Earthmasters: The Dawn of the Age of Climate Engineering. By Clive Hamilton. Yale University Press; 247 pages; $28 and £20. Buy from Amazon.com, Amazon.co.uk

A Case for Climate Engineering. By David Keith. MIT Press; 194 pages; $11.95 and £10.95. Buy from Amazon.com, Amazon.co.uk

EVERYONE knows what must be done about climate change, but no one is doing anything about it. More than two decades of speeches and summitry have failed to thin out emissions of greenhouse gases. In fact, emissions are accelerating: a quarter of all the carbon dioxide ever pumped into the air by humans was put there in the decade between 2000 and 2010. It will hang around for centuries, meaning that the future is sure to be hotter, even if all greenhouse-gas emissions cease overnight. The official ambition of limiting the global temperature rise to 2°C looks increasingly like a bad joke.

Officially, the plan is still to fix the problem by cutting emissions at their source; cleaning up the factories, cars, ships and homes whose engines and heating systems pump greenhouse gases into the air. But what if there were another way? In 2006, depressed by the lack of progress on emissions, Paul Crutzen, an atmospheric researcher, broke a long-standing taboo among climate scientists by publicly pointing out that if humans have the power to heat the planet, then they also have the power to cool it down again. While environmentalists wring their hands about the effects of industrial civilisation, a number of “geo-engineers” are advocating seizing control of the climate, tinkering with Earth’s atmosphere or its surface in an attempt to slow the planet’s heating.

It sounds like some impossible technocratic fever-dream from the 1950s. But two books argue that it is not. Among the ideas about planetary cooling are artificially brightening ocean clouds to increase the amount of sunlight they reflect into space and building machines that suck greenhouse gases directly from the air. Most are poorly understood, expensive or both. But one idea looks so cheap and technologically convenient that, as far as anyone can tell, there is nothing to stop it being started almost immediately. This would be to use fleets of aircraft to slightly dim the Sun, mimicking the after-effects of volcanic eruptions by filling the upper atmosphere with a fine haze of sulphate particles.

Both Clive Hamilton, an Australian philosopher, and David Keith, a Canadian physicist, agree that the physics and the engineering of such a global dimming are plausible. Both also agree that it would be a bitterly contentious move, politically tortuous and possibly even self-defeating in the long run. There, though, the agreement ends. Mr Hamilton believes geo- engineering is a bad idea: politically unworkable, hubristic and ethically dubious. Mr Keith argues that it may be a good idea: morally attractive, workable and affordable. Applied with caution, it may buy time to build a low-carbon civilisation.

To say that geo-engineering is controversial is an understatement. Mr Keith points out that shrouding the sky would be an uneven, imperfect fix. It could certainly reduce average global temperatures. But it would do nothing to stop other consequences of greenhouse-gas emissions, such as making the oceans more acidic. And although average temperatures would fall, that would hide a lot of regional variation. Reflecting sunlight only works during the day, whereas greenhouse gases warm the planet all the time. So a globally warmed and geo-engineered world would have warmer nights and cooler days than one in which levels of greenhouse gases had never risen in the first place. Sulphate-cooling is most effective in summer and least effective in winter.

That, in turn, would lead to variations in local weather patterns. It is here that some of the political difficulties become apparent. Climate change is contentious enough as an unintended side-effect of burning fossil fuels. How much more rancorous would it become if humans were seen openly to be controlling the weather?

Mr Keith imagines a geo-engineered world in which India has a run of dry years, with its seasonal monsoon rains falling short. It would be impossible to prove that geo-engineering was responsible, just as you cannot ascribe any particular typhoon to a warmed climate. But this would not matter much to the millions of hungry people looking for someone to blame. If humans are to take charge of controlling the climate, then they will be held responsible, fairly or otherwise, for its effects. Whose hands will be on the controls, and how will that person be chosen?

Another common worry is that geo- engineering is simply a way for the present generation to duck its responsibilities and dump them on its children. Mr Hamilton makes many important points, not least that whereas geo-engineering is seen as a new and dubious idea in the West, it is treated much more matter-of-factly elsewhere in the world, especially in China, which has a long history of attempts at weather control. But overall his book is weakest, oddly, on the ethical questions. His tone when discussing supporters of the idea too often veers into the contemptuous, which risks alienating curious readers. And non-expert readers may struggle with the occasional dollops of philosophical jargon (“the spatial metaphysics of the world”, for example) which should not have survived the editing process.

Mr Keith manages to keep the tone sober without ever sounding dull. His chapter on ethics deftly summarises some of the competing moral claims: many people may be richer in the future, for example, so cutting emissions will be relatively less costly for them. Meanwhile, today, billions of people aspire to the clean water, plentiful food and advanced health care common in the rich world; mandating clean energy will make obtaining those more expensive. And it is the poor—who lack the cash and resources to adapt—who are likely to feel climate change most keenly. Even immediate emissions cuts will do little to help them. Carbon dioxide lingers for so long in the atmosphere that Earth already faces centuries of warming. Geo-engineering, for all its many risks, might offer some immediate, temporary relief.

Reading about proposals to alter the climate of an entire planet on purpose is dizzying. Yet scientists already talk of the dawning of a new geological age, the Anthropocene, named because humans, or rather, the industrial civilisation they have created, have become the main factor driving the evolution of Earth. Both these books emphasise just how seriously the idea of deliberately altering the climate is being considered, both in scientific journals and among some governments. Mr Hamilton is an effective critic of a breathtaking idea. But Mr Keith is a better guide for the undecided.