Some scientists and policy makers are therefore proposing that we take seriously the idea of geoengineering - that is, large-scale manipulations of the earth, such as spraying sulfate aerosols into the stratosphere to alter the reflectivity of the planet or fertilising the ocean with iron to spur blooms of carbon-sucking plankton. However, geoengineering seems too risky. Many of the technologies involved have never been employed on such a large scale, which means that we could be endangering ourselves or future generations. Indeed, spraying sulfate aerosols could destroy the ozone layer and iron fertilisation could promote toxic planktons and destroy all forms of marine life.

I propose that we consider another solution to the problem of climate change that has not been considered before and that is potentially less risky than geoengineering. Elsewhere my colleagues and I have called this solution ''human engineering''. It involves the biomedical modification of humans to make us better at mitigating, and adapting to the effects of, climate change.

Before I explain the proposal, let me make clear that human engineering is intended to be a voluntary activity - possibly supported by incentives such as tax breaks or sponsored healthcare - rather than a coerced, mandatory activity. My colleagues and I are positively against any form of coercion of the sort that the Nazis perpetrated in the past (segregation, sterilisation and genocide).

Also, this proposal is intended for those who believe that climate change is a real problem, and who, as a result, are willing to take seriously geoengineering. Someone who doesn't believe that climate change is a real problem is likely to think that encouraging people to recycle more is an overreaction to climate change.

Finally, the main claim here is a modest one, namely, human engineering should be considered alongside other solutions such as geoengineering. The claim is not that human engineering ought to be adopted as a matter of public policy. This is an attempt to encourage ''outside the box'' thinking vis-a-vis a seemingly intractable problem.

Here are four examples of human engineering that seem feasible to implement in the near future and potentially have broad appeal.

Pharmacological meat intolerance

The UN Food and Agriculture Organisation estimates that 18 per cent of the world's greenhouse emissions (in CO2 equivalents) come from livestock farming, a higher share than from transport. Close to 9 per cent of human CO2 emissions are due to deforestation for expansion of pastures, 65 per cent of nitrous oxide is due to manure and 37 per cent of methane comes directly or indirectly from livestock.

Some experts estimate that each of the world's 1.5 billion cows alone emit 100 litres to 500 litres of methane a day. Since a large proportion of these cows and other grazing animals are meant for consumption, reducing the consumption of these kinds of red meat could have significant effects on the environment. Indeed, even a minor (21 per cent to 24 per cent) reduction of red meat consumption would achieve the same reduction in emissions as the total localisation of food production; that is, having zero ''food miles''.

Now, some people will simply refuse to give up eating red meat. However, there are others who may be willing to give up eating red meat, but they lack the motivation or willpower to do so. After all, many people find the taste of red meat simply irresistible, which may explain why many vegetarian restaurants offer dishes that taste like meat.

Human engineering could help here. Just as some people have a natural intolerance to milk or crayfish, it is possible artificially to induce mild intolerance to red meat by stimulating the immune system against common bovine proteins. The immune system would then become primed to react to them, and henceforth eating ''eco-unfriendly'' food would induce unpleasant experiences. Even if the effects would not last a lifetime, the learning effect is likely to persist for a long time. A potentially safe and practical way of inducing such intolerance may be to produce ''meat'' patches - akin to nicotine patches. People can then wear these patches before they eat to curb their enthusiasm for red meat. To ensure that these patches have the broadest appeal, we can produce patches that just target animals that contribute the most to greenhouse gas emissions.

Making humans smaller

Human ecological footprints are partly correlated with our size. We require a certain amount of food and nutrients to maintain each kilogram of body mass. The larger one is, the more food and energy one requires. Larger people also consume more energy in less obvious ways. A car uses more fuel each kilometre to carry a heavier person than a lighter person; more fabric is needed to clothe larger than smaller people; heavier people wear out shoes, carpets and furniture more quickly than lighter people.

A way to reduce this ecological footprint would be to reduce size. Since weight increases with the cube of length, even a small reduction in height might produce a significant effect in size. (To reduce size, we could also reduce average weight. But I shall use the example of height.) Reducing the average height in the US by just 15 centimetres would mean a mass reduction of 23 per cent for men and 25 per cent for women, with a corresponding reduction of the metabolic rate (15 per cent/18 per cent).

How could height reduction be achieved? Height is determined partly by genetic factors and partly through diet and stressors. One possibility is to use preimplantation genetic diagnosis, which is now employed in fertility clinics as a means of screening out embryos with inherited genetic diseases. One might be able to use preimplantation genetic diagnosis to select shorter children. This would not involve modifying or altering the genetic material of embryos in any way. It would simply involve rethinking the criteria for selecting which embryos to implant.

Also, one might consider hormone treatment either to affect growth hormone levels or to trigger the closing of the growth plate earlier than normal. Hormone treatments are already used for growth reduction in excessively tall children.

Finally, there is a strong correlation between birth size and adult height. Gene imprinting - where only one parent's copy of the genes is turned on and the other parent's copy is turned off - has been found to affect birth size. So drugs or nutrients that either reduce the expression of paternally imprinted genes or increase the expression of maternally imprinted genes could potentially regulate birth size.

The last two methods are controversial as they involve making irreversible choices for one's children. But parents are permitted to give hormone treatments to their children, who are otherwise perfectly healthy, so that, for example, a daughter predicted to be 195 centimetres tall could instead be 183 centimetres tall. On what grounds then should we forbid other parents who want to give hormone treatments to their children so that their children could be 152 centimetres tall instead of 165 centimetres tall? It might be thought that in the case of the former, the daughters would later appreciate and consent to the parents' decision. But if climate change would seriously affect the well-being of millions of people including one's children, then these children may also later appreciate and consent to the parents' decision. It is also worth remembering how fluid human traits - such as height - are. A hundred years ago people were much shorter on average, and there was nothing wrong with them medically. We should be wary of the idea that there is an optimal height, namely, the average height in our society today, since this may simply reflect a status-quo bias.

Lowering birth rates through cognitive enhancement

Another obvious way to reduce ecological footprints is to lower birth rates. There are, of course, many available methods of curbing birth rates, such as the use of contraception. But there is strong evidence that birth rates decline as more women receive adequate access to education.

While the primary reason for promoting education is to improve human rights and well-being, fertility reduction may be a positive side-effect in tackling climate change. In general, there seems to be a link between cognition and lower birth rates. In the US, for example, women with low cognitive ability are more likely to have children before age 18. Hence, another possible human engineering solution is to use cognition enhancements, such as Ritalin and Modafinil, to achieve lower birth rates.

As with education, there are many other, more compelling reasons to improve cognition, but the fertility effect may be desirable as a means of tackling climate change. Even if the direct cognitive effect on fertility is minor, cognition enhancements may help increase the ability of people to educate themselves, which would then affect fertility and, indirectly, climate change.

Pharmacological induction of altruism and empathy

Many environmental problems are collective action problems, in which individuals do not co-operate for the common good. But if people were generally more willing to act as a group, we may be able to enjoy the sort of benefits that arise only when large numbers of people act together.

Pharmacological induction of altruism and empathy may help here. There is evidence that altruism and empathy have biological underpinnings. For example, test subjects given the prosocial hormone oxytocin were more willing to share money with strangers and to behave in a more trustworthy way. Also, a noradrenaline reuptake inhibitor increased social engagement and co-operation with a reduction in self-focus. Furthermore, oxytocin appears to improve the capacity to read other people's emotional state, which is a key capacity for empathy. This suggests that interventions affecting the sensitivity in these neural systems could increase the willingness to co-operate with social rules or goals.

Again, I am not proposing that we coerce someone to take up these pharmacological measures. Instead, there might be someone who wants to do the right thing, but owing to a weakness of will, cannot get himself to do the right thing. Having the option to use pharmacological means to increase altruism and empathy may allow this person voluntarily to overcome his weakness of will and enable him to do the right thing.

These examples are intended to illustrate some possible human engineering solutions. Others might include increasing our resistance to heat and tropical diseases, and reducing our need for food and water.

Let me now explain why we should take human engineering seriously. It should be clear that human engineering is less risky than geoengineering. In addition to the fact that much of the technology involved in human engineering - such as preimplantation genetic diagnosis and oxytocin - is already safely available for other uses, human engineering applies at the level of individual humans. This means that we can better manage these risks than those imposed by something such as geoengineering, which takes place on a global, scale.

Human engineering could also be liberty enhancing. In response to climate change, some people have proposed we adopt something akin to China's one-child policy. For example, a group of doctors in Britain has advocated a two-child maximum. But suppose that the relevant issue is some kind of fixed allocation of greenhouse gas emissions for each family. If so, given fixed allocations of greenhouse gas emissions, human engineering could give families the choice between having one large child, two medium-sized children or three small children. Human engineering seems more liberty enhancing than a policy that says you can have only one or two children.

It may turn out that human engineering is not the best way of tackling climate change.

But to concede this now would be to ignore the widely acknowledged fact that we do not know which solutions to climate change will be the most effective.

To combat climate change, we can either change the environment or change ourselves. Given the enormous risks associated with changing the environment, we should take seriously the idea that we may need to change ourselves.

Matthew Liao is the director of the bioethics program and an associate professor in the Centre for Bioethics in the Department of Philosophy at New York University.