How much warmer is the planet presently than during the MWP?

Question

I asked a similar question earlier, but I hadn't read the full Loehle paper because I was having problems downloading it.

http://answers.yahoo.com/question/index;_ylt=Ahj_9Ts6XCPlEPR8x0ZiSSPsy6IX;_ylv=3?qid=20080131103827AAvI6b0

This paper has been cited as proving that the MWP was significantly warmer than today. This is because the abstract of the paper claims:

"...the MWP being approximately 0.3°C warmer than 20th century values at these eighteen sites."

But the conclusion of the paper is:

"the MWP peak remains 0.07°C above the end of the 20th Century values, though the difference is not significant."

And in a correction published by the authors, if you look at his final plot on page 12, you'll see that the weighted and culled plots show the MWP peak 0.1°C warmer than 1935, which was 0.55°C cooler than today.

http://www.econ.ohio-state.edu/jhm/AGW/Loehle/SupplementaryInfo.pdf

So now I ask a similar question - how much warmer are we now than during the MWP?

Answer 1

I think it’s difficult to say since the data collection methods are different, but it’s probably < 0.25 degrees on either side of the MWP high.

But I have a hypothesis that deals strictly with temperature charts without trying to establish a cause for GW ...

Much of the GW concern is centered on graphs showing dramatic temperature upswings in modern times. The thinking being that when extrapolated they are precursors of earth’s climate reaching potentially unlivable warmth. Indeed, if these are an accurate representation of global temperature it is a reason for concern; whether humans are causing or not; and whether humans can do something about it or not. Computer models have been created in an attempt to validate this thinking. But computer models – and manual calculations as well – are only as good as the data they rely upon. Hence my hypothesis:

Hypothesis:

Much of the reason temperature charts show such dramatic increases is due to better and more accurate data collection methods. As a result, either 1) modern temperatures are represented higher than they should be (less likely), or 2) historical temperatures are represented lower than they should be (more likely).

Observation:

I base this hypothesis on the now discredited “Hokey Stick” (misspelling intentional) of Michael Mann that appeared as key evidence in IPCC AR3 (2001).

http://meteo.lcd.lu/globalwarming/hockey...

In this chart all temperature reconstructions from before 1880 are necessarily from proxies (ice cores, tree rings, etc.) whereas after that point the records are from instrument measurement. Obviously most of these post-1880 readings are from ground-based thermometers, since the more accurate satellite readings were not available until 1960. But ground-based readings are notoriously unreliable due to urban heat island effects; a paucity of reporting stations; and the poor placement, condition, and maintenance of the Stevenson screens (thermometers).

http://wattsupwiththat.wordpress.com/cat...

But back to one of the problems of Mann’s chart that is relevant to this discussion: the proxy data of the 20th century plots a curve divergent from the instrument temperatures (instrument data climbs sharply at the divergence point). This is even more apparent when you look closely at the IPCC “spaghetti graph” (the one with all the colors representing different reconstructions)

http://en.wikipedia.org/wiki/Image:1000_...

This would signify, in my mind at least, that if the instruments read higher than proxies now, had we had the benefit of instrument readings in the past they would show higher temperatures than the proxies then as well. This would of course significantly smooth the temperature curve – perhaps even to the point that it would show a much less severe warming trend. (Mann’s mean temperature curve actually showed a 900 year cooling trend until the inflection point around 1880 where instrument data was combined with the proxy data. This does not mean it didn’t begin to warm there however.)

Before you send me off to realclimate, I have already read Hansen’s reply to this problem. He states (in a nutshell) that the proxy and thermometer readings were 'close enough' that there was no longer any need to consider proxy data. And indeed they do appear close for a time, although that timeframe was one where due to the x axis (time) being necessarily compressed, the scale of the graph is such that the slope of the curve is very steep. This creates, for lack of a better term, a parallax effect that may make the proxy and instrument data look closer together than they really are. (If the actual data was available this could be verified.) As the proxy curve begins to plateau in the late 1900s, there is a marked divergence. Interestingly, Briffa (lead author) in IPCC AR4 (2007) truncates the proxy data in what might be considered an effort to mask this divergence. But the divergence problem remains ...

http://www.climateaudit.org/?p=2532

So, simply stated, if we had been able to use the same methods of data collection all along, wouldn’t we perhaps find that the either the current temperature is artificially high, or the MWP temperature too low, and total warming is possibly less than presently thought?

Answer 2

The paper is clear:

"...the difference is not significant."

It's unknown... less than the margin of error for the data involved.

Note that the paper also refers to comparing the MWP with "end of the 20th-century values."

The last data point for the 29 year averages used as end of the 20th-century values was 1992, biasing the 20th century towards cooler times then we're in today.

I'll have to revist the corrected 2008 version of the paper so see what you're referring to about the MWP being graphed as 0.45°C coller than today.

Answer 3

1. Peak MWP temps occurred 850(!) AD.

2. Peak MWP temp was +.56 +- .14 above anomaly.

3. 1935 (enddate) temp was +.13 +-.14 above anomaly.

4. GISS records put 1935 temp at +.10 above anomaly.

5. Correcting for base anomaly differences puts MWP peak at +.53 above GISS anomaly.

6. Current (2007) GISS temp is .54 (2006) or .57 (2007) above anomaly.

7. Current temps are therefore .01 to .04 +- .14 C above peak MWP temps.

And rising.

With no end in sight.

Answer 4

As Benjamin pointed out in your earlier post on this, anything in Energy and Environment is not worth discussing.  It is to real peer-review what Weekly World News is to the New York Times.  

Here are some real relevant recent publications on millenial scale temperature reconstructions that don't use tree rings.  Talk about these, not garbage.  

[Most interesting one, in my opinion]

Title: On the long-term context for late twentieth century warming

Author(s): D'Arrigo R, Wilson R, Jacoby G

Source: JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES 111 (D3): Art. No. D03103 FEB 7 2006

Document Type: Article

Language: English

Cited References: 51      Times Cited: 12      Find Related Records Information

Abstract: [1] Previous tree-ring-based Northern Hemisphere temperature reconstructions portray a varying amplitude range between the "Medieval Warm Period'' (MWP), "Little Ice Age'' (LIA) and present. We describe a new reconstruction, developed using largely different methodologies and additional new data compared to previous efforts. Unlike earlier studies, we quantify differences between more traditional ( STD) and Regional Curve Standardization (RCS) methodologies, concluding that RCS is superior for retention of low-frequency trends. Continental North American versus Eurasian RCS series developed prior to merging to the hemispheric scale cohere surprisingly well, suggesting common forcing, although there are notable deviations ( e. g., fifteenth to sixteenth century). Results indicate clear MWP ( warm), LIA ( cool), and recent ( warm) episodes. Direct interpretation of the RCS reconstruction suggests that MWP temperatures were nearly 0.7 degrees C cooler than in the late twentieth century, with an amplitude difference of 1.14 degrees C from the coldest ( 1600 - 1609) to warmest ( 1937 - 1946) decades. However, we advise caution with this analysis. Although we conclude, as found elsewhere, that recent warming has been substantial relative to natural fluctuations of the past millennium, we also note that owing to the spatially heterogeneous nature of the MWP, and its different timing within different regions, present palaeoclimatic methodologies will likely "flatten out'' estimates for this period relative to twentieth century warming, which expresses a more homogenous global "fingerprint.'' Therefore we stress that presently available paleoclimatic reconstructions are inadequate for making specific inferences, at hemispheric scales, about MWP warmth relative to the present anthropogenic period and that such comparisons can only still be made at the local/regional scale.

Title: Summer temperature trend over the past two millennia using air content in Himalayan ice

Author(s): Hou S (Hou, S.), Chappellaz J (Chappellaz, J.), Jouzel J (Jouzel, J.), Chu PC (Chu, P. C.), Masson-Delmotte V (Masson-Delmotte, V.), Qin D (Qin, D.), Raynaud D (Raynaud, D.), Mayewski PA (Mayewski, P. A.), Lipenkov VY (Lipenkov, V. Y.), Kang S (Kang, S.)

Source: CLIMATE OF THE PAST 3 (1): 89-95 2007

Document Type: Article

Language: English

Cited References: 33      Times Cited: 0      Find Related Records Information

Abstract: Two Himalayan ice cores display a factor-two decreasing trend of air content over the past two millennia, in contrast to the relatively stable values in Greenland and Antarctica ice cores over the same period. Because the air content can be related with the relative frequency and intensity of melt phenomena, its variations along the Himalayan ice cores provide an indication of summer temperature trend. Our reconstruction point toward an unprecedented warming trend in the 20th century but does not depict the usual trends associated with "Medieval Warm Period" (MWP), or "Little Ice Age" (LIA).

Title: Temperature patterns over the past eight centuries in Northern Fennoscandia inferred from sedimentary diatoms

Author(s): Weckstrom J, Korhola A, Erasto P, Holmstrom L

Source: QUATERNARY RESEARCH 66 (1): 78-86 JUL 2006

Document Type: Article

Language: English

Cited References: 44      Times Cited: 5      Find Related Records Information

Abstract: Establishing natural climate variability becomes particularly important in remote polar regions, especially when considering questions regarding higher than average warming. We present a high-resolution record of temperature variability for the past 800 yr based on sedimentary diatoms from a treeline lake in Finnish Lapland. The BSiZer multiscale smoothing technique is applied to the data to identify significant features in the record at different temporal levels. The overall reconstruction shows relatively large multi-centennial temperature variability with a total range of about 0.6-0.8 degrees C. At millennial scales, the temperatures exhibit a statistically significant long-term cooling trend prior to industrialization (Delta T= -0.03 degrees C/century). At the centennial timescale, three warm time intervals were identified around AD 1200-1300 (terminal phase of the Medieval Warm Period, MWP), 1380-1550 and from 1920 until the present. Pronounced coolness occurred between AD 1600 and 1920, indicative of the Little Ice Age (LIA). At the decadal level, certain shorter-term climate excursions were revealed. The warmest similar to 10-30 yr, non-overlapping periods occurred in AD 1220-1250, 1470-1500 and 1970-2000, respectively. The classic events of MWP and LIA are evident in our record, as is also the 20th century warming.

Answer 5

About 1*C.

Yes, there were no thermometers back during the middle ages, but scientists have been about to reconstruct temperatures using proxy data. Your question is hard to answer, because 1) there is no set definition as to when the “Medieval Warm Period began or ended, and 2) there is no agreement on which “average” would be used in comparison. The data is best reviewed as a graph: http://books.nap.edu/openbook.php?record...

Answer 6

There is a little bit for everyone on your second link.  You implication that we are now 0.55 minus 0.1 C warmer now is unfounded in my opinion.  You have to look at the evidence and understand that conclusions like absolute determination are not possible and therefore exact comparisons are not really possible either.  It may have been warmer or colder in my opinion.  It was probably similar.

Answer 7

Nobody can say for sure because thermometers were not invented until long after the MWP ended. So there are no temperature records and it all has to be inferred by other means which I do not trust to be very accurate.

Answer 8

Unless we lived back in those times and had an accurate way to measure temperature would we know that the MWP was warmer than what we have experienced in our life times. The thing we do know is that was when most of the Great Basin area turned into a desert. So what we could do is, a temperature graph of the Great Basin and see what happened with the temperature for that region over the past 30 years.

But I'd expect that wouldn't tell us anything, except if the region was getting hotter, staying the same, or getting cooler again.

What I expect is that at least in the Great Basin region during the MWP that it did warm up back than more then it is doing in that region today.

As for other locations it's really hard to say, since we don't have global temperature readings going back that far.

The other thing is that we do know that we have been warming up since the little Ice age during this warming spell, so how much of that .6 degree Celsius increase is from the planet trying to get back to a more balanced average global temperature?