Good practice would then be to present all the curves that cannot be ruled out because of errors. When asked why he didn’t present the other cures too, he said that he only wanted to show the one curve. Not a very convincing answer, and not very reassuring.

Found at RealClimate
http://snipurl.com/sewjt

Just tell me what this “good evidence” in favor of CAGW is in a few sentences.

Is it possible for you to answer this question by simply summarizing the evidence? Or will you simply appeal to authority again?

http://minnesotansforglobalwarming.com/m4gw/2009/10/the-mystery-of-hockey-stick-chart-revealed.html

Feel free to comment If I got any part of this wrong

I’m a little confused . . . are you saying that you have no personal evaluation of the evidence on this point?

“Presently available proxy evidence indicates that temperatures at many, but not all, individual locations were higher during the past 25 years than during any period of comparable length since A.D. 900. The uncertainties associated with reconstructing hemispheric mean or global mean temperatures from these data increase substantially backward in time through this period and are not yet fully quantified”

So it’s basically like I said. The primatey evidence that the MWP was cooler than the present is that a small group of carefully selected trees grew at least as much between 1950 and 1990 as some other group of carefully selected trees during the Medieval Warm Period. And how exactly were these thermomoter trees selected? Nobody really knows except for a few “scientists” who assure us we can trust them despite their demonstrated lust for shrouding their work in secrecy.

Firstly how can you say that statement is correct. Has any poll been conducted among scientists with the question “do you believe that the MWP was warmer than the late 20th century”? Naturally since they are scientists they will also have to asked the question why do you believe as you do.

Well, “most scientists” was a shorthand for saying that most of the peer-reviewed papers in the field have said that the current period is likely warmer, as has the IPCC in the last two assessment reports, and the NAS in its report on temperature reconstructions (with some caveats in regard to not being able to assign a degree of certainty to the statement).

Why I believe it was warmer is
1. Because people who lived during the period left sagas that could be taken as a “proxy” clues (a better one than Briffa’s) that it was indeed so
2. The ice-core records of Greenland say so.
3. There is other evidence that says so.
4. The IPCC itself said so till 2001
5. The National Academy of Sciences still says so in some of its crucial publications as well as the Woods Hole Oceanographic Institution.

(1) Local records tell us only what happened at that location, not what happened globally (let alone the amount of synchronicity in the times at which the different parts of the globe experienced their warmest temperatures during the broad period of time defined by the MWP).

(2) Do you have quotes from the earlier IPCC reports? My impression is that the first report had a schematic graphic taken from Lamb that did seem to show a warmer MWP than at the end of the period, which was before the modern (post 1970) warming, but that if you extended it to temperatures up to the present would have it about the same as the MWP. And, at any rate, this was only schematic since noone had attempted to do a globe-wide or hemispheric-wide reconstruction. And, while the Mann et al. work was pioneering, I don’t think they were the first ones to suggest that the warm periods identified in different areas sometime during the broad period of the MWP appeared to be asynchronous.

(3) Where do the NAS and Woods Hole make these claims? Here is the NAS report on temperature reconstructions: http://books.nap.edu/catalog.php?record_id=11676

Where is the evidence for this? It certainly does not show up in the records of the past 10,000 years.

As I noted, it is hard to tell during that period since CO2 has been quite constant.

The ice core records show the CO2 kept going down when temperatures started rising to take us out of the ice ages and kept rising when we plunged into them, so CO2 was not a driver either of the rise in temperatures nor could it save us from repeatedly plunging into them.

It is strange that the people who have written the papers that have provided this data don’t seem to agree with your conclusions. One problem is that you are likely taking the data well beyond where it can be taken. You do know that the temperature data from ice cores is a proxy for that given location (or region)…and there are various issues with it and, in fact, at least one paper that talks about important corrections that must be made during (I believe) the drop in temperature from interglacial into glacial periods? You also have to look at temperature behavior in both hemispheres since one tends to warm or cool before the other. (And, in fact, I believe that the one that does so more slowly actually does not have a lag between the temperature change and CO2 change.)

Since the temperature fluctuations have been far more rapid than the “slow change over thousands of years” that you are attributing to orbital parameters.

What I am referring to are the general triggers for the glacial and interglacial transitions, as well as, for example, the gradual cooling on the Arctic over the last ~8000 years that the recent temperature reconstruction showed.

brazil84 says:

Why didn’t he say something like “the evidence indicates that the Medieval Warm Period was less warm than now.”?

I didn’t say that because I like to attribute statements on scientific issues that I haven’t worked in, or carefully examined and analyzed the data myself, to the scientists who actually have. (Some people, by contrast, seem to think they are experts on something because they have read Steve McIntyre’s and his cohorts’ accounts of it.)

And what exactly is the evidence that the MWP is cooler than the present?

According to the National Academy of Sciences report: “Presently available proxy evidence indicates that temperatures at many, but not all, individual locations were higher during the past 25 years than during any period of comparable length since A.D. 900. The uncertainties associated with reconstructing hemispheric mean or global mean temperatures from these data increase substantially backward in time through this period and are not yet fully quantified.” ( http://books.nap.edu/openbook.php?record_id=11676&page=3 ) Of course, some further progress in that quantification has been made since the report was issued in 2006)

That’s a good point, but it’s worth noting that like most warmists, Joel Shore is attempting to divert the discussion into a debate over how many scientists believe what.

Why didn’t he say something like “the evidence indicates that the Medieval Warm Period was less warm than now.”?

Because that would invite a request to explain and provide such evidence.

And what exactly is the evidence that the MWP is cooler than the present? Primarily, it seems that a small group of carefully selected trees grew at least as much between 1950 and 1990 as some other group of carefully selected trees during the Medieval Warm Period. And how exactly were these thermomoter trees selected? Nobody really knows except for a few “scientists” who assure us we can trust them despite their demonstrated lust for shrouding their work in secrecy.

It’s completely unsurprising that warmists want to shift the debate into scientist counting contests since the actual evidence on their side is thin — to put it mildly.

Richard: Well, most scientists do not believe that the MWP was warmer than the late 20th century, although obviously the issues with temperature reconstructions make it difficult to assert that with a high degree of confidence.

Joel can you please explain:

What you mean by a “scientist”?
How you know that most scientists do not believe that the MWP was warmer than the late 20th century?
How you decide the degrees of confidence with which you make your speculative assertions?

Thanks in anticipation.

S

Firstly how can you say that statement is correct. Has any poll been conducted among scientists with the question “do you believe that the MWP was warmer than the late 20th century”? Naturally since they are scientists they will also have to asked the question why do you believe as you do. Secondly beliefs are not a substitute for the truth.

I presume that you yourself are a scientist? (I am not). If you believe that, why do you, specially when you also say that there is not a “high degree of confidence” in that statement.

Why I believe it was warmer is
1. Because people who lived during the period left sagas that could be taken as a “proxy” clues (a better one than Briffa’s) that it was indeed so
2. The ice-core records of Greenland say so.
3. There is other evidence that says so.
4. The IPCC itself said so till 2001
5. The National Academy of Sciences still says so in some of its crucial publications as well as the Woods Hole Oceanographic Institution.

In fact the only evidence which says to the contrary is the temperature reconstructions of Mann, Briffa et al, which, as you point out, have “issues” with their confidence.

CO2 is the main driver of temperatures when it changes rapidly while other things (such as orbital parameters) don’t change very much. And, CO2 is not the main driver of temperatures when it is nearly constant while other things are changing.

Where is the evidence for this? It certainly does not show up in the records of the past 10,000 years.

During the glacial – interglacial oscillations, the understanding is that the trigger for glaciation and de-glaciation was provided by orbital changes. However, these orbital changes resulted in almost no change in global mean annual radiative forcing (unproven assumption. You simply do not know the radiation of the Sun to any accuracy before the satellite era)…They just changed the distribution of the radiative forcing in latitude and seasonality. Hence, almost all the temperature change was as a result of feedbacks. The most important feedback was simply the change in albedo from the growth and shrinkage of the land ice sheets (and, to a lesser extent changes in vegetation) and this accounted for about half the temperature change. Changes in CO2 levels accounted for about one third of the temperature change, with a little bit more added by the other greenhouse gases, and these greenhouse gas changes also likely played an important role in synchronizing the change in the two hemispheres. Changes in aerosol loading in the atmosphere contributed a bit too (maybe 10-15%).

The ice core records show the CO2 kept going down when temperatures started rising to take us out of the ice ages and kept rising when we plunged into them, so CO2 was not a driver either of the rise in temperatures nor could it save us from repeatedly plunging into them.

During the Holocene (and up until the industrial revolution), CO2 levels have been pretty constant and so much of whatever temperature change has occurred is likely due to the changes in orbital parameters (which is a slow change over thousands of years) and changes in solar irradiance or volcanic activity.

Since the temperature fluctuations have been far more rapid than the “slow change over thousands of years” that you are attributing to orbital parameters.

But, the more general answer to your question is that the world is not so black-and-white. CO2 is the main driver of temperatures when it changes rapidly while other things (such as orbital parameters) don’t change very much. And, CO2 is not the main driver of temperatures when it is nearly constant while other things are changing.

During the glacial – interglacial oscillations, the understanding is that the trigger for glaciation and de-glaciation was provided by orbital changes. However, these orbital changes resulted in almost no change in global mean annual radiative forcing…They just changed the distribution of the radiative forcing in latitude and seasonality. Hence, almost all the temperature change was as a result of feedbacks. The most important feedback was simply the change in albedo from the growth and shrinkage of the land ice sheets (and, to a lesser extent changes in vegetation) and this accounted for about half the temperature change. Changes in CO2 levels accounted for about one third of the temperature change, with a little bit more added by the other greenhouse gases, and these greenhouse gas changes also likely played an important role in synchronizing the change in the two hemispheres. Changes in aerosol loading in the atmosphere contributed a bit too (maybe 10-15%).

During the Holocene (and up until the industrial revolution), CO2 levels have been pretty constant and so much of whatever temperature change has occurred is likely due to the changes in orbital parameters (which is a slow change over thousands of years) and changes in solar irradiance or volcanic activity.

It is not false. In fact, both of them drive each other.

According to the AGW hypothesis. If this is true however please explain the warmer MWP than the CWP, most of the warmer Holocene interglacial period and the little ice age on the basis of CO2 as the main driver of the temperatures.

Al Gore’s “graph for CO2 does start at 0, or at least pretty close.”

Okey dokey. Now let’s take a look at some other graphs that have a zero y-axis: click1, click2, click3.

Not too scary, are they? That’s why the alarmists use charts like this: click. They are deliberately designed to scare the scientifically illiterate populace. That’s why they don’t work around here.

I have just watched it and, unless my eyes are deceiving me, you’re wrong.

I am confused then … Why does the RealClimate screen capture show him on the lift in front of the graph that I described?