Medieval Warm period due to NAO

Medieval Warm period due to NAO

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Discussion

Jasandjules

69,825 posts

228 months

Saturday 4th April 2009
quotequote all
So the planet can get warm due to nature?

How odd that it can't these days... What has changed? Hint - Money.

dibbly_dobbler

11,257 posts

196 months

Saturday 4th April 2009
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nigelfr said:
Breaking news from Physics.org..."In the April 3rd edition of Science a collaborative group of scientists from Switzerland, California and the UK report that medieval climate over Europe was heated by the North Atlantic Oscillation (NAO). This oscillation pattern, defined as the pressure difference between the Icelandic Low and the Azores High, also influences modern-day weather conditions and has contributed to the recent droughts in North Africa and floods in North-Central Europe."

So I guess that makes a big hole in the "It's the Sun" theory of Global Warming.
Nigel - I am genuinely interested in this (and am getting more confused, the more I read !) surely if it puts a hole in 'The Sun did it' argument, it puts an equal sized one in 'MMGW did it' ?

turbobloke

103,742 posts

259 months

Saturday 4th April 2009
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The North Atlantic Oscillation. Medieval Warm Period. Interesting topics.

There's a new paper from Eichler et. al., in Geophysical Research Letters (2009) looking at temperature reconstructions for a period starting in 1250 which is at the heart of the Medieval Warm Period. they state:

Spectral analysis of the Belukha temperature reconstruction shows significant periods at 205, 86, and 10.8 years, which can be related to the solar Suess, Gleissberg, and Schwabe cycles, respectively. The 8.3 and 2.3 periodicities are typical of the North Atlantic Oscillation (NAO) index [e.g., Cook et. al., 1998; Garcýa et. al., 2005], whereas the 5.2 year cycle is observed in both, solar and NAO data series.

We also have the Bucha research on auroral oval impacts on global circulation patterns, driven by solar eruptivity (not irradiance). Planetary circulations like the jet stream alter their paths and intensities as a result of auroral oval effects. Such a mechanism (jet stream shift) was cited by the Hydrology and Ecology Centre for the 2007 summer floods in the UK.

These types of change alter oceanic circulation as evidenced by the changing domination of the Aleutian Low and North Pacific High. In papers by Václav Bucha & Václav Bucha Jr such as: "Geomagnetic Forcing of Changes in Climate and in the Atmospheric Circulation", Journal of Atmospheric and Solar-Terrestrial Physics, 60, 2, 145-169 (1998) and Bucha Sr "Influence of Corpuscular Radiation on Changes in the Middle Atmosphere and Troposphere", Advances in Space Research, 8, 7, 205-210 (1988) they say:

"The results enable us to test a causal link of the Sun-climate processes, to explain strong interannual climate and weather changes in several key regions of the northern hemisphere, mainly in winter, and to study possible causes of the North Atlantic Oscillation."

then

"The results obtained contribute also to the study of the occurrence of long-term cyclic changes which were observed both in solar and geomagnetic activity and temperature T, as well as in the radioactive 14C, and have a similar trend. A composite curve was suggested by summing up sinusoidal curves with periods 70, 200, 800 and 2400 years roughly representing changes of all three mentioned parameters (aa, T and 14C) during the past 1600 years and their probable trend for the next 800 years. The results seem to imply that global warming could be slowed down in next decades, because the natural component influencing the increase of temperature in the 20th century will most probably decrease in the next century due to the weaker external geomagnetic forcing which was suggested to modify natural meteorological processes."

The geomagnetic aa index is a measure of solar eruptivity, the forcing ignored by the IPCC and True Believers, perhaps not for much longer. The 70 and 200 and 800 year periodicities ink to Gleissberg, De Vries Suess and Bond solar cycles respectively. The past 1600 years includes the Medieval Warm Period (MWP).

Note that the above is an accurate projection of a minimum in solar activity inducing global cooling via global atmospheric and oceanic circulation patterns driven by solar forcing - made 20 years ago. The work also covers the MWP as a historical natural global temperature maximum, and links solar to NAO.

In their proposed mechanism for explaining the erutivity based Sun-climate links, downward winds following a geomagnetic storm onset are generated in the polar cap of the thermosphere (hence the relevance of the auroral oval) and penetrate to the stratosphere and troposphere, where the atmospheric response can be observed as a sudden increase of pressure and temperature.

Processes in the auroral oval modulate the alternation of meridional and zonal type atmospheric circulation, and cause temperature changes. Their data-based investigations over daily, monthly and yearly timescales confirmed that fluctuations in climate - including the above mentioned zonal and meridional circulations - plus blocking, invasions of arctic air and the southern oscillation, can be accounted for by the processes in the auroral oval.

NAO, and solar forcing of climate involving NAO, is indeed a fascinating topic.

DocJock

8,341 posts

239 months

Saturday 4th April 2009
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Thanks for the link nigel. I was looking on the wrong site rolleyes

I have only read the report (and the one above regarding coral records)
No mention in either of checking for any correlation between solar cycles and the NAO though. wink

ETA : dammit, too slow. More reading, thanks TB.

Edited by DocJock on Saturday 4th April 19:15

turbobloke

103,742 posts

259 months

Saturday 4th April 2009
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nelly1 said:
Medieval you say?

Well I guess that's the Man Made CO2 theory pretty much up the Swanney as well!
Good spot hehe

Following my previous post about the NAO and the solar cycle influences on climate, including via NAO, a few more general comments.

nigelfr said:
Breaking news from Physics.org..."In the April 3rd edition of Science a collaborative group of scientists from Switzerland, California and the UK report that medieval climate over Europe was heated by the North Atlantic Oscillation (NAO). This oscillation pattern, defined as the pressure difference between the Icelandic Low and the Azores High, also influences modern-day weather conditions and has contributed to the recent droughts in North Africa and floods in North-Central Europe."

So I guess that makes a big hole in the "It's the Sun" theory of Global Warming.
Brave conclusion, its basis already evaporated into thin but not so hot air. See previous post.

In any case, what drives the NAO? Tax gas? Not a hope, the NAO is claimed by your article to have stronger amplitude in the past (no manmade carbon dioxide) than now. Also see my previous post, I'm not repeating that even for you!

No big hole at all. There are a number of natural forcings. Solar forcings affect the radiation budget (hence climate, the amplification of TSI was quantified recently by Shaviv and I posted about that already) and low level cloud formation (Svenskark, confirmed even by Sloan and Wolfendale as >/= 20% causation). Global circulation patterns are affected by auroral oval impacts (Bucha). These remain as natural climate forcings, acting on different aspects of the climate system via different mechanisms.

There is another obvious key question but an unanswered question in the article: it says nothing about what causes NAO to cycle. See my previous post, again no repeats.

NAO isn't hanging on carbon dioxide for sure. Your post simply shows natural climate variation - no human influence is visible, which make it a curious thread and post for somebody with your views as it says nothing about removing solar forcing from the picture.

One of the authors is even quoted on PhysOrg as follows:

Article said:
Valerie Trouet, first author of this study, points out that “the modern-day effects of the NAO are relatively small and short-lived compared to those during the Middle Ages”. This study demonstrates that climate has undergone large changes long before humans started releasing large amounts of greenhouse gases into the atmosphereindicating that natural forcings should be taken into account when trying to understand the climate of the future.
Quote a pro-climate realism piece as it happens.

Also we should celebrate the acceptance of the Medieval Warm Period as a major, naturally warmed, global climate event, particularly as Mann et al tried to remove it from the history books, so at least we can look forward to no more hockey sticks woohoo

More bang-up-to-date stuff on NAO, which I mentioned previously but not in detail, now has more relevance, and it puts the NAO in context: Tsonis et. al. (Tsonis, Wang and Swanson) have a paper in press in Geophysical Research Letters (GRL) entitled: ‘The pacemaker of major climate shifts.’ This expands on the very important but curiously ignored Tsonis et al (2007) GRL paper, which demonstrated a new dynamical mechanism for major climate shifts. The Abstract and Conclusion of the new paper states, amongst other things:

New paper by Tsonis et al said:
Models and data suggest that the interplay of major climate modes may result in climate shifts [Tsonis et al., 2007]. More specifically it has been shown that when the network of North Atlantic Oscillation (NAO), Pacific Decadal Oscillation (PDO), El Nino/Southern Oscillation (ENSO) and North Pacific Index (NPI) synchronizes, an increase in the coupling between these oscillations destroys the synchronous state and leads the climate system to a new state. These shifts are associated with significant changes in global temperature trend and in ENSO variability. Here we probe the details of this network’s dynamics to investigate if a certain oscillation is the culprit in these shifts. From a total of 12 synchronization events observed in three climate simulations and in observations we find that the instigator of these shifts is NAO...(The paper concludessmile...Here we were able to identify that the major participant in this coupling strength increase is NAO, which we found to be behind all climate shifts observed in observations as well as in three climate simulations. Understanding variability of our extremely complex climate system is far from complete as new and often contradicting views are proposed. In this realm we hope that our results will provide some direction and focus to this perpetual quest for understanding climate variability.
Not that good an agreement between data and models (again), but well on the way to accounting for major climate shifts without carbon dioxide anywhere in sight, which matches the situation with global climate data.

We live in interesting times.

nigelfr

Original Poster:

1,658 posts

190 months

Saturday 4th April 2009
quotequote all
Guam said:
nigelfr said:
Sorry for the delay in replying, DocJock. I haven't seen the paper yet, just the report from
http://www.physorg.com/news157963913.html

You make a valid point about the measurements: I wondered how CRF was measured in that period too. Especially as the CRF/cloud theory requires energies in the 10GeV range according to Shaviv and the Be or C proxies require much lower energies.( <1 GeV IIRC)

Now the responses so far have been interesting: I see some of the ditto bots have turned up and revealed that they haven't learnt anything about the "It's the Sun" theory of GW even though it has been mentioned here a lot by the sceptics as an alternative to AGW.

In brief: some people have compared proxies of sun spot counts against temperature and have used the medieval warm period in Europe to "prove" a correlation. Now there is no proven mechanism to show cause and effect and also Europe is not the world, so a warm Europe doesn't really show global temperature.

Sceptics of this sunspot theory have suggested that an oscillation in the Atlantic (NAO) like the ENSO in the Pacific could explain the MWP. The report as mentioned on Physics.org sounds interesting.

Edited by nigelfr on Saturday 4th April 16:54
You really are Myopic Nige you dont get any of the counter arguement do you (guess you cant afford to). THe Countercase has been consistently that warming and cooling phases are completely normal variations in a natural system affected (if at all) by human activity to a microscopic extent (certainly not measurable in any meaningful sense) you on the other hand selectively cherry pick the arguement whenever data comes out trying (specifically with TB) to rubbish ANY counter debate.

What is too funny for words is that trying to shoot TB down on one aspect of "Natural Forcings" you have actually re-inforced him with a generally supportive postulation. You do see how hilarious that is and how inane it makes your whole position dont you smile


Cheers

Tom
Sorry, Tom, but I think it's you that don't get the point. The sceptics use the MWP to show a global warm period. If it's an oscillation it's not global and it's not a forcing.

Obviously you don't understand the AGW position either Tom. Every time someone says "Cold today innit, where's yer global warming now?", they are told that AGW is a signal imposed on natural climate variation.

nigelfr

Original Poster:

1,658 posts

190 months

Saturday 4th April 2009
quotequote all
Swanson and Tsonis have this to say "Finally, it is vital to note that there is no comfort to be gained by having a climate with a significant degree of internal variability, even if it results in a near-term cessation
of global warming. It is straightforward to argue that a climate with significant internal variability is a climate that is very sensitive to applied anthropogenic radiative anomalies (c.f. Roe [209]). If the role of internal variability in the climate system is as large as this analysis would seem to suggest, warming over the 21st century may well be larger than that predicted by the current generation of models, given the propensity of those models to underestimate climate internal variability [Kravtsov and Spannagle 2008]."
http://www.uwm.edu/~kswanson/publications/2008GL03...

mybrainhurts

90,809 posts

254 months

Sunday 5th April 2009
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Nigel in shock Worse Than Previously Thought Mk XXXXXXXXIV revelation...

smile

turbobloke

103,742 posts

259 months

Sunday 5th April 2009
quotequote all
mybrainhurts said:
Nigel in shock Worse Than Previously Thought Mk XXXXXXXXIV revelation...

smile
rotate

Following my two posts yesterday we had one of the longest GoogleWikiBlog intervals in recorded threaddom, and after remarks in the sea level discussion I was waiting to be told I wasn't doing well on this thread either hehe

But since it's always the same judge and they're biased to kingdom come and on to the next ice age, it means absolutely nothing except their wishful thinking biggrin

No evidence or - heaven forbid - data to counter anything as yet (same old).

turbobloke

103,742 posts

259 months

Sunday 5th April 2009
quotequote all
guam said:
The Countercase has been consistently that warming and cooling phases are completely normal variations in a natural system affected (if at all) by human activity to a microscopic extent (certainly not measurable in any meaningful sense) you on the other hand selectively cherry pick the arguement whenever data comes out trying (specifically with TB) to rubbish ANY counter debate.
yes

True Believers need data to support their case. They have no such data. They are all at sea, though the level isn't changing much wink

thinfourth2

32,414 posts

203 months

Sunday 5th April 2009
quotequote all
nigelfr said:
Guam said:
nigelfr said:
Sorry for the delay in replying, DocJock. I haven't seen the paper yet, just the report from
http://www.physorg.com/news157963913.html

You make a valid point about the measurements: I wondered how CRF was measured in that period too. Especially as the CRF/cloud theory requires energies in the 10GeV range according to Shaviv and the Be or C proxies require much lower energies.( <1 GeV IIRC)

Now the responses so far have been interesting: I see some of the ditto bots have turned up and revealed that they haven't learnt anything about the "It's the Sun" theory of GW even though it has been mentioned here a lot by the sceptics as an alternative to AGW.

In brief: some people have compared proxies of sun spot counts against temperature and have used the medieval warm period in Europe to "prove" a correlation. Now there is no proven mechanism to show cause and effect and also Europe is not the world, so a warm Europe doesn't really show global temperature.

Sceptics of this sunspot theory have suggested that an oscillation in the Atlantic (NAO) like the ENSO in the Pacific could explain the MWP. The report as mentioned on Physics.org sounds interesting.

Edited by nigelfr on Saturday 4th April 16:54
You really are Myopic Nige you dont get any of the counter arguement do you (guess you cant afford to). THe Countercase has been consistently that warming and cooling phases are completely normal variations in a natural system affected (if at all) by human activity to a microscopic extent (certainly not measurable in any meaningful sense) you on the other hand selectively cherry pick the arguement whenever data comes out trying (specifically with TB) to rubbish ANY counter debate.

What is too funny for words is that trying to shoot TB down on one aspect of "Natural Forcings" you have actually re-inforced him with a generally supportive postulation. You do see how hilarious that is and how inane it makes your whole position dont you smile


Cheers

Tom
Sorry, Tom, but I think it's you that don't get the point. The sceptics use the MWP to show a global warm period. If it's an oscillation it's not global and it's not a forcing.

Obviously you don't understand the AGW position either Tom. Every time someone says "Cold today innit, where's yer global warming now?", they are told that AGW is a signal imposed on natural climate variation.
A true beliver sees the weather like this

Its warmer then yesterday = global warming
its colder then yesterday = global warming
It's sunny = Global warming
It's not sunny = Global warming
It's snowing = Global warming
Fish falling from the sky = Global warming
Earthquake = Global warming
Stub your toe on the bed = Global warming
Someone farts = Global warming


Normal people just see weather


turbobloke

103,742 posts

259 months

Sunday 5th April 2009
quotequote all
thinfourth2 said:
Stub your toe on the bed = Global warming
Maybe I just contributed to the scorching heat.....ouch hehe

Still. We can all relax now, not least because:

Guardian in Climate Change NOT Worse Than Previously Thought shock

http://www.guardian.co.uk/environment/2009/mar/31/...

Jasandjules

69,825 posts

228 months

Sunday 5th April 2009
quotequote all
How does that reconcile with the IPPC and all other scaremongerers saying it's worse than previously thought!!?!?! FFS you think they'd at least keep the story straight... Poor form.....

BTW Nigel, how can they be sure that the rest of the world was cooler in the MWP? Have they data which supports that? (Just at the back of my mind I thought I read there were some tree rings whjich were checked and it was also warm(er) in the USA and North Asia during the MWP..).

HUW JONES

1,985 posts

202 months

Sunday 5th April 2009
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Cut to the chase please...how much more tax do I have to pay for this? Thanks.

Jasandjules

69,825 posts

228 months

Sunday 5th April 2009
quotequote all
HUW JONES said:
Cut to the chase please...how much more tax do I have to pay for this? Thanks.
Lots for the next couple of years, then hopefully solar activity will show clearly that it's not CO2 and their lies will have to cease. Look out for new ways of raising tax coming along to a motorway network near you.

turbobloke

103,742 posts

259 months

Sunday 5th April 2009
quotequote all
Jasandjules said:
BTW Nigel, how can they be sure that the rest of the world was cooler in the MWP? Have they data which supports that? (Just at the back of my mind I thought I read there were some tree rings whjich were checked and it was also warm(er) in the USA and North Asia during the MWP..).
You're right Jasandjules. There is a considerable body of research evidence showing overwhelmingly the global nature of MWP, crossing both hemispheres. A sample:

Evidence for the Existence of the Medieval Warm Period in China.
Abstract: "it can be estimated that the annual mean temperature in south Henan Province in the thirteenth century was 0.9–1.0°C higher than at present."
http://www.springerlink.com/content/gh98230822m7g0...

Evidence for a 'Medieval Warm Period' in a 1,100 year tree-ring reconstruction of summer temperatures in New Zealand.
Abstract: "This record is the longest yet produced for New Zealand and shows clear evidence for persistent above-average temperatures within the interval commonly assigned to the MWP."
http://www.agu.org/pubs/crossref/2002/2001GL014580...

Environmental change in eastern Greenland during the last 1300 years - evidence from foraminifera and lithofacies in Nansen Fjord.
Abstract: "The evidence suggests that the climate in the region of Nansen Fjord was warmer and more stable than today during a 'Medieval Warm Period'"
http://hol.sagepub.com/cgi/content/abstract/6/2/17...

There's more.

Using thermal and hydrological demand for crops crown by the first agropastoralists of southern Africa, Huffman (1996) constructed a climate history of the region based on evidence acquired from various Iron Age settlements. Relic evidence of the presence of cultivated sorghum and millets proved that the climate of the subcontinent-wide region must have been warmer and wetter than it is today from approximately AD 900-1300 - indeed these crops cannot be grown in that part of southern Africa under current climatic conditions, which are far too cool and dry. Support for this result came from Tyson et. al. (2000), who obtained a quasi-decadal record of oxygen and carbon-stable isotope data from a well-dated stalagmite of Cold Air Cave in the Makapansgat Valley (30 km southwest of Pietersburg, South Africa). See also Holmgren et. al. (2001 & 2003) also Lamb et. al. (2003).

In N America, Arseneault and Payette (1997) analysed tree ring and growth-form series in Quebec to produce a climate record for this region between 690 and 1591 AD - revealing a MWP. The scientists concluded that the Medieval Warm Period there was approximately 1°C warmer than the 20th century. Same continent but over a bit, Calkin et al. (2001) assessed 'the most current and comprehensive research of Holocene glaciation' along the Gulf of Alaska between the Kenai Peninsula and Yakutat Bay, where they detected a Medieval Warm Period that lasted for "at least a few centuries prior to A.D. 1200."


Add that to the rest list (it's getting tedious citing the obvious) and the denialists, which have includede the IPCC in its reports, are nowhere.

nigelfr

Original Poster:

1,658 posts

190 months

Sunday 5th April 2009
quotequote all
Guam said:
I love that TB the line is that they overspecced the design in essence, rather than oh look Global warming is cobblers so we will be fine with what we have smile These guys have no shame smile

Cheers
You guys make me laugh... the Thames Barrier was designed in 1974 on the basis of data and predictions then.

The Dutch have had sea water defences for hundreds of years and they are currently considered adequate for a few more decades: so were they planning for AGW in the 1500's or did they just put a safety margin in the design?

nigelfr

Original Poster:

1,658 posts

190 months

Sunday 5th April 2009
quotequote all
turbobloke said:
Following my two posts yesterday we had one of the longest GoogleWikiBlog intervals in recorded threaddom, and after remarks in the sea level discussion I was waiting to be told I wasn't doing well on this thread either hehe
Some of us have a life on Saturday night.

Interestingly your Tsonis post had nothing to do with the MWP, but al least you mentioned the NAO.

I notice that the denierscepticwebblogramblingssphere is quiet on the issue so I guess I'm going to have to wait before I hear anything substantive on the MWP/NAO/CRF connection.

turbobloke

103,742 posts

259 months

Sunday 5th April 2009
quotequote all
I quoted Tsonis for my valid reasons, not anybody else's different reasons of their own device.

You already got everything substantive you need on MWP and NAO in my posts on this thread, ignoring them is your choice but it's hardly going to be missed by others.

nigelfr

Original Poster:

1,658 posts

190 months

Sunday 5th April 2009
quotequote all
Guam said:
nigelfr said:
Guam said:
You really are Myopic Nige you dont get any of the counter arguement do you (guess you cant afford to). THe Countercase has been consistently that warming and cooling phases are completely normal variations in a natural system affected (if at all) by human activity to a microscopic extent (certainly not measurable in any meaningful sense) you on the other hand selectively cherry pick the arguement whenever data comes out trying (specifically with TB) to rubbish ANY counter debate.

What is too funny for words is that trying to shoot TB down on one aspect of "Natural Forcings" you have actually re-inforced him with a generally supportive postulation. You do see how hilarious that is and how inane it makes your whole position dont you smile


Cheers

Tom
Sorry, Tom, but I think it's you that don't get the point. The sceptics use the MWP to show a global warm period. If it's an oscillation it's not global and it's not a forcing.

Obviously you don't understand the AGW position either Tom. Every time someone says "Cold today innit, where's yer global warming now?", they are told that AGW is a signal imposed on natural climate variation.
Oh Come on now Nigel thats not what the Pro AGW camp have been saying for YEARS now is it smile

The fact is its got beyond laughable now, as effectively if its warm, then its warmer than it should otherwise be and if its cold it should be colder smile

The arguement is spent with anyone other than the most blinkered believer now, furtunately the economic climate will shift focus and resources and politicians will eventually do what they have always done and move towards the view that will gain them the most votes (and that is increasingly NOT going to be AGW imho) smile


Cheers
Really Tom, I don't know what to make of this post. Maybe you had too much life last night smile

(I'm not trying to be offensive)

You obviously are aware of the concept of forcings. Here's what Hansen et al had to say in 1998 "The forcings that drive long-term climate change are not known with an accuracy sufficient to define future climate change. Anthropogenic greenhouse gases (GHGs), which are well measured, cause a strong positive (warming) forcing. But other, poorly measured, anthropogenic forcings, especially changes of atmospheric aerosols, clouds, and land-use patterns, cause a negative forcing that tends to offset greenhouse warming. One consequence of this partial balance is that the natural forcing due to solar irradiance changes may play a larger role in long-term climate change than inferred from comparison with GHGs alone. Current trends in GHG climate forcings are smaller than in popular “business as usual” or 1% per year CO2 growth scenarios. The summary implication is a paradigm change for long-term climate projections: uncertainties in climate forcings have supplanted global climate sensitivity as the predominant issue...

Natural Forcings

Natural climate forcings are limited to factors “imposed” on the climate system. Thus fluctuations of soil dust aerosols that occur with drought conditions are an internal climate feedback process. The dominant known natural climate forcings that may be important on global and decadal time scales are changes of the sun and stratospheric aerosols from large volcanoes. Both of these forcings have been measured accurately from satellites during the past two decades and are estimated for the preceding century from indirect measures.
Solar Irradiance.

Measured changes of solar irradiance since 1979 reveal a cyclic variation of amplitude ~0.1% in phase with the sunspot cycle (38). The variation is largest at UV wavelengths that are absorbed in the stratosphere, but ~85% of the variation occurs at wavelengths that penetrate into the troposphere.

Variations of solar irradiance on longer time scales are hypotheses based on ad hoc relationships between irradiance and observed solar features such as sunspot number or the length of the solar cycle. The estimated solar forcing of 0.3 W/m2 for the period 1850 to the present (Fig. 2) is based on the analysis of Lean et al. (39).

Additional indirect solar forcings have been hypothesized, but so far only a small effect via ozone changes has been quantified. The indirect solar forcing via ozone change is in phase with the direct solar forcing and approximately one-third of its magnitude (1). Thus, this indirect solar effect may have added a positive forcing of ≈0.1 W/m2 over the past 150 years.
Volcanos.

Stratospheric aerosols from large volcanoes can cause a large negative forcing. This forcing decays approximately exponentially with a time constant of ~1 year. The eruption of Mt. Pinatubo in 1991, which caused a peak forcing just over 3 W/m2 with an uncertainty of ~20% (13), was probably the largest volcanic aerosol forcing this century.

The climate forcing due to volcanoes in the century preceding satellite data can be estimated from measurements of atmospheric transparency (40). But, because of limitations in the spatial, temporal, and spectral coverage of these ground-based data, the accuracy of the global climate forcing is probably not better than a factor of two.

We calculate the decadal mean of this episodic forcing because our interest is long-term climate change. Fig. 2 shows the range of volcanic aerosol forcings between a decade with no large volcanoes and the decade estimated to have the greatest aerosol amount during the past 150 years (the 1880s). The forcings are calculated relative to the mean aerosol forcing for the period since 1850 (40). Note that use of the decadal mean for this episodic forcing differs from the other forcings in Fig. 2, which each represent the change since 1850. "

http://www.pnas.org/content/95/22/12753.abstract

and the IPCC reports have whole sections devoted to the natural forcings. See this from the technical summary TAR "Any human-induced changes in climate will be embedded in a background of natural climatic variations that occur on a whole range of time- and space-scales. Climate variability can occur as a result of natural changes in the forcing of the climate system, for example variations in the strength of the incoming solar radiation and changes in the concentrations of aerosols arising from volcanic eruptions. Natural climate variations can also occur in the absence of a change in external forcing, as a result of complex interactions between components of the climate system, such as the coupling between the atmosphere and ocean. The El Niño-Southern Oscillation (ENSO) phenomenon is an example of such natural "internal" variability on interannual time-scales. To distinguish anthropogenic climate changes from natural variations, it is necessary to identify the anthropogenic "signal" against the background "noise" of natural climate variability." http://www.ipcc.ch/ipccreports/tar/vol4/english/08...



BTW Tom you say
Guam said:
... you on the other hand selectively cherry pick the arguement whenever data comes out trying (specifically with TB) to rubbish ANY counter debate.
Tom, believe it or not, in a debate the object is to rubbish the counter argument: it takes all the fun out of it if all I'm allowed to do is agree with the other side. Interestingly, the reason I argue against TB's points is that he is often the only one who comes with anything that even resembles a plausible argument.