospheric CO2 concentration minimum of a glacial period and the atmospheric concentration maximum of an interglacial period.
References:
1. A model of glacial-interglacial cycles
2. Climate Models and Predictions for the Future
3. Feedback Loops in Global Climate Change Point to a Very Hot 21st Century
4. The riddle of atmospheric carbon dioxide (this is a fairly large Powerpoint lecture, summarizing many of the proposed theories)
5. CO2 and T again
6. Glacial-Interglacial Changes in Deep-Sea CO3= and Atmospheric d13C: Predictions from Competing Biogeochemical Mechanisms
7. Investigating the Southern Ocean’s role in the uptake of atmospheric CO2 along glacial-interglacial timescales (PDF)
8. Glacial-interglacial variations in atmospheric carbon dioxide (PDF)
9. What caused the glacial/interglacial atmospheric pCO2 cycles? (PDF)
10. A possible sequence of events for the generalized glacial-interglacial cycle (Abstract only)
11. Link to a discussion of the scientific accuracy of the "Great Global Warming Swindle" (basic discussion of the CO2 lag issue)
12. 800-year CO2 lag explained (a "confirming" resource)
Lucky 13 (New in late April): The lag between temperature and CO2
Summarizing the putative causes of glacial-interglacial CO2 variability, and identifying the most promising full explanations, is difficult. In the following, I will only talk about the increase in CO2 in a glacial-to-interglacial transition, for simplicity and because this is where the misconceptions are usually focused. A thematic reversal of process and result is required for an interglacial-glacial transition when atmospheric CO2 levels are decreasing. I will briefly summarize each process and discuss the amount of the effect, and the pros and the cons regarding it as the explanation for the atmospheric CO2 change.
1. Decreased solubility of CO2 in warmer surface ocean waters and warming continents. This is widely (and mistakenly) given as the ultimate cause of the CO2 increase. It is even proposed as an explanation for the Industrial Age CO2 increase, which is, to put it mildly, ludicrous. For a long time, I also thought this was the simple explanation for the differences in atmospheric CO2 concentration between glacial and interglacial periods.
Reference #8 discusses this well in the "Ocean Temperature" section (reference 8 is the primary reference for what follows). Polar waters change by about +2.5 C, tropical waters by about +5 C. This would cause an increase (atmospheric) of about 30 ppm. There are two counter-processes; at the same time, oceanic salinity is decreasing. This effect causes a decrease in atmospheric CO2, because the solubility of CO2 increases in less saline water, of about -6.5 ppm. While the Earth is warming, there is also a net transfer of CO2 from the oceans to the land biosphere (more growing area), amounting to -15 ppm*. Total: +8.5 ppm -- an order of magnitude less than required.
2. * Ocean carbon content. One of the reasons for this value is that the decrease in the inorganic carbon content of the oceans decreases the dissolution of deep-sea calcium carbonate (CaCO3). The dissolution of CaCO3 in the oceans is controlled by the concentration of carbonate ion, which is determined by the equilibrium balance of the dissolved inorganic carbon content and the ocean alkalinity. Transferring CO2 from oceans to land causes the depth at which CaCO3 begins to dissolve to become shallower. However, in warmer oceans, more CaCO3 is produced by "calcareous" organisms compared to non-calcareous organisms, so more CaCO3 is buried in shallower waters, and more will dissolve in deeper waters. This could account for as much as 55 ppm, BUT only if the dissolution depth changes enough. According to the data from oceanic sediments, it doesn't. BUT... this effect is large enough that it must be part of the full explanation.
3. Changes in the ocean's "biological pump" rate, and the effect of less iron (less dusty conditions in warmer, more humid, periods). Changes in the ocean's nutrient content will affect the amount of phytoplankton growth than can support. In a warming world, counter-intuitively perhaps, phytoplankton growth is reduced. Briefly, the ocean's nutrient content decreases for a number of reasons; cold periods are drier and dustier, potentially providing more iron where iron is limited, so warmer conditions provide less iron to the oceans. The oceanic nitrate concentration is also sufficiently changeable to account for a significant change. (This effect is most pronounced in the polar regions, though it must be noted that the Arctic is significantly different than the Antarctic.) The problem with nitrate is that it doesn't appear to be the most important nutrient limiting phytoplankton growth on the time-scales of glacial-interglacial transistions, phosphate is -- and phosphate in the oceans doesn't change sufficiently fast enough.
4. More on the polar regions. Two major oceanic processes can affect CO2 in the polar regions as the world warms. The first is physical: the rate at which deep water exchanges with surface water increases; this increases the movement of deep water with more dissolved CO2 (due to CaCO3 dissolution and organic matter respiration) to the surface, where the CO2 is released to the atmosphere. THIS IS NOT THE SAME AS THE EFFECT OF WARMER OCEAN TEMPERATURES ON CO2 SOLUBILITY!!! The second is biological: nutrient utilization by phtoplankton decreases, so there is less phytoplankton activity, meaning less CO2 is removed from the atmosphere by biological activity. The uncertainties in this process are primarily due to the data which is used to determine the level of biological productivity.
So those are the various potential mechanisms. Because none of them "gets the job done" completely, reference 10 describes how these mechanisms in combination would actually accomplish the full range of CO2 atmospheric variability. Feel free to read the whole thing: here is the critical section regarding the warming scenario:
"The first part of the transition out of full glacial conditions is achieved through increased temperature and increased mixing in the Southern Ocean. The final part of the atmospheric pCO2 rise up to full interglacial conditions is accomplished through rising sea level and the subsequent change in mean ocean alkalinity and phosphate, and a rise in the Northern Hemisphere temperature and ocean mixing."
And so now we are back at feedback augmenting an initial temperature increase, ascribed to Milankovitch forcing. It is suggestive that the regions which are most affected by the small temperature changes accompanying Milankovitch superposition minima or maxima are the polar regions. The "mixed mechanism" explanation proposes that the initiation event is warming and increased ventilation of Southern Ocean waters, which due to the large volumes and amounts of dissolved CO2 can initiate a significant increase in atmospheric CO2 relatively quickly -- certainly 800 years, the observed lag time. The increasing atmospheric CO2 maintains the increasing temperature trend, further enhancing oceanic ventilation, and then initiating the other feedback mechanisms (rising sea level, oceanic chemistry changes). Thus, after the initiation step, increasing atmospheric CO2 and the corresponding temperature increase are the main drivers of the processes which complete the transition from glacial conditions to interglacial conditions.
To summarize and reiterate: because the initiation event is a small warming, the temperature increase will ALWAYS "lead" the increase in atmospheric CO2. But once the atmospheric CO2 increase has been initiated, it is the primary feedback mechanism "on itself" that maintains and completes the transition.
#6. 17,000 scientists signed a petition that said global warming is not being caused by humans, and also that increased atmospheric CO2 concentrations would be beneficial.
It isn't clear than anything close to 17,000 real practicing scientists signed the "petition". Furthermore, the petition was deeply deceptive. It was put out by the Oregon Institute of Science and Medicine (OISM).
First, here's what the petition "signers" were actually supporting, as given in 1998:
"We urge the United States government to reject the global warming agreement that was written in Kyoto, Japan in December, 1997, and any other similar proposals. The proposed limits on greenhouse gases would harm the environment, hinder the advance of science and technology, and damage the health and welfare of mankind. ... There is no convincing scientific evidence that human release of carbon dioxide, methane, or other greenhouse gasses is causing or will, in the foreseeable future, cause catastrophic heating of the Earth's atmosphere and disruption of the Earth's climate. Moreover, there is substantial scientific evidence that increases in atmospheric carbon dioxide produce many beneficial effects upon the natural plant and animal environments of the Earth."
A signer could wish to oppose the Kyoto Treaty, and yet still think that greenhouse gases are causing warming. A signer could also think that anthropogenic greenhouse gases are contributing to warming, but won't cause "catastrophic heating" (see note below). So even a statement that the 17,000 signers don't believe that human activities are causing global warming is unsupportable; the wording of the petition clearly says "catastrophic heating".
Note: Senator James Inhofe frequently refers to the "global warming hoax" as indicating that greenhouse gases will cause catastrophic climate change. By saying it this way, he either deliberately or accidentally has given himself an "out clause", because he can state, accurately, that he has never said greenhouse gases won't cause climate change. Just not catastrophic climate change.
More information on the Oregon Institute of Science and Medicine and the "petition" drive (from Sourcewatch):
The Oregon Institute of Science and Medicine (OISM) describes itself as "a small research institute" that studies "biochemistry, diagnostic medicine, nutrition, preventive medicine and the molecular biology of aging." It is headed by Arthur B. Robinson, an eccentric scientist who has a long history of controversial entanglements with figures on the fringe of accepted research."
"The Oregon Petition, sponsored by the OISM, was circulated in April 1998 in a bulk mailing to tens of thousands of U.S. scientists. In addition to the petition, the mailing included what appeared to be a reprint of a scientific paper." [Checking the Web site, they still claim that this is a peer-reviewed paper!] "... the paper was titled "Environmental Effects of Increased Atmospheric Carbon Dioxide" and was printed in the same typeface and format as the official Proceedings of the National Academy of Sciences." ... "A cover note signed "Frederick Seitz/Past President, National Academy of Sciences, U.S.A./President Emeritus, Rockefeller University", may have given some persons the impression that Robinson's paper was an official publication of the academy's peer-reviewed journal. The blatant editorializing in the pseudopaper, however, was uncharacteristic of scientific papers." ... "The NAS issued an unusually blunt formal response to the petition drive. "The NAS Council would like to make it clear that this petition has nothing to do with the National Academy of Sciences and that the manuscript was not published in the Proceedings of the National Academy of Sciences or in any other peer-reviewed journal," it stated in a news release. "The petition does not reflect the conclusions of expert reports of the Academy." In fact, it pointed out, its own prior published study had shown that "even given the considerable uncertainties in our knowledge of the relevant phenomena, greenhouse warming poses a potential threat sufficient to merit prompt responses. Investment in mitigation measures acts as insurance protection against the great uncertainties and the possibility of dramatic surprises." ...
When questioned in 1998, OISM's Arthur Robinson admitted that only 2,100 signers of the Oregon Petition had identified themselves as physicists, geophysicists, climatologists, or meteorologists, "and of those the greatest number are physicists." This grouping of fields concealed the fact that only a few dozen, at most, of the signatories were drawn from the core disciplines of climate science - such as meteorology, oceanography, and glaciology - and almost none were climate specialists. The names of the signers are available on the OISM's website, but without listing any institutional affiliations or even city of residence, making it very difficult to determine their credentials or even whether they exist at all. When the Oregon Petition first circulated, in fact, environmental activists successfully added the names of several fictional characters and celebrities to the list, including John Grisham, Michael J. Fox, Drs. Frank Burns, B. J. Honeycutt, and Benjamin Pierce (from the TV show M*A*S*H), an individual by the name of "Dr. Red Wine," and Geraldine Halliwell, formerly known as pop singer Ginger Spice of the Spice Girls. Halliwell's field of scientific specialization was listed as "biology." Even in 2003, the list was loaded with misspellings, duplications, name and title fragments, and names of non-persons, such as company names.
More info: The Leipzig Declaration on Climate Change (from Sourcewatch)
#7. If "they" can't predict the weather accurately (choose "a week", "a month", "three months", or any other preferred short time interval) from now, why should we believe that "their" climate predictions will be accurate (choose "10", "25", "50", "73", "100" or any other multi-year interval longer than a decade) from now?
Weather models are not the same as climate models.
We can't even predict the weather next week
Why climate models are imperfect and why they are crucial anyway
Climate models vs. weather models
"Predicting the weather is hard because you have to get the exact details of a weather system right. If your prediction of a storm track is 100 km off, then a giant snowstorm predicted to bury a city might fall harmlessly offshore. If your temperature is 3 deg C off, then what you predicted as rain turns into snow. If your initial conditions are off, then precipitation predicted to fall during rush hour falls at midnight. All of these things mean that you've blown the forecast, and people will mumble about how weather forecasters don't know what they're doing.
For the climate, these things generally don't matter. What matters is that, in the long run, one gets the statistics of the weather right. If one storm in a climate model is 100 km too far East, that won't matter if the long-term statistics of the storm track is right. This is quite a different problem than predicting the EXACT evolution of a single atmospheric disturbance."
#8. The Vikings came up with the name Greenland because during the Medieval Warming Period it was quite green and amenable to agriculture.
Not quite. Greenland's climate during the Medieval Warm Period was apparently warmer than subsequent Little Ice Age conditions and could be getting nearly that warm again. But Greenland was named "Greenland" by Erik the Red, who wanted to attract other Vikings to follow him there, not because it was particularly green.
Shouldn't Greenland be known as Iceland and vice versa?
Erik the Red's Saga: Chapter 2 (from the Viking Sagas)
Viking settlements indicate that it was easier to live in Greenland during the MWP than now; it's getting warm enough now for agriculture again.
The Norse settlers in Greenland - a short history
