There is international consensus that 'dangerous" climate change must be avoided. Yet without radical changes in energy sources and usage and global economies, changes that so far society has been unable or unwilling to make, it seems highly likely that we will start to experience unacceptably damaging and/or societally disruptive global environmental change later this century. What actions can be taken to safeguard future environmental quality, ecosystems, agriculture, economy, and society? A new science--'geoengineering" that until recently would have seemed pure science fiction, promises an alternative way of temporarily regaining control of climate. Colossal engineering schemes to shade the sun, make the atmosphere hazier, modify clouds, even throw iron into the ocean, are all being promoted as possible ways out of our dilemma. This article considers the state of this new science, and its" implications for society.
Keywords: geoengineering, bio-geoengineering, climate, global warming, C[O.sub.2], mitigation, terraforming, carbon sequestration, albedo
The idea of deliberately manipulating the climate of a planet has long been a recurring theme in science fiction writings. Known as 'terraforming', cold planets such as Mars, or even the Moon, are warmed by the addition of greenhouse gases to the atmosphere, or given increased sunlight by the positioning of giant mirrors in space, with the aim of making the planet habitable for life (Figure 1). In reverse, too-hot planets such as Venus would have their atmosphere progressively stripped of excessive greenhouse gases and the strength of solar radiation reduced by sunshades. The increasing awareness and concern about the potentially dire consequences of a much warmer future Earth has led to a recent explosion of interest in applying terraforming to Earth--'geoengineering'--the deliberate modification of the Earth's climate to counter-act global warming and climate change.
The history of geoengineering actually pre-dates much of the science fiction literature of terraforming. The Swedish scientist Svante Arrhenius (1859-1927), who first recognised the important link between carbon dioxide (C[O.sub.2]) in the atmosphere and climate (later termed the 'greenhouse effect'), apparently considered the climatic implications of industrial activities and the burning of coal rather good things because of the harsh winters in Sweden (1). He logically wondered whether coal should thus be burned more quickly to accelerate the warming! The first serious consideration of geo-engineering had to wait until the 1960s and 70s, when at the height of the Cold War the former USSR considered ways to warm its vast icy tundra in the hope of generating fertile farm land. The spirit of this planned intervention is captured in "Man Versus Climate", a book describing the weather and climate modification plans of the USSR at the time (1). Today, the feasibility and desirability of geo-engineering are being seriously assessed by researchers and governments as a means of altering the Earth's climate system to ameliorate (reduce) the global warming impacts of continuing fossil fuel C[O.sub.2] emissions (1).
In this paper, we review the engineering technologies that might give us the ability to retake control of our planet's climate and reduce global temperatures. We will discuss the climate benefits of these schemes, as well as the side-effects and risks they pose. We will also touch on the economics and ethics of geoengineering--critical facets of the debate, yet ones that have to date very much lagged behind the maturation of the physical science. We will also briefly outline the possibilities for addressing the root cause of global warming and how C[O.sub.2] might be removed from the atmosphere. But we will start by summarising the underlying science of climate change, which provides the background to how geoengineering schemes actually 'work' (i. …