Climate crisis: Paint the roads white and give all houses grass roofs

The best way to tackle global warming is to reduce CO2 emissions – but what other ways are there to cool the planet down? Steven Cutts investigates

Drought and global warming have contributed to severe water shortages in Tehuacan Valley in Mexico (Getty)

The climate crisis looks like a huge potential threat to mankind, and the future doesn’t seem very bright. The mainstream scientific community has a list of credible reasons why we need to be afraid, and most of them have a list of measures that might be able to save us from catastrophe. But what if they’re wrong? What if we do everything that the mainstream climate change scientists advise and the Earth’s temperature continues to rise? When we have no more cards left to play, surely we’re going to have to look elsewhere?

Other than cutting CO2 emissions, is there anything else we can do to cool things down?

When heat from the sun strikes this planet, it isn’t necessarily absorbed. Freshly fallen snow reflects about 90 per cent of the incident sunlight. In contrast, a modern road surface might reflect as little of 4 per cent, whereas grass reflects about 25 per cent. This means that if we lay a lot of concrete roads over land that was previously covered in grass then the amount of heat energy absorbed in that area will go up. Some scientists believe that by changing the surface finish of a modern city, we ought to be able to reflect more sunlight back out into space and reduce the ambient temperature.

One idea gaining traction in America is to decrease the temperature in individual towns and cities. It has been calculated that by painting rooftops white in urban areas, we could actually reduce peak temperatures in that environment by 1-2C. So convinced are the proponents for this idea, that a number of American cities are now actively encouraging the painting of roofs and even road surfaces with either white or grey paint.

About 10 per cent of the Earth’s land surface area is used as farmland and only about 2 per cent is part of the urban environment. Even if this kind of policy were introduced, it’s hard to imagine how it could influence global temperatures although it might be possible to slightly reduce temperatures within a large, concreted area. For example, a city. It has long been understood that the temperature in New York City is around 1-3C warmer than the surrounding countryside. This urban/concrete effect was recognised in the 19th century but it takes an event like the recent heatwave to make us give it any real attention. 

Quite apart from the issues surrounding global warming, a particularly severe heatwave can easily claim thousands of lives, especially the lives of frail, elderly people. Some years ago – during a very hot summer in Europe – tens of thousands of elderly people died, mostly in homes that weren’t air conditioned. If we could reduce temperatures by just 1 to 3 degrees, the benefits for city dwellers might be substantial.

Spurred on by this philosophy, cool-roofs initiatives have gained followers in such places as Melbourne, Chicago and Los Angeles. If you go onto YouTube, you can see a man who owns a company that paints the road white and he seems pretty convinced it’s going to work. Other scientists feel uneasy about this idea. The proponents of the cool-roofs philosophy seem to be suggesting that we can find a way to live with the impact of climate change rather than control and reverse it.

In addition, it’s difficult to foresee how the broader environment might respond to such minor innovations. One study acknowledged that reducing local temperatures in a specific area might reduce evaporation of water from that region, which in turn might reduce rainfall down wind. On the other hand, it might not. The complexities of the systems involved are so great that it’s hard to know whether any computer model can accurately predict the impact of any manmade attempt to alter the climate.

Green rooftops can help to reflect the sun’s radiation (AFP/Getty)

Supposing you want to know what the weather will be like tomorrow morning. One option is walk outside your house and check the weather right now. If you predict that the weather tomorrow will be the same as the weather today, then your chances of being right are about 70 per cent. However, a 70 per cent confidence level might not be enough for you. 

You might instead want to contact a meteorologist with access to immense computer power and data from all kinds of scientific equipment, including satellites that have been launched into low orbit at the cost of many billions of dollars. If you do all of this, then your chances of being able to predict the weather tomorrow are about 80 per cent, a full 10 percentage points better than predicting that nothing very much is going to change between now and tomorrow morning.

Predicting global change on a broader timescale is much harder than this.

We’re going to have to whitewash an awful lot of rooftops to gain any convincing evidence that white buildings reduce temperature. In spite of this, the authorities in San Francisco are actively encouraging the construction of green roofs with rooftop gardens becoming compulsory on some new buildings. It’s widely understood that grass and other plants reflect much more incident radiation than dull grey concrete that covers most modern buildings. But how many new roofs would you have to resurface to have any discernible effect on overall city temperatures? Answer, we don’t actually know and it’s going to take a lot of research to find out.

More outlandish still are attempts to obstruct the rays of the sun in outer space. In 2007, the Guardian claimed to have found a leaked American document that openly discussed the option of space mirrors to deviate the amount of heat energy reaching the earth from the sun.

If anything, this has been looked at in more detail than the cool-roofs initiative. The technology required to manufacture space mirrors on this scale is not yet available but it’s relatively easy to calculate how and where a space mirror should be constructed.

You’d need to manufacture multiple mirrors many miles wide and locate them in places that would obstruct the sun’s rays before they hit the Earth. In 1989, an American physicist by the name of James Early was working at the Lawrence Livermore National Laboratory. Early proposed using a “space shade” 2,000km in diameter and leaving it in outer space, about a million miles from the Earth.

In practise, such a mirror would need to be as thin as possible and would probably resemble tin foil. Imagine a sheet of tin foil just a few atoms thick, mass-produced in outer space using new techniques such that it could be stretched between a more solid frame and spread out over many miles. It might sound like a fantasy but many of the techniques involved are already under development.

In the 19th century, the French mathematician Lagrange demonstrated that there are midway points between the Earth and the moon where a satellite might appear to linger in space without actually being in orbit. These are called Lagrange points and Lagrange was careful enough to number them. James Early suggested L1 as the optimum location for a space mirror, although other scientists have suggested completely different positions.

In comparison to the space mirrors or even sulphur dioxide release, cloud brightening really does sound like a bargain

James Early and his co-workers calculated that if we were to block just 2 per cent of the incident radiation from the sun then we would decrease the temperature of the Earth by enough to counter the effects of CO2 emissions. One advantage of this system is that it would grow very slowly with sections of the mirror being added every day over many years. If any adverse effects began to emerge then scientists could stop or downgrade its construction. However, the costs of such a system would be enormous. Even by using raw materials that we had successfully mined in outer space, it would require a monumental effort and we would have absolutely no confidence that it would work until it was completed.

A number of fledgling research groups are now looking at negative emissions technology. Machines which can effectively suck carbon dioxide out of the Earth’s atmosphere and turn it into something else are appearing in more than one research laboratory. (It’s called negative because the amount of CO2 they extract from the atmosphere is more than they inject.) 

There are even people claiming to have manufactured petrol by mixing atmospheric CO2 with water and a few other readily available ingredients. This kind of thing is relatively easy to do on the pages of a chemistry textbook, or even in a test tube, but it’s much harder to get it working on an industrial scale. Ever since the Yom Kippur Air War, there have been governments everywhere desperate to make petrol out of the Earth’s atmosphere and nobody’s pulled it off. Even if we can make it happen, the proponents of negative emissions technology may not be able to deliver on the massive scale that is necessary to reduce global warming.

But all is not lost. Yet another group of far sighted scientists are looking at volcanic eruptions. In 1991 Mount Pinatubo erupted in the Philippines. During this event, nearly 20 million tons of sulphur dioxide, dust and ash entered the Earth’s atmosphere. This dust circulated around the planet causing temperatures to drop by an average of half a degree globally. Since the forces of nature have already done this experiment for us, it looks like pumping lots of additional dust into the atmosphere might really do the trick, dragging global temperatures back down to something where we can get back into our much-cherished cars and not have to evacuate the hotter cities around the equator. 

Stratospheric engineering would be cheaper than emissions reductions. You can rest assured that there is now a reasonably coherent plan to drop huge amounts of sulphur dioxide into the stratosphere and increase the reflectivity of the Earth’s atmosphere in much the same way that volcanic eruptions have already done. Such a project might cost around $2.5bn (£1.96bn) dollars a year to run, small change in comparison to the cost of most of the other techniques that have been suggested to solve global warming. It has the backing of a Yale-based scientist, who acknowledged that we would have to build a new fleet of hi-tech aircraft to actually deliver the particles to the requisite altitude. If you threw the dust out of the back of an ordinary jet airliner, it would fall back to Earth in a few days but if we released it in the stratosphere (about 20km high) it might linger in position for some 18 months, although the dust itself would require continuous replenishment.

If the idea of turning the Earth’s atmosphere into a chemistry set is starting to bother you, try taking a look at another – British – proposal. Stephen Salter at Edinburgh University suggested using seawater spray to increase the reflectivity of the clouds in the earth’s atmosphere. This technique (sometimes referred to as cloud brightening) could be introduced for a fraction of the cost of some other proposals with one estimate putting the bill at a paltry $100m. In comparison to the space mirrors or even sulphur dioxide release, cloud brightening really does sound like a bargain. One advantage of cloud brightening is our ability to introduce it in stages and then gradually scale it up. If adverse side effects start to appear, we could simply switch the machines off.

Incredibly some of these ideas are close to being tested in the Earth’s atmosphere, albeit on a relatively modest scale. But if they were introduced, how would we know if they were even working? One obvious problem with most of these concepts is that they don’t attempt to reduce the amount of CO2 in the Earth’s atmosphere. If CO2 levels continue to increase, there is a significant risk that CO2 – an eminently water-soluble gas – might enter the earth’s oceans and begin to acidify the sea.

The principle challenge to all these calculations is that they are so complex that it’s difficult to know whether any intervention would be effective in changing a major outcome variable (for example, the temperature when you walk outside your house). Even if we began to implement some of these proposals, would the change in global temperature and weather pattern that then followed be a consequence of our new policy or a completely random event? 

Remember that during the last ice age the ice cap of the North Pole extended as far south as modern London. Later on, the ice age ended and the ice sheets receded to their current position. All of these changes occurred in the complete absence of human activity on the Earth. On the same note, it’s worth remembering that in the geological history of this planet, there have been several ice ages.

It is a sign of human nature that many of these alternative solutions to global warming have been shouted down so loudly. At times, it seems to hark back to our allegiance to the early Christian church and the countless religious conflicts that stemmed from this. One batch of clergy or another claim an absolute insight into the holy scriptures and insist that the order of service is theirs to dictate. Strange individuals who refused to listen are publicly tied to the stake.

Blue sky thinking has taken us to some odd places on this matter but it doesn’t do any harm and shouldn’t be regarded as heresy in itself.