Why Paleoclimate Rules
An abridged, reader-friendly version of this paper (and published here) is now available. Here’s the article and opening section:
The past is the key to the future. Contrary to popular belief, climate models are not the principal basis for assessing human-made climate effects. Our most precise knowledge comes from Earth’s paleoclimate, its ancient climate, and how it responded to past changes of climate forcings, including atmospheric composition. Our second essential source of information is provided by global observations today, especially satellite observations, which reveal how the climate system is responding to rapid human-made changes of atmospheric composition, especially atmospheric carbon dioxide (CO2). Models help us interpret past and present climate changes, and, in so far as they succeed in simulating past changes, they provide a tool to help evaluate the impacts of alternative policies that affect climate
Paleoclimate data yield our best assessment of climate sensitivity, which is the eventual global temperature change in response to a specified climate forcing. A climate forcing is an imposed change of Earth’s energy balance, as may be caused, for example, by a change of the sun’s brightness or a human-made change of atmospheric CO2. For convenience scientists often consider a standard forcing, doubled atmospheric CO2, because that is a level of forcing that humans will impose this century if fossil fuel use continues unabated.
We show from paleoclimate data that the eventual global warming due to doubled CO2 will be about 3°C (5.4°F) when only so-called fast feedbacks have responded to the forcing. Fast feedbacks are changes of quantities such as atmospheric water vapor and clouds, which change as climate changes, thus amplifying or diminishing climate change. Fast feedbacks come into play as global temperature changes, so their full effect is delayed several centuries by the thermal inertia of the ocean, which slows full climate response. However, about half of the fast-feedback climate response is expected to occur within a few decades. Climate response time is one of the important ‘details’ that climate models help to elucidate.
H/T: Andy Revkin’s indispensable Twitter feed.
Please Keith.
Hansen.
NOT peer reviewed – just conference proceedings.
So who cares what it says.
That said, I do agree with your basice premise about recognizing climate history as a window on the future.
A glance at “Fig. 1. Global temperature relative to peak Holocene temperature, based on ocean cores” for the last 800,000 puts the lie to the ‘unprecedented’ story.
But not this kind of twisted and self-serving version.
I see they ‘forgot’ to note that CO2 LAGGED these temperature increases.
It would help if you started your blogs with something credible, not more junk from Hansen et al.
Instead of using this Hansen opinion piece as a starting point, why don’t you try something different:
http://blogs.forbes.com/larrybell/2011/07/19/nasas-inconvenient-ruse-the-goddard-institute-for-space-studies/
Hansen’s a bright guy and a good scientist. I doubt if anybody commenting here has the chops to challenge him.
But to get that from paleoclimate data alone (as quoted in your post) seems a real stretch. Not saying he did the math wrong–I just find it hard to justify such confidence in paleoclimate data.
Well, let me offer slight spin to that tom.
I think hansen’s paper is a wonderful starting point for an important argument. If we had good paleo data then its clear that our best estimate of ECR would come from that. So the real question is how good is hansens estimate here? what assumptions does he make and how sensitive is his final number to the uncertainties in the data and the choices he made.
The doubling has nothing to do with what we’ll do this century, it’s used because of the logarithmic nature of the rsponse.
BTW, which of those paleo cultures was anthropogenically increasing CO2.
Furthermore, can someone interpret this: Fast feedbacks come into play as global temperature changes, so their full effect is delayed several centuries by the thermal inertia of the ocean, which slows full climate response.
Why are they called “fast”.
@Tom
I agree with your first bit. Your second bit not so much. Have you ever taken a course in paleoclimatology? If not then your argument from personal incredulity is somewhat unsatisfying…
@steve mosher
see question asked of Tom.
@kdk33
‘slow’ feedbacks are ice/albedo changes etc. It’s useful to remember that climate is something that is usually quantified on millenial and longer timescales…
@ Marlowe,
My guess is that Tom & Steve are likely to old to have taken Intro to Paleoclimatology; how long have there been courses offered on this? Even now finding places that will even offer a course is not easy; Pitt, Baylor, Northwestern, University of Wyoming look likely, maybe.
A search of the UCAS site that lists all courses offfered in UK Universities didn’t show a single listing for paleoclimatology.
Wonder how much this little piece of self-serving “history” and fortune telling cost the bankrupt US Treasury? Doesn’t anyone know that our unsinkable ship of state is plowing full speed through a field of humongus icebergs; that the captain and crew are as high as a roomfull of stupid college freshmen football players before their first game; and the wait staff is lowering the only lifeboats onboard silently into the sea? Oh, did I mention that the ship’s rats all got off at the last port of call and are learning Chinese as fast as they can?
MJ,
Not sure I’m with you.
If ice/albedo changes work on milleniel scales, then the arctic ice scare, and the disappearing glaciers scare, are both unrelated to AGW, I take it.
I thought (per previous discussion with MT) that it was the deep ocean that was slow.
ray has nice code
http://geosci.uchicago.edu/~archer/Pierrehumbert.mp4
my understanding: slow feedbacks, on annual to whatever. fast feedbacks are on days to weeks – so not instantaneous, but close.
@ob
No. Equilibrium climate sensitivity is on timescales of hundreds of years (10^2 yr) and quicker. That’s long enough for water vapor, clouds, sea ice, and other “fast feedbacks”, as well as a substantial amount of deep ocean mixing of heat. Longer term sensitivity, which is increasingly being referred to as Earth System sensitivity, is on timescales of thousands of years and longer (> 10^3 yr), which allows for the response of changes in ice sheets and vegetation dynamics (although changes in these dynamics begin to operate on much quicker scales, they don’t reach new equilibria for much longer). Contra kdk33, that does not mean that we don’t expect vegetation changes or significant melting of ice sheets in the present and near future.
There’s also transient warming, which is what you seem to be talking about when you say “fast”.
@ mosher
what assumptions does he make and how sensitive is his final number to the uncertainties in the data and the choices he made?
That’s exactly the question that needs to be asked. In case you wonder, MJ, I have taken classes in paleoclimatology, so I am entitled to be skeptical.
ok. Didn’t read thouroghly. Then let’s change my point to: there are even “faster” feedbacks. There should be a paper in review on those and their implications for “non-equilibrium” climate sensitivity.
“In case you wonder, MJ, I have taken classes in paleoclimatology”
Please forgive me if I’m skeptical of that particular claim…
@ Sashka,
Re taking the Paleoclimatology courses, I have to ask, where? Not being a smartass, but definitely curious since it’s not a course as available as say Intro to Basket Weaving.
@ Marlowe,
I totally agree about being skeptical about anyone saying they have taken paleoclimatology courses, because what institutions offer it? What are the standard texts? Where is the professional society? IS IT EVEN A SCIENCE?
Can an inductive reasoning excursion with blatantly little deductive reasoning (with a chunk of deductive reasoning ignored as a “divergence” problem) to back it up be called “science”? Is this even a question?
re 13. It’s a really cool field. at the last AGU my favorite sessions were those that combined GCMs and proxy recons. The nice thing about conferences ( as opposed to papers) is you get the unvarnished truth.
Contra kdk33, that does not mean that we don’t expect vegetation changes or significant melting of ice sheets in the present and near future.
OK, then let me rephrase more carefully. if ice sheets take 10^3 years to respond, then is it reasonable for me to NOT worry about several feet of sea level rise, due to melting continental ice sheets, in, say, the next 2 centuries?
Not long ago, in an exchange with MJ, regarding sea level rise, he referred to melting ice sheets as “the elephant in the room” – the obvious, but ignored, reason for sea level rise due to AGW that we should worry about.
What percentage of continental ice sheets would have to melt to produce 3 feet of SLR, and is that percentage consistent with a millenial (10^3) ice sheet time constant?