Clouds of uncertainty

The debate between climate change scientists and climate change skeptics has raged on for years. Justin Gillis reports that clouds pose the greatest difficulty when it comes to analysing climate change.

For decades, a small group of scientific dissenters has been trying to shoot holes in the prevailing science of climate change, offering reasons for the wrong outlook. Over time, nearly every one of their arguments has been knocked down by accumulating evidence, and polls say 97 per cent of working climate scientists now see global warming as a serious risk. In recent years, climate change skeptics have seized on one last argument that cannot be so readily dismissed. Their theory is that clouds will save us. They acknowledge that the human release of greenhouse gases will cause the planet to warm.

But they assert that clouds – which can either warm or cool the earth, depending on the type and location – will shift in such a way as to counter much of the expected temperature rise and preserve the equable climate on which civilisation depends. Their theory exploits the greatest remaining mystery in climate science, the difficulty that researchers have had in predicting how clouds will change. The scientific majority believes that clouds will most likely have a neutral effect or will even amplify the warming, but the lack of unambiguous proof has left room for dissent.  

Richard S Lindzen, a professor of meteorology at the Massachusetts Institute of Technology, is the leading proponent of the view that clouds will save the day. Lindzen says the earth is not especially sensitive to greenhouse gases because clouds will react to counter them, and he believes he has identified a specific mechanism. On a warming planet, he says, less coverage by high clouds in the tropics will allow more heat to escape to space, countering the temperature increase. His idea has drawn criticism from other scientists, who cite errors in his papers and say proof is lacking.

Natural thermostats

Scientists say clouds have an enormous effect on the climate. The energy that drives life on earth arrives as sunlight. To remain at a steady temperature, the earth has to return the energy it receives back to space, primarily as heat. Clouds alter the energy flow in both directions. On balance, in today’s climate, clouds cool the earth. Dense, low-lying clouds are responsible for most of that effect, because they reflect considerable sunlight back to space. Many high, thin clouds have the opposite influence, allowing incoming sunshine to pass through but effectively trapping heat that is trying to escape. Humans are perturbing the earth’s heat balance by releasing greenhouse gases. Chemists proved in the 19th century that these gases work like an invisible blanket in the atmosphere. In the mid-20th century, as it became clear how fast carbon dioxide levels were rising, some scientists began to predict a warming of the planet. But they also realised that an exact forecast was difficult for several reasons, especially the question of how clouds would react. Scientists use sophisticated computer programs to forecast future climates, but the computers are not yet powerful enough to predict the behaviour of individual clouds across the whole earth over a century, which forces the researchers to use rough approximations.

The most elaborate computer programs have agreed on a broad conclusion: clouds are not likely to change enough to offset the bulk of the human-caused warming. Other computer analyses foresee a largely neutral effect. The result is a big spread in forecasts of future temperature, one that scientists have not been able to narrow much in 30 years of effort. The earth’s surface has already warmed about 1.4 degrees Fahrenheit since the Industrial Revolution, most of that in the last 40 years. It is an average for the whole planet, representing an enormous addition of heat. An even larger amount is being absorbed by the ocean. The increase has caused some of the world’s land ice to melt and the ocean to rise. By midcentury, the level of greenhouse gases in the atmosphere is expected to double compared with the value that prevailed before the Industrial Revolution. At the low end, computers predict that the earth could warm in response by another 2 degrees Fahrenheit. The likelier figure, the analyses say, is 4 degrees. At the high end, the warming could exceed 8 degrees. In all possible outcomes, the warming over land would be roughly twice the global average, and the warming in the Arctic greater still. Even in the low projection, many scientists say, the damage could be substantial.

A major goal of climate research is to improve the way clouds are represented in the computer analyses, which should narrow the range of predicted temperatures. And some of the most important data that researchers need to do so are streaming from a hilltop in rural Oklahoma, near the town of Lamont, where the Department of Energy runs the world’s largest facility for measuring cloud behaviour. The questions that scientists still need to answer are voluminous. For instance, they want a better idea of how clouds form at a microscopic scale, how their behaviour varies under different atmospheric conditions, and how sensitive they are to warmer temperatures. Recently, $30 million worth of new radars have been installed in Oklahoma and at other research facilities, promising a better view of the innards of clouds. Satellites are also supplying better data, and theories of the atmosphere are improving.

Feedback loop?

Lindzen agrees that the level of greenhouse gases is rising because of human activity and that this should warm the climate. But for over a decade, Lindzen has said that when surface temperature increases, the columns of moist air rising in the tropics will rain out more of their moisture, leaving less available to be thrown off as ice, which forms the thin, high clouds known as cirrus. Like greenhouse gases, these cirrus clouds act to reduce the cooling of the earth, and a decrease of them would counteract the increase of greenhouse gases.

Lindzen calls his mechanism the iris effect, after the iris of the eye, which opens at night to let in more light. Here, the earth’s “iris” of high clouds would be opening to let more heat escape. When Lindzen first published this theory, in 2001, he said it was supported by satellite records over the Pacific Ocean. But other researchers quickly published work saying that the methods he had used to analyse the data were flawed and that his theory made assumptions that were inconsistent with known facts. Using what they considered more realistic assumptions, they said they could not verify his claims.

Today, most mainstream researchers consider Lindzen’s theory discredited. He does not agree, but he has had difficulty establishing his case in the scientific literature. Lindzen published a paper in 2009 offering more support for his case that the earth’s sensitivity to greenhouse gases is low, but once again scientists identified errors, including a failure to account for known inaccuracies in satellite measurements.

Lindzen acknowledged that the 2009 paper contained “some stupid mistakes” in his handling of the satellite data. Last year, he tried offering more evidence for his case, but after reviewers for a prestigious American journal criticised the paper, Lindzen published it in a little-known Korean journal. Lindzen blames groupthink among climate scientists for his publication difficulties, saying the majority is determined to suppress any dissenting views. They, in turn, contend that he routinely misrepresents the work of other researchers.