Does carbon dioxide really attack ice cracks?
We know that rising temperatures can lead to melting of some of earth’s great bodies of ice – and given enough time, even their disappearance.
But a new study suggests that greenhouse gas emissions could affect the stability of ice in a much more fundamental way – at the atomic level. Theoretical modelling by physicists at the Massachusetts Institute of Technology suggests the presence of carbon dioxide makes ice weaker, and more prone to crack.
The modelling simulates on a tiny scale how molecules of carbon dioxide can interfere with water crystals in ice. But what the consequences are for larger scale ice melt remains to be seen.
The theory
Normally, water molecules in ice are attracted to each other because their constituent parts have a small electrical charge. The hydrogen in one molecule of water, which is slightly positive, is attracted to the oxygen in another molecule, which is slightly negative. These attractions help give ice its structure.
Image - Water (note)Image - Water Water (note)
Left: a water molecule. Right: attractions between water molecules. Source: The Carbon Brief Infographotron
When ice is damaged it cracks open. The researchers believe that this disrupts the normal attractions between water molecules in a particular way. When these attractions are disrupted, water molecules attract carbon dioxide instead, because carbon dioxide’s building blocks are also slightly charged. So the crack provides an opportunity for positive hydrogen in water to attract the negative oxygen in carbon dioxide.
Image - Carbon Dioxide (note)Image - Water Carbon (note)
Left: a carbon dioxide molecule. Right: attractions between water molecules and carbon dioxide molecules.
Once there’s an electrical attraction between the damaged water molecule and a carbon dioxide molecule, carbon dioxide chemically bonds to the newly exposed surface of ice. The more carbon dioxide available, the more of these bonds that can form.
The carbon dioxide molecule then moves towards the innermost part of the crack, where the ice is under greatest strain. In the process, the carbon dioxide bonds with, then breaks with, a lot of molecules of water. These broken water molecules weaken the ice and make it more brittle.
At the innermost part of the crack, where the attractions between water molecules are under the greatest stress, carbon dioxide accumulates. Its presence weakens the interactions between water molecules, and helps to break them open – propagating the crack deeper into the ice.
So how does this apply in the real world?
The authors suggest that this breakup of ice at the atomic level could alter the rate that ice melts. In theory, more carbon dioxide could lead to more ice fracturing, and as ice breaks into smaller pieces, more of it is exposed to warm air and it can melt more quickly.
Scale this process up, and there could be some implications for the climate, perhaps? Lead author of the study, Markus Buehler, explained:
“The consequences of these changes remain to be explored by the experts, but they might contribute to changes of the global climate”
Of course, scientists’ ability to simulate these changes doesn’t necessarily mean they are actually playing out in the real world. While the press release does make it seem as though the study has implications for global climate change, the authors say that was never the intention of the research – Buehler clarified in an interview:
“We look at tiny crystals of ice. Glaciers are much, much larger, so our research might or might not be relevant at that scale. We’re not addressing climate change in this paper.”
The theory suggests that any amount of carbon dioxide helps to propagate cracks in the ice. But it’s worth noting that the levels of carbon dioxide simulated in this study are much higher than concentrations in the atmosphere, or even levels projected by the end of this century.
We asked a range of scientists to comment on the paper. Most said that it was outside of the scope of their expertise. But Professor Peter Sammonds from University College London, who has previously worked on ice fracturing, said:
“A small change in carbon dioxide is unlikely to make a big difference.”
Early days
This is an interesting study, but as with a lot of early-stage scientific research, it needs to be discussed carefully. The conclusions are drawn solely from a model and haven’t been tested out on the ground yet – it’s unlikely that modelling cracking ice by looking only at the impact of increased levels of carbon dioxide will give a complete picture.
However, no matter how significant this particular process ends up being, carbon dioxide and other greenhouse gas emissions will continue to affect earth’s big bodies of ice in a less direct but much more well established way, driving temperature rise which puts the planet’s ice sheets under pressure.
Carbon dioxide enhances fragility of ice crystals
Zhao Qin & Markus Buehler