Dr. Michael Asten writes to alert me of this article in the Australian, with this advice:
The overwhelming conclusion ought to be that cycles should be built into at least some of the climate models in order to explore scenarios. But I fear we might still be another decade away from seeing that happen.
I suspect he is right, as I do Stephan Rahmstorf who says:
“if the system (is cyclic), we’d expect the opposite. In 30 years’ time we will know for sure.”
From: The Australian
Today’s global warming is well within historic range
by: Michael Asten January 28, 2013 12:00AM
US President Barak Obama vows action on climate change with the declaration “none can avoid the devastating impact of raging fires and crippling drought and more powerful storms”.
It was an appeal using rhetoric and not science because the most severe impacts of these natural disasters come from the challenge of managing increased population or changed population demands, not changes in the events per se.
Great fires are a regular feature of North American and Australian landscapes, and their human impact is worst when they reach housing or infrastructure built among trees, on the edge of bushland that has not been cleared by “cool” burn-offs.
Civilisations have been hit by droughts since the Nile delta drought of 4200 years ago destroyed Egypt’s old kingdom, leaving the pyramids as witness.
As global citizens, we still have much to learn about the management of water, it seems, whether in our Murray-Darling backyard or in the Sahel of Africa.
The term “powerful storms” summons up graphic images of Hurricane Sandy, which devastated New York – except it was no longer hurricane-strength when it, like a dozen others in written history, struck.
But the Manhattan area it flooded contained huge areas of high-density development on reclaimed swamps, ponds and what was riverbed before civilisation drained, dozed and filled to provide for the population of one of the world’s greatest cities. When New York has absorbed the lessons from this, it will be able to pass on advice and technology to places such as Bangladesh.
…
A growing number of mainstream scientists agrees there is evidence for such cycles as drivers of climate change, although debate on causes and mechanisms is strong. By way of example, I note three recent papers that find evidence for long-term cycles influencing the Earth’s climate.
Weichao Wu of the Peking University and colleagues studied sea-surface temperature records preserved in deep-sea sediments near Okinawa in the Pacific Ocean, and found evidence for multiple cyclic temperature variations over the past 2700 years.
The most interesting temperature peaks correspond to medieval, Roman and possibly Minoan warming periods of about 900, 1800 and 2500 years ago.
The paper is significant in that it concludes that the current rate of global temperature change lies in the same range as that of those historical warming periods.
This suggests we have evidence that challenges current climate orthodoxy on two grounds, first by suggesting that such warming events were global not local European phenomena, and second that current warming is not unprecedented in the historical record.
While we read many claims by oceanographers of an increasing rate of rise in sea-levels associated with increased atmospheric carbon dioxide, an alternative interpretation of observed data is made in a recent analysis by Don Chambers of the University of South Florida and colleagues.
Chambers poses the question: “Is there a 60-year oscillation in sea-level?” and shows evidence that the answer is probably yes.
I read his data and find it is arguable that the upswing of that oscillation is responsible for about half of the current 3mm/year rate of rise, leaving the background rate of rise at about 1.7mm, where it has been for 110 years.
Intergovernmental Panel on Climate Change lead author Stefan Rahmstorf, writing in the climate scientists’ blogsite Real Climate this month, commented on whether the data supports an interpretation of cycles, or non-cyclic shifts associated with changes in aerosols and current increases in greenhouse gases.
Rahmstorf concludes in favour of the latter but ends with the objective and open-minded comment “if the system (is cyclic), we’d expect the opposite. In 30 years’ time we will know for sure.”
…
A third work that may eventually prove immensely important in understanding cycles in climate change is a study by JA Abreu of the prestigious Swiss university ETH, with co-authors including Australia’s 1995 Australia Science Prize winner Ken McCracken.
Abreu reconstructs a history of solar sunspot cycles over the past 10,000 years from elemental isotopes created by cosmic rays impinging on the atmosphere, subsequently preserved in Greenland ice-core records.
…
The mechanisms of sun-spot, solar magnetic field and cosmic ray interactions are complex and will be intensely studied, but the associations illustrated here demand consideration when we seek to model our future climate.
The devastating impacts of extreme climate events of which Obama speaks have always been with us, and we have to expect that the human tragedies they bring will be exacerbated by growth in global population.
==========================================================
Full story here: http://www.theaustralian.com.au/national-affairs/opinion/todays-global-warming-is-well-within-historic-range/story-e6frgd0x-1226563061202
Any scientist who denies the presence of cycles in earth’s climate is not credible.
Why do people keep ignoring the obvious…especially in the US…….the dust bowl
” In 30 years’ time we will know for sure.”
That’s why we must continue to put CO2 into the atmosphere and continue this experiment so that our great great grandchildren will know for sure. Otherwise they might never know and even be fooled into believing that the cooling was caused by CO2 reduction rather than natural causes.
This debate about cycles was another of those sidelined by the IPCC “settling” climate science. In the 1970s and 80s the debate about cycles were a function of the Milankovitch cycles. If anyone presented a paper suggesting this as a causation factor it was quickly attacked. The main debate was about the orbital eccentricity as the major cycle evident in the ice age pattern. I remember a conference in Ottawa around 1990 at which Milankovitch was mentioned without engendering an attack. Critical to this was the appearance of Antarctic ice core records. It was the first time a relatively detailed long term record was available for spectral analysis.
The main pattern of thought at the time was set out at a conference in 1990, as I recall, in Warsaw. Eastern Bloc nations, particularly the Soviets argued for overlapping cycles as the major pattern of climate change. They have along history of studying cycles of climate, especially as they relate to crop production and the economy. The best known is the Kondratiev Cycle used by many market analysts.
http://en.wikipedia.org/wiki/Kondratiev_wave
The western nations led by the US were into chaos theory at the time and argued this was the reality. I remember this well because I suggested at one conference they better hope it is chaos theory because their forecasts are so wrong.
The pundits and media incorrectly assumed the difference between the two blocs was purely political, a result of the Cold War. It wasn’t at all, rather, it was a very important difference still needing resolution. I think cycles predominate but the challenge is identifying even the major ones and more challenging how they interact. I struggled with this because it almost cost me approval of my doctorate. I had a very long record of precipitation events for southern Hudson Bay and ran a spectral analysis that showed a very strong 22-year drought cycle. This tallied with a similar length cycle of droughts and sunspots identified by Douglass in tree rings. I argued for retention of the data and speculation in my thesis, despite being told it might jeopardize approval.
The big problem with any resolution of the issue goes back to the problem of insufficient records that Lamb identified in his autobiography as his objective for establishing the Climatic Research Unit. “…it was clear that the first and greatest need was to establish the facts of the past record of the natural climate in times before any side effects of human activities could well be important. A worldwide record was needed particularly on the timescale of human history…” Lamb realized his plan was thwarted when Wigley took over as director and began the work that was to culminate in the settled science of the IPCC. Lamb wrote “Since my retirement from the directorship of the Climatic Research Unit there have been changes there and in the direction of my own efforts. My immediate successor, Prof. Tom Wigley, was chiefly interested in the prospects of world climates being changed as a result of human activities primarily through the burning of wood gold oil and gas reserves and by changing the face of the earth…”
Kyoto expired; now we must eliminate the political machinations of the IPCC and move forward with meaningful, accountable, research as far removed from government as possible.
Excellent epistemological analyis of the issue. Unfortunately not only in ’Climatism’ is this core of the scientific method habitually breached.
Whenever a scientific issue is apt to raise public fears – and thereby funding for the scientists and regulatory controls for the funding politicians – Occam’s razor becomes blunt. A case in point with similar societal import is radiophobia, which is generated and nourished by the postulate that any minute amount of ionizing radiation is cancerogenic, the so called LNT-postulate which is wrongly sold as LNT-’theory’. The German “Energiewende“ is a calamitous offspring of this phobia – well nourished by the media and highly cherished by the political classes.
As an aside: here in Old Europe we know Occams’s razor as: „Entia non sunt multiplicanda praeter necessitatem [or: sine necessitate]“
Rainer Facius
Science in politics is addressed in Roger Pielke Jr’s editorial in this morning’s Wall Street Journal:
http://online.wsj.com/article/SB10001424127887323539804578264460952281772.html?mod=googlenews_wsj
[Begin Quote]
Dear Expert, Please Cook the Books
Governments love to refer to experts—until their advice doesn’t suit.
By ROGER PIELKE JR.
Last week the Greek government brought charges against Andreas Georgiou, the head of its independent statistical agency Elstat, and two of his colleagues for allegedly overstating the country’s 2009 debt.
The debt calculations were a critical factor in characterizing the magnitude of the nation’s financial crisis and the subsequent responses by the European Union and the International Monetary Fund. For his part, Mr. Georgiou complained after the investigation began: “I am being prosecuted for not cooking the books.” By contrast, Greek politicians have argued that the statistical agency was working counter to national interests. One politician said that Elstat was “too focused on the numbers and not enough on serving the country and the government.”
This situation is but one of a growing number of recent conflicts found where expertise meets politics.
For instance, last year in L’Aquila, Italy, six scientists and one government member of the Italian National Commission for the Forecast and Prevention of Major Risks were sentenced to six years in prison for misleading the public about the likelihood of an earthquake. At an ill-timed news conference held before the devastating 2009 earthquake that killed 308 people, local residents were told by one of the experts that they should enjoy a glass of red wine instead of worrying about a natural disaster.
Closer to home, in the immediate aftermath of Hurricane Sandy, New Jersey Gov. Chris Christie issued an executive order classifying the storm as a “post-tropical cyclone” rather than a hurricane, pre-empting the scientific evaluation of the National Weather Service, which has yet to make its final determination. Whether Sandy was a hurricane or not makes a big difference in insurance payouts to individual homeowners. If a hurricane, the payouts would be much smaller. In a letter to the Weather Service, New York Sen. Charles Schumer reminded the agency that its scientific judgments could cost his constituents a lot of money.
Each of these very different cases shares a common characteristic. An institution—Elstat in Greece, the Major Risks Commission in Italy and the U.S. National Weather Service—was tasked with rendering expert judgments as an input to policy making. In each case, that input was thwarted in some way.
Ironically, Elstat was created in 2010 to improve the provision of statistical data to Greek politicians. Before that, “the practice was for the finance ministry’s general accounts office to collude with the Bank of Greece TELL.AT 0.00% to come up with deficit and debt figures ignoring surveys carried out by the statistical service,” one economist told the Financial Times.
In Italy, the earthquake experts stand accused of colluding with politicians to convey a reassuring message to the public via a “media operation.” The message being sent was motivated, at least in part, by the experts’ desire to discredit an amateur earthquake forecaster who had heightened public alarm by predicting a big earthquake.
Dozens of U.S. states have defined a tiered “hurricane deductible” for insurance payouts, several of which rely on scientific judgments of the Weather Service, an agency that was not established for such a purpose. Given the political pressure, it seems highly unlikely that Sandy will be classified as a hurricane in the agency’s final characterization.
Political challenges to the use of expertise know no national boundaries and can be found across the political spectrum. And those who diagnose the problem and issue calls to cleanly separate science and politics fail to recognize that the challenge actually lies in the better integration of the two.
Improving the ability of experts to provide input to decision making will require leadership. Politicians must unambiguously and publicly clarify what questions they wish to have the experts address, and the experts must commit to limiting their role to answering those questions—including all of the associated ambiguities and uncertainties—and refrain from using their platform to place a thumb on the political scales.
Strengthening advisory institutions offers the promise of improving the quality and use of expert advice. Rather than sending experts to jail, politicians should be making it easier for their advice to clearly be heard.
Mr. Pielke is a professor of environmental studies and a fellow of the Cooperative Institute for Research in Environmental Sciences at the University of Colorado.
A version of this article appeared January 30, 2013, on page A11 in the U.S. edition of The Wall Street Journal, with the headline: Dear Expert, Please Cook the Books.
[End Quote]
@ Rainer Facius. Very well put, other than “cancerogenic”. You what? Spare us from another pseudoword that the MSM latch onto. It’s ‘carcinogenic’, please.
William:
Rahmstorf’s comment is disingenuous.
“if the system (is cyclic), we’d expect the opposite. In 30 years’ time we will know for sure.”
It is a fact that the earth’s climate changes cyclically. We will not need to wait 30 years to determine if solar magnetic cycle changes affect planetary temperature and to determine the true magnitude of AGW.
There are past cycles of warming and cooling in the paleo record. The past cycles of warming and cooling correlate with cosmogenic isotope changes (Dansgaard-Oeschger events). There are also abrupt climate changes (Heinrich events) which also correlate with cosmogenic isotope changes. The interglacial phases are terminated by a Heinrich event.
Earth climate drivers are chaotic and hence cannot explain the periodicity of the cyclic gradual warming. Earth climate drivers cannot abruptly cause the cyclic abrupt climate changes, the Heinrich events.
There is currently no explanation to what causes the glacial/interglacial cycle. (A back of the envelope calculation and hundreds of papers support the assertion that earth based changes such as the amount of insolation at 65 N in the summer is more than an order of magnitude less than what is required to cause the glacial/interglacial cycle.) The paleo temperature data does not support Milankovich’s hypothesis that orbital changes to insolation at 65N drives the glacial/interglacial cycle.
http://en.wikipedia.org/wiki/Milankovitch_cycles
“Identifying dominant factor
Milankovich himself believed that reductions in summer insolation in northern high latitudes was the dominant factor leading to glaciation, which led to him (incorrectly) deducing a 41ish-kyr period for ice ages.[12] Subsequent research has shown that the 100kyr eccentricity cycle is more important, resulting in 100,000-year ice age cycles of the Quaternary glaciation over the last few million years.
The 100,000-year problem is that the eccentricity variations have a significantly smaller impact on solar forcing than precession or obliquity and hence might be expected to produce the weakest effects. However, observations show that during the last 1 million years, the strongest climate signal is the 100,000-year cycle. In addition, despite the relatively great 100,000-year cycle, some have argued that the length of the climate record is insufficient to establish a statistically significant relationship between climate and eccentricity variations.[8] Some models can however reproduce the 100,000 year cycles as a result of non-linear interactions between small changes in the Earth’s orbit and internal oscillations of the climate system.[9][10] (William: The computer models have massive amplifications factors and can therefore be tuned to create ONE!!! interglacial/cycle. If there really was a massive amplification factor there would not be a cycle of 23 glacial interglacial cycles. The glacial interglacial cycle would be chaotic. It is not.
The driver of the glacial/interglacial cycle is a cyclic significant solar magnetic. The affects of the significant solar magnetic cycle changes is modulated by the position of the earth (tilt), eccentricity of the earth’s orbit and the hemispheric timing –Whether the Northern or the Southern Hemisphere is closest to the sun at perihelion when the earth tilt at perihelion.)”
http://one.geol.umd.edu/www/preprints/Bond_et_al.pdf
Persistent Solar Influence on North Atlantic Climate During the Holocene
By: Gerard Bond, Bernd Kromer, Juerg Beer,Raimund Muscheler,Michael N. Evans, William Showers, Sharon Hoffmann,Rusty Lotti-Bond,Irka Hajdas, Georges Bonani
Surface winds and surface ocean hydrography in the subpolar North Atlantic appear to have been inßuenced by variations in solar output through the entire Holocene. The evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon-14 and beryllium-10 and centennial to millennial time scale changes in proxies of drift ice measured in deep-sea sediment cores. A solar forcing mechanism therefore may underlie at least the Holocene segment of the North Atlantic’s 1500-year cycle.
http://www.agu.org/pubs/crossref/2002/2000PA000571.shtml
On the 1470-year pacing of Dansgaard-Oeschger warm events
The oxygen isotope record from the Greenland Ice Sheet Project 2 (GISP2) ice core was reanalyzed in the frequency and time domains. The prominent 1470-year spectral peak, which has been associated with the occurrence of Dansgaard-Oeschger interstadial events, is solely caused by Dansgaard-Oeschger events 5, 6, and 7. This result emphasizes the nonstationary character of the oxygen isotope time series. Nevertheless, a fundamental pacing period of ∼1470 years seems to control the timing of the onset of the Dansgaard-Oeschger events. A trapezoidal time series model is introduced which provides a template for the pacing of the Dansgaard-Oeschger events. Statistical analysis indicates only a ≤3% probability that the number of matches between observed and template-derived onsets of Dansgaard-Oeschger events between 13 and 46 kyr B.P. resulted by chance. During this interval the spacing of the Dansgaard-Oeschger onsets varied by ±20% around the fundamental 1470-year period and multiples thereof. The pacing seems unaffected by variations in the strength of North Atlantic Deep Water formation, suggesting that the thermohaline circulation was not the primary controlling factor of the pacing period.
William:
See figure 3 at this site which shows the cyclic warming and cooling that occurs during the Holocene interglacial period and the Wisconsin glacial period.
http://www.climate4you.com/
Fig.3. The upper panel shows the air temperature at the summit of the Greenland Ice Sheet, reconstructed by Alley (2000) from GISP2 ice core data. The time scale shows years before modern time. The rapid temperature rise to the left indicate the final part of the even more pronounced temperature increase following the last ice age. The temperature scale at the right hand side of the upper panel suggests a very approximate comparison with the global average temperature (see comment below). The GISP2 record ends around 1855, and the two graphs therefore ends here. There has since been an temperature increase to about the same level as during the Medieval Warm Period and to about 395 ppm for CO2. The small reddish bar in the lower right indicate the extension of the longest global temperature record (since 1850), based on meteorological observations (HadCRUT3). The lower panel shows the past atmospheric CO2 content, as found from the EPICA Dome C Ice Core in the Antarctic (Monnin et al. 2004). The Dome C atmospheric CO2 record ends in the year 1777.
Re Tim Ball says: January 30, 2013 at 7:55 am
Russian interest in climate cycles with respect to its fisheries is seen in UN FAO Technical Paper 410:
“Climate change and long-term fluctuations of commercial catches: the possibility of forecasting”
Professor Leonid B. Klyashtorin of the Federal Institute for Fisheries and Oceanography, Moscow, Russian Federation (e-mail: Klyashtorin@mtu-net.ru)
http://www.fao.org/fi/oldsite/eims_search/1_dett.asp?calling=simple_s_result&lang=en&pub_id=61004
[Begin Quote]
Abstract
The main objective of this study was to develop a predictive model based on the observable correlation between well-known climate indices and fish production, and forecast the dynamics of the main commercial fish stocks for 5–15 years ahead. Spectral analysis of the time series of the global air surface temperature anomaly (dT), the Atmospheric Circulation Index (ACI), and Length Of Day (LOD) estimated from direct observations (110-150 years) showed a clear 55-65 year periodicity. Spectral analysis also showed similar periodicity for a reconstructed time series of the air surface temperatures for the last 1500 years, a 1600 years long reconstructed time series of sardine and anchovy biomass in Californian upwelling areas, and catch statistics for the main commercial species during the last 50-100 years. These relationships are used as a basis for a stochastic model intended to forecast the long-term fluctuations of catches of the 12 major commercial species for up to 30 years ahead. According to model calculations, total catch of Atlantic and Pacific herring, Atlantic cod, South African sardine, and Peruvian and Japanese anchovy for the period 2000–2015 will increase by approximately two million tons, and will then decrease. During the same period, total catch of Japanese, Peruvian, Californian and European sardine, Pacific salmon, Alaska pollock and Chilean jack mackerel is predicted to decrease by about 4 million tons, and then increase. The probable scenario of climate and biota changes for next 50-60 years is considered.
[End Quote]
The report was dated 2001 and includes predictions for commercial catches for the next 30 years. Twelve years have passed. How successful have the predictions been so far?
Well, its happening here in Toronto:
http://www.thestar.com/news/city_hall/2013/01/29/torontos_environment_chair_again_expresses_doubt_about_global_warming.html
No doubt that, as Gore says, there being a lot of opportunity in this “crisis”, this $250,000 will turn into $250 million fairly easily for those who “need” to study this closer.
A relatively short period of warming coincided with a significant increase in atmospheric CO2 levels. The “consensus” of scientists then assumed causation based on no evidence at all. Worse, the notion that “CO2 causes warming” became the Holy Grail, common sense was abandoned, and nobody was interested in looking at alternative explanations.
Personally, I am inclined to believe that climate is subject to cyclical effects (as well as other trends, and random/chaotic factors).
But I know that fashion, politics, and mass hysteria are subject to cyclical effects. I think this needs study and want to know where I can submit my grant applications.
There are not many linear systems in nature.
Claiming to study weather, while denying natural cycles does make one look stupid.
“none can avoid the devastating impact of raging fires and crippling drought and more powerful storms”
The following came from National Geographic July 1998
Population: Moving Toward Trouble
“A hazard only becomes a disaster when it occurs where people live,” says NOAA meteorologist Joe Golden. Unfortunately, people are moving into harm’s way at a fast clip. Mexico City has nearly 20 million people living in a region at risk from quakes and volcanoes. The U.S. South and West – prone to drought, fire, hurricanes, quakes, and mudslides – are expected to grow by 32 and 51 percent respectively by the year 2025. Half the U.S. population lives in coastal states, with some 34 million in Texas and Florida, two favorite hurricane targets. And three of Canada’s largest cities – Montreal, Vancouver and Ottawa – lie in seismically active zones. Do people worry? Not much. “There’s a certain fatalism about disaster,” says Mario Ordaz-Schroeder of Mexico’s National Center for Disaster Prevention. Indeed, by desire or lack of choice people still build on volcanic slopes and barrier islands. Says NOAA’s Golden, “We don’t always learn from our mistakes.”
– – – – – – – –
I think Rahmstorf has no objective basis when he tries to eschew two complete ~60 year variations in sea level data in favor of a set of radiative model projections (guesses) which scientists know have not been even close to being reasonably correct in the last several decades. Rahmstorf finds against the ~120 yrs of empirical observations based on his arbitrary positing that we need another half of one of the observed 60 year variations (30 years) to know if the radiative models are wrong.
Well, we already know the radiative models are fine constructs of intentional circular logic. The models obediently give an answer that they were told to give and we know those model answers do not agree with observations.
I politely suggest that Rahmstorf’s strategy, in his RC article, is to stall for more time. Looks like he is stalling for more time to continue the claims of high / alarming climate impact of AGW from CO2; more time for him to forestall the implications of real world data (sea level data) that falsify a claim of alarming impact from AGW by CO2. Specifically, I suggest Rahmstorf, as a significant leader of an important part of the IPCC’s AR5, is stalling to maintain in AR5 some of the intentional previous bias that was seen in AR4.
Alternatively, I counter Rahmstorf’s stalling strategy with an idea of science researchers demanding immediate different focus going forward. I suggest scientists immediately demand the scientific community give a much more balanced effort in understanding the manifold behavior of the earth-atmospheric system and not continue the past >2 decades of myopic / biased focus on an ‘a priori’ premise that there is dangerous AGW by CO2.
John
Our recent bad fires come from the suppression of natural, brush-clearing fires for 40 years. Fires that normally would not reach the top of the forest canopy, and thus facilitate a rejuvenation of the forest, could reach the tops and kill everything. Being hotter, with the excess of accumulated brush and dead wood, these fires travel faster and are harder to extinguish both naturally and by man.
Combine this forest mismanagement with the green requirement that you cannot clear an area around your own home and you have a real recipe for disaster. Fire breaks are only common sense. I have never heard the reasoning for the idea that one cannot protect your own home from forest fires.
@CaligulaJones January 30, 2013 at 9:23 am
Thanks for the reference to the (regrettable) item on Toronto (http://www.thestar.com/news/city_hall/2013/01/29/torontos_environment_chair_again_expresses_doubt_about_global_warming.html). There are so many errors in it, both the writer’s and of those he quotes. I am now drafting a reply to the paper.
IanM
Thanks Ian, I was going to attempt, but a little too busy to do a worthwhile job (and while I work in data, its not my expertise). Please let us know how it turns out.
Yes, there is a 60 year cycle in our climate, and some others.
That is the subtitle of my climate pages: The cyclic nature of Earth’s climate
The BY scale is fairly disturbing and portends a very dark future for Earth. As if that were not enough, the MY scale is a serious short term red alert. We need to leave Earth or all is lost. Dark, dark future for humans. And instead of reaching for the stars, we, the supposed moderns, shoe gaze, worship the Great Earth Mother Spirit and call ourselves a virus. It is beyond suicidal.
They were told that is about 1968 by engineers they dismissed within their hearing as ignorant spanner mongers.
William Astley says:
January 30, 2013 at 8:46 am
“…There has since been an temperature increase to about the same level as during the Medieval Warm Period and to about 395 ppm for CO2…”
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Archaelogical facts would suggest that that statement cannot be correct since as the Greenland glacier presently recedes (ie., as it receded/recedes in the late 20th century/early 21st century), we are still finding the remains of Viking settlements. Those settlements were not built under ice, and therefore the Greenland glacier must have been less extensive during the Viking Warm Period/MWP than it is today.
Further, farming was far more difficult some 700 to 1200 years ago. Tools were less capable and fertilizers and greenhouses did not exist. Grains, crops and animals were not anthropogenically genetically engineered specifically to cope with harsh conditions. With the farming technology possessed by the Vikings, they could not have maintained settlement in Greenland for a period of a few hundred years unless Greenland was substantially warmer than today, maybe 4 to 8 degreesC warmer. One bad winter would have killed off a settlement if it was surviving on subsistence level. No emergency supplies could have been airlifted in. when the Vikings settled conditions in Greenland must have been such that it was bountiful and a time of plenty so that food could be stored to cope with a few consecutive bad winters/poor summers.
We know of no particular mechanism that would account for only a small part of Greenland being uniquely warmer, and it is therefore likely that high Northern Latitudes in general were warmer and that there was less Arctic Ice. Of course, it is impossible to rule out some micro climatic influence (eg perhaps temperature and routing differences in the Gulf Stream) but there is no reliable evidence of any such micro climatic conditions that would explain how the Vikings were able to settle and maintain those settlements in Greenland for such a lengthy period unless Northern latitudes in general were considerably warmer than today. The fact that there is plausible evidence that the Vikings sailed in open boats to Northern Canada would also suggest that conditions in high Northern latitudes generally were more benign than today.
Chambers poses the question: “Is there a 60-year oscillation in sea-level?” and shows evidence that the answer is probably yes….
Given that this is close to a multiple of the difference between a lunar and earth year which makes the cycles correspond every 33 years approximately there is almost certain to be a cycle at around that period.