The IPCC and the term "most"

BCO

An online "sparring partner" brought up an interesting point during one of our regular (some would say never-ending) climate debates the other day. He asked me for clarity on what the IPCC means by "most" in their Fourth Assessment Report when they say:

"Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations."

The reports are quite clear in defining quite specific definitions for "very likely," "likely" etc. but there seems to be no attempt at all to quantify the term "most". Does "most" mean 51% or does it mean closer to 90%? Or is it somewhere in between? Maybe most is 71.2% - who knows? Does it matter? Obviously there's a lot of "wiggle room" in this definition, unless it's clarified somewhere in another IPCC document and I have just not looked in the right places.

Any assistance on this would be great. Thanks,

BCO

mgb_phys

It doesn't have a specific meaning it's just a summary, it's there for the usual legal wiggle reasons.
if they said "All the observed increase in global average temperatures since the mid-20th century ...' then some country could come along and say:
- But the temperature rise in one area of Hawaii is due to a volcano erupting, therefore the report contains a false statement, therefore it's all wrong and we can all get new bigger SUVs

Xnn

Since 1750 anthropogenic warming has resulted in
a net positive forcing of +1.6 [+0.6 to +2.4] W m–2.

Over the same time, solar changes have contributed +0.12 [0.06 to 0.3] W m-2.

[1.6]/[1.6+.12] => 93%

So, since 1750, humans have most likely caused about 93% of the warming.

Most of the solar increases occurred prior to 1950 or so and
solar activity is currently very low. So, the amount of
warming from human causes since 1950 maybe greater
than 100%. In other words, there may have been
global cooling since the 1950's without human intervention.

Count Iblis

Would it be possible to pevent an ice age in the future by deliberately pumping large amounts of CO2 in the atmosphere, or would that not work?

BCO

Thanks for the answers, both.

Xnn

Yes; that is essentially what we are doing.

Ruddiman has a paper showing that we humans have actually
been doing that with CH4 and CO2 for several thousand years.

http://earth.geology.yale.edu/~avf5/...ddiman2003.pdf

Not everyone agrees with him, but I suspect he is mostly correct.

seycyrus

This doesn't explain why the IPCC failed to quantify the term "most" in the same manner that they did for "likely" and "very likely" etc.

Would you say that "most" means >90% ?

ibnsos

Not to mention it implies the only two possible causes of warming are solar and man made.

mgb_phys

It's a quote from a press release of an executive summary.
You could have the entire report totally correct if it was entirely written in lambda calculus but its message might be a little more difficult to follow.

Xnn

No; Generally "most" is considered to mean >50%.

Notice the IPCC has defined "very likely" as >90% probability.

Since more than 93% of the warming is manmade, perhaps
the IPCC decided it fair to use the "very likely most" term.

However, it probably could have been equivalently worded:

"Likely the predominate amount of warming since the mid 20th century is man-made."

However, in that case they'd have to define what "most" and "predominate" meant
and they decided it was easier to define the other set of words instead.

DrClapeyron

"Most" means whatever I would like it to mean. It could be anywhere from 0-100%. Most people agree with this definition.