THE LIVING SKIES:
CLOUD BEHAVIOUR AND ITS ROLE IN CLIMATE CHANGE
Oliver Morton
April 2002
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ABSTRACT
The clouds above have always been a worldwide source of poetic inspiration, quotidian joy and curious speculation. They are also, at the moment, central to questions of great environmental, economic and thus political importance. The behaviour of clouds and the processes that produce them in the face of increased atmospheric greenhouse gas concentrations [1] will be a crucial element in future climate change, and as yet that behaviour is not well understood. It is possible that changes in cloud formation and structure could act to reduce the climatic effects of greenhouse gases quite markedly, and thus reduce the need to abate their production. Were this the case, it would have a profound influence on the current efforts in many countries to implement the Kyoto Protocol (the global accord outlining actions to be taken to minimize climate change) and develop global successor regimes to further reduce greenhouse gas emissions. However it is also possible that clouds will not reduce the effects of increased greenhouse gases, or will do so no more than is envisioned in current predictions, or will even exacerbate them.

The problem, briefly stated, is this. It is not yet possible to have confidence in the predictions made by General Circulation Models (GCMs) — computer driven representations of the climate system — about how cloud cover will change in a world with significantly higher levels of atmospheric greenhouse gases. It is possible that these models, which make use of a physics-based understanding of the processes that govern the movement of radiation and moisture into, through and out of the atmosphere, are simply not yet detailed enough in their representation of the processes involved to capture their fundamental behaviours under changed conditions. Since they cannot model the climate raindrop by raindrop, or even cloud by cloud, these models have to simplify, and in their current form that simplification may be producing quite large errors.

Another possibility is that processes and factors not yet dealt with in the models, processes which, to the extent they are understood, are understood as topics for chemistry or biology, may play an important role. While it is with this second type of consideration that interdisciplinary issues arise most clearly, a realistic look at future directions in cloud research needs to deal with both possibilities for two reasons. The first is that it is not possible, a priori, to say that the solution to the problem cannot come from within the community of scientists already studying it; that is, without interdisciplinary input. The second is that it is impossible to understand the role interdisciplinary input might have without first understanding the issues within the field and the approaches being taken to address them.

While the representation of clouds in models of greenhouse-gas rich climates is a recognised research priority, and of fundamental importance to policy, this report will also discuss the fact that changes in cloud cover brought about by non greenhouse-gas processes–notably changes in the production and behaviour of aerosol particles due to a variety of human, geological, chemical, biological and even, conceivably, astrophysical factors–may lead to climate changes at the regional level as significant in some cases as the changes due to greenhouse gases. To understand clouds one needs to understand the sources of the dust they form on, the chemistry of the droplets within them, the degree of control over their environment exercised by any microbes living in the cloud water, and the role, if any, of cosmic rays that ionise the lower atmosphere. Here the need for interdisciplinary approaches is obvious from the start.

Clouds, as has been pointed out by both Hamlet and Charlie Brown, are the natural world’s great Rorschach tests. What they look like depends on what you want to see in them, or what’s suggested to you, or what you think other people want you to see. They can be camels or weasels or whales. They can look like an expression of purely physical processes, or the interaction of dust and dew, or a fog of chemical reactions, or the abode of life, or the result of far-off astrophysical events. The truth is that they are complex enough to be all of those things. That does not mean all approaches to studying clouds are equally valid, but it does mean that they are worth exploring and bringing together, to see if they offer insights to each other. Thus it may be that microbiology has little of use to add to the debate among climate modelers about the accurate representation of clouds, or to predictions of regional climate changes - but current understanding cannot say that for sure. The best way to find out is to explore the possible microbiology of clouds with climate issues clearly in mind. This is the sort of interdisciplinary approach that seems most likely to add to the understanding needed to help with policy formulation.

The question of how clouds behave in future climates is of primary importance to all predictions of climate change, and answering that question -and applying the answer to consequent regulatory questions about the need for control of greenhouse gases, industrial aerosols and changes in land use - will require the ability to combine an understanding of clouds gained from various different disciplinary approaches. This monograph seeks to set out how such combinations are currently developing, and to suggest paths that might fruitfully be followed in the future.