Important New Study: Esper et. al 2012

[Snow_Miser]

This is big news in the scientific community: Esper et. al 2012.

Esper et. al 2012 documented that the Medieval Warm Period and the Roman Warm Period have both been underestimated in previous studies, and according to the author, Dr. Jan Esper:

“Such findings are also significant with regard to climate policy, as they will influence the way today’s climate changes are seen in context of historical warm periods.”

They also find a cooling of -0.3 Degrees C per millenia:

Their findings demonstrate that this trend involves a cooling of -0.3°C per millennium due to gradual changes to the position of the sun and an increase in the distance between the Earth and the sun.”This figure we calculated may not seem particularly significant,” says Esper. “However, it is also not negligible when compared to global warming, which up to now has been less than 1°C. Our results suggest that the large-scale climate reconstruction shown by the Intergovernmental Panel on Climate Change (IPCC) likely underestimate this long-term cooling trend over the past few millennia.”

http://www.uni-mainz.de/eng/15491.php

The paper can be viewed here:

http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate1589.html

Solar insolation changes, resulting from long-term oscillations of orbital configurations¹, are an important driver of Holocene climate^{2, 3}. The forcing is substantial over the past 2,000 years, up to four times as large as the 1.6 W m⁻² net anthropogenic forcing since 1750 (ref. 4), but the trend varies considerably over time, space and with season⁵. Using numerous high-latitude proxy records, slow orbital changes have recently been shown⁶ to gradually force boreal summer temperature cooling over the common era. Here, we present new evidence based on maximum latewood density data from northern Scandinavia, indicating that this cooling trend was stronger (−0.31 °C per 1,000 years, ±0.03 °C) than previously reported, and demonstrate that this signature is missing in published tree-ring proxy records. The long-term trend now revealed in maximum latewood density data is in line with coupled general circulation models^{7, 8} indicating albedo-driven feedback mechanisms and substantial summer cooling over the past two millennia in northern boreal and Arctic latitudes. These findings, together with the missing orbital signature in published dendrochronological records, suggest that large-scale near-surface air-temperature reconstructions^{9, 10, 11, 12, 13} relying on tree-ring data may underestimate pre-instrumental temperatures including warmth during Medieval and Roman times.

The 1000 year solar natural cycle can clearly be seen in this reconstruction.

[PhillipS]

Interesting paper, thank you for sharing it. It will be interesting to see how the authors' findings hold up. Several things caught my eye as I read it:

• The area of study was limited to Scandinavia, making it very regional. Their paleoclimate reconstruction can't really be extrapolated to the NH, much less global paleoclimate.
  
• They used only one type of proxy, tree rings, and all of their non-living trees were recovered from lakes and lakeshores. They were not found in-situ. I confess my knowledge of dendrochronology is limited, but my understanding is that once a tree is removed from its growing site it is impossible to assess other factors which affect tree growth, such as soil condition, precipitation, and sunlight. And, as Trixie's post pointed out, there is a spectrum of various types of proxies - so why did they focus on only MXD and ignore the others? If their approach was corroborated by other proxies I think it would have leant strength to their findings.
  
• In the second paragraph of the paper there is the sentence "Indeed an evaluation of long-term temperature reconstructions, even over the past 7,000 years from across northern Eurasia, demonstrates that TRW-based records fail to show orbital signatures found in low-resolution proxy archives and climate model simulations". I read that to mean that the TRW data doesn't match their model simulation so they chose to ignore that data. Correct me if I'm wrong, but isn't emphasizing models over data a no-no for the skeptic community?
  

And, the biggest eye-opener from that paper is their assertion that the 0.31 C/1000 year cooling trend they calculated is greater than AGW warming. Let's do a quick reality check on that claim. The global warming during the instrumental period has been about 0.15 C/decade (according to Dr Roy Spencer). That works out to 15 C/1000 years - which is about 50 times the rate of cooling they calculated. Hmmm. Looked at another way, since 1972 AGW has erased 2,000 years of cooling due to orbital changes. Hmmm, again. I think they made a big claim with very limited data to support it.

[donsutherland1]

Interesting paper, thank you for sharing it. It will be interesting to see how the authors' findings hold up. Several things caught my eye as I read it:

• The area of study was limited to Scandinavia, making it very regional. Their paleoclimate reconstruction can't really be extrapolated to the NH, much less global paleoclimate.
  
• They used only one type of proxy, tree rings, and all of their non-living trees were recovered from lakes and lakeshores. They were not found in-situ. I confess my knowledge of dendrochronology is limited, but my understanding is that once a tree is removed from its growing site it is impossible to assess other factors which affect tree growth, such as soil condition, precipitation, and sunlight. And, as Trixie's post pointed out, there is a spectrum of various types of proxies - so why did they focus on only MXD and ignore the others? If their approach was corroborated by other proxies I think it would have leant strength to their findings.
  
• In the second paragraph of the paper there is the sentence "Indeed an evaluation of long-term temperature reconstructions, even over the past 7,000 years from across northern Eurasia, demonstrates that TRW-based records fail to show orbital signatures found in low-resolution proxy archives and climate model simulations". I read that to mean that the TRW data doesn't match their model simulation so they chose to ignore that data. Correct me if I'm wrong, but isn't emphasizing models over data a no-no for the skeptic community?
  

And, the biggest eye-opener from that paper is their assertion that the 0.31 C/1000 year cooling trend they calculated is greater than AGW warming. Let's do a quick reality check on that claim. The global warming during the instrumental period has been about 0.15 C/decade (according to Dr Roy Spencer). That works out to 15 C/1000 years - which is about 50 times the rate of cooling they calculated. Hmmm. Looked at another way, since 1972 AGW has erased 2,000 years of cooling due to orbital changes. Hmmm, again. I think they made a big claim with very limited data to support it.

I'm reading the paper. The proverbial devil may lie in the details. The paper compares 2000-years of orbital forcing of around 6 watts per square meter to about 250-years of anthropogenic forcing of 1.6 watts per square meter. On a 250-year basis, the orbital forcing would amount to 0.75 watts per square meter for such a period, which would be less than half of the 250-year anthropogenic forcing since 1750. In other words, even assuming that the authors' calculations are reasonable, the most recent 250-year period has seen anthropogenic forcing exceed orbital forcing.

[donsutherland1]

http://www.realclima...t-developments/

Many thanks for sharing this, Wxtrix. This helps put the new paper into a larger context.

[PhillipS]

I'm reading the paper. The proverbial devil may lie in the details. The paper compares 2000-years of orbital forcing of around 6 watts per square meter to about 250-years of anthropogenic forcing of 1.6 watts per square meter. On a 250-year basis, the orbital forcing would amount to 0.75 watts per square meter for such a period, which would be less than half of the 250-year anthropogenic forcing since 1750. In other words, even assuming that the authors' calculations are reasonable, the most recent 250-year period has seen anthropogenic forcing exceed orbital forcing.

Thank you for clarifying that for me.

[WeatherRusty]

There has got to be a misconception regarding the -6 watts per square meter figure. I suspect this is a flat plane measure of incident radiation change rather than a measure of radiative forcing. Accordingly, when allowing for Earth's geometry and albedo we must divide by 4 and multiply times 0.7 to get a global radiative forcing of about -1w/m^2 and an equilibrium temperature response of about -0.3C over the 2000 year period which fits the results of this study.

Remember, a doubling of CO2 gives 3.7W/m^2. Notice the difference?

EDIT:

Noted also that the -6 watt figure applies to 60-70 north latitude and during the summer months rather than a globally averaged annual radiative forcing.

When they mention the anthropogenic 1.6w forcing figure in comparison they are comparing to a globally averaged annual radiative forcing.

[PhillipS]

I'm reading the paper. The proverbial devil may lie in the details. The paper compares 2000-years of orbital forcing of around 6 watts per square meter to about 250-years of anthropogenic forcing of 1.6 watts per square meter. On a 250-year basis, the orbital forcing would amount to 0.75 watts per square meter for such a period, which would be less than half of the 250-year anthropogenic forcing since 1750. In other words, even assuming that the authors' calculations are reasonable, the most recent 250-year period has seen anthropogenic forcing exceed orbital forcing.

Don - now that I've had some time to think about the paper and your explanation of the 6 W/m2 forcing they postulate, I may be more confused then ever. 6 W/m2 over a 2,000 year period isn't the same magnitude of forcing as 0.75 W/m2 over a 250-year period. Is it?

Could the 6 W/m2 value be the min to max delta in insolation between the depth of the glacial period and the peak of the interglacial? That would make it about four times the solar cycle TSI variability.

[donsutherland1]

Don - now that I've had some time to think about the paper and your explanation of the 6 W/m2 forcing they postulate, I may be more confused then ever. 6 W/m2 over a 2,000 year period isn't the same magnitude of forcing as 0.75 W/m2 over a 250-year period. Is it?

Could the 6 W/m2 value be the min to max delta in insolation between the depth of the glacial period and the peak of the interglacial? That would make it about four times the solar cycle TSI variability.

PhillipS,

You might be right about the 6 W/m2 value pertaining to the minimum to maximum delta. Unfortunately, the paper doesn't provide the level of detail to be sure.

[WeatherRusty]

Don - now that I've had some time to think about the paper and your explanation of the 6 W/m2 forcing they postulate, I may be more confused then ever. 6 W/m2 over a 2,000 year period isn't the same magnitude of forcing as 0.75 W/m2 over a 250-year period. Is it?

Could the 6 W/m2 value be the min to max delta in insolation between the depth of the glacial period and the peak of the interglacial? That would make it about four times the solar cycle TSI variability.

If the 6W/m^2 value where to represent a global radiative forcing in the same sense as it is for CO2, then we would expect a Planck temperature response of about 1.8C. That's before any feedback. The total radiative forcing due to orbital changes is much, much less than 6 watt/meter squared.

I believe what they have done is to ascertain the direct radiation difference due to orbital variation during the summer months (JJA) and between 60 and 70 degrees north latitude. This is not the same thing as a planet wide radiative forcing. Yet they have compared the 6 watt value to the 1.6 watts of NET ANTHROPOGENIC RADIATIVE FORCING.

[Snow_Miser]

Thanks for your input everyone. Really nice discussions.

[Snow_Miser]

Don - now that I've had some time to think about the paper and your explanation of the 6 W/m2 forcing they postulate, I may be more confused then ever. 6 W/m2 over a 2,000 year period isn't the same magnitude of forcing as 0.75 W/m2 over a 250-year period. Is it?

Could the 6 W/m2 value be the min to max delta in insolation between the depth of the glacial period and the peak of the interglacial? That would make it about four times the solar cycle TSI variability.

I don't know if the 6.4 w/m^2 they attribute to the solar/orbital forcing is necessarily over the 2000 year timeframe, because temperatures go up AND down during this timeframe, or over the timeframe where temperature changes relatively rapidly over the reconstruction (short term temperature increases and decreases over the last 2000 years). Note how there appears to be a cyclical pattern going on with the reconstruction of temperatures.

[Msalgado]

There is not enough data there to establish a pattern on a 1000 year time frame.

[Snow_Miser]

There is not enough data there to establish a pattern on a 1000 year time frame.

I'm not sure what you are trying to argue here.

[donsutherland1]

I don't know if the 6.4 w/m^2 they attribute to the solar/orbital forcing is necessarily over the 2000 year timeframe...

Here's what the authors wrote in their paper:

...we propose that the millennial scale cooling trend retained in N-scan is forced by JJA insolation changes of ~6 W m^-2 over the past 2,000 years.

Note how there appears to be a cyclical pattern going on with the reconstruction of temperatures.

The interannual and multicentennial cyclical pattern also shows up in other climate proxies. The possibility that the longer-term summer cooling trend in the Arctic zone/northern Scandanavia may be stronger than earlier research indicated and that some earlier warmer periods were warmer than earlier research indicated is new. The paper also noted that the CGCM (climate global circulation model) simulations revealed similar patterns over the Arctic zone/northern Scandanavia, but a weaker cooling trend toward lower latitudes.

It will be interesting to see if this research holds up to the kind of rigorous scrutiny that takes place in the scientific arena.

[Snow_Miser]

...we propose that the millennial scale cooling trend retained in N-scan is forced by JJA insolation changes of ~6 W m^-2 over the past 2,000 years.

Okay thank you for the clarification.

This would give you a very low climate sensitivity actually, FWIW, if their results were to be correct. You would get a sensitivity of 10 w/m^2/K, which would give you a climate sensitivity of around 0.37 Degrees C.

It will be interesting to see if this research holds up to the kind of rigorous scrutiny that takes place in the scientific arena.

It sure will be.

[Snow_Miser]

Also, if the forcing that was 4 times as much as the anthropogenic forcing over a 2000 year scale produced a total temperature change of -0.6 Degrees C over the past 2000 years, this means that the total anthropogenic forcing would produce a temperature change of around 0.15 Degrees C, which is consistent with the 10-11 w/m^2 sensitivity I talked about in my prior post.

FWIW, this would actually mean that only about 18-19% of the recent warming of 0.8 Kelvin can be attributed to the anthropogenic forcing.

[Carvers Gap]

This is big news in the scientific community: Esper et. al 2012.

Esper et. al 2012 documented that the Medieval Warm Period and the Roman Warm Period have both been underestimated in previous studies, and according to the author, Dr. Jan Esper:

“Such findings are also significant with regard to climate policy, as they will influence the way today’s climate changes are seen in context of historical warm periods.”

They also find a cooling of -0.3 Degrees C per millenia:

Their findings demonstrate that this trend involves a cooling of -0.3°C per millennium due to gradual changes to the position of the sun and an increase in the distance between the Earth and the sun.”This figure we calculated may not seem particularly significant,” says Esper. “However, it is also not negligible when compared to global warming, which up to now has been less than 1°C. Our results suggest that the large-scale climate reconstruction shown by the Intergovernmental Panel on Climate Change (IPCC) likely underestimate this long-term cooling trend over the past few millennia.”

http://www.uni-mainz.de/eng/15491.php

The paper can be viewed here:

http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate1589.html

Solar insolation changes, resulting from long-term oscillations of orbital configurations¹, are an important driver of Holocene climate^{2, 3}. The forcing is substantial over the past 2,000 years, up to four times as large as the 1.6 W m⁻² net anthropogenic forcing since 1750 (ref. 4), but the trend varies considerably over time, space and with season⁵. Using numerous high-latitude proxy records, slow orbital changes have recently been shown⁶ to gradually force boreal summer temperature cooling over the common era. Here, we present new evidence based on maximum latewood density data from northern Scandinavia, indicating that this cooling trend was stronger (−0.31 °C per 1,000 years, ±0.03 °C) than previously reported, and demonstrate that this signature is missing in published tree-ring proxy records. The long-term trend now revealed in maximum latewood density data is in line with coupled general circulation models^{7, 8} indicating albedo-driven feedback mechanisms and substantial summer cooling over the past two millennia in northern boreal and Arctic latitudes. These findings, together with the missing orbital signature in published dendrochronological records, suggest that large-scale near-surface air-temperature reconstructions^{9, 10, 11, 12, 13} relying on tree-ring data may underestimate pre-instrumental temperatures including warmth during Medieval and Roman times.

The 1000 year solar natural cycle can clearly be seen in this reconstruction.

.

Thank you for sharing and a good read.

[WeatherRusty]

Also, if the forcing that was 4 times as much as the anthropogenic forcing over a 2000 year scale produced a total temperature change of -0.6 Degrees C over the past 2000 years, this means that the total anthropogenic forcing would produce a temperature change of around 0.15 Degrees C, which is consistent with the 10-11 w/m^2 sensitivity I talked about in my prior post.

FWIW, this would actually mean that only about 18-19% of the recent warming of 0.8 Kelvin can be attributed to the anthropogenic forcing.

Wrong

The 6 watt figure is not a radiative forcing like the 1.6 watt figure is. One can not be compared with the other as they are not measures of the same parameter. See my above posts.

The 6 watt figure represents changed insolation between 60-70 north latitude for the months of JJA.

The 1.6 watt figure represents a global radiative forcing as measured at the tropopause.

[Snow_Miser]

Wrong

The 6 watt figure is not a radiative forcing like the 1.6 watt figure is. One can not be compared with the other as they are not measures of the same parameter. See my above posts.

The 6 watt figure represents increased insolation between 60-70 north latitude for the months of JJA.

The 1.6 watt figure represents a global radiative forcing as measured at the tropopause.

So why did they compare the two as being forcings in the abstract of the paper, then?

Solar insolation changes, resulting from long-term oscillations of orbital configurations¹, are an important driver of Holocene climate^{2, 3}. The forcing is substantial over the past 2,000 years, up to four times as large as the 1.6 W m⁻² net anthropogenic forcing since 1750 (ref. 4),

[Snow_Miser]

.

Thank you for sharing and a good read.

Thank you, I'm glad you liked the paper.

[WeatherRusty]

So why did they compare the two as being forcings in the abstract of the paper, then?

Solar insolation changes, resulting from long-term oscillations of orbital configurations¹, are an important driver of Holocene climate^{2, 3}. The forcing is substantial over the past 2,000 years, up to four times as large as the 1.6 W m⁻² net anthropogenic forcing since 1750 (ref. 4),

I don't know, but they definitely are comparing two separate values as if they were the same. Not sure if this has any affect on the results of the study indicating a warmer past, probably not. The study is not invalidated, but the comparison to antropogenic radiative forcing being 1/4 that of high latitude radiation changes appears to be based on a misunderstanding of what a radiative forcing represents.

Here in the body of the paper they state this:

Over recent millennia, orbital forcing has continually reduced summer insolation in the Northern Hemisphere⁵. Peak insolation changes in Northern Hemisphere high latitudes, at ~65° N between June–August (JJA), have been identified as the prime forcing of climate variability over the past million years

and

As suggested previously^{2, 6}, we propose that the millennial scale cooling trend retained in N-scan is forced by JJA insolation changes of ~ −6 W m⁻² over the past 2,000 years

------

The 6 watts is a radiation change over high latitudes during JJA. Not the same thing as a global radiative forcing.

What this means is that more/less of the total global insolation is falling in the far northern latitudes. This is actually the mechanism thought to be governing the glacial cycles of the past 3 million years. Glaciation is enhanced when the summer melt is less than the cold season snowfall and this in turn is modulated by incident radiation falling near 70 degrees north.

[cmc0605]

I was just able to skim through this paper (very busy week). WeatherRusty is absolutely correct that these two "forcing" values are completely incomparable, and it's a shame that the authors were so irresponsible in what looks like little more than a sound byte for their paper. The two relevant references they provide are (4) The IPCC 2007 report, (5) Berger and Loutre's paper tracking insolation values back 10 million years.

None of these support a completely implausible 6 W/m2 forcing over the last millennia, and it's not surprising that the precessional cycle has changed local, seasonal insolation...this is well known, and is incorporated into any modeling study of the last millennia or Holocene.

That said, not being a tree ring person, the paper itself appears interesting enough and I'm sure will generate discussion in the dendro community. That RealClimate critique looks to be spot on.

[Vergent]

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