Physicists at CERN (the European Organization for Nuclear Research) created something of an online kerfuffle last month when they sought to improve our understanding of cosmic rays and clouds.
While their experiment made novel findings on particle formation, it was the blogosphere’s claims about the implications for climate change that were really interesting.
Previous articles on The Conversation have outlined why the greenhouse effect is real and why increasing greenhouse gases drive global warming.
Against this understanding, some commentators argue for a range of speculative and unsubstantiated mechanisms that will supposedly counteract future global warming, or that provide an alternative explanation for 20th century climate change.
While these arguments do not stand the scrutiny of the peer-reviewed literature, they are prominent in the blogosphere.
There are two common arguments levelled against the risk associated with a doubling or more of atmospheric carbon dioxide.
“Don’t worry - the climate system will stop itself from warming too much.”
The first argument is based on the idea that negative feedback processes within the climate system will limit or cap future warming – to a level not much warmer than present.
In the context of human-induced climate change, a positive feedback is a natural mechanism that enhances the warming caused by greenhouse gases. A negative feedback is one that reduces the warming caused by greenhouse gases.
Increasing water vapour in the atmosphere is the most important positive feedback mechanism for the enhanced greenhouse effect. Like carbon dioxide and methane, water vapour is a powerful greenhouse gas. As the climate system warms, the atmosphere can hold more water vapour, greatly enhancing the amount of warming from carbon dioxide.
So increasing carbon dioxide concentrations are the agent of change, or the primary driver of warming, while increasing water vapour is the climate system’s response to that warming. Most estimates suggest that water vapour feedback approximately doubles the warming from increasing carbon dioxide alone.
Increased moisture in the atmosphere also leads to changes in cloud type and cloud cover.
Exactly what will happen to clouds is one of the major sources of uncertainty when it comes to future climate change. The uncertainty is due to the sheer complexity of clouds and their relationship to the Earth’s radiation budget.
We expect that some changes in cloud properties will slightly reduce the impact of greenhouse warming. For example, clouds will most likely be more reflective as the atmosphere warms, reflecting more incoming solar energy back out to space.
However, it is also possible that the heat-trapping effect of high clouds will increase in response to warming, offsetting the effect of more reflection of solar energy. So the net effect of these cloud feedbacks is very uncertain.
Again, it is important to keep in mind that feedback processes do not drive climate change. Rather, they can accentuate or moderate changes associated with external influences on the climate system.
Changes to cloud are regularly cited in the blogosphere as a factor that will halt future warming at a level we can cope with. However this argument is entirely inconsistent with another that is regularly presented on blogs: that the Earth was much hotter in the past.
The latter argument is true; the Earth has indeed been much hotter in the geological past, especially when greenhouse gas concentrations have been high.
In this way, we know very clearly that clouds contain no magical mechanism that limits future warming to a level that is benign to human civilisation. Unfortunately, the climate system is not cognisant of what will cause us disruption and does not self-regulate for our own interests.
“Don’t worry, something else will stop the climate system warming too much”
The other argument put forward against climate change is that of a “missing climate forcing”.
Climate forcing is an external mechanism, such as large volcanic eruptions, that can alter the climate system and cause warming or cooling. These are distinct from feedbacks, but frequently confused on the web.
The “missing climate forcing” argument contends climate scientists have completely botched their understanding of what factors push and pull the climate system. Alternatively, they have neglected to account for some very important, additional mechanism that either explains 20th century changes or limits future warming.
Again, the natural limit on future warming to benign levels is disingenuous. But does the idea of “a missing forcing” have any credence when it comes to explaining 20th century changes?
One of the more popular “missing forcings” in the blogosphere is the cosmic ray hypothesis.
The cosmic ray hypothesis
The base proposition is that cosmic rays, or charged particles from space (predominantly protons), enhance the formation of tiny new particles in the upper atmosphere.
Clouds need particles (aerosols) to form. Cosmic rays have been proposed as a factor that assists cloud formation and that enhances the reflectivity of clouds.
The idea of cosmic rays as a missing forcing goes a little bit like this …
Before the 20th century, the Earth was bombarded with more cosmic rays than at present. This resulted in greater cloud cover and higher cloud reflectivity. This meant less sunlight at the surface of the Earth and a cooler climate system.
Over the course of the 20th century, changes in the sun’s magnetic field caused a reduction in cosmic rays reaching the Earth, which in turn meant fewer clouds. Fewer clouds mean more sunlight reaching the surface. This chain of causality is proposed as an explanation for the global warming trend measured by instruments over the past century.
While observations do not support this claim, an interesting experiment was recently carried out by physicists at CERN seeking to improve our understanding of cosmic rays and clouds.
The actual science regarding new particle formation (nucleation) is interesting in itself. But this experiment is particularly interesting because of the reaction it attracted in the blogosphere, as to the implication of these results for climate change science.
The cosmic ray experiment at CERN
First, let’s take a look at the experiment and what the researchers discovered.
The design of the experiment was to keep a chamber (shielded from cosmic rays) at a fixed relative humidity and then introduce increasing concentrations of sulphuric acid at three different temperatures (approximately -20, 5 and 20° Celsius, representing upper middle and lower levels of the atmosphere). Sulphuric acid is an important substance in the formation of particles (sulphate aerosols) in the real atmosphere.
The researchers found the production of new particles increased with increasing sulphuric acid concentrations and decreasing temperature. The cosmic ray shielding was removed and the nucleation rate increased by up to a factor of ten. This is a new and novel result.
Even more interesting was that, even though they took measures to evacuate all other substances from the chamber, the nucleated particles all contained nitrogen, which is absent in sulphuric acid and water. They found that the nitrogen had originated from ammonia, which was present in undetectable quantities, but had participated in the nucleation process.
The CERN team subsequently repeated the experiment, but this time added small (although measurable) quantities of ammonia to the chamber. Again, the cosmic rays resulted in a tenfold increase in the nucleation rate. The addition of ammonia, however, increased the nucleation rate by 100-1000 fold.
Therefore, ammonia is obviously very important for the production of new particles, as are cosmic rays.
All in all, this was a well-constructed experiment. The results have provided unprecedented observations of the initial stages of new particle formation and invaluable empirical information for scientists working on theoretical and computer models of atmospheric nucleation.
Is this significant for our understanding of climate change?
Now let’s turn our attention to the blogosphere, where it has been claimed these results strengthen the hypothesis that recent warming can be attributed to cosmic rays; or at least a lack of them.
In fact, by itself, the CERN experiment showed there may be a link between cosmic rays and the formation of aerosols in the atmosphere.
The experiment did not demonstrate cosmic rays influence cloud production specifically. Further, the experiment did not show that increases or decreases in cosmic rays cause a measurable impact on climate.
Further still, the experiment in no way demonstrated that decreases in cosmic rays over the 20th century caused recent global warming.
To evaluate the impact of cosmic rays on climate, their effect on clouds needs to be weighed against the many other processes that we know affect cloud cover and cloud reflectivity.
The link between new particle production and cloud seeds
The particles the CERN researchers produced in the laboratory were a few nanometres in radius (one billionth of a metre). These particles are known as condensation nuclei (CN).
Out in the real atmosphere, under typical conditions, a particle must be (at least) 50 nanometres in radius to act as a “seed” for a cloud droplet. These particles are known as cloud condensation nuclei (CCN).
For a condensation nuclei to become a CCN, and capable of seeding a cloud, it must increase its radius approximately tenfold and its volume 1000-fold.
Did the increase in CN concentration which CERN researchers observed in the laboratory also imply an increase in the number of CCN? We still don’t know the answer to that.
Moreover, if there is a significant increase in CCN from cosmic rays, does it provide comparable numbers of CCN to those produced by other sources? There are many natural and human sources of aerosols. These include forest fires, volcanic eruptions and industrial pollution, all of which produce CCN in the atmosphere.
In fact, there is no shortage of CCN in the atmosphere. Based on our current understanding, the direct emission of CCN from all other sources would far outweigh the possible production of CCN from cosmic rays.
The link between increased CCN and clouds
The link between changes in aerosol particles and subsequent changes in the reflectivity and the area of clouds is worthy of separate discussion in itself.
But we can summarise here that satellite and upper atmosphere measurements have demonstrated that increased particles in the atmosphere can modify the reflectivity of clouds. There is also evidence that aerosols can increase local cloud amount.
If global increases or decreases in cloud reflectivity or cloud amount are large enough, they will impact on the Earth’s radiation budget.
In terms of climate change in the past 100 years or more, our current understanding is that observed increases in atmospheric aerosols – mostly from industrial pollution and biomass burning – have had a net cooling effect on climate; partially counteracting warming due to increasing greenhouse gases.
Some of this is due to the direct effects of aerosols on solar radiation, and some is due to the effects of aerosols on cloud properties (the so-called “indirect” effects of aerosols).
But such particles are short lived, and greenhouse gases will remain in the atmosphere long after the aerosols have disappeared.
To date, there is no body of work or convincing individual study that shows that changes in the amount of cosmic rays reaching Earth have an appreciable effect on cloud extent or cloud reflectivity.
So what’s the conclusion?
Perhaps the most conclusive piece of evidence against a role for cosmic rays in explaining 20th century changes, is the direct observations of cosmic rays and clouds themselves.
It has long been recognised within the field (as opposed to the blogosphere) that trends in cosmic rays are in the wrong direction to explain global warming in recent decades. That is, even if the relationships in the laboratory can be extrapolated to the real world, the effect of cosmic ray changes is more likely to have cooled the planet.
It is also a fact that there is no observable downward trend in global cloud cover.
Considering all the evidence then, one has to jump through many hoops to place cosmic rays as a significant driver of 20th century climate. The required leaps of logic are both unconvincing and unpublished.
So there is little evidence to suggest that observed global warming has been caused by cosmic rays.
The hypothesis also studiously ignores the large range of evidence showing increasing carbon dioxide as the dominant cause of 20th century global warming.
Such claims on the web are, therefore, perhaps hopeful at best and misinformation at worst. Linking these claims to the recent work at CERN does a great disservice to the interesting and important results from those researchers.
Taken in overall context, the impact of cosmic rays is not likely to have a large or significant influence on future climate change either.
As with all arguments put forward against our understanding of the enhanced greenhouse effect, cloud uncertainties and the cosmic ray hypothesis provide absolutely nothing to suggest it is OK to go ahead and double atmospheric carbon dioxide concentrations over the next century.
Thanks to Dr Leon Rotstayn of CSIRO Marine and Atmospheric Research for providing an expert review and valuable suggestions for this article.