I recall a conversation I had with Dr. Bob Carter at a restaurant in Townsville, QLD after our public presentations there in June 2010 where he lamented the fact that many of the AGW proponents and many of his critics, “really don’t integrate the earth’s geologic timeline into their critical thinking”. I’ve had dozens of similar comments posted on WUWT. It only takes one look at this graph from Lorraine Lisiecki’s most recent paper in Geophysical Research Letters to get a handle on the geologic timeline of CO2 in recent Earth history. The title and x axis annotations are mine. Compare the peaks of CO2 and Sea Surface Temperature change over the last 1.5 million years.
Figure 3. Proxy comparison. (top) pCO2 (red) [Petit et al., 1999; Monnin et al., 2001; Siegenthaler et al., 2005; Lüthi et al., 2008 , Dd13CP−NA 2 (blue), alkenone concentration (green dashed) [Martínez‐Garcia et al., 2009″], boron‐based estimates with error bars (black dots [Hönisch et al., 2009]; gray circles [Tripati et al., 2009]; triangles [Seki et al., 2010]), and alkenone d13C estimates (squares) [Seki et al., 2010]. Dd13CP−NA 2 and alkenone proxies are scaled to ppm using the mean and standard deviation of pCO2 from 800–0 ka. (See auxiliary material for ODP 1090 age model.) (bottom) Changes in Dd13CP−NA 2 (blue), WEP SST [Medina‐Elizalde and Lea, 2005], and a tropical SST stack (purple) [Herbert et al., 2010] with trend reduced by 0.29°C/Myr to match the WEP. Dd13CP−NA 2 is scaled to °C using the standard deviation of the SST stack from 500–100 ka. – click for larger image”]
Granted, there’s not enough resolution on this graph to see the present (at far left) clearly, and I’m sure there will be arguments complaining it doesn’t show the current measured CO2 ppm value, at ~390ppm, but I’m not posting this to try to dispel current measurements, only to help others gain an understanding of the longer geologic record. Here’s the abstract and conclusion, along with another graph of interest:
Abstract: (emphasis mine)
A high‐resolution marine proxy for atmospheric pCO2 is needed to clarify the phase lag between pCO2 and marine climate proxies and to provide a record of orbital‐scale
pCO2 variations before the oldest ice core measurement at 800 ka. Benthic d13C data should record deep ocean carbon storage and, thus, atmospheric pCO2. This study finds that a modified d13C gradient between the deep Pacific and intermediate North Atlantic (Dd13CP−NA2) correlates well with pCO2. Dd13CP−NA 2 reproduces characteristic differences between pCO2 and ice volume during Late Pleistocene glaciations and indicates that pCO2 usually leads terminations by 0.2–3.7 kyr but lags by 3–10 kyr during two “failed” terminations at 535 and 745 ka. Dd13CP−NA 2 gradually transitions from 41‐ to 100‐kyr cyclicity from 1.3–0.7 Ma but has no secular trend in mean or amplitude since 1.5 Ma. The minimum pCO2 of the last 1.5 Myr is estimated to be 155 ppm at ∼920 ka. Citation: Lisiecki, L. E. (2010), A benthic d13C based proxy for atmospheric pCO2 over the last 1.5 Myr, Geophys. Res. Lett., 37, L21708, doi:10.1029/2010GL045109.
That minimum pCO2 920,000 years ago of 155ppm comes dangerously close to the value at which photosynthetic function shuts down, said to be around 140-150ppm. Earth came close to losing its plant life then.
Here’s another graph, again annotated by me, showing her data:
Figure 2. Comparison of pCO2 (gray) [Petit et al., 1999; Monnin et al., 2001; Siegenthaler et al., 2005; Lüthi et al., 2008 with (top) benthic d18O (black) [Lisiecki and Raymo, 2005 and (bottom) Dd13CP−NA 2 (black). Glacial stages are labeled by MIS number. In Figure 2 (bottom), pCO2 has been smoothed with a 2‐kyr boxcar filter.
I also found this passage of interest:
An anomalous phase relationship between ice volume and pCO2 may explain why these two warming events [Termination 6 (535 ka) and MIS 18 (745 ka)] are weaker than most Late Pleistocene terminations. During both “failed” terminations, the initial d18O change is approximately half the amplitude of most Late Pleistocene terminations; d18O spends ∼20 kyr at intermediate values of 3.8–4.2‰ and then briefly returns to more glacial values before achieving full interglacial conditions ∼40 kyr after the initial warming. The Dd13CP−NA2 lag during these two failed terminations suggests that full deglaciation requires an early pCO2 response.
This is along the lines of Andrew Lacis CO2 knob idea, but it is clear that CO2 isn’t fully in control, but one of many control knobs for climate. There’s also some discussions about the role of polar ice in climate regulation:
The initial trigger for terminations and the mechanistic link between pCO2 and northern hemisphere ice volume remain controversial [e.g., Huybers, 2009; Denton et al., 2010]. Variability in the phase between d18O and Dd13CP−NA2 supports the hypothesis of Toggweiler [2008] that glacial changes in pCO2 are controlled by southern hemisphere processes only weakly linked to northern hemisphere insolation and ice volume. However, tighter coupling between the hemispheres appears to develop at ∼500 ka, as suggested by smaller phase differences between Dd13CP−NA 2 and d18O (Table S3), an increase in pCO2 amplitude, and the phase lock between Antarctic temperature and northern hemisphere insolation during the last five terminations [Kawamura et al., 2007].
Conclusions
[19] In conclusion, Dd13CP−NA2 correlates well with ice core pCO2 from 800–0 ka and reproduces many features of the pCO2 record. Comparison of Dd13CP−NA
2 and pCO2 suggests that marine and ice core age models [Lisiecki and Raymo,
2005; Parrenin et al., 2007; Loulergue et al., 2007] differ by ≤2.7 kyr at terminations. Within the marine sedimentary record Dd13CP−NA2 usually leads d18O by 0.2–3.7 kyr at terminations but lags by 3–10 kyr during “failed” terminations at 535 and 745 ka. Thus, an early pCO2 response appears necessary for complete deglaciation, and pCO2 appears less tightly coupled to northern hemisphere ice volume before 500 ka. [20] Several proxies that correlate with pCO2 (Dd13CP−NA2 , South Atlantic productivity [Martínez‐Garcia et al., 2009], and WEP SST [Medina‐Elizalde and Lea, 2005]) and a carbon
cycle box model [Köhler and Bintanja, 2008] suggest that glacial pCO2 minima do not decrease during the MPT. Moreover, the minimum pCO2 concentration of the last
1.5 Myr is estimated to occur at 920 ka. Dd13CP−NA2 gradually shifts from 41‐kyr cycles to 100‐kyr cycles from 1.3–0.7 Ma but shows no secular trend in mean or amplitude over the last 1.5 Myr, whereas tropical SST records suggest warmer glacial maxima before 1.3 Ma [Herbert et al., 2010]. This likely indicates that at least one of these proxies is affected by factors other than pCO2 before 1.3 Ma; thus, additional high resolution proxies are needed.
======================================================
The thing to bear in mind is that these are proxies, not empirical measurements, and there’s no error/uncertainty shown. Of course at the present, we have ~ 390ppm of CO2 in the atmosphere, and that is nothing I dispute, not does any other skeptic I know of. What is clear from this study though is that our current period of increased CO2 is riding on the back of natural variability of CO2 concentration, which has been observed to occur with regularity over the past 1.5 million years. Of course the question arises as to how much the present concentrations will affect our slide into the next glaciation, if at all. If we are lucky, our “geoengineering” of the planet with some extra CO2 may very well be a lucky break for humanity. Notice that those peaks in CO2 and SST, the most recent of which is the very brief period of the rise of man, are quite short compared with the much longer periods of cooler temperatures.
h/t to Dr. Leif Svalgaard, who has the full paper here
walt man says:
“Hmm! Proxy! How come we can believe these proxies but not Mann’s?”
We cannot believe Michael Mann because he used the Tiljander proxy — even after being informed that it was upside down. And as we see in the link, Mann repeatedly massages other data to support his repeatedly debunked hockey stick chart.
There are numerous articles falsifying Mann’s use of the Tiljander proxy. Do a search for “Tiljander” here, or at Climate Audit, or at Bishop Hill, and at many other sites.
Dr Michael Mann deliberately used a corrupted proxy because it gave his graph the desired shape. That is why we cannot believe Mann’s proxies, or the mann himself.
That minimum pCO2 920,000 years ago of 155ppm comes dangerously close to the value at which photosynthetic function shuts down, said to be around 140-150ppm. Earth came close to losing its plant life then.
I’m not sure how this idea spread on here, as I’ve seen it repeated a few times now, but it’s incorrect.
The pCO2 compensation point for C3 plants varies generally within the range of ~40-150 uatm, depending on taxon, and is subject to a reasonable amount of plasticity, depending on other environmental variables, and particulars of a plant’s physiology.
C4 and CAM plants have much lower pCO2 compensation points (e.g., ~10 uatm for corn). For example, see http://jxb.oxfordjournals.org/content/27/1/98.abstract
Most algae (marine and freshwater) have effective carbon concentrating mechanisms, and are able to perform photosynthesis effectively even at much lower pCO2 levels than C4 and CAM plants.
So, if atmospheric CO2 were to drop to 150 or 140 uatm, that doesn’t mean that “photosynthetic function shuts down” or anything close to it. It atmospheric CO2 dropped to that level it would mean that some C3 plants are going end just breaking even on their carbon budget some to much of the time. It wouldn’t be good for those particular plants, but they’d most likely just be outcompeted by other C3 or C4 plants with lower CO2 compensation points.
In order to shut down photosynthesis of land plants, you’d have to drop atmospheric CO2 to < ~ 10 uatm. Even so, marine algae would still be able to perform photosynthesis. To shut down photosynthesis on the planet almost entirely (and therefore have insufficient DIC for even marine algal photosynthesis), you really have to have very, very low atmospheric CO2 (<< 1 uatm). There's no way to get that low anytime soon (or even not soon).
While atmospheric pCO2 of ~155 uatm probably did make some C3 plants a lot less competitive, the suggestion that the planet was remotely close to the global shutdown of photosynthesis is totally baseless.
Best,
Chris
Proxies are what they are. They may be empirically measured but that are not direct measurements of what they are proxy for. A proxy is not the real thing we all know that. Some are reasonably good others less so. In this case they appear to be reasonable but you will notice the absence of error bars. This is a big red flag for me. I also question the true representativeness of the proxy and empirical data distributions. I have the haunting thought that behind all these compilations on geological time scales are some models. Think GISS.
I guess what I am trying to say is for sure we simply don’t know.
Ammonite says:
November 13, 2010 at 2:11 pm
crosspatch says: November 13, 2010 at 12:01 pm
If Earth’s atmosphere was pure CO2 and no water…
You are describing Venus.
Or Mars? (pressure anyone?)
Mike says:
November 13, 2010 at 10:04 am
You mean so say proxy measurements are not direct measurements of temperature or CO2. They are still empirical.
Yes, of course. In the same way that a knife may be used to inflict damage.
It might be minor, but it may still causes damage.
But how much damage depends upon BOTH the placement and severity of the inflicted injury.
In the end, your argument loses for the following reasons:
— (A) Proxies are just that: Stand ins for something else, i.e., they are NOT a direct measurement, and ARE YET affected by OTHER effects NOT mentioned.
— (B) Proxies –not being directly translatable to REAL scientific measurements– are essentially nothing more than the opinions of the proxies themselves. That is, how did the proxies respond to their environment? What ELSE was involved in that decision to respond in that certain way?
— (C) Proxies then, cannot be said to be accurate, for they are NOT direct measurement devices, UNLESS the quality being measure is THE ONLY thing which affects their indications.
Chris
Nov 13, 7:57 pm
Trees are among the most climatically influential plants. Trees – and 85 % of all plants – are C3. For low CO2 to influence climate via vegetation, it would not need the extreme of photosynthesis failure. C3 plant stress and changed ratios of plant species (e.g. More grass less trees) would suffice.
If in fact the lowest CO2 concentrations shown in the proxy graph would put severe stress on the ability of plants to do photosynthesis, that is not so much an indication that the earth’s biosphere had a close call in the recent geological past as it is an indication that the CO2 values of the proxy graph are too low. Note that if the graph is recalibrated so that the highest levels of the interglacials more closely match today’s CO2 levels, then the lowest levels do not go low enough for photosynthetic stress to occur.
@Mike November 13, 2010 at 10:04 am
“proxy measurements are not direct measurements of temperature or CO2 [but they] are still empirical.” Yes, Mike, that’s also my understanding of the word.
“based on observation and experiment: based on or characterized by observation and experiment instead of theory”
http://encarta.msn.com/dictionary_1861608199/empirical.html
Conversely, for example, computer projections of global temperature – are based on theory alone and not observation and experiment, there being none.
Some authorities claim that 180 ppmv CO2 is the point when plant life starts to die. Either way, 155 or 180, plant life is the basis for ALL life. No plants-no life.
As far as present CO2 concentrations are concerned, 390 ppmv is certainly the correct measure, where it is measured but may not be for all the planet’s surface. CO2 is a heavy gas and will form in pockets as has been known in volcanically active areas causing deaths of people by asphyxiation.
CO2 measurements for the past 100 plus years show that up to, or even over, 500 ppmv were not unusual so this fixation with 285 ppmv as ‘normal’ by the alarmists has no foundation in fact at all.
Can I just point out that in the second graph, covering 800,000 years, the thickness
of the line is about 2500 years?
The fact is, humans will burn all carbon based fuel until we cannot find any more, well, at least all of it until it becomes too expensive.
Nothing short of the mass murder of billions of people will change that fact.
Why do the climatists waste time on it?
Shouldn’t they advocate preparations to adapt?
Perhaps instead of worrying about high lakes in the Andes, we could begin designs for adding water reserves and pipelines in the vicinity. Or also, hurricanes, they are coming anyway, shouldn’t we be trying to prepare the areas better and start moving away from places like New Orleans and other periodically inundated locals?
Politics is about making change and gaining advantage. Science is about searching for truth. Whenever you see a ‘scientist’ trying to make change (social or monitary;-) and gain advantage, s/he is more about politics than science. Lorraine Lisiecki’s latest paper is all about the science.
AGW is a dead theory that is stinking up the global atmosphere big time. The politicians of the world (and a few unscrupulous “scientists” who always liked the dark side of the force) are still trying to make a dime off it before the flys and maggots have completely devoured it and it’s all gone; they’ll then have to find some other corpse to use for gain and change.
When you try to step back and get a wider perspective of what is happening these days, if you’re any kind of history buff at all, you get the feeling that all this has happened many, many times before. Today, it’s about something called AGW and/or CO2; or it is if you only glance at the surface. What is it really? It’s more about fear than anything else. The proxey is not the problem, the problem is the fear. People have and will fear everything under the Sun till the end of time. When they get hyper they pick out something that seems really bad and use it for all it’s worth. In the end, they tend to do all kinds of mean and stupid things via their proxey to quench their fears. They usually get to the point that they start killing each other in great numbers, run out of resources and energy, relax for 20-30 years, and then their kids pick it up again with some other proxey. What do we fear? Over population? Diminishing resources and raw materials? Destruction of the environment? Loss of our way of life? Who do we fear? What’s a good excuse to jump up and down about and get everyone excited?
There are other things that are part of the human equation of fear. One is that there are those within our midst that simply hate (and fear?) the way things are and they just want to get everyone fired up and marching behind them and their little red flag; and if their mob stomps all over some other mob, well that’s just part of the cost, and the fun, of politics.
R. Gates says:
November 13, 2010 at 10:34 am
–
On the other (C02 climate control knob) hand, we have not (nor will we ever be able to) sustain burning of fossil fuels to keep the C02 levels up long enough to hold off the 10-20 kyr fall into the next glaciation.
To put it into better words, we can only keep the C02 levels up for the plant life we depend upon, not the cozy temperatures we enjoy now. Geologic sequestration has a life of its own, and an appetite that will far outlast our reserves.
Socialism is the fraudulent offer to live at the expense of others if only they would allow others to do the same, all facilitated and controlled by unlimited government. Christians should easily see which of the Ten Commandments that are violated by socialism. For the others, it is the Commands that forbid telling lies, coveting the property of another and theft.
Socialism holds out that society will really be better for everyone if only it were allowed to achieve its goals. However, that is the core lie. The fatal flaw in socialism is that when government sets wages and prices, there cannot be any “economic calculation”, and thus scarce resources (all resources are in finite supply), cannot be efficiently allocated.
Economist Joseph Salerno wrote of this in an essay, Postscript: Why a Socialist Economy is “Impossible”
“In “Economic Calculation in a Socialist Commonwealth,” Ludwig von Mises demonstrates, once and forever, that, under socialist central planning, there are no means of economic calculation and that, therefore, socialist economy itself is “impossible” –not just inefficient or less innovative or conducted without benefit of decentralized knowledge, but really and truly and literally impossible.”
see: http://mises.org/econcalc/post.asp
R. Gates says:
November 13, 2010 at 10:34 am
“Do we really want to take the gamble that sending the CO2 levels off the long term chart by our use of fossil fuels will be harmless or simply forestall the next glaciation?”
Do we really want to take the gamble that sheer idiocy will cause economic collapse? Thats where I think the Precautionary Principle should apply.
JPeden says:
November 13, 2010 at 12:45 pm
R. Gates says:
November 13, 2010 at 10:34 am
Do we really want to take the gamble that sending the CO2 levels off the long term chart by our use of fossil fuels will be harmless or simply forestall the next glaciation? This is certainly not prudent. Reducing our addiction to fossil fuels is prudent on many levels, and in my mind, the only real question is how to reduce them in a manner that spreads out the cost of this reduction in an economically equitable way between the wealthy and poor countries of today while preserving a viable planet (for human habitation) for tomorrow.
There you have it, the sum of “progressive” Post Normal Climate Science: demonizing Man’s use of fossil fuel, disasterizing GW, and achieving “equality” or equitable-ness between “rich” and “poor” via central, superimposed control, which will somehow magically cause Nature to become amicable. And all in the face of the fact that Communism has never worked and is essentially a dead end kind of human and societal evolution = Slavery
________
Interesting way to meld the idea of being prudent in our use of fossil fuels (which should be an environmental and health issue) into a warning on the dangers of slipping into “communism”. Indeed, nature doesn’t care if you are rich or poor as the laws of physics will apply the same. A rich man and a poor man alike will die when then fall off a cliff, and if there is chance that we are headed for a cliff with the rapid rise of CO2 that humans have caused over the past few hundred years (a geological blink of the eye) then the prudent thing to do is to put lots of resources in place to see if there is a cliff there, and how we might put the brakes on in time so as not to take the plunge over it.
Also, just a comment about the notion that there is no way that there could be tipping points in the effects of rising CO2 levels. As the climate is a system existing on the edge of chaos, we know that there are always tipping points. Yes, only a few parts per million of CO2 are being added every year by human activities, but like grains of sand added to a sand pile, eventually there is a tipping point and the pile changes through a small landslide until a new equalibrium point is reached. The issue with CO2 is that we don’t know is where those tipping points are…400 ppm, 450 ppm, 600 ppm? Humans are conducting an experiment on a grand scale and it would be wise to try and truly understand what the sensitivity of the climate is to this experiment, i.e. where the tipping points might be…
DCC says:
November 13, 2010 at 3:05 pm
nc ask on November 13, 2010 at 11:43 am:
“R.Gates how much of the present C02 levels are caused by man? Also how long does that CO2 hang around?”
Ay, there’s the rub. The answer depends on who you ask and how you ask it. Do a Google search and you find very few answers that agree with one another. The IPCC put a value of 100 years on CO2 residence time in the atmosphere while many, perhaps most other scientific studies show a value of 5-15 years. Man’s contribution to CO2 levels is also hotly debated.
The 5-15 years is the residence time of any CO2 molecule in the atmopshere (whatever the origin), before being catched by one of the other reservoirs (mainly oceans and vegetation). Nothing to do with how much time it costs to reduce an excess amount of CO2 (whatever the origin) above the 1.5 million years temperature-CO2 equilibrium as seen in the introduction. The residence time is mainly seasonal with a 150/800 GtC exchange rate, the excess CO2 decay is based on a 4 GtC/800 sink rate, quite a difference… The latter gives a half time decay rate of about 40 years. The IPCC has a different approach with several short to very long decay rates, depending of the reservoir where the extra CO2 sinks. But the long rates are based on a saturation of the deep oceans (if we burn all oil and gas and a lot of coal), which still is not for tomorrow…
And humans are responsible for the the increase in the atmosphere: over the 160 year period we have emitted near double the amount of CO2 which is found as an increase in total quantity present in the atmosphere (as mass, not as individual molecules). In how far that affects temperature, is a complete different question…
John Marshall says:
November 14, 2010 at 4:38 am
As far as present CO2 concentrations are concerned, 390 ppmv is certainly the correct measure, where it is measured but may not be for all the planet’s surface. CO2 is a heavy gas and will form in pockets as has been known in volcanically active areas causing deaths of people by asphyxiation.
CO2 measurements for the past 100 plus years show that up to, or even over, 500 ppmv were not unusual so this fixation with 285 ppmv as ‘normal’ by the alarmists has no foundation in fact at all.
The current 395 ppmv and the ancient 285 ppmv are what is measured in the bulk of the atmosphere everywhere over the oceans and above a few hundred meters over land. Near ground over land, levels at night and morning are elevated, due to soil bacteria and plant respiration, especially without wind under an inversion. In the afternoon, sunlight warms the soil and better mixing with the overlying background CO2 levels occur.
The net effect is that many historical measurements refer to local CO2 levels and that plants may survive even very low CO2 levels as measured in the bulk atmosphere, as the local CO2 levels are high enough for at least a few hours a day of photosynthesis.
A number of correspondets here seem to think that ‘communism’ and ‘social democracy’ are identical. They are not. Have a good look at Australia. We regularly have ‘social democrat’ governments, but we have never had ‘communism’ Capitalism is well and very healthy here – more healthy, perhaps because better regulated, that in the USA. If you use labels, like ‘fascism’, ‘Nazism’, or ‘communism’, it would be a good idea to know what they mean. I have worked with old-generation American scientists who maintained that ‘Australia is a Communist country’. THAT is sheer BS. Private enterprise prospers here, unless it clearly doing thgings that are illegal.
Malcolm Miller says:
“A number of correspondets here seem to think that ‘communism’ and ‘social democracy’ are identical. They are not.”
I agree. The main difference is that communists are socialists in a hurry.
Both systems are based on confiscating the earned assets of productive workers, and redistributing that wealth to those who have not earned it.
Socialism is a foot in the door, and it is based entirely on coveting of the property of others, which is a bad idea that always causes problems. As a previous reader put it:
1. Government is force
2. Good ideas do not have to be forced on others
3. Bad ideas should not be forced on others
4. Liberty is necessary for the difference between good ideas and bad ideas to be revealed, which is why socialism is ultimately opposed to freedom and democracy
You could pay $100,000 for an Econ education and never learn those simple facts.
So, by your definition, the People’s Republic of China is a social democratic government with a capitalist economy and no Communism….
R. Gates says:
November 14, 2010 at 11:45 am
“Indeed, nature doesn’t care if you are rich or poor as the laws of physics will apply the same. A rich man and a poor man alike will die when then fall off a cliff, and if there is chance that we are headed for a cliff…”
==============================
The only thing that is headed for a cliff, is the herd of lemmings that comprise the CAGW religion.
And it can’t happen fast enough!
http://www.google.com/imgres?imgurl=http://www.cartoonstock.com/newscartoons/cartoonists/rni/lowres/rnin632l.jpg&imgrefurl=http://www.cartoonstock.com/directory/j/jump_off_a_cliff.asp&h=400&w=243&sz=33&tbnid=AGP7fbxFFVpTAM:&tbnh=288&tbnw=175&prev=/images%3Fq%3Dlemmings%2Bcliff&zoom=1&q=lemmings+cliff&usg=__eBCe3WMdKVXuANuyg46Wt-_01kw=&sa=X&ei=Io3gTOKeEoLWtQOTvpC8Cg&ved=0CBkQ9QEwAQ
Chris
Norfolk, VA, USA
“Why does global temp spike up sharply after long gradual decline during recent glaciations?”
Well known orbital mechanics?
Ref – JK says:
November 14, 2010 at 8:39 pm
“Why does global temp spike up sharply after long gradual decline during recent glaciations?”
Methane (and other manmade gases) in caves reach a critical density and explode when two stick are rubbed together?
R. Gates says:
November 14, 2010 at 11:45 am
Interesting to see you still working the sandpile model and “edge of chaos” argument. It seems a little opportunistic since generally the orthodox AGW position is “the physics” i.e. just radiative balance without any consideration of dynamic system complexity – to to a cynic your selective appeal to chaos is somewhat lacking consistency.
From what I have been reading recently it I’m starting to think that CO2’s effect on climate is more biology than physics. Take Lovelock’s Gaia hypothesis – everyone looks at this at a sort of new-age mysticism but in fact it is a serious scientific hypothesis and probably largely correct. Earth is a living planet and the biosphere is a very strong if not dominant climate driver.
Take this paper by Beerling and Berner 2005 (PNAS):
http://www.pnas.org/content/102/5/1302.full
For a period during the Devonian – Carboniferous there was a positive feedback between trees sucking CO2 out of the atmosphere and the tree’s leaves evolving larger wider shape and more efficiency to fix CO2 even as their own activity decreased atmospheric CO2 and forced them to increase efficiency further.
Overall during the early phanerozoic (600-200 Mya) global temperature fell sharply and I think its arguable that this was largely due to plant evolution and development of soils, forests and spreading the hydrological cycle to land (plus dimethyl sulphides from photosynthesising plankton nucleating cloud at sea). So in general plant and tree increase means lower temperatures.
There is no reason to believe that the effect of plants on CO2 and climate was confined to the Devonian/Carboniferous. What if increasing CO2 now gave a boost to plant photosynthesis and tree / plant natural assemblages in such a way as to reduce temperature via the hydrological cycle? This could be another negative feedback of CO2. In fact CO2 would then exert two opposing effects – warming due to radiative balance, but cooling due to enhanced plant metabolism (providing enough land is left non-urbanised). And a variable producing opposing effects is a recipe for chaotic dynamics and nonlinear pattern.