If humans pump enough carbon dioxide (CO₂) into the atmosphere, the stratocumulus clouds(层积云 ) could disappear, and the earth's temperature could climb sharply to heights not predicted in current climate models. It would burn the planet. That's the conclusion of a paper published in the journal Nature Geoscience and described in detail by Natalie Wolchover for Quanta Magazine.

As Wolchover explained, clouds have long been one of the great uncertainties of climate models. Computer models that easily capture the complexity and detail of most climate systems just aren't powerful enough to predict worldwide changes in cloud behavior. But clouds are important. They reflect sunlight away from the earth's surface. And stratocumulus clouds are those white blankets you might have seen as you looked out the window of arm airplane, rolling out below you and hiding the ground Researchers suspect that certain sudden, past jumps in temperature may have been caused by changes to clouds like these.

For the new research, scientists modeled just a small patch of sky using a supercomputer. They found that if carbon dioxide levels reach about 1, 200 parts per million(ppm) in the atmosphere, stratocumulus clouds break up. That's a very high carbon dioxide concentration.   Right now, levels have climbed past 410 ppm--a dangerous change from 280 ppm before the Industrial Revolution.

But humans put more and more CO₂ into the atmosphere every year. If current trends continue, the earth could reach 1, 200 ppm within 100 to 150 years. This could happen if our society doesn't follow through on any of its commitments to reduce emissions(排放), Wolchover reported. And even if it does, the result would be another 8 degrees Celsius of heat added to the global average, on top of the dangerous changes already underway due to greenhouse gases.

That’s an enormous change, and it goes beyond predictions of worldwide ice melt and catastrophic sea level rise. And, once the stratocumulus clouds are gone, Wolchover reported, they likely wouldn't reappear until atmospheric carbon dioxide levels dropped below where they are currently.

There's still some uncertainty in the data. The 1, 200 ppm figure could change as scientists look into the issue further.

The Effects of a Warmer World Are Visible in Animals’ Bodies

For humans, adapting to climate change will mostly be a matter of technology. More air conditioning, better-designed houses and bigger flood defences may help mitigate the effects of a warm world. 【小题1】 In a paper published in Trends in Ecology & Evolution, a team led by Sara

Ryding, a phd candidate at Deakin University, in Australia, shows that is already happening. Climate change is already altering the bodies of many animal species, giving them bigger beaks, limbs and ears. In some species of Australian parrot, for instance, beak size has increased by between 4 % and 10 % since 1871.

All that dovetails (吻合)nicely with evolutionary theory. “Allen’s rule”, named for Joel Asaph Allen, who suggested it in 1877, holds that warm-blooded-animals in hot places tend to have larger appendages (附属物) than those in less hot regions.【小题2】 For example, Fennec foxes, which are native to the Sahara desert, have strikingly large ears, especially compared with their Arctic cousins.

Ms Ryding examined museum specimens, comparing their bodies to those of their modern counterparts. She is not the first researcher to take that approach. 【小题3】 All sorts of other factors, from changes in prey to the evolving reproductive preferences of males or females, might possibly have been driving the changes. Her team combined data from different species in different places.Since they have little in common apart from living on a warming planet, she says, climate change is the most reasonable explanation.

【小题4】 Researchers have already seen changes in the geographical range of many species, from insects to fish. Another evolutionary rule-of-thumb, Bergmann’s rule, holds that animals in hotter places tend to have smaller bodies, another way to boost the ratio between surface area and volume. Other animals may alter their behaviour as well as their bodies, such as by seeking shelter at hot times of the day.

Studying a broader range of animals will help firm up exactly what is happening. Much of Ms Ryding’s data concern birds, with less information available for other taxa (类群). But it seems clear that the world of the future is not just going to be hotter than humans are used to.【小题5】

Population and climate

The human population on Earth has grown to the point that it is having an effect on Earth’s atmosphere and ecosystems. Burning of fossil fuels, deforestation, urbanization, and cultivation of rice and cattle are increasing the concentration of carbon dioxide, nitrogen oxides and dust in the atmosphere. About 70 percent of the Sun’s energy passes through the atmosphere and strikes Earth’s surface. This radiation heats the surface of the land and ocean, and these surfaces then reradiate infrared radiation back into space. This allows Earth to avoid heating up too much. However, not all of the infrared radiation makes it into space; some is absorbed by gases in the atmosphere and is reradiated back to Earth’s surface. A greenhouse gas is one that absorbs infrared radiation and then reradiates some of this radiation back to Earth. Carbon dioxide and nitrogen oxides are greenhouse gases. In fact, without greenhouse gases in the atmosphere, scientists calculate that Earth would be about 33℃ cooler than it currently is.

The current concentration of carbon dioxide in the atmosphere is about 360 parts per million. Human activities are having a major influence on atmospheric carbon dioxide concentrations, which are rising so fast that current predictions made by scientists are that atmospheric concentrations of carbon dioxide will double in the next 50 to 100 years.

Some scientists predict that a doubling of carbon dioxide concentration will raise global temperatures anywhere between 1.4℃ and 4.5℃. The increase in temperature will not be uniform, with the smallest changes at the equator and changes two or three times as great at the poles. The local effects of these global changes are difficult to predict, but it is generally agreed that they may include alterations in ocean currents, increased winter flooding in some areas of the Northern Hemisphere, a higher incidence of summer drought in some areas, and rising sea levels, which may flood low-lying countries.