Past regional cold and warm periods linked to natural climate drivers

November 26, 2009,

Intervals of regional warmth and cold in the past are linked to the El Niño phenomenon and the so-called "North Atlantic Oscillation" in the Northern hemisphere's jet stream, according to a team of climate scientists. These linkages may be important in assessing the regional effects of future climate change.

"Studying the past can potentially inform our understanding of what the future may hold," said Michael Mann, Professor of meteorology, Penn State.

Mann stresses that an understanding of how past natural changes have influenced phenomena such as El Niño, can perhaps help to resolve current disparities between state-of the-art climate models regarding how human-caused climate change may impact this key climate pattern.

Mann and his team used a network of diverse climate proxies such as tree ring samples, ice cores, coral and sediments to reconstruct spatial patterns of ocean and land surface temperature over the past 1500 years. They found that the patterns of temperature change show dynamic connections to such as El Niño. They report their findings in today's issue of Science.

Mann and his colleagues reproduced the relatively cool interval from the 1400s to the 1800s known as the "Little Ice Age" and the relatively mild conditions of the 900s to 1300s sometimes termed the "Medieval Warm Period."

"However, these terms can be misleading," said Mann. "Though the medieval period appears modestly warmer globally in comparison with the later centuries of the Little Ice Age, some key regions were in fact colder. For this reason, we prefer to use 'Medieval Climate Anomaly' to underscore that, while there were significant climate anomalies at the time, they were highly variable from region to region."

The researchers found that 1,000 years ago, regions such as southern Greenland may have been as warm as today. However, a very large area covering much of the tropical Pacific was unusually cold at the same time, suggesting the cold La Niña phase of the El Niño phenomenon.

This regional cooling offset relative warmth in other locations, helping to explain previous observations that the globe and on average were not as warm as they are today.

Comparisons between the reconstructed temperature patterns and the results of theoretical climate model simulations suggest an important role for natural drivers of climate such as volcanoes and changes in solar output in explaining the past changes. The warmer conditions of the medieval era were tied to higher solar output and few volcanic eruptions, while the cooler conditions of the Little Ice Age resulted from lower solar output and frequent explosive volcanic eruptions.

These drivers had an even more important, though subtle, influence on regional temperature patterns through their impact on climate phenomena such as El Niño and the . The modest increase in solar output during medieval times appears to have favored the tendency for the positive phase of the NAO associated with a more northerly jet stream over the North Atlantic. This brought greater warmth in winter to the North Atlantic and Eurasia. A tendency toward the opposite negative NAO phase helps to explain the enhanced winter cooling over a large part of Eurasia during the later Little period.

The researchers also found that the model simulations failed to reproduce the medieval La Nina pattern seen in the temperature reconstructions. Other focused more specifically on the mechanisms of El Niño do however reproduce that pattern. Those models favor the "Thermostat" mechanism, where the tropical Pacific counter-intuitively tends to the cold La Niña phase during periods of increased heating, such as provided by the increase in solar output and quiescent volcanism of the medieval era.

The researchers note that, if the thermostat response holds for the future human-caused change, it could have profound impacts on particular regions. It would, for example, make the projected tendency for increased drought in the Southwestern U.S. worse.

Provided by Penn State

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3.2 / 5 (9) Nov 26, 2009
Mann himself saying: "climate anomalies at the time, .. were highly variable from region to region."
Given the paucity of historical climate proxies, and the highly localized nature of such climate proxies, what confidence then on their ability to determine the overall global temperature?
Yet another nail in the coffin of those who would have us believe that 'climate scientists' can actually say anything meaningful with any degree of confidence!!
3.1 / 5 (10) Nov 26, 2009
Models are simplified descriptions of reality. They will never be able to mimic exactly the regional changes in climate.

However, I am unsure if any of you change deniers have noticed, but computers keep getting more powerful and capable. Scientists keep adding more and more variables to the models as they learn more, and they are able to run them on finer and finer grids because the computers are faster and have more memory.

As a consequence, the trend line for the models is to get increasingly more accurate, and reflect reality with greater precision as time goes on. The biggest problem scientists have right now is that measured warming is greater than can be explained by most of the models. You guys need to pay attention because new data and models are getting more refined all the time.
3.4 / 5 (10) Nov 26, 2009
You guys need to pay attention because new data and models are getting more refined all the time.

I have a theory today, not refined well, but it says that you will be responsible for climate change. So, I am going to tax you personally at 50% of all of your income.

I know, my theory isn't very sound, but give me five decades to refine it. If it isn't sound then, then I'll stop taxing you.
3 / 5 (4) Nov 27, 2009
How can we expect to dump billions of tons of hydrocarbon contaminated
air into the upper atmosphere each year (jet planes) and it not have any effect?
3.9 / 5 (7) Nov 27, 2009
After the UEA/CRU hacking scandal, how can anything Mann says be taken to be anything but ideologically driven fraud?
Nov 27, 2009
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3.9 / 5 (11) Nov 27, 2009
You are quite misinformed, dear "yafizicist". 3 different publicly available satellite measurement datasets show that there has been no more warming since 2001. That "warming faster than ever" drivel is just political propaganda that has nothing to do with reality.
5 / 5 (3) Nov 27, 2009
"3 different publicly available satellite measurement datasets show that there has been no more warming since 2001." q]
......but since the natural climate oscillations are added on top of any underlying trend, you ABSOLUTELY HAVE TO FOCUS ON THE LONG-TERM TREND, beyond the time span of the El Nino-Southern Oscillation. Analogy: Some weeks in 2008 the stock market was good, but the overall trend was "recession".
3.7 / 5 (6) Nov 27, 2009
In strict formal logic, arguments by analogy are invalid.

You absolutely have to focus on the long-term trend and you absolutely need a reliable and proven method of prediction plus the appropriate hardware to run it on, which is still decades away.
What has been observed so far are 3 decades of warming followed by 1 decade of stagnation. Nowhere near an "accelerating" trend toward catastrophe and not nearly enough data to speculate about what will happen in the next century.
2.7 / 5 (7) Nov 27, 2009
An excellent piece that shows clearly the so called LIA and MWP were Atlantic phenomemon,as shown by global evidence.
To the deniers that think the world is is not. This year will be amongst the 5 hottest on record. Another important indication is that the peak of global sea ice was the lowest ever recorded http://arctic.atm...rend.jpg
2.7 / 5 (7) Nov 27, 2009
...It should also be noted that AMSU measurements show that the lower Tropospheric temperatures were regularly at record high temperatures, exactly as predicted by global warming models.http://discover.i...sutemps/
2.7 / 5 (7) Nov 27, 2009
Another installment of fiction presented by the Marvellous Mann aka 'Hockey stick hack', brought to you by the publishers of Marvell comix and other assorted fiction aka 'Peer reviewed' science fiction!
Nov 27, 2009
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3 / 5 (2) Nov 27, 2009
Many good comments and a few from people obviously off their meds.
What you say is true but the data is still robust enough to show that there has been little temp. change over the time period. In fact overlapping data from 2 satellites shows that the instruments give virtually the same temp. readings leaving little doubt that all data is accurate and the instruments have not drifted as they aged. In fact, what we are really interested in is the change in temperature not the absolute temp.
One can argue, like NASA, that we have to wait for the new satellite before we can rely on the data but that doesn't hold water because the satellites check against each other.
When the evidence doesn't fit your theory you have to alter the theory NOT the evidence.
1 / 5 (1) Nov 29, 2009
The naivety of man never fails to amuse me!
This planet is continually changing its climate; sometimes quickly in relative terms sometimes slowly; it has done so over millions of years.
Now we have supper fast computers and so called climate experts modelling climate data much of which is unproven science. Before you can factor in so called man made elements you much be sure of the natural background on which all this data lays.
Otherwise many will assume that so called man made pollution is the cause of the present mini climate anomaly as that is what is really happening. 200 years in climate change is a mere drip in the ocean as far as the earth is concerned.
Many of us clear headed scientists are coming to the conclusion that this so called man made climate alteration is akin to a new religion, those who beg to differ will be cast as heretics. Many of us now believe there is strong evidence to suggest that volcanic activity and the solar cycles are the main cause of climate
2.3 / 5 (3) Nov 29, 2009
I fail to see how reducing carbon emissions could be anything other than a positive...
And to those of you who maintain that human-related carbon emissions are 'negligible' are relying on pure mysticism.
4 / 5 (1) Nov 30, 2009
I agree with marjon above - while it's all well and good to debate this stuff, we have to remember that there's no reason not to invest in alternative/renewable energy sources. Nothing but good will come of this, especially as we start to run out of fossil fuels. Whether you believe in AGW or not, we must all agree on the sensibility of diversifying our energy sources.
not rated yet Dec 07, 2009
I fail to see how reducing carbon emissions could be anything other than a positive...
And to those of you who maintain that human-related carbon emissions are 'negligible' are relying on pure mysticism.

The issue is the 'how'. Why won't the AGW believers promote nuclear power as a solution instead of using government coercive taxes?

Well nuclear power for starters could be as catastrophic as global warming itself. To be honest i'm coming from a neutral perspective. I don't support or discredit AGW. What I DO know is that we, as a society have been spoon-fed lies for too long. But, there is no way around this.
True and honest scientific research from credible sources has paled into insignificance behind a torrent of false data and controversy, by scientists with TOO MUCH funding. But I really don't think it's that black and white.
I just want some clarity for once, shit...
not rated yet Dec 08, 2009
The issue is the 'how'. Why won't the AGW believers promote nuclear power as a solution instead of using government coercive taxes?

I think you'll find that a lot of the independent, non-government funded groups DO in fact support alternative energy sources. The government on the other hand appears to be money-grubbing, but realistically a lot of politicians do have a clear conscious, they just lack the backbone to actually go ahead with plans on alternative energy sources, because they run the risk of losing votes.

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