Cosmic rays are charged particles that come from outer space and bombard the Earth’s atmosphere. These particles usually decay in the early stages of the Earth’s atmosphere, raining down a shower of product particles onto the Earth’s surface. If you have never heard about cosmic rays, this may seem very alarming. But the fact is that these particles have been showering over you for your entire life, and can cause little harm. However, a collaboration of scientists from across the globe have been wondering whether this continuous barrage of extra terrestrial particles has an effect on the Earth’s atmosphere, and more particularly its cloud formation. To investigate this question, the Cosmics Leaving Outdoor Droplets (CLOUD) experiment, was formed.
Since 1750 the Earth’s surface temperature has risen by 0.8 degrees, a deceivingly small number which has had a very large effect on the climate of the Earth. Since then, and more fervently in the past few decades, scientists have been trying to predict how the surface temperature of the Earth will rise in the next century. At the moment, our current estimate is that by 2096 the surface temperature of the Earth will have risen by 1.5-4.5 degrees since 1750.
The keen eyed reader might notice that this is a very large error, seeing as the temperature has only risen by 0.8 degrees in the past 250 years. The difference in these temperatures is huge, and will effect what precautions, and preparations the human race must put in to survive the different temperatures. For scaling purposes, one would expect that the lower estimation would be an ‘okay’ situation. Some areas of the Earth would become too hot for production, and will be essentially inhabitable, however other areas will grow in production and will become more habitable, almost balancing each other out. However, the upper estimation is a very different story indeed. This temperature rise would be comparable to that of the temperature rise since the last Glacial Maxima: a time when most of the Northern Hemisphere was under 2km of ice. Not a promising outlook for the Earth.
So it is clear that scientists must make their estimation more precise, so that the human race can prepare for the clear track that the Earth’s climate will take. In order to do this, scientists must understand something called the climate sensitivity. This is the ratio of temperature change to a radiative forcing (essentially the difference in the amount of radiation absorbed, to the amount of radiation emitted by the Earth and its atmosphere).
Since 1750, the pre-industrial era, the human race has increased the amount of CO2 in the atmosphere. This molecule has a good ability of absorbing heat from the Sun and warming the Earth’s atmosphere. It should be noted that CO2 is natural in the atmosphere, but the increased amount produced by the human race is not, and causes the Enhanced Greenhouse Effect. However, since 1750, the human race has also released a lot of aerosols into the atmosphere, which form clouds. Clouds cool the atmosphere by reflecting radiation from the Sun. Measurements of ice from Antarctica can tell us the CO2 levels in the atmosphere of the pre-industrial world. However, we do not have a way of telling how cloudy it was at this time. This is the main source of the uncertainty in our calculation of climate sensitivity.
Atmospheric aerosols are particles that are suspended in the air, and can be liquid or solids. They come in two types: primary, which include dust particles and carbon particulates from burning; or secondary, formed by particle nucleation. These aerosols are the seeds of clouds: cloud droplets can form around the particles, which then clump together to make clouds. Without aerosols, there would be no clouds, and without clouds there would be no life! However, the mechanism in how aerosols are created in the atmosphere is poorly known. In fact, scientists know very little about aerosol’s effect on clouds themselves.
This is where CLOUD comes into play. In its first five years of operation, the CLOUD experiment has already identified the vapours responsible for the formation of many aerosols. The CLOUD experiment is a 3 metre high cloud chamber which is situated in one of the test beam facilities at CERN.
This is the first experiment that has been able to study cloud formation under precise conditions, and is lighting the way in understanding cloud formation. This is the first goal of CLOUD: exploring how cloud formation has changed since the industrial era, and cracking down on the uncertainty of climate predictions. As a particle physicist I find CLOUD’s second goal extremely interesting – do cosmic rays effect cloud formation in the atmosphere? The barrage of charged particles from outer space changes over time with the Sun’s activity. Events known as Forbush decreases happen when the Sun releases a lot of plasma into the solar system. This blocks galactic cosmic rays, and thus the number of cosmic rays incident on the atmosphere is less. Could this be an unexpected form of radiative forcing?
CLOUD investigates this by using the proton beam from the Proton Synchrotron (PS) at CERN as an artificial source of cosmic rays. By firing this beam of high energy charged particles at the cloud chamber, scientists working on CLOUD can analyse the effect of this beam on the formation of the clouds within the chamber. Several recent papers have linked a change in cloud formation and characteristics with Forbush decreases, but the results from CLOUD should confirm whether our hypothesis that cosmic rays effect clouds is correct or not.
This ambitious experiment looks to change our view on climate change completely, and could dramatically improve the predictions of climate change. An extremely important task, and one which will benefit the human race in generations to come, CLOUD is one of a kind!
With thanks to CERN, and the CLOUD experiment. •