Preface

Interest in the Earth's climate is at an all-time high, due in large part to concerns that it may be changing to a degree that will soon adversely affect the habitability of the planet. This realization coincides with rapid improvements in the tools needed to study and understand the physical processes that produce and maintain the climate, including remote-sensing instruments and observing platforms in space, and fast computers and sophisticated algorithms to analyse large amounts of data and to run advanced theoretical models.

The radiation that reaches the Earth from the Sun and its interaction with the Earth's atmosphere plays a critical role in determining the climatic conditions that prevail over the globe. The Earth's surface-radiation budget drives the water content of the atmosphere and with it the dominant component of the natural greenhouse effect that serves to maintain the benign climate on our planet. Variations in the incoming solar radiation and in the composition and cloud content of the atmosphere and ice cover of the surface result in climatic variability on timescales of billions of years to less than a century. Recently, human activity has begun to modify the Earth's climate, through emissions of greenhouse gases, like carbon dioxide and methane, and of aerosols, that alter the radiative balance and generally warm the planet, but can also cool it depending on the exact circumstances.

The net radiation at the top of the atmosphere is now extensively monitored by satellites but the radiation budget at the Earth's surface (which determines the surface temperature, the single most important climate variable) can only be indirectly derived from radiation-transfer models that require detailed clima-tological data, such as cloud and aerosol radiative properties, water and temperature profiles in the atmosphere, and surface properties. These quantities can be measured in different ways, but the only approach that results in global and seasonal coverage with sufficient detail is the use of remote-sensing instruments on satellites, i.e. by the detailed spectral analysis of the outgoing radiation field as well as its energy flux.

Climate models aim to quantify the radiation changes or forcings that arise from natural variations, for example in solar luminosity, and anthropogenic effects such as pollution-induced changes in atmospheric composition, cloudiness and aerosol distribution. Long-term prediction and monitoring of the Earth's radiation budget and climatic change therefore require reliable radiation-transfer computations. The methods by which this can be achieved require not only a detailed understanding of the quantum-mechanical interactions of molecules and photons of various kinds, but also the development of computationally efficient algorithms that quantify the transfer of energy as radiation throughout the atmosphere, especially in the vertical direction and in the presence of clouds and aerosols.

Atmospheric composition itself controls both the solar and terrestrial radiation fields and the thermal structure, and in turn the radiation field determines the photochemistry and composition of the atmosphere. There is thus a strong coupling between atmospheric photochemistry, atmospheric composition and climate.

The purpose of this book is to provide advanced students and researchers with an understanding of the radiation processes that control the Earth's climate and climatic change, the methods used by remote-sensing systems for climate measurements and monitoring, and the theoretical principles involved in incorporating radiative processes into climate models. It aims to provide a picture of the recent and current state of the Earth's radiation budget and the physico-chemical processes that determine it.

Radiation and climate is a field of research that encompasses many subdisciplines in Physics, Chemistry, and Astronomy. Lessons learned for the Earth are also valuable to the understanding of climate on other planets, and, as we can now study the latter in detail, vice versa. The book concludes by setting the study of the contemporary terrestrial climate in context with the conditions that are thought to have developed over the history of the planet, and with those recently revealed on our neighbours in the Solar System.

I.M. Vardavas and F.W. Taylor

Heraklion

October 2006

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