Extensive water and ice clouds are a unique feature of our planet. On average, more than half of the globe is covered by visible clouds. They control major parts of the energy balance of the Earth system, both with respect to the incoming short-wave solar radiation and the outgoing long-wave thermal radiation. Latent heating and cooling related to cloud processes modify the atmospheric circulation. By modulating sea surface temperatures, clouds affect oceanic circulation as well. Clouds are an essential component of the global water cycle, on which all terrestrial life depends.
Humankind is perturbing the Earth's cloud system through industrial activities and surface changes. Regionally, the most obvious (but not necessarily most relevant) perturbations are the frequently visible contrails resulting from aircraft emissions. Anthropogenic cloud perturbations can even be seen from space through ship tracks in persistent low marine clouds. The main recognized mechanisms of anthropogenic perturbations of clouds are:
In the current climate epoch, clouds are the dominant controllers of the albedo of Earth because of their high reflectivity for sunlight, or albedo (ca. 0.5), and their geographical coverage (over 50%). Along with greenhouse gases (CO2, CH4, etc.), clouds play a major role in the control of outgoing long-wave radiation (OLR). Because of the enormous emphasis and effort devoted to greenhouse gases, their concentrations are accurately monitored and their climate forcing is now precisely predicted. In contrast, clouds are still poorly represented in climate models, and parameterizations of their various feedbacks on the climate system are still missing. Despite the generally accepted importance of clouds for the Earth system and their high susceptibility to anthropogenic forcings, our knowledge of the spatio-temporal cloud distribution is poor. Global cloud climatologies are under development but are far from definitive. The total effect of clouds on the energy balance is highly uncertain. The counteracting short- and long-wave radiative effects of clouds as functions of time and space are not well understood. In terms of numerical weather prediction, forecast skills are lowest for clouds and precipitation; significant progress has not been achieved over the past decade. Whereas human effects on clouds can be clearly discerned in a few extreme cases, the subtle but much more important large-scale and long-term effects of anthropogenic emissions are incredibly difficult to separate from the inherent high variability of cloud properties. Even the definition of what clouds are can be debated. The textbook definition states that clouds are "visible aggregations of minute water and ice particles." Also visible are haze aerosols — even from space — with ensuing global effects on the energy balance. Hygroscopic aerosol growth below 100% relative humidities causes a continuum of atmospheric hydrometeors reaching from dry aerosols to what are traditionally called clouds. Presently, we lack the necessary sensitive and globally available instrumentation. Also, techniques for transferring small-scale measurements to larger scales are missing. Clouds thus remain the largest source of uncertainty in the prediction of climate change.
There are solid reasons for our very limited knowledge of clouds and related processes. Clouds involve spatial scales from sub-millimeter to hemispheric and temporal scales from sub-second to seasonal. Thus, cloud climatologies ideally require three-dimensional global monitoring systems, and these are not presently available. Clouds also comprise the most complex geophysical multiphase systems in which microphysical, chemical, dynamic, and radiative processes need to be considered concurrently. Thus, the totality of cloud processes requires multidisciplinary analytical systems that do not yet exist, at least not for each cloud environment, with all the necessary sensitivities and resolutions. The space-borne possibilities for cloud studies that are required for global monitoring can only address a very limited subset of cloud parameters necessitating complementary in situ investigations.
To meet this challenge, we propose this Forum to assess the limits of our understanding of clouds and anthropogenic perturbations of them and to formulate strategies for reducing the critical uncertainties. The interdisciplinary character of an Ernst Strüngmann Forum offers a unique opportunity for critical assessments of present knowledge concerning a highly complex key issue in the discussion of global change. This assessment is urgently needed to delineate future research directions.Top of page
The Earth's radiation budget is very sensitive to the amount of clouds, their liquid-water and ice-water paths, their crystal types and drop sizes, their base and top heights, their horizontal extent and horizontal variability, and their diurnal cycles. Top-of-atmosphere radiative forcing by doubled CO2 concentrations can be balanced by the increased reflection of sunlight that would be caused by a modest increase of ~15 – 20% in the amount of low clouds. Consequently, for the quantification of the absolute amount of low clouds, Slingo has stated an accuracy requirement of ~1%.
In general, clouds form when air is cooled and the relative humidity rises above a critical value. Cooling is caused mainly by upward vertical motions and radiative loss, and cloud particles form only on pre-existing aerosol particles. Clouds are thus complex multiphase systems within the atmosphere controlled by dynamical, microphysical, chemical, and radiative processes. Possibly, even biological processes need to be considered. This multitude of cloud-related processes and the presence of clouds throughout the troposphere allow for a large number of feedbacks between cloud processes and the rest of the Earth system that are not fully understood. The complexity of clouds impedes their understanding because most often only the net effects of all cloud processes can be observed without being able to separate individual controlling factors. Consequently, atmospheric observations need to be complemented by numerical modeling and laboratory simulations.
Anthropogenic perturbations of clouds have long been a focus of global climate research. Since the work of Twomey, increasing interest has focused on the role of the atmospheric aerosols, although the basic question of how clouds respond to a warming climate remains unresolved. Because of the inherent variability of clouds and the high number of controlling parameters, attempts to quantify these effects are semi-quantitative at best. Global results have been sought based on numerical models with crudely parameterized aerosol/cloud interactions. To move beyond simulations of the first aerosol indirect effect, more complex parameterizations are being developed. There is an obvious need to integrate better the extensive amounts of surface laboratory and satellite-acquired data into this process. Regarding intentional modification of clouds, careful analyses of mechanisms are required, along with assessment of complex, often fragmentary observational evidence of the efficacy of field tests or operations.
Cloud parameterizations in climate models are frequently tuned to match satellite-acquired radiances with anthropogenic forcings limited to the indirect effects of aerosols on cloud albedo lifetime and extent and feedbacks limited to those that can act through changes to the large-scale state of the system. The next generation of numerical models will allow for investigations of feedbacks mediated by cloud-induced changes to the natural aerosol, or land surface, as well as effects associated with changes to cloud- and meso-scale circulations. This will result from rapidly improving spatial resolution (global cloud resolving models) and more extensive representation of the Earth system. Both advances require revisions in our conception of the parameterization problem for clouds and cloud-related aerosols.
Jost Heintzenberg and Robert J. Charlson
Winner of the 2009 Atmospheric Science Librarians International Choice Award