By Eric Vandenbroeck

In a few days world leaders will gather in Paris for a grand conference on climate change, the 21st such get-together since the United Nations began to grapple with the issue. A torrent of pronouncements and promises has already issued forth—from Pope Francis, Xi Jinping, Barack Obama and many others. The IMF warns that human fortunes will “evaporate like water under a relentless sun” if climate change is not checked soon.

Barring a global catastrophe or the spectacular failure of almost every climate model yet devised, though, emissions of greenhouse gases will warm the world by more than 2°C.

Changes in the atmospheric level of carbon dioxide, the biggest contributor to global warming, persist for centuries. So it is useful to imagine that mankind has a fixed carbon budget to burn through. Pierre Friedlingstein, a climatologist at Exeter University, calculates that if temperature rises are to be kept below 2°C, the world can probably emit about 3,200 gigatonnes of carbon dioxide in total. The tally so far is 2,000 gigatonnes. If annual emissions remain at present levels, the budget will be exhausted in just 30 years’ time.

Global greenhouse-gas emissions might also hold steady for a while. Total man-made emissions in 2014 were about the same as in 2013, according to the International Energy Agency.

The bad news is that even if greenhouse-gas emissions are stabilizing, they are doing so at an exalted level, and there is little reason to suppose that the plateau will be followed by a downward slope.

But of course climate change will not be bad for everything and everyone. Some cold countries will find that their fields can grow more crops; others will see fish migrate into their waters. With its ocean-moderated climate, Britain stands out as exceptionally favored. Yet bad effects will increasingly outnumber benign ones almost everywhere. Some organisms will run into trouble well before the 2°C limit is breached.

But in the end climate change will have to be tackled more intelligently and more economically than it has been so far. Renewable energy is crucial. Contrary to what many claim, though, it is not true that existing solar and wind technologies could cheaply save the planet while also creating lots of green jobs if only they were subsidized for just a few more years. Those renewable power sources have cost consumers dear and mangled energy markets. Paying for yet more wind turbines and solar panels is less wise than paying for research into the technologies that will replace them.

Mankind will also have to think much more boldly about how to live under skies containing high concentrations of greenhouse gases. It will have to adapt, in part by growing crops that can tolerate heat and extreme weather, in part by abandoning the worst-affected places. Animals and plants will need help, including transporting them across national and even continental boundaries. More research is required on deliberately engineering the Earth’s atmosphere in order to cool the planet.

It is often said that climate change is an urgent problem. If that were true, it might be easier to tackle. In fact it is a colossal but slow-moving problem, spanning generations.

What is known about global warming—and what remains dark

Greenhouse warming sets off a cascade of effects known as feedback's, which are harder to measure. On balance, warming begets more warming. Higher temperatures enable the atmosphere to hold more water vapor. Oceans absorb huge amounts of carbon dioxide, keeping a lid on climate change—but as they warm up, their absorption capacity declines. Melting ice produces dark pools of water that absorb more energy. Partly for this reason, the Arctic is warming faster than other places.

Inadvertently, though, humans also cool the Earth. Although the overall effect of deforestation is to warm the planet, replacing trees with crops or grassland makes the land paler and more reflective. Particles created from sulphur dioxide-the cause of acid rain-reflect lots of light back into space. China has probably been shielded from higher temperatures by air pollution, and might heat up quickly if it gets serious about scrubbing its skies.

The greatest mystery is the effect of human activity on clouds. Because clouds grow on aerosol particles, more of them are likely to form in a more polluted atmosphere. Clouds are also affected by temperature changes. But precisely how is unclear-and this matters, because whereas high clouds tend to keep the Earth warm, low clouds tend to cool it. Part of the problem in measuring their effect is that many clouds are small. Climate models tend to simplify the world by dividing the atmosphere and the oceans into boxes, perhaps 50km by 50km in the horizontal plane, and treating these as pixels in a giant three-dimensional computer simulation. To capture cloud processes properly might require climate models with cells just tens of metres square. No computer in the world could handle that.

Add up all these difficulties; throw in some problems with measuring temperatures, and you get a lot of uncertainty. The chart on the previous page, which shows the estimates by the International Panel on Climate Change (IPCC) of “radiative forcing”-perturbations to the Earth’s energy system from human and other activities-contains black bars showing 95% confidence ranges. Some of those bars are long. It is especially hard to be sure of the effect of aerosols.

If the past is a little hazy, the future is more so. Not only does it depend on the outcome of physical processes that are inadequately understood. It also depends on human actions. How many people will be living in 2100? How rich will they be? Will they make strenuous efforts to cut greenhouse-gas emissions, do nothing, or something in between?

If mankind makes heroic efforts, the Earth system will remain within familiar bounds, making predictions easier. If concentrations of greenhouse gases increase steeply, though, things become highly unpredictable. Passing irreversible tipping points, such as the collapse of the Greenland ice sheet, becomes more likely. If nothing were done to avert climate change by 2200, the IPCC estimates, the world would probably warm by between 3°C and 10°C. That enormous range is manageable at one end, unimaginable at the other.

Much remains unknown, then. But, equally, much has been settled—it is just that the settled stuff generates fewer papers and conference panels, because researchers have moved on. Some possibilities that seemed troubling a few years ago have been probed and revealed to be less so. It now appears unlikely, for example, that climate change will lead to the irreversible collapse of the Gulf Stream. Melting permafrost will emit methane, but not as much as some once feared.

Even those mysterious clouds are giving up some of their secrets. Satellite-based radar and laser measurements have enabled scientists to peer into clouds; small-scale models designed to capture their behavior have been refined and plugged into global models. It seems increasingly likely that low cloud cover will diminish as the Earth warms, speeding the process.

Most important, the basic proposition of climate change—the causal relationship between greenhouse-gas emissions and higher temperatures—has become almost unassailable. As it happens, the interesting debate about the global-warming hiatus has a boring coda: 2013 turned out slightly hotter than 1998, and 2014 was roasting, setting a new record. That will not stand for long.

The biggest single cause of the fossil-fuel boom is China, which is examined in the next article. But rich Western countries are more culpable than they think. They have transformed their rural landscapes with wind farms and pushed up electricity prices for consumers, yet have managed to drive surprisingly little carbon out of the energy system. The record would look even worse if Western countries had not simultaneously exported much of their heavy industry, and thus much of their pollution, to China and other emerging countries.

Today we large wind farms like for example in Germany. Helped by some big storms, these turbines produced 41,000 gigawatt-hours of electricity in the first half of this year, 15% of Germany’s total electricity output. Add hydro-electric power stations, solar farms and biomass, and the country derived 35% of its electricity from renewable sources. Germany has become a world leader in green power, but also a warning about what can go wrong.

Wind and sunshine have two big drawbacks as sources of power. First, they are erratic. The sun shines weakly in winter when it shines at all, and the wind can drop. On January 20th this year the output from all of Germany’s solar and wind farms peaked at just over 2.5 gigawatts-a small proportion of the 77 gigawatts Germany produced that day. A few months later, during a sunny, windy spell in early June, the combined wind and solar output jumped to 42 gigawatts.

The second problem with wind and solar energy, oddly, is that it is free. Wind turbines and solar panels are not free, of course. Although the cost of solar photovoltaic panels has plunged in the past few years, largely because Germany bought so many, wind and solar farms still tend to produce more expensive electricity than coal or gas power stations on a “levelised cost” basis, which includes the expense of building them. But once a wind or solar farm is up, the marginal cost of its power output is close to zero.

The problem lies with the effect of renewable on energy markets. Because their power is free at the margin, green-power producers offer it for next to nothing in wholesale markets (they will go on to make money from subsidies, known as feed-in tariffs). Nuclear power stations also enter low bids. The next-lowest bids tend to come from power stations burning lignite coal—a cheap but especially dirty fuel. They are followed by the power stations burning hard coal, then the gas-fired power stations. The energy companies start by accepting the lowest bids. When they have filled the day’s requirements, they pay all successful bidders the highest price required to clear the market.

Climate-change denial is strikingly rare among China’s political leaders, some of whom trained as engineers. They understand that their country is expected to suffer some of the worst consequences of global warming: northern China, which is increasingly hot and dry, will probably become hotter and drier still. The politicians are also well aware that their country’s urbanites are fed up with breathing toxic air.

Nobody quite knows how much coal is burned in China. Misreporting is common: earlier this year official statistics were amended to suggest that the country had consumed 14% more coal between 2000 and 2013 than had been thought. Yet the quantity might now be falling. Consumption seems to have dropped very slightly between 2013 and 2014. In the first seven months of this year China’s mines produced 5% less coal than they did during the same period last year. If this trend were to continue, it would make the government’s pledge to reach peak greenhouse-gas emissions by 2030 seem unduly modest.

China will remain a heavy polluter. Though steel and cement factories will probably use less energy in future, ordinary people will doubtless consume more. As they grow richer, they demand air-conditioning, cars and bigger homes: in 2012 the average city-dweller inhabited 33 square meters, compared with 25 square meters a decade earlier. Still, the astonishing surge in dirty, coal-fired energy consumption has probably subsided, thinks Mr Grubb. It might just be a little hard to see, through the hazy, choking air.

Countries in danger

Few countries of any size are more gravely threatened by climate change than Bangladesh (which has more than 110m people).

To counter the most spectacular threat to human life, Bangladesh’s government has built several thousand cyclone shelters—at best, sturdy buildings sitting atop pillars of reinforced concrete, which in normal times are often used as schools. One new shelter a few kilometers from Bujbunia could accommodate more than 1,000 people if they were to press closely together, and might even hold a few hundred cows on the ground floor. Women and children will rush there if a big cyclone threatens; men will head for the nearest brick-built mosque.

Farmers are also preparing for storm surges in a humbler way. Scooping up grayish mud, they build plinths up to a meter high. Leveled and packed down, these become the floors of their homes; walls and roofs are made of palm fronds, bamboo and corrugated iron. The aim is to build the plinth higher than the flood waters will reach, to prevent the family’s food and possessions being swept away. Even stoves would be destroyed; they are only made of earth.

But to protect themselves against diverse dangers, the farmers of southern Bangladesh need to make drastic changes. They are doing that by investing in their children’s education. In a secondary school in the settlement of Sreefaltola, a class of eighth-graders, mostly farmers’ children, shout out their plans for the future.

Almost every one of them aspires to be an electrical engineer or a doctor, or at least to find a job in a nearby city. Not all will succeed; many will probably stay in farming. But those who make it might be able to move their parents out of one of the most hazardous places in Asia.

This is not normally what is meant by adaptation to climate change. All the same, it is the most effective method of adaptation in Bangladesh, says Mr Alam. Some have taken it even further. In the village of Gobindapur, a grand new house is being built, two stores high and made of solid brick. It belongs to Reshma Begum, an imperious woman in a pink sari. Clutching two mobile phones, she complains that it is becoming increasingly hard to find domestic workers now that so many young women in the village are running small businesses. Some of her income comes from a son who works in Malaysia.

Migrations such as these are beginning to show up in official statistics. Between 2001 and 2011 the population of Barisal division in southern Bangladesh fell slightly, even as numbers in the country as a whole went up. Within the district, people moved from the countryside to cities, so that Barisal’s rural population dropped from 1.96m to 1.81m. By removing people from the most flood-prone areas, urbanization may be doing as much to preserve life as any number of cyclone shelters.

A decade ago adaptation was almost taboo in international discussions about climate change, because it was believed to distract attention from the task of stopping global warming altogether. Now both are recognized as important. Rich countries are trying to rustle up $100 billion a year by 2020 to help poor countries cope with climate change—a bribe to keep them coming back to the climate talks, to be sure, but also a welcome sign of changing priorities. Even China is chipping in.

It is not yet clear whether the money will be spent on better crops and fertilizers or on solar panels and other green-energy schemes that will help poor countries hold down their greenhouse-gas emissions. Britain, which spends an unusually large share of its budget on foreign aid, suggested in September that its contribution to the Green Climate Fund would be divided evenly between those two things.

That is the wrong balance. Solar panels are nice to have; many Bangladeshi farmers already possess small ones which they use for charging mobile phones and powering a couple of light bulbs. But these are no substitute for reliable electrical power, and there are plenty of more important things. Diverting money that would otherwise be spent on health and sanitation to expensive forms of clean energy will make it harder, not easier, for the world’s poorest people to cope with climate change.

What about carbon tax

A global carbon tax—or even one involving many countries—is likely to remain an economic theory for a long time. Certainly, nothing of the sort will be seriously discussed in Paris. Even so, there are three perfectly good things everybody could be getting on with right away. Two are humdrum, though no less worthwhile for that. The third requires greater ambition.

First, countries should be nudged to upgrade their promises for cutting emissions. In advance of the Paris conference, an untidy mess of pledges has been dumped on the table. Some countries say, fairly straightforwardly, that they will cut greenhouse-gas emissions by such-and-such a percentage compared with a particular year. But they pick different base years—invariably ones in which their emissions were very high—to make their promises look better. Australia goes for 2005; Russia plumps for 1990, just before its heavy industries collapsed. Other countries do not even propose to hold emissions to a specific level. Some countries go in for statistical tricks, arguing, for example, that their efforts to prevent deforestation should be weighed more generously. The pledges should be made more comparable.

A good second move would be to ditch the carbon monomania. Tackling carbon dioxide, the most important greenhouse gas, is essential. Yet aside from its effect on ocean chemistry and the fact that it is warming the world, though so gradually that most people cannot detect it, carbon dioxide is innocuous. And the effects of emissions persist in the atmosphere for so long that even a drastic cut would have only a slight effect on climate change in the short term.

Carbon dioxide is not, however, the only greenhouse pollutant. Methane, black carbon (ie, soot) and hydrofluorocarbons also warm the world a good deal. It has been estimated that if strong action were taken to suppress them, the world might be 0.6°C cooler by 2050 than it would otherwise be—a quick, fairly noticeable change. There is talk of regulating hydrofluorocarbons under the Montreal Protocol, which cracks down on ozone-depleting chlorofluorocarbons—an excellent idea.

The best target is black carbon, which comes off open wood fires and out of the exhaust pipes of unsophisticated diesel vehicles. This is an immediate killer, and it is easier to tackle than carbon dioxide.

Man-made global cooling?

A report published in 2012 for the Copenhagen Consensus Centre estimated that marine-cloud brightening would prevent global warming more effectively than a carbon tax.

If spraying seawater into the air would probably cool the Earth, spraying sulfur into the stratosphere would be almost certain to do so. It has been done, after all. Volcanoes spew out sulfur that creates particles which reflect sunlight back into space; those particles also bounce light around the atmosphere, producing wonderful sunsets. These can cool the Earth significantly, albeit briefly (see chart): within a year or so the particles are washed out of the atmosphere.

Sulfur could be sprayed at precisely the right height and in very fine droplets, which would reflect more light for longer. It might take only a small fleet of high-altitude aircraft flying in relays to put enough in the stratosphere to cancel out the entire temperature rise resulting from human greenhouse-gas emissions. The sulfur would eventually fall as acid rain, but not in alarming quantities: the amount of sulfur required would be much less than is currently thrown up into the air by vehicles and factories.

Both marine-cloud brightening and stratospheric aerosols carry risks. One is that cooling the Earth without removing carbon dioxide does not quite return the climate to normal. The more carbon dioxide that is present in the air, the less plants perspire, affecting the water cycle. And the heat-trapping greenhouse effect would still operate, just with less heat in the system. With temperatures more evenly distributed in the atmosphere, there would be less convection and, presumably, less precipitation. So a cooler world with lots of greenhouse gases would probably be a drier world. Any country that suffered a drought would surely blame the geoengineers.

But the biggest problem is what would happen if the engineering stopped. Assuming that greenhouse-gas emissions continued while the ships or aeroplanes were doing their work, abruptly ending the artificial shielding would lead to a sudden jump in temperatures, which would be disastrous for people and the natural world alike. Ken Caldeira of the Carnegie Institution points out that an abrupt “termination shock” could be avoided if geoengineering were used only to slow global warming and then gradually wound down. But that assumes the nations of the world can agree on how to manage the climate. The history of United Nations climate talks suggests they can’t.

Still, these methods ought to be developed and even—very carefully—tested. The Earth might need a drastic intervention, particularly if it became clear that something alarming was about to happen, such as a breakdown of the Indian monsoon. Marine-cloud brightening could be deployed on a small scale to avert specific disasters. Mr Latham suggests that cooling just a few hundred square kilometers of ocean in the right place could make a hurricane less severe. If the climate-modellers are right that hurricanes will become more intense as the ocean warms, this will become increasingly tempting.

The most persuasive reason for investigating geoengineering further is that somebody is likely to try it. Countries will have different ideas about when global warming becomes truly dangerous: Britain, for instance, is a lot more sanguine than the Maldives. Some of the more skittish states might start injecting aerosols into the lower stratosphere, perhaps in a clumsy way. If no formal experiments had been carried out and thus scientists in other countries did not know what to look for, it might not be obvious for some time that this was going on.

Of course one can doubts that nations would ever formally agree to engineer the Earth’s climate: their interests are too diverse. What is more likely is that a country would just go ahead and try it. That would put the others in a quandary. Should they forcibly stop that country from acting, or should they step in with superior geoengineering techniques? Before long they could find themselves acting as zookeepers to the planet.

The most important thing of all however is to innovate.

Hence there is a crying need to develop technologies that are cheaper and more dependable than today’s wind turbines and solar farms. Bur one can also hold out hope for artificial photosynthesis, which uses solar energy to make hydrogen from water.