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Discovery of ice’s rough surface may help explain water’s true role in future climate change

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Discovery of ice’s rough surface may help explain water’s true role in future climate change

Cirrus or Mares Tails Clouds against a blue sky

•    First observation of kinks in cubic ice that forms high altitude cirrus clouds may affect our ideas of how the Earth reflects the Sun’s energy to keep cool

•    Findings from Universität Göttingen researchers working at Institut Laue-Langevin, published in PNAS, further demonstrates the need to review accuracy of today’s climate models

The contribution of clouds to global warming is more than 50 times that of man's contribution of CO2 to the atmosphere. Thin, wispy cirrus clouds found in the upper atmosphere play an important, but uncertain role in the Earth’s climate system. Covering around 25% of the Earth, they are known to be very effective absorbers of outgoing infrared radiation, trapping heat within our atmosphere, a process, which has a significant effect on global warming. What is less well understood is how this balances with the amount of the Sun’s radiation that cirrus clouds ‘backscatter’ back into space before it even reaches Earth, a mystery listed in the IPCC report for future climate prediction as one of the major unknowns in climate modelling.

At these high altitudes and low temperatures some of the H20 that makes up Cirrus clouds can take the form of cubic ice, where the oxygen atoms are arranged in a diamond structure, as opposed to the more well-known hexagonal ice crystals that dominate at lower altitudes and form the majority of other clouds.

Whilst lower altitude clouds with rain droplets have been modelled successfully, limited understanding of the structure and properties of cubic ice which cannot be reproduced exactly, has led to hexagonal ice being used in climate models to represent cloud formation at both high and low altitudes. This is despite increasing evidence suggesting Cirrus clouds ‘backscatter’ more solar radiation back into space than lower altitude clouds formed from truly hexagonal ice, calling into question the accuracy of these models.

In order to address uncertainty around the structure of cubic ice, a research team from the Universität Göttingen and the Institut Laue-Langevin (ILL), lead by Prof Werner Kuhs, analysed samples of cubic ice using ILL’s powder diffractormeter known as D20, considered the world’s best machine for this type of measurement. This analysis was conducted at temperatures found in the upper atmosphere, ranging from 175 to 240K, and over timescales of several hours.

Normal hexagonal ice crystals have a very smooth surface and it had been generally assumed that cubic ice was composed of neat crystals of cubic symmetry. However the team revealed for the first time a series of kinks within the cubic ice structure that result from its layers of hydrogen and oxygen stacking on top of each other with a slight offset which gives the ice a rough surface.

Whilst the exact effect of cubic ice’s rough surface on the Earth’s heat balance is uncertain it may help explain the different interactions between solar radiation and clouds in the upper and lower atmosphere.  It also confirms the need to update today’s climate models to reflect the change in the structural nature of water as you move higher in our atmosphere.

Prof Kuhs’ findings may answer a second problem in climate science known as the super saturation puzzle describing the unexpected existence of highly super-saturated air in the upper atmosphere.

Kuhs and his colleagues also looked at changes in the defect structure over time and demonstrated that it decreases as you increase the temperature. The cubic ice they observed was continuously turning into the more stable hexagonal structure, a process they believe must be repeated within cirrus clouds in the upper atmosphere.


Prof Werner F. Kuhs said: “Whilst one might assume that water would be one substance that we know almost everything about, this study of its various natural forms shows we have still much to learn. If science is to accurately predict the future of our climate and how our actions as humans might affect it, we must start to improve our understanding of the basic properties of this key component of the Earth’s global heating system.”

Dr Thomas Hansen who worked with Prof Kuhs on this paper said: “We needed a rather high angular resolution of the diffraction patterns in order to see the fine details of the defects in the ice. At the same time we also needed a good time resolution in order to observe the change in structure in response to temperature. Usually these are mutually exclusive, but D20 at ILL is a place which can give you sufficiently good data for both.”

Re.: PNAS, vol 109 n° 52 : Extent and relevance of stacking disorder in “ice Ic"

Contact: Mr James Romero  +44 8456801866