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• 07/01/14--11:36: A friend has passed away
• Blog topic:
In 1977 he made the movie "the weather machine" about how Earth is doomed to enter a new ice-age. Since those apocalyptic predictions did not materialize, he knew better once the wheels have turned and people talked about imminent warming. That is, his immune system developed immunity against blindly accepting alarmist predictions.

When he learned of my friend Henrik Svensmark's work, he of course found it interesting, which is why several years later they ended up writing a book together, the Chilling Stars.

As for myself, I met him just once. We had lunch in the restaurant opposite to the British Museum, but had plenty of e-mail correspondence. He will be fondly remembered. This is his last blog post.

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Blog topic:
An artists conception of the Milky Way with its spiral arms
Last month I visited the U of Washington to give a talk in which I discussed the effects of cosmic rays on climate. At the end of it, not one, but two people independently asked me about Overholt et al., which supposedly ruled out the idea that passages through the galactic spiral arms affect the appearance of glaciations on Earth (see summary I wrote a few years ago, which includes links to PDFs the actual papers). I told them that the paper had really stupid mistakes and it should be discarded in the waste bin of history, but given that Overholt et al. is still considered at all, I have no choice but to more openly euthanize it.

Before I get into the technical details (which will cause many of the readers to click their way out of here), I do want to say something general about the refereeing process and how it can easily break down, as it did here.

Given that there are many more people who are eager to shoot down the cosmic ray climate link than people researching it, very often I find that the criteria used to accept papers which refute the link are way more lenient than the criteria needed to accept papers that supposedly refute it. This is because the refuters don't have an incentive to find errors in refuting papers (e.g., as I demonstrated with the Shakun et al. paper supposedly finding that CO2 leads the average global temperature). This is because any paper has a much higher probability of getting refereed by the refuter camp than the proponents camp. Simple statistics.

The second comment is that I wrote them politely, it ended up with an erratum, but to save face they continued claiming the that their paper supports the erroneous claim that my original conclusions are unsubstantiated. One can show that even those leftover criticisms are plagued with errors.

#### Main Problem

In the their paper, Overholt et al. try to estimate previous passages through the galactic spiral arms, and compare those passages to the appearance of ice-age epochs on Earth, over the past billion years. The gravest error is that the analysis was carried out using a spiral arm pattern speed that was totally different from the range of pattern speed they actually wrote they used!

They wrote that they take 10.1 to 13.7 (km/s)/kpc as the possible range for the relative motion of the spiral arm pattern speed with respect to the solar angular velocity (i.e., a nominal value of about 11.9 (km/s)/kpc). However, if one looks at the average spiral arm crossing as obtained in their analysis, it is about 100 Myr (the first of their group of 4 arm passages is at roughly at 275 Myr and their second group is at roughly 670 Myr). This implies an average spiral arm passage every (670 Myr - 275 Myr)/4 ~ 100 Myr, which is inconsistent with the above pattern speed. In fact, an average spiral arm passage every 100 Myr implies a relative pattern speed of about: $$\Omega_\odot - \Omega_\mathrm{arm} \approx 15.4 (km/s)/kpc,$$ I am pretty sure that the source of error is the fact that they accidentally took the absolute pattern speed when calculating the spiral arm passages. For their nominal 217 km/s solar velocity at 8 kpc, one gets $\Omega_\odot = 27.1 (km/s)/kpc$, such that the absolute pattern speed obtained for the nominal 11.9 (km/sec)/kpc relative speed, is: $$\Omega_\mathrm{arm} = \Omega_\odot - (\Omega_\odot-\Omega_\mathrm{arm}) = 27.1 − 11.9 = 15.2 (km/s)/kpc$$ i.e., roughly the value I read by eye from their graph describing the spiral arm passages.

Because they accidentally took the absolute pattern speed, they obtained spiral arm crossings which are much more frequent that the climatic data or meteoritic data. If they would have taken the 10.1 to 13.7 range instead, the spiral arm passages would have been 112 to 152 Myr, which includes the ice-age epoch occurrences and the cosmic ray flux variations based on iron meteorites. The phase would have agreed as well.

I then suggested that they publish an erratum to that paper, which they did. However, to save face, they claimed that their re-calculated spiral passages are still inconsistent with the meteoritic data or with the climate record, which brings me to the additional problems still present in their analysis.

A few more problems relate to the way Overholt et al. derive the spiral arm crossing (which is why even with their erratum, their manuscript is pointless).

First, they take the spiral arm model of Englmaier et al. 2009, but they trace the spiral arms by eye, and as a consequence get a distorted result which gives highly unlikely “tight” clusters of 4 consecutive passages each rotation. In fact, it is so distorted that 2 consecutive arm crossings are of the same arm, which even with the radial epicyclic motion of the solar system is ridiculous.

Second, they assume that their highly asymmetric (and unlikely) spiral arm configuration should remain as such for many spiral arm passages, but because the arms are dynamic, without more information, a more reasonable assumption would have been that the arms would have tended to be separated by 90 degrees instead of the “tight” clusters of 4 consecutive passages.

Third, they didn’t consider the fact that supernovae are biased to take place about 10 to 20 million years after the spiral arm passage because of the finite lifetime of the stars that end their lives as supernova explosions.

Fourth, they don’t actually carry out any statistical analysis how likely or unlikely is it to find all the ice-age epochs as close to the estimated spiral arm passages, not that a statistical analysis would have helped any given their problematic determination of the arm passages.

Last, and perhaps most important. The cosmic ray flux can be shown using Iron meteorites to be periodic with a roughly constant 145 Myr period, and in phase with the appearance of ice-age epochs, which means that any distorted reconstruction such as that of Overholt et al. is inconsistent with the data, which they have totally ignored.

#### Summary

Overhaul et al. analysis of the spiral arm passages is bad at so many levels it is not really worth ever considering again. However, will people still quote it and claim that it refutes the galactic spiral arm explanation for the appearance of spiral arm passages? Probably, but now you know better.

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Blog topic:
Bill "the science guy" Nye says that I am a denier.
I recently stumbled on a transcript of Bill “the science guy” Nye’s interview on CNN last week. In it, he said that climate skeptics (i.e., people like myself), are at least as bad as people who deny that smoking causes cancer. There are quite a few things he misses, in fact, he got things totally wrong, but I do like the his analogy to smoking and cancer as you’ll see.

First, what did he actually say? During his appearance on CNN, Bill Nye compared the link between denying a link between climate change and anthropogenic activity to denying a link between smoking and cancer:

“I just want to remind voters that suppose you had somebody running for congressional office in your district who insisted there was no connection between cigarette smoking and cancer. Would you vote for that person? You might, but if this person were adamant — ‘No, the scientists who studied cigarette smoking, they don’t know what…’ — if they were adamant, would you vote for them? And so, in the same way the connection between climate change and human activity is at least as strong as cigarettes and cancer. And so, I just want everybody to keep this in mind: that it’s very reasonable that the floods in Texas, the strengthening storms, especially — the president was in Florida — these things are a result of human activity making things worse. It’s very bad. I get this that people died in Texas, and I am reminding you what else. This is a very expensive business. When you flood the fourth largest city in the United States, somebody is going to pay for it, and it’s you and me. And so, the sooner we get to work on climate change, the better.”

Here’s the actual video:

These short comments and comparison he made are inappropriate for several major reasons.

To begin with, he follows the usual alarmist assertion that every calamity is necessarily due to global warming (now known as “climate change” so that they can cover more ground) and that all the climate change is necessarily anthropogenic. Since bizarre weather patterns can happen all the time with some probability, and since part of the climate change is also natural, these implicit “logical” steps would not make sense, at least in any other scientific discipline. I won’t dwell on this since others already did.

Second, the global warming debate is mostly a quantitative one. While the IPCC claims that climate sensitivity can be very large (e.g., a 4.5°C increase per CO2 doubling which would be catastrophic), I claim that such high values are ruled as they are inconsistent with a lot of empirical data (e.g., see this). In other words, skeptics like myself don’t argue that CO2 has no effect on climate, only that the IPCC story is highly exaggerated. On the other hand, whether smoking contributes 90% or 10% of the cases of lung cancer would be good enough reason for any rational person to quit smoking (well, rational and able to overcome his or her addiction). On the other hand, if a given emission scenario over the 21st century will cause a 1°C or 4°C could make a huge difference. This means that denying that smoking causes cancer is a yes/no question while most skeptics don’t “deny anthropogenic global warming”, instead, they only claim it is minor.

The third point is the main one I want to make. The reasoning behind the attribution of lung cancer to smoke and behind attributing global warming to anthropogenic activity is conceptually different, so different, that putting both on the same pedestal is simply wrong. The evidence linking lung cancer to smoking is in the form highly statistically significant increase in the incidence rate of lung cancer when comparing smoking to non-smoking groups. So, either smoking or something very closely related to it is clearly carcinogenic. The assertion that climate sensitivity is large, that most of the 20th century warming is necessarily anthropogenic and that temperature increase over the 21st century will be large are the result of model predictions, not on empirical evidence. In fact, empirical evidence such as the small response to volcanic eruptions (e.g., Lindzen and Giannitsis 1998, see also note on climate effects of volcanoes here), or the lack of correlation between the order of magnitude CO2 variations over geological time scales and the global temperature (e.g., this discussion on cosmic rays and climate), or of course, the lack of warming over the past nearly two decades, counter to all the model predictions (e.g., see this discussion of the hiatus long before the term was used).

If we use the cancer analogy it would be as if we had a model that can predict, at the biochemical level that some of the chemicals in cigarettes are carcinogenic, and that smoking should cause cancer, but, while the model would say that the incidence rate should be high and should explain most of the lung cancer cases, comparing the actual incidence rate would show that only some modest fraction of the lung cancer cases are attributed to smoking, but the rest are not. Yet, even with the evidence showing only a small difference in the incidence rate between the smoker and non-smoker group, the model will still be believed and it would be used to make predictions to other types of cancers, e.g., saying that they happen at a huge rate while they don’t. Sounds ridiculous? Well, this is what’s happening in climate science.

Anyway, the last time I had any thought about the “science guy”, was when I heard he was to give the commencement speech at my wife’s graduation from Caltech (roughly when the global warming hiatus began!). Back then I thought of how uninspiring it is. I am not saying that publicizing science to the public is not important (which is what Bill Nye did so well), but I didn’t think that this is the kind of figure that will push the young cadet of scientists and engineers into new frontiers. Its isn’t as if he is doing rocket science. In retrospect, I now think it was even less appropriate to have him.

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Blog topic:
After being asked by 5 independent people about the new sunspot number reconstruction and that it doesn’t show that the sun should have contributed any warming to the 20th century, I decided to write about it here. I have one word to describe it – irrelevant. It is also a good opportunity to write about new results (well, one that saw the light of day a few months ago) showing again that the sun has a large effect on climate. Yet, the world will still continue to ignore it. Am I surprised? No I’m not.

First, what’s the story? A group led by Frédéric Clette had a presentation at the IAU assembly in Hawaii. In it, they argued that the sunspot number suffers from various systematic errors as it is a subjective measurement. Because those systematic errors vary with time (with the different observers and observational methods), the SN reconstruction can exhibit a fictitious long term trend. They also attempted to calibrate the data, and obtain a more homogeneous dataset. This is described at length in their arXiv preprint.

The most interesting aspect about their new sunspot reconstruction is that there is significantly less variation in the sunspot number between the different solar maxima since the Maunder minimum. This implies, according to them, that there wasn't a significant increase in solar activity over the 20th century (no "20th century Grand Maximum"), and therefore the sun should not have contributed anything towards increased temperatures. This point was of course captured by the media (e.g., here).

Figure 1: The old (red) and new (blue) sunspot number reconstructions of Clette et al.

So, what do I think about it? First, I have no idea whether the calibration is correct. They do make a good argument that the SN reconstruction is problematic. Namely, some corrections are probably necessary and there is no reason a priori to think that what they did is invalid. However, their claim about solar activity in general not varying much since the sun came out from the Mounder minimum is wrong. There are other more objective ways to reconstruct solar activity than subjective sunspot counting, and they do show us that solar activity increased over the 20th century. So at most, one can claim that solar activity has various facets, and that the maximum sunspot number is not a good indicator of all of them. This is not unreasonable since the number of sunspots would more directly reflect the amount of closed magnetic field lines, but not the open ones blowing in the solar wind.

The two important objective proxies for solar activity are cosmogenic isotopes (14C and 10Be), and the geomagnetic AA index. The AA index (measured since the middle of the 19th century) clearly shows that the latter part of the 20th century was more active than the latter half of the 19th century. The longer 10Be data set reveals that the latter half of the 20th century was more active than any preceding time since the Maunder minimum. (The 14C is a bit problematic because human nuclear bombs from the 1940's onwards generated a lot of atmospheric 14C so it cannot be used to reconstruct solar activity in the latter part of the 20th century).

Figure 2: The AA geomagnetic index showing a clear increase in solar activity over the 20th century (From here).

Figure 3: The 10Be production showing again, that the sun was particularly active in the latter half of the 20th century. The sunspot number is the "old" reconstructions without Clette's et al. corrections.

What does it tell us? Given that long term variations in Earth's climate do correlate with long term solar activity (e.g., see the first part of this) and given that some solar activity indicators (presumably?) don't show an increase from the Maunder minimum, but some do, it means that climate is sensitivite to those aspects of the solar activity that increased (e.g., solar wind), but not those more directly associated with the number of sunspots (e.g., UV or total solar irradiance). Thus, this result on the sunspots maxima (again, if true), only strengthens the idea that the solar climate link is through something related to the open magnetic field lines, such as the strength of the solar wind or the cosmic ray flux which it modulates.

The second point I wanted to write about is a recently published analysis showing that the sun has a large effect on climate, and quantifying it. In an earlier work, I showed that you can use the oceans as a calorimeter to see that the solar radiative forcing over the solar cycle is very large, by looking at various oceanic data sets (heat content, sea surface temperature and tide gauges). How large? About 6-7 times large than one can naively expect from changes in the solar irradiance.

More recently, Daniel Howard, Henrik Svesmark and I looked at the satellite altimetry data. It is similar to the tide gauge records in that it measures how much heat goes into the ocean by measuring the sea level change (most of the sea level on short time scales is due to thermal expansion). Unsurprisingly, we found that the satellite altimetry showed the same solar-cycle synchronized sea level change as the tide gauge records. However, because the satellite data is of such high quality, it is has a higher temporal resolution than the tide gauge records which allows singling out the thermal expansion component from other terms (e.g., associated with trapping of water on land). This allows for an even better estimate of the solar forcing, which is 1.33±0.34 W/m2 over the last solar cycle. You can see in fig. 4 how much the sun and el-Niño can explain a large fraction of the sea level change over yearly to decadal time scales.

Figure 4: Sea level data and the model fit. The blue dots are the linearly detrended global sea level measured with satellite altimetry. The purple line is the model fit to the data which includes both a harmonic solar component and an ENSO contribution. The shaded regions denote the one sigma and 1% to 99% confidence regions. The fit explains 71% of the observed variance in the filtered detrended data.

The bottom line is that the sun appears to have a large effect on the climate on various time scales. Whether or not the sunspots reflect the increase in solar activity since the Maunder minimum (as reflected in other datasets) is not very important. At most, if they don't reflect, it only strengthen's the idea that something associated with the solar wind does (such as the cosmic rays which they modulate).

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Blog topic:

Last week I participated in an interesting debate that was held at the Cambridge Union, the oldest debating club in the world (dating back to 1815. The invite was to be on the side opposing the proposition “This house would rather cool the planet than warm the economy”.

Although I think the phrasing of the question is problematic to begin with, since it assumes that “warming the economy” necessary would cool the climate, I should applaud the Cambridge Union for supporting free speech and allowing people on both side to voice their arguments, especially given how many on the alarmist side refuse to do so, claiming that there is nothing to debate anymore.

I should also add that I was quite shocked to see how the audience was so one sided (though far less than the ridiculous 97:3 ratio we hear about!) and unwilling to listen to scientific arguments. I am actually quite lucky to be living in Israel where free speech and free thought are really more than lip service. Having honest debates in Israeli academia or in the media is actually the norm.

Below you will find the summary I wrote myself before the debate. Since it is rather concise I thought it would be a good idea to bring it here as well.

Have fun

— Nir

Let me begin by asking you a question. What is the evidence that people, like the proponents here, use to prove that we humans are responsible for global warming and that future warming will be catastrophic if we don’t get our act together?

The fact is that this idea is a misconception and the so called evidence we constantly hear is simply based on fallacious arguments

To begin with, any one who appeals to authority or to a majority to substantiate his or her claim is proving nothing. Science is not a democracy and the fact that many believe one thing does not make them right. If people have good arguments to convince you, let them use the scientific arguments, not logical fallacies. Repeating it ad nauseam does not make it right!

Other irrelevant arguments may appear scientific, but they are not. Evidence for warming is not evidence for warming byhumans. Seeing a poor polar bear floating on an iceberg does not mean that humans caused warming. (Actually, the bear population is now probably at its highest in modern times!). The same goes to receding glaciers. Sure, there was warming and glaciers are receding, but the logical leap that this warming is because of humans is simply an unsubstantiated claim, even more so when considering that you can find Roman remains under receded glaciers in the Alps or Viking graves in thawed permafrost in Greenland.

Other fallacious arguments include using qualitative arguments and the appeal to gut feelings. The fact that humanity is approaching 10 billion people does not prove that we caused a 0.8°C temperature increase. We could have just as much caused an 8°C increase or an 0.08°C. If all of humanity spits into the ocean, will sea level rise appreciably?

In fact, there is no single piece of evidence that proves that a given amount of CO2 increase should cause a large increase in temperature. You may say, “just a second, we saw Al Gore’s movie, in which he presented a clear correlation between CO2 and temperature from Antarctic ice cores”. Well, what he didn’t tell you is that one generally sees in the ice cores that CO2 lags the temperature by typically a few hundred years, not vice versa! The simple truth is that Al Gore simply showed us how the amount of CO2 dissolved as carbonic acid in the oceans changes with temperature. As a matter of fact, over geological time scales, there were huge variations in the CO2  (a factor of 10) and they have no correlation whatsoever with the temperature. 450 million years ago there was 10 times as much CO2 in the atmosphere but more extensive glaciations.

When you throw away the chaff of all the fallacious arguments and try to distill the climate science advocated by the IPCC and alike, you find that there are actually two arguments which appear as legitimate scientific arguments, but unfortunately don’t hold water. Actually, fortunately! The first is that the warming over the 20th century is unprecedented, and if so, it must be human. This is the whole point of the hockey so extensively featured in the third assessment report of the IPCC in 2001. However if you would google “climategate” you would find that this is a result of shady scientific analysis - the tree ring data showing that there was little temperature variation over the past millennium showed a decline after 1960, so, they cut it off and stitched thermometer data. The simple truth is that in the height of the middle ages it was probably just as warm as the latter half of the 20th century. You can even see it directly with temperature measurements in boreholes.

The second argument is that there is nothing else to explain the warming, and if there is nothing else it must be the only thing that can, which is the anthropogenic contribution. However, as I mention below, there is something as clear as daylight… and that is the sun.

Before explaining why the sun completely overturns the way we should see global warming and climate change in general. It is worth while to say a few words on climate sensitivity and why it is impossible to predict ab initio the anthropogenic contribution.

The most important question in climate science is climate sensitivity, by how much will the average global temperature increase if you say double the amount of CO2. Oddly enough, the range quoted by the IPCC, which is 1.5 to 4.5°C per CO2 doubling was set, are you ready for this, in a federal committee in 1979! (Google the Charney report). All the IPCC scientific reports from 1990 to 2013 state that the range is the same. The only exception is the penultimate report which stated it is 2 to 4.5. The reason they returned to the 1.5 to 4.5 range is because there was virtually no global warming since 2000 (the so called “hiatus”), which is embarrassingly inconsistent with a large climate sensitivity. What’s more embarrassing is that over almost 4 decades of research and billions of dollars (and pounds) invested in climate research we don’t know the answer to the most important question any better? This is simply amazing I think.

The body of evidence however clearly shows that the climate sensitivity is on the low side, about 1 to 1.5 degree increase per CO2 doubling. People in the climate community are scratching their heads trying to understand the so called hiatus in the warming. Where is the heat hiding? While in reality it simply points to a low sensitivity. The “missing” heat has actually escaped Earth already! If you look at the average global response to large volcanic eruptions, from Krakatoa to Pinatubo, you would see that the global temperature decreased by only about 0.1°C while the hypersensitive climate models give 0.3 to 0.5°C, not seen in reality. Over geological time scales, the lack of correlation between CO2 and temperature places a clear upper limit of a 1.5°C per CO2 doubling sensitivity. Last, once we take the solar contribution into account, a much more consistent picture for the 20th century climate changes arises, one in which the climate drivers (humans AND solar) are notably larger, and the sensitivity notably smaller.

So, how do we know that the sun has a large effect on climate? If you search on google images “oceans as a calorimeter”, you would find one of the most important graphs to the understanding of climate change which is simply ignored by the IPCC and alarmists. You can see that over more than 80 years of tide gauge records there is an extremely clear correlation between solar activity and sea level rise - active sun, the oceans rise. Inactive sun - the oceans fall. On short time scales it is predominantly heat going to the oceans and thermal expansion of the water. This can then be used to quantify the radiative forcing of the sun, and see that it is about 10 times larger than what the IPCC is willing to admit is there. They only take into account changes in the irradiance, while this (and other such data) unequivocally demonstrate that there is an amplifying mechanism linking solar activity and climate.

The details of this mechanism are extremely interesting. I can tell you that it is related to the ions in the atmosphere which are governed by solar activity and in fact, there are three microphysical mechanisms linking these ions to the nucleation and growth of cloud condensation nuclei. Basically, when the sun is more active, we have less clouds that are generally less white.

So, the main conclusion is that climate is not sensitive to changes in the radiative forcing.

This means that we are not required to “cool the economy” in order to cool earth. In Paris and Copenhagen the leaders of the world said that we should make sure that the total global warming will be less than 2°C. It will be less than 2°C even if we do nothing. There are several red flags that people do their best to ignore. The lack of warming in the past 2 decades is a clear sign that sensitivity is low, but people ignore it.

Last point. People say that we should at least curb the emissions as a precautionary step. However, resources are not infinite. Most people in developed nations can pay twice for their energy, but for third world nations? It would mean more expensive food, hunger and poverty, and many in the developed world actually freezing in winter. So in fact, taking unnecessary precautionary steps when we know they are unnecessary is immoral. It is even committing statistical murder.

Now the really last point, I am also optimist that humanity will switch to alternative energy sources in less than 2-3 decades just because they will become cheap enough, and just for the reason that people want to save money. Just like the price of computers has plummeted exponentially (Moore’s law— number of transistors doubles every 18 months) so does the cost of energy from photovoltaic cells (cost halves every 10 years). Once they will be really cost effective, without subsidies, suddenly we won’t be burning fossil fuels because it would be the expensive thing to do!

Let us use our limited resources to treat real problems.

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Blog topic:

By Henrik Svensmark and Nir shaviv

Our new results published today innature communications provide the last piece of a long studied puzzle. We finally found the actual physical mechanism linking between atmospheric ionization and the formation of cloud condensation nuclei. Thus, we now understand the complete physical picture linking solar activity and our galactic environment (which govern the flux of cosmic rays ionizing the atmosphere) to climate here on Earth though changes in the cloud characteristics. In short, as small aerosols grow to become cloud condensation nuclei, they grow faster under higher background ionization rates. Consequently, they have a higher chance of surviving the growth without being eaten by larger aerosols. This effect was calculated theoretically and measured in a specially designed experiment conducted at the Danish Space Research Institute at the Danish Technical University, together with our colleagues Martin Andreas Bødker Enghoff and Jacob Svensmark.

Background

It has long been known that solar variations appear to have a large effect on climate. This was alreadysuggested by William Herschel over 200 years ago. Over the past several decades, more empirical evidence have unequivocally demonstrated the existence of such a link, as exemplified in the examples in the box below.

The fact that the ocean sea level changes with solar activity (see Box 1 above) clearly demonstrates that there is a link between solar activity climate, but it can be used to quantify the solar climate link and show that it is very large. In fact, this “calorimetric” measurement of the solar radiative forcing is about 1 to 1.5 W/m2 over the solar cycle, compared with the 0.1-0.2 W/m2 change expected from just changes in the solar irradiance. This means that a mechanism amplifying solar activity should be operating—the sun has a much larger effect on climate than can be naively expected from just changes in the solar output.

Over the years, a couple of mechanisms were suggested to explain the large solar climate link. However, one particular mechanism has accumulated a significant amount of evidence in its support. The mechanism is that of solar wind modulation of the cosmic rays, which govern the amount of atmospheric ionization, and which in turn affect the formation of cloud condensation nuclei and therefore how much light do the clouds reflect back to space, as we now explain.

Cosmic Rays are high energy particles originating from supernova remnants. These particles diffuse through the Milky Way. When they reach the solar system they can diffuse into the inner parts (where Earth is) but lose some energy along the way as they interact with the solar wind. Here on Earth they are responsible for most of the ionization in the Troposphere (the lower 10-20 km of the atmosphere where most of the “weather” takes place). We now know that this ionization plays a role in the formation of cloud condensation nuclei (CCNs). The latter are small (typically 50nm or larger) aerosols upon which water vapor can condense when saturation (i.e., 100% humidity) is reached in the atmosphere. Since the properties of clouds, such as their lifetime and reflectivity, depends on the number of CCNs, changing the CCNs formation rate will impact Earth’s energy balance.

The full link is therefore as follows: A more active sun implies a lower CR flux reaching Earth and with it, lower ionization. This in turn implies that fewer cloud condensation nuclei are produced such that the clouds that later form live shorter lives and are less white, thereby allowing more solar radiation to pass through and warm our planet.

Figure 5: The link between solar activity and climate: A more active sun reduces the amount of cosmic rays coming from supernovae around us in the galaxy. The cosmic rays are the dominant source of atmospheric ionization. It turns out that these ions play an important role in (a) increasing the nucleation of small condensation nuclei (a few nm) and (b) increasing the growth rate of the condensation nuclei (which is the effect just published). The larger growth rates imply that they are less likely to stick to pre-existing aerosols and thus have a larger chance of reaching the sizes of cloud condensation nuclei (CCNs, typically > 50 nm in diameter). Thus, a more active sun decreases the formation of CCNs, making the clouds less white, reflecting less sunlight and therefore warming Earth.

Until today we had just empirical results which demonstrate that this link is indeed taking place. The main results are summarized in Box 2 below. In particular, we have seen correlations between solar activity and cloud cover variations, as well as between cosmic ray flux variations arising from changes in our galactic environment and long term climate change using geological data.

The first suggestion for an actual physical mechanism was that ions increase the nucleation of small (2-3 nm sized) aerosols called condensation nuclei (CNs). The idea is that small clusters of sulfuric acid and water (the main building blocks of small aerosols) are much more stable if they are charged. That is, the charge allows the aerosols to grow from a very small (few molecule) cluster to a small stable CN without breaking apart in the process. This effect was first seen in our lab (Svensmark 2006). The effect was seen again in the CLOUD experiment running at CERN (Kirkby 2011). Later experiments have shown that ions accelerate also other nucleation routes in which the small clusters are stabilized by a third molecule (such as Ammonia). That is, ions play a dominant role in accelerating almost all nucleation routes (as long as the total nucleation rate is lower than the ion formation rate).

Figure 7: The Ion induced nucleation effect measured in the lab. Left: The first demonstration in our SKY experiment showing that increased ionization increases the nucleation of small aerosols (typically 3 nm in size). Right: Corroboration of the results in the CLOUD experiment at CERN.

In the meantime, a number of research groups aimed at testing the idea that cosmic ray ionization could help the formation of cloud condensation nuclei (CCN). This was done by using large global circulation models coupled with aerosol physics. The idea was to see if an added number of small aerosols would grow into more CCNs. All of the numerical models gave the result that the small aerosols were lost before they could become large enough, leading to the conclusion that the effect of cosmic rays on the number of CCN over a solar cycle was insignificant (e.g., Pirece and Adams 2009). This could also be explained analytically (Smith et al. 2016). It was therefore proclaimed that the theory was dead.

Given the empirical evidence, it was clear to us that a link must be present, even if the ion induced nucleation mechanism itself is insufficient to explain the link. Thus, our response was to address the same question without using models but instead to test it experimentally. Therefore, in 2012 we tested if small nucleated aerosols could grow into CCN in our laboratory and discovered that without ions present, the response to increased nucleation was severely damped, just like the above-mentioned models; however with ions present, all the extra nucleated particles grew to CCN sizes, in contrast to the numerical model results (Svensmark et al. 2013). So, experiments contradicted the models. The logical conclusion was that some unknown ion-mechanism is operating, helping the growth.

Figure 8: Left: When injecting small aerosols, the relative increase decreases with aerosol size because as aerosols grow they tend to coagulate with larger aerosols. Right: However, when increasing the ionization in the chamber, not only are more aerosols nucleated, the relative increase survives to larger sizes implying that some mechanism is increasing the survivability of the aerosols as they grow.

Following the experimental results showing that increased ionization does indeed increase the number of large CCNs, the natural question to ask was whether these results were caused by the particular experimental conditions—perhaps this mechanism does not work in the real atmosphere. It is therefore fortunate that our Sun carries out natural experiments with the whole Earth.

On rare occasions, “explosions” on the Sun called coronal mass ejections result in a plasma cloud passing the Earth, with the effect that the cosmic rays flux decreases suddenly and remains low for about a week. Such events, with a significant reduction in the cosmic ray flux, are called Forbush decreases, and are ideal to test the link between cosmic rays and clouds. Finding the strongest Forbush decreases and using three independent cloud satellite datasets and one dataset for aerosols, we clearly found a response to Forbush decreases. These results validated the whole chain from solar activity, to cosmic rays, to aerosols (CCN), and finally to clouds, in Earth’s atmosphere (Svensmark et al 2009Svensmark et al. 2016).

Figure 9: The average effect of the 5 strongest Forbush decreases in the 1987-2007 period on cloud properties. Plotted in red is the reduction in the cosmic ray flux following “gusts” in the solar wind (from Coronal Mass Ejections). In black we see the reduction in aerosols over the oceans and three different cloud parameters from three different datasets (Svensmark et al 2009). These results provide an in situ demonstration of the effect of cosmic rays on aerosols and cloud properties.

With the accumulating empirical and experimental evidence, it was clear that atmospheric ionization is playing a role in the generation of the aerosols needed for cloud formation, however, the exact mechanism proved to be elusive. For this reason, we decided to setup another laboratory experiment mimicking conditions found in the real atmosphere and study how atmospheric ions may be affecting the production of CCNs. This also led us to look for alternative mechanisms which will increase the survivability of the CNs as they growth. Indeed, after several years of research, one was found.

The discovery

A little more than 2 years ago, we made the realization that charge will play a role in accelerating the growth rate of small aerosols. When more ions are present in the atmosphere, more of them end up sitting on sulfuric acid clusters of a few molecules. Moreover, the charge makes the sulfuric acid clusters stick to the growing aerosols much faster, as we explain in the box below. Since faster growing aerosols have lower chances of coagulating with larger aerosols, more of the growing aerosols can then survive to reach larger sizes. In other words, when the ionization rate is higher, more CCNs can are formed.

After realizing that this effect should be taking place we did two things. First, we calculated how large it should be and found that for the typical conditions present in the pristine air above oceans, in which the typical sulfuric acid density is a few 106 molecules/cm3, the ions accelerate the growth by typically 1 to 4%. However, because the number of aerosols surviving the growth is exponentially small (typically several e-folds), the relative change in the CCN density is a few times larger still (by the number of e-folds in the exponential damping to be precise). Thus, over the solar cycle (which changes the tropospheric ionization by typically 20%), we expect a several percent variation in the CCN density and with it, the cloud properties, as is observed.

The second thing we did was go to the lab and design an experiment in which we can see this effect taking place (and also validate our theoretical calculations). This is not trivial because the effect is larger for lower sulfuric acid levels (as a larger percentage of the molecules would be charged). However we cannot measure at very low sulfuric acid levels because the aerosols then grow very slowly such that they stick to the chamber walls before their growth can be reliably measured. This forced us to measure at high sulfuric acid levels for which the effect is smaller. This posed a formidable technological challenge. To overcome this, we designed an experiment which can keep relatively stable conditions over long periods (up to several weeks at a time) during which we could automatically increase or decrease the ionization rate at the chamber. This allowed us to collect a large amount of data and get high quality signals (e.g., see fig. 11 in the box below).

We found that aerosols indeed grow faster when the ionization rate is higher, totally consistent with the theoretical predictions (as can be seen in fig. 12 in the box below). This allows them to survive the growth period without coagulating with larger aerosols.

So, what do the results imply? Until now we had significant amount of empirical evidence which demonstrated that cosmic rays affect climate, but we didn't have the actual underlying physical mechanism pinned down. Now we have. It means that we not only see the existence of a link, we now understand it. Thus, if the solar activity climate link was until now ignored under the pretext that it cannot be real, this will have to change. But perhaps more interestingly, it also explains how long term variations in our galactic environment end up affecting our climate over geological time scales.

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Blog topic:
Two months ago I wrote about the Cambridge Union Debate I participated in. The Cambridge Union has meanwhile posted the debate online. Here is my part of it. (It starts at 32m31s.)

I should add that the debate was a real eye opener. By living in Israel, I have had the luxury of experiencing a mostly diverse society, open to a wide range of scientific (and other) opinions. This has allowed me to carry out research without having to care about what other people think. It stands however in stark contrast to the body of Cambridge students I addressed. They are well intentioned but unfortunately completely brainwashed. They cite the 97% polemic about most scientists believing in anthropogenic global warming without stopping for a second to think about it, or the evidence that supposedly supports it. They want to think of themselves as liberals, but in fact, they have the most conservative mindset unable to even attempt objective thinking.

The only exception, which stood out as remarkable contrast to the rest, was the remark made by a historian named Josh from Christ college (at 1h 21m 21s).

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