THE stratosphere—specifically, the lower stratosphere—has, it seems, been drying out. Water vapour is a greenhouse gas, and the cooling effect on the Earth's climate due to this desiccation may account for a fair bit of the slowdown in the rise of global temperatures seen over the past ten years. These are the somewhat surprising conclusions of a by Susan Solomon of America's and her colleagues, which was published online by on January 28th. Whether the trend will continue, stop or reverse itself, though, is at present unknown.
The stratosphere sits on top of the troposphere, the lowest, densest layer of the atmosphere. The boundary between the two, the tropopause, is about 18km above your head, if you are in the tropics, and a few kilometres lower if you are at higher latitudes (or up a mountain). The tropopause separates a rowdy below from a sedate above. In the troposphere, the air at higher altitudes is in general cooler than the air below it, an unstable situation in which warm and often moist air below is endlessly buoying up into cooler air above. The resultant commotion creates clouds, storms and much of the rest of the world's weather. In the stratosphere, the air gets warmer at higher altitudes, which provides stability
The stratosphere—which extends up to about 55km, where the mesosphere begins—is made even less weather-prone by the absence of water vapour, and thus of the clouds and precipitation to which it leads. This is because the top of the troposphere is normally very cold, causing ascending water vapour to freeze into ice crystals that drift and fall, rather than continuing up into the stratosphere.
A little water manages to get past this cold trap. But as Dr Solomon and her colleagues note, satellite measurements show that rather less has been doing so over the past ten years than was the case previously. Plugging the changes in water vapour into a climate model that looks at the way different substances absorb and emit infrared radiation, they conclude that between 2000 and 2009 a drop in stratospheric water vapour of less than one part per million slowed the rate of warming at the Earth's surface by about 25%.
Such a small change in stratospheric water vapour can have such a large effect precisely because the stratosphere is already dry. It is the relative change in the amount of a greenhouse gas, not its absolute level, which determines how much warming it can produce, and this change was about 10% of the total.
By comparison with the greenhouse effect caused by increases in carbon dioxide, the stratospheric drying is hardly massive. Dr Solomon and her colleagues peg the 2000-2009 cooling effect at about a third of the opposite effect they would expect from the carbon dioxide added over the same decade, and only a bit more than a twentieth of the warming expected from the rise in carbon dioxide since the industrial revolution. But it is surprising, nonetheless.
It is for the most part only in the tropics that tropospheric air can be drawn up into the stratosphere; it is also in the tropics that one finds the most spectacular thunderstorms, and these can reduce the temperature at the top of the troposphere, deepening the cold trap that ascending water vapour must pass through and thus impeding its rise. Over the past decade this stormy effect seems to have been pronounced, with the coldest parts of the tropical troposphere getting about a degree colder. But why this should be is not clear. Sea-surface temperatures, which drive the big tropical storms, have been high, and during the past few years have seemed to correlate with increased coldness aloft. At other times, though, they have seemed to predict a wetter stratosphere.
Dr Solomon cannot say what is driving the change she and her colleagues have studied, nor how long it will last. It may be one of many aspects of the climate that flop around, seemingly at random, over periods of years to decades. Or it might be something driven by a long-term change, such as the build-up of greenhouse gases (or, conceivably, layers of sooty smog). Dr Solomon suspects the former, because of the way the relationship between the stratosphere and the sea-surface temperature has changed. Patterns of sea-surface temperature which come and go, rather than absolute levels that continue to rise, may be the important thing.
That said, it is possible that the changes in the stratosphere are linked to the effects humans are having on the atmosphere at large, and that the drying may persist in providing a brake on warming. Or it may be, as others have suggested in the past, that the long-term trend, as the troposphere warms up, will be to a wetter, more warming lower stratosphere, too. Whether this is the case depends on physical subtleties that are currently undecided, but it is not implausible. If it were true, then the current drying would be more a blip than a trend.
A better understanding of matters as diverse as how water vapour actually gets across the tropopause and how the stratosphere circulates at the global scale might help sort the question out, and Dr Solomon's high profile contribution may help focus researchers on those problems. Meanwhile, the good news (if further research bears it out) that the world's warming has been slowed, at least for a few years, needs to be leavened with the realisation, yet again, that there are significant uncertainties in science's understanding of the climate — and thus unquantifiable risks ahead.
1. What is the order of layers in the atmosphere, starting from the lowermost and going to the topmost?
A. Tropopause, Troposphere, Mesosphere, Stratosphere.
B. Troposphere, Tropopause, Stratosphere, Mesosphere.
C. Troposphere, Tropopause, Mesosphere, Stratosphere.
D. Troposhere, Stratosphere, Tropopause, Mesosphere.
2. What is the passage has been cited as the main reason affecting global temperatures?
A. Relative change in water vapour content in the Stratosphere.
B. Drop in Stratospheric water vapour of less than one part per million.
C. The extreme dropness in the Stratosphere.
D. Absorption and emission of infrared radiation by different substances.
3. Why is the situation in the troposphere defined as unstable?
A. Because, unlike the Stratosphere, there is too much water vapour in the Troposphere.
B. Because the Troposphere is not directly linked to the Stratosphere, but through the Tropopause which creates much of the world‘s weather.
C. Because of the interaction between warm and cool air which is unpredictable in nature and can leads to storms.
D. Because this layer of the atmosphere is very cloudy and can lead to weather related disruptions.
4. What accounts for the absence of water vapour in Stratosphere?
A. The layer of Stratosphere is situated too far above the water vapour to reach.
B. Rising global temperatures, leading to reduced water vapour that get absorbed in the Troposphere.
C. The greenhouse gas gets absorbed by the cloudes in the Troposphere and comes down as rain.
D. Before the vapour can rise up, it has to pass through below freezing temperatures and turns into ice.