by Craig and Sherwood Idso
Special Issue
This week we announce the release of our newest major report, Carbon Dioxide and Earth’s Future: Pursuing the Prudent Path. Based on the voluminous periodic reports of the Intergovernmental Panel on Climate Change (IPCC), the ongoing rise in the atmosphere’s CO2 concentration has come to be viewed as a monumental danger — not only to human society, but to the world of nature as well. But are the horrific “doomsday scenarios” promulgated by the climate alarmists as set-in-stone as the public is led to believe? Do we really know all of the complex and interacting processes that should be included in the models upon which these scenarios are based? And can we properly reduce those processes into manageable computer code so as to produce reliable forecasts 50 or 100 years into the future? At present, the only way to properly answer these questions is to compare climate model projections with real-world observations. Theory is one thing, but empirical reality is quite another. The former may or may not be correct, but the latter is always right. As such, the only truly objective method to evaluate climate model projections is by comparing them with real-world data.
In what follows, we conduct just such an appraisal, comparing against real-world observations ten of the more ominous model-based predictions of what will occur in response to continued business-as-usual anthropogenic CO2 emissions: (1) unprecedented warming of the planet, (2) more frequent and severe floods and droughts, (3) more numerous and stronger hurricanes, (4) dangerous sea level rise, (5) more frequent and severe storms, (6) increased human mortality, (7) widespread plant and animal extinctions, (8) declining vegetative productivity, (9) deadly coral bleaching, and (10) a decimation of the planet’s marine life due to ocean acidification. And in conjunction with these analyses, we proffer our view of what the future may hold with respect to the climatic and biological consequences of the ongoing rise in the air’s CO2 content, concluding by providing an assessment of what we feel should be done about the situation.
Click on the links below to read the report, or download the full report in a pdf file (2.5 mb in size) below.
Carbon Dioxide and Earth’s Future: Pursuing the Prudent Path
1. Unprecedented Warming of the Planet
2. More Frequent and Severe Floods and Droughts
3. More Frequent and Severe Hurricanes
4. Rising Sea Levels Inundating Coastal Lowlands
5. More Frequent and Severe Storms
7. Widespread Plant and Animal Extinctions
8. Declining Vegetative Productivity
10. Marine Life Dissolving Away in Acidified Oceans
Executive Summary
As presently constituted, earth’s atmosphere contains just slightly less than 400 ppm of the colorless and odorless gas we call carbon dioxide or CO2. That’s only four-hundredths of one percent. Consequently, even if the air’s CO2 concentration was tripled, carbon dioxide would still comprise only a little over one tenth of one percent of the air we breathe, which is far less than what wafted through earth’s atmosphere eons ago, when the planet was a virtual garden place. Nevertheless, a small increase in this minuscule amount of CO2 is frequently predicted to produce a suite of dire environmental consequences, including dangerous global warming, catastrophic sea level rise, reduced agricultural output, and the destruction of many natural ecosystems, as well as dramatic increases in extreme weather phenomena, such as droughts, floods and hurricanes.
As strange as it may seem, these frightening future scenarios are derived from a single source of information: the ever-evolving computer-driven climate models that presume to reduce the important physical, chemical and biological processes that combine to determine the state of earth’s climate into a set of mathematical equations out of which their forecasts are produced. But do we really know what all of those complex and interacting processes are? And even if we did — which we don’t — could we correctly reduce them into manageable computer code so as to produce reliable forecasts 50 or 100 years into the future?
Some people answer these questions in the affirmative. However, as may be seen in the body of this report, real-world observations fail to confirm essentially all of the alarming predictions of significant increases in the frequency and severity of droughts, floods and hurricanes that climate models suggest should occur in response to a global warming of the magnitude that was experienced by the earth over the past two centuries as it gradually recovered from the much-lower-than-present temperatures characteristic of the depths of the Little Ice Age. And other observations have shown that the rising atmospheric CO2 concentrations associated with the development of the Industrial Revolution have actually been good for the planet, as they have significantly enhanced the plant productivity and vegetative water use efficiency of earth’s natural and agro-ecosystems, leading to a significant “greening of the earth.”
In the pages that follow, we present this oft-neglected evidence via a review of the pertinent scientific literature. In the case of the biospheric benefits of atmospheric CO2 enrichment, we find that with more CO2 in the air, plants grow bigger and better in almost every conceivable way, and that they do it more efficiently, with respect to their utilization of valuable natural resources, and more effectively, in the face of environmental constraints. And when plants benefit, so do all of the animals and people that depend upon them for their sustenance.
Likewise, in the case of climate model inadequacies, we reveal their many shortcomings via a comparison of their “doom and gloom” predictions with real-world observations. And this exercise reveals that even though the world has warmed substantially over the past century or more — at a rate that is claimed by many to have been unprecedented over the past one to two millennia — this report demonstrates that none of the environmental catastrophes that are predicted by climate alarmists to be produced by such a warming has ever come to pass. And this fact — that there have been no significant increases in either the frequency or severity of droughts, floods or hurricanes over the past two centuries or more of global warming — poses an important question. What should be easier to predict: the effects of global warming on extreme weather events or the effects of elevated atmospheric CO2 concentrations on global temperature? The first part of this question should, in principle, be answerable; for it is well defined in terms of the small number of known factors likely to play a role in linking the independent variable (global warming) with the specified weather phenomena (droughts, floods and hurricanes). The latter part of the question, on the other hand, is ill-defined and possibly even unanswerable; for there are many factors — physical, chemical and biological — that could well be involved in linking CO2 (or causing it not to be linked) to global temperature.
If, then, today’s climate models cannot correctly predict what should be relatively easy for them to correctly predict (the effect of global warming on extreme weather events), why should we believe what they say about something infinitely more complex (the effect of a rise in the air’s CO2 content on mean global air temperature)? Clearly, we should pay the models no heed in the matter of future climate — especially in terms of predictions based on the behavior of a non-meteorological parameter (CO2) — until they can reproduce the climate of the past, based on the behavior of one of the most basic of all true meteorological parameters (temperature). And even if the models eventually solve this part of the problem, we should still reserve judgment on their forecasts of global warming; for there will yet be a vast gulf between where they will be at that time and where they will have to go to be able to meet the much greater challenge to which they aspire.
Idso – CO2 and Earth’s Future 1-31-11 (PDF 2.5MB)
h/t to Bob Feguson, SPPI
(continuation)
First, we choose a mean climate state similar to the present. The last 2000 years are good, since the mean climate state is almost equal to present and there are abundant paleoclimate reconstructions.
Using the Thomas J. Crowley data in the paper Causes of Climate Change
Over the Past 1000 Years (download the excel data and make the plots yourself):
http://www.ncdc.noaa.gov/paleo/pubs/crowley.html
After making the plots, we have:
ΔF between the MWP and the LIA: 0.4W/m^2 (upper range)
ΔT between the MWP and the LIA: 0.4 ºC (upper range)
This make the climate sensitivity 1ºC/ (W/m^2)
Using the formula:
ΔF (W*m^-2) = 5.35*ln([CO2]/[CO2]o)
One obtain the value of 3.7 ºC/(doubling of CO2)
But this ΔT is just from the northern hemisphere. If we consider the whole planet, the ΔT is likely lower, resulting in a lower climate sensitivity. The value obtained, around 3 ºC per doubling of CO2, is consistent with the predictions of climate models.
Now assume that the Medieval Warm Period was in effect warmer than most climate scientists estimate. For example, if we extrapolate to the whole globe the results of FREDRIK CHARPENTIER LJUNGQVIST for the Northern Extratropics in the paper:
A NEW RECONSTRUCTION OF TEMPERATURE VARIABILITY IN THE EXTRA-TROPICAL NORTHERN HEMISPHERE DURING THE LAST TWO MILLENNIA
http://agbjarn.blog.is/users/fa/agbjarn/files/ljungquist-temp-reconstruction-2000-years.pdf
That was shown by Craig and Sherwood Idso as an alternative to the classic “hockey stick” reconstruction of Northern Hemisphere temperatures in the article that is the subject of this WUWT post.
The result is:
ΔF between the MWP and the LIA: 0.4W/m^2 (upper range)
ΔT between the MWP and the LIA: 0.7 ºC (upper range)
This make the climate sensitivity 1.75 ºC/ (W/m^2)
Using the formula:
ΔF (W*m^-2) = 5.35*ln([CO2]/[CO2]o)
One obtain the value of 6.5 ºC/(doubling of CO2)
An enormous value of climate sensitivity (more than twice the mean value of the IPCC estimates) !
This, I think, is unlikely, but if all those “skeptics” are right, then climate sensitivity was severely UNDERestimated, and it is more than twice the current estimate.
Two points of caution:
1)This is likely not equilibrium response, because the climate probably do not reached equilibrium during the MWP and the LIA. The farther from equilibrium, the more the “equilibrium ΔT” is , resulting in a HIGHER climate sensitivity.
2)The dominants forcings during the MWP and the LIA were solar+volcanic, and both affect shortwave radiation. Greenhouse warming affects longwave radiation. So the sensitivity to those different types of forcings may be different. Maybe the CO2 sensitivity is higher, maybe lower, than the shortwave forcing sensitivity.
Comparing recent longwave forcing to the recent temperature response (from 1970s to today) will be the subject of another comment.
From Peru : “Huh?”
“Would be”, not “is”. It then goes on to explain why it isn’t.
Many of the comparisons of temperature change with CO2 change in order to estimate climate sensitivity make the mistake of assuming that the temperature change is caused by the CO2 change. The principal source of this error is the IPCC Report not allowing enough for natural factors such as ENSO (esp. on the shorter time scales) , solar variation (esp. on the longer time scales), and clouds.
Some comparisons which do take more account of natural factors have resulted in much lower values for climate sensitivity. From memory, values down to 0.3 (a tenth of the IPCC’s value), but I don’t have links to hand.
Mike Jonas says:
February 17, 2011 at 8:30 pm
“From Peru : “Huh?””
If you don’t know the interjection, try here:
http://www.merriam-webster.com/dictionary/huh
“Definition of HUH
—used to express surprise, disbelief, or confusion, or as an inquiry inviting affirmative reply ”
So I am expressing my surprise after you state that climate sensitivity only applies to CO2. That is false: Climate sensitivity apply to any radiative forcing.
Then you said:
“Many of the comparisons of temperature change with CO2 change in order to estimate climate sensitivity make the mistake of assuming that the temperature change is caused by the CO2 change.”
That is not true. All climate models incluse ALL the forcings: solar activity, volcanic aerosols, antropogenic tropospheric aerosols and greenhouse gases (that include CO2 plus CH4, CFCs, NOx, etc).
“The principal source of this error is the IPCC Report not allowing enough for natural factors such as ENSO (esp. on the shorter time scales)”
ENSO add noise to the year-to-year global temperature, but do not have a long-term effect. The IPCC makes predictions about decadal temperature averages, that are not affected by ENSO.
” solar variation (esp. on the longer time scales)”
The IPCC DOES include solar variation as a forcing. Have you read the IPCC report?
“and clouds”
This is a feedback to temperature, not a source of forcing.
“Some comparisons which do take more account of natural factors have resulted in much lower values for climate sensitivity. From memory, values down to 0.3 (a tenth of the IPCC’s value), but I don’t have links to hand.”
I have some links. Check my previous comment. The result is the opposite of what you are saying. Higher natural variability in the past imply a climate sensitivity much HIGHER than in the IPCC report: a sensitivity of more than 6ºC to a doubling of CO2!
We have now truly gone full circle.
Solar variation is included alone in the IPCC Report (no feedbacks), leaving much warming unaccounted for. Clouds are not understood (as the IPCC Report states many times), and the IPCC assumes without evidence that clouds only react to climate change – thus you have no basis for asserting this. The IPCC augments CO2 (climate sensitivity 1.2) with feedbacks (para 8.6.2.3 eg.) in order to make it supply the unaccounted warming, even though there is no mechanism and contra logic for most of the feedback.
There is logic and there is evidence for feedbacks to solar variation, via GCRs and clouds, yet the IPCC chooses to ignore it. Your assertion that the IPCC does include solar variation as a forcing is therefore technically correct but misleading.
On ENSO : I think you are probably right when you say that ENSO has no long term effect, provided you mean, say, century-plus. To say that ENSO “add noise to the year-to-year global temperature” is way off the mark, since ENSO clearly operates on a timescale of several decades. The IPCC’s treatment of global 20thC temperature rise makes no allowance for the 20thC having contained two ENSO warming periods and only one ENSO cooling period.
PS. Yes I have read the IPCC Report. Repeatedly. It was the IPCC Report that first made me sceptical of AGW – until I read the report I was happy to accept AGW as fact.
Mike Jonas says:
February 18, 2011 at 6:02 pm
“Clouds are not understood (as the IPCC Report states many times), and the IPCC assumes without evidence that clouds only react to climate change – thus you have no basis for asserting this. The IPCC augments CO2 (climate sensitivity 1.2) with feedbacks (para 8.6.2.3 eg.) in order to make it supply the unaccounted warming, even though there is no mechanism and contra logic for most of the feedback.”
You are concerned with the part of the IPCC report that uses Global Climate (computer) Models (GCM) to calculate the feedback parameters. The cloud feedback is the most uncertain of those feedback parameters, and so being skeptical on this is legitimate.
One way to test if the GCM are getting it right or not is to use an entirely different method to calculate the climate sensitivity: PALEOCLIMATE
You just need to recontruct both past temperatures and past forcings. Then you divide the temperature change by the forcing change and you get a rough value of the climate sensitivity. Note that this method authomatically includes both direct effects and feedbacks at once, so you don’t need to parametrice the feedbacks.
For this, I strogly recommend you to read the Paleoclimate chapter of the IPCC report:
http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter6.pdf
Where there are shown the paleoclimate recontructions of the last 2000 years along with the forcings (ALL of them: solar, volcanic, antrogenic tropospheric aerosols and greenhouse gases).
A source for the chapter is the paper:
Thomas J. Crowley: Causes of Climate Change Over the Past 1000 Years (download the excel data and make the plots yourself):
http://www.ncdc.noaa.gov/paleo/pubs/crowley.html
Using the data just do this:
Compute the climate sensitivity by this simple division:
climate sensitivity (ºC/(W*m^-2)) = [ΔT between the MWP and the LIA (ºC)] / [ΔF between the MWP and the LIA (W/m^2)]
And then convert it to sensitivity to doubling of CO2 using the formula:
ΔF (W*m^-2) = 5.35*ln([CO2]/[CO2]o)
Do the EXCEL graphs and the math yourself. You get a value for climate sensitivity of roughly 3.5 ºC per doubling of CO2, entirely consistent with the results of the computer models.
Mike Jonas says:
February 18, 2011 at 6:02 pm
“Solar variation is included alone in the IPCC Report (no feedbacks), leaving much warming unaccounted for.
(…)
There is logic and there is evidence for feedbacks to solar variation, via GCRs and clouds, yet the IPCC chooses to ignore it. Your assertion that the IPCC does include solar variation as a forcing is therefore technically correct but misleading.”
Feedback parameters in computer global climate models are needed to calculate the climate sensitivity to ANY forcing (including solar). The parameters are themselves part of the model results. These feedbacks are mainly:
-water vapor: a warmer atmosphere holds more water vapor, and because water vapor is itself a strong greenhouse gas, this causes more warming (a positive feedback)
-snow/ice: a warmer surface means less springtime and summertime snow and sea ice cover and as the earth surface loses ice and snow becomes darker, abosorbing more sunlight and causing more warming (a positive feedback)
-infrared radiation: a warmer surface and atmosphere radiate more heat as infrared, making the planet lose more heat to space ( a negative feedback)
-clouds: this is the more tricky feedback. More water vapor means more clouds. More clouds means that more incoming solar radiation is reflected back to space (negative feedback) but also that more infrared radiation from the earth is radiated back to the ground (more greenhouse effect, so a positive feedback). What effect wins depends of the type of clouds. Low altitude clouds reflect more sunlight than the ground radiation that is re-radiated to the ground. High altitude clouds reflect less sunlight than re-radiate more ground radiation back to the ground. The computer models predict less low altitude clouds and more high altitude clouds, resulting in a net positive feedback.
All this effect occur in consecuence of warming, no matter if the forcing that caused is shortwave (solar, volcanic or antropogenic aerosols) or longwave (greenhouse gases). So obviously the computer models compute the feedback in response to solar or volcanic forcing as well as in response to greenhouse forcing.
In conclusion: The climate models used by climate scientists include feedbacks in response to solar and volcanic forcings, not only in response to greenhouse forcings.
If you don’t like the computer models, then use the paleoclimate data (that obviously do NOT assume anything about the feedback parameters). You will found an answer that confirms the results of computer models.
Finally you commented about ENSO:
“On ENSO : I think you are probably right when you say that ENSO has no long term effect, provided you mean, say, century-plus. To say that ENSO “add noise to the year-to-year global temperature” is way off the mark, since ENSO clearly operates on a timescale of several decades.”
What are you saying?
ENSO swithches from El Niño to La Niña and then back to El Niño in a timescale of years. See this graph:
http://www.climatewatch.noaa.gov/2009/articles/climate-variability-oceanic-nino-index
“The IPCC’s treatment of global 20thC temperature rise makes no allowance for the 20thC having contained two ENSO warming periods and only one ENSO cooling period”
Huh?
There are dozens of ENSO warming (El Niños) and ENSO cooling (La Niñas) periods in the 20th century. In just the last 10 years we have had 3 El Niños (2002-2003, 2006-2007 and 2009-2010) and 2 La Niñas (2007-2008 and 2010-2011). This is short term noise that do not affect the medium-term, decadal average temperature predictions of the IPCC.
And if you are thinking about periods dominated by warm ENSO events (more frequent and more intense El Niños and more weak and less frequent La Niñas) or cool ENSO events (more frequent and more intense La Niñas and more weak and less frequent El Niños), I have already show to you (see my comments above) that there is no correlation between a La Niña-dominated period and global temperatures: During the Medieval Warm Period, the Pacific was dominated by La Niña !
From Peru :
“The climate models used by climate scientists include feedbacks in response to solar and volcanic forcings, not only in response to greenhouse forcings.”
As I am sure I have said before, they do not include feedbacks in response to solar forcing. They only include feedbacks to surface temperature changes (this is how the feedbacks to CO2 forcing are arrived at). They assume that any feedback to solar variation will be via surface temperature, but this ignores possible feedback via, eg., GCRs and clouds.
re ENSO : apologies, I have been using “ENSO” when I should have been using “PDO”. The PDO is recognised as having warming and cooling phases of varying length. Recent such phases have been around 30 years each. The 20th century contained two PDO warming phases and only one PDO cooling phase.
The fact that there were La Ninas in the MWP doesn’t mean ENSO / PDO has no effect, only that the sun can be a more powerful influence. All factors operate at all times. Analogy : waves on an ocean swell.
Mike Jonas says:
February 20, 2011 at 12:49 pm
“As I am sure I have said before, they do not include feedbacks in response to solar forcing. They only include feedbacks to surface temperature changes (this is how the feedbacks to CO2 forcing are arrived at). They assume that any feedback to solar variation will be via surface temperature”
The feedbacks are by definition the response of the climate system to temperature change. Solar variation of course will cause temperature change, and then the climate models authomatically include feedback to solar variation.
” but this ignores possible feedback via, eg., GCRs and clouds.”
Galactic Cosmic Rays(GCR) is not a feedback, but a possible forcing resulting from the effect of them on cloud cover, causing a change in albedo. Since the effect of GCR on clouds is poorly understood, the IPCC does not included it.
Cloud cover actually respond to temperature change (this is the cloud feedback), and since any forcing (including solar) cause temperature change, the cloud feedback is included in the computer climate models in response to any forcing, including solar.
“re ENSO : apologies, I have been using “ENSO” when I should have been using “PDO”. The PDO is recognised as having warming and cooling phases of varying length. Recent such phases have been around 30 years each. The 20th century contained two PDO warming phases and only one PDO cooling phase.”
Apologies accepted. Talking about decadal timescales on PDO now makes sense (after all, is the Pacific DECADAL Oscillation). But the terminology “warm phase” and “cool phase” is misleading. See those maps:
http://www.usgcrp.gov/usgcrp/images/ocp2006/pages/ocp06-fig9.htm
See the large cool areas in the Extratropical Pacific during the so-called “warm” phase and the strongly warm areas in the extratropical Pacific in the so-called “cool” phase. Because of this, it is better to talk about positive and negative phases instead of warm and cool phases.
“The fact that there were La Ninas in the MWP doesn’t mean ENSO / PDO has no effect, only that the sun can be a more powerful influence.”
The importance of this fact is that shows that ocean cycles are NOT strong drivers of climate trends (they just add cyclic noise to the data). The driver of climate trends is radiative forcing. In the case of the MWP and the LIA, the forcings were mainly solar and volcanic. Now the main forcings are greenhouse gases and athropogenic tropospheric aerosols.
The unavoidable implication of the fact that the weak forcing of the Sun caused the MWP and the LIA in the past is that the much stronger current greenhouse forcing necessarily is today by far the biggest driver of Climate Change.
I agree that ocean cycles are probably not strong drivers of long term climate trends. (I added a bit of qualification to be on the safe side).
GCRs are a feedback to solar variation, because their levels are affected by solar activity. ie, as solar activity changes, so GCR activity changes. GCRs are also a factor in their own right, in that the level of GCRs reaching the solar system can vary.
In your last sentence, you draw the wrong conclusion. The fact that weak changes in solar irradiation caused the significant MWP and LIA in the past shows that there are indeed multiplying factors – ie, positive feedbacks to solar activity. No such feedbacks have been recognised in the IPCC Report or allowed for in the climate models.
Mike Jonas says:
February 22, 2011 at 2:12 pm
“I agree that ocean cycles are probably not strong drivers of long term climate trends”
Totally agree. A lot of so-called climate “skeptics” blame ocean cycles for climate change. But these only redistribute heat between the ocean and the atmosphere, but do not create heat. Only external radiative forcing can do it.
“GCRs are a feedback to solar variation, because their levels are affected by solar activity. ie, as solar activity changes, so GCR activity changes.”
It is not a climate feedback, it is a forcing. The effect of solar wind on GCR (it reduces the level of GCR radiation) occurs in outer space, then the level of GCR could teorethically affect cloud cover, changing earth albedo.
The effect on cloud cover, however, it is still speculative, and the main variation in GCR occurs in the 11-year solar cycle. No big variations in cloud cover are seen with a period of 11 years.
“In your last sentence, you draw the wrong conclusion. The fact that weak changes in solar irradiation caused the significant MWP and LIA in the past shows that there are indeed multiplying factors – ie, positive feedbacks to solar activity.No such feedbacks have been recognised in the IPCC Report or allowed for in the climate models.”
This is not true. The climate feedbacks are intrinsic to any climate change, no matter of the cause of it. I made a list of the most important climate feedbacks before (water vapor, infrared radiation, snow-ice albedo, cloud cover). All respond to temperature change, without any difference between the forcing that caused it (solar, volcanic, greenhouse,etc).There is no way that a feedback that works over solar forcing induced climate change does not act also over greenhouse gases forcing.
If you do not agree with this, could you mention a feedback that is unique to solar forcing?
Note: not cite GCR, they are not a feedback, but a theoretical forcing with little empirical evidence that it is significant or even existent. If you know about a paper showing that actually there is an 11-year cycle in cloud cover (GCR radiation peaks at the solar minimum every 11 years), please give me the link.
From Peru : Maybe the problem is the word “feedback” I am using it to describe indirect effects on climate (more a “feed-on” perhaps than a “feed-back”), whereas the IPCC use it specifically for effects initiated by surface temperature changes. So let’s just look at the mechanisms:
Solar variation affects the level of GCRs reaching Earth.
GCRs can alter the level of cloud-forming nuclei in the atmosphere.
Therefore GCRs may alter the cloud cover.
Changes in cloud cover can have a very large impact on climate.
The effects so far are unlikely to be linear in nature, or they would very likely have been clearly identified by now.
The IPCC Report and the climate models make absolutely no allowance for any of this.
Some mechanism, like a solar-GCR-cloud link, is needed to explain the MWP and LIA, because change in solar irradiation is insufficient to explain them.
…
Events in outer space also affect the level of GCRs reaching Earth.
There may be other indirect effects of solar variation on climate which have not yet been identified.
Mike Jonas says:
February 22, 2011 at 4:28 pm
“From Peru : Maybe the problem is the word “feedback” I am using it to describe indirect effects on climate (more a “feed-on” perhaps than a “feed-back”), whereas the IPCC use it specifically for effects initiated by surface temperature changes”
That IPCC definition is the universal definition used by climate scientists to call the “effects initiated by surface temperature changes”. Anything else is NOT a feedback.
About the solar influence on GCR, it is clearly not a feedback, but an indirect FORCING.
To assess the magnitude of the solar-cycle induced GCR variations on cloud cover, we must look at the existence and magnitude of an 11-year cycle in cloud cover. This is because GCR radiation peaks every 11 years at each solar minimum.:
http://neutronm.bartol.udel.edu/catch/cr3.html
A study of the correlation between 1983 and 2001:
The possible connection between ionization in the atmosphere by cosmic rays and low level clouds
http://www.arm.ac.uk/preprints/433.pdf
Shows that in effect there is a correlation between 1983 and 1994, but after 1995 the
correlation brokes down. The cause could be:
1) The correlation between GCR and low cloud cover does not exist
2) The new (post-1994) ISCCP data may have a calibration error (Marsh and Svensmark, 2003), however, no such error has been reported by the ISCCP team so far
3)Other climatic parameters are acting on cloudiness in addition to atmospheric ionization.
Assuming that the correlation is real, the paper shows that the forcing from the increased solar activity effect on GCR during the 20th century is of 0.38 W/m^2, slightly less than the direct effect of increased solar irradiance 0f 0.44 W/m^2.
So in effect the GCR-solar forcing, if real, is significant, and will mean that the ΔF(W/m^2) between the MWP and the LIA is bigger than estimated, and as a consecuence the climate sensitivity is lower.
However, the GCR radiation link with cloud cover is still open to debate, because, as I have shown, the correlation breaks down after 1995, meaning that either the effect does not exist or that the data on cloud cover is flawed.
However, independent of all of this, a warmer MWP will make the numerator ΔT (ºC) in the expression:
climate sensitivity (ºC/(W*m^-2))= [ΔT (ºC)] /[ΔF (W/m^2)]
Bigger, meaning that the climate sensitivity is bigger than previously estimated. All the discussion in this comment is about the denominator ΔF (W/m^2).