Excerpt from an essay on carbon dioxide and the greenhouse effect.
The whole essay:
aip.org
During the 1990s, further ice core measurements indicated that during past glacial periods, temperature changes had preceded CO2 changes by several centuries. Was it necessary to give up the simple hypothesis that had attracted scientists ever since Tyndall in the 19th century — that changes in CO2 were a simple and direct cause of ice ages? Some scientists doubted that dates could be measured so precisely, but most of the evidence pointed to a time lag.(54) This confused many people. If changes in CO2 lagged behind changes in temperature (and likewise for methane, another greenhouse gas measured in the ice cores), didn’t that contradict the greenhouse theory of global warming? But in fact the lag was not good news.
It seemed that rises or falls in carbon dioxide levels had not initiated the glacial cycles.In fact most scientists had long since abandoned that hypothesis. In the 1960s, painstaking studies had shown that subtle shifts in our planet's orbit around the Sun (called "Milankovitch cycles") set the timing of ice ages. The amount of sunlight that fell in a given latitude and season varied predictably over millenia, altering how long snow ands sea ice lingered in the spring, which crucially affected how much sunlight the surface absorbed. The fact that carbon dioxide levels lagged behind the orbital effect should have been no surprise, since a change in the temperature would change the gas level. For one thing, warmer oceans would evaporate out more gas. For another, as Arctic tundra warmed up it would likewise emit CO2 and methane. The ice cores now showed, as theorists had predicted since the 19th century, that a powerful feedback cycle was amplifying the effect of the cyclical changes in sunlight. Even a small change in the gas level would bring further changes in the global heat balance, which would in turn alter the gas level, which... and so forth. This suggested how tiny shifts in the Earth’s orbit had set the timing of the enormous swings of glacial cycles.
Or, more ominously, how a change in the gas level initiated by humanity might be amplified through a temperature feedback loop. The ancient ice ages were the reverse of our current situation, where humanity was initiating the change by adding greenhouse gases. As the gas level rose, temperature would rise with a time lag — although only a few decades, not centuries, for the rates of change were now enormously faster than the orbital shifts that brought ice ages.
There were many ways temperature or other climate features could influence the carbon dioxide level one way or another. Perhaps variations of temperature and of weather patterns caused land vegetation to release extra CO2, or take it up... perhaps the oceans were involved through massive changes in their circulation or ice cover... or through changes in their CO2-absorbing plankton, which would bloom or decline insofar as they were fertilized by minerals, which reached them from dusty winds, rivers, and ocean upwelling, all of which could change with the climate... or perhaps there were still more complicated and obscure effects.
A key point stood out. The cycling of carbon through living systems was not something to trifle with. In the network of feedbacks that made up the climate system, CO2 was a main driving force. This did not prove by itself that the greenhouse effect was responsible for the warming seen in the 20th century. And it did not say how much warming the rise of CO2 might bring in the future. What was now beyond doubt was that the greenhouse effect had to be taken very seriously indeed.(55)
By now there were a dozen teams around the world using computers to integrate every advance in observation or theory. As the 21st century arrived, the growing agreement among the rival teams, and the consistency of their models' results with many different kinds of observations, became overwhelmingly convincing. Scarcely any reputable expert now doubted that CO2 and other greenhouse gases were at least partly responsible for the unprecedented warming all around the world since the 1980s. A final nail in the skeptics' coffin came in 2005, when a team compared computer calculations with long-term measurements of temperatures in the world's ocean basins (it was not in the air but the massive oceans, after all, that most of any heat added soon wound up). In each separate ocean basin, they showed a close match between observations of rising temperatures at particular depths, and calculations of where the greenhouse effect should appear. This was telling evidence that the computer models were on the right track. Nothing but greenhouse gases could produce the observed ocean warming — and other changes that were now showing up in many parts of the world, as predicted.
The computations pinned down an imbalance. The Earth was now taking in from sunlight nearly a watt per square meter more than it was radiating back into space, averaged over the planet’s entire surface. That was enough energy to cause truly serious effects if it continued. James Hansen, leader of one of the studies, called it "smoking gun" proof of greenhouse effect warming.(56)
Yet amid all the uncertainties about how carbon cycles operated, how much could we trust the computer models? Scientists are more likely to believe something if they can confirm it with an entirely independent line of evidence, preferably from somewhere nobody had looked before. Just such new evidence came up in the 1990s, thanks to an unexpected alliance of paleontology and plant physiology. Studies of plant species that had changed little since the rise of the dinosaurs (magnolia for one) showed that if you exposed them to a higher level of CO2, the structure of their leaves changed. Ancient fossil leaves showed just such changes. Several kinds of chemical studies confirmed that the level of the gas had swung widely over geological ages, and the temperature too.
Eventually geochemists and their allies managed to get numbers for the “climate sensitivity” in ancient eras, that is, the response of temperature to a rise in the CO2 level. Over hundreds of millions of years, a doubled level of the gas had always gone along with a temperature rise of three degrees, give or take a couple of degrees. That agreed almost exactly with the numbers coming from many computer studies.
It was good to see that the models had not missed something huge. There seemed scant possibility of a runaway greenhouse catastrophe. It was less reassuring to notice what the climate had looked like in certain ancient times when CO2 had stood at a high level — a level that humanity would eventually reach if we went on burning all available oil and coal. The Earth had been virtually a different planet, with tropical forests near the poles and sea levels a hundred meters higher. Worse, as one group pointed out, unchecked emissions seemed bound to bring not only "a warming unprecedented in the past million years," but changes "much faster than previously experienced by natural ecosystems..."(57)
If the planet warmed up by several several degrees during the 21st century, as paleontologists and computer modelers agreed was likely, what would be the consequences? This became the new center for most of the research. It was becoming clear that the consequences would be severe in many parts of the world, perhaps in some places catastrophic.
Through all these discoveries, Keeling and others had kept on monitoring and analyzing the ongoing changes in atmospheric CO2 levels. Since the 1980s, a cooperative international program had been measuring the gas at land stations around the world and along shipping lanes. The baseline continued to rise ominously, but not smoothly. There had been years when the world's atmosphere had gained one billion metric tonnes of the gas, while in other years it gained as much as six billion. How much did changes in the world's industries and agricultural practices affect the rate of the rise? Economic statistics allowed a good reckoning of how much gas humanity emitted in burning fossil fuels — and also of some significance, in the manufacture of cement — but the effects of deforestation and other land use changes were not so easy to figure.
Beyond that, how much did changes in the level of CO2 reflect changes in the growth or decay of plants, perhaps related to some fluctuation in the oceans or on the Sun? What could one learn from the way the curve reacted to temporary climate changes brought on by El Niño events, volcanic eruptions, and so forth?(58) Further clues came from world-wide measurements of other biologically active gases, especially oxygen (the exacting techniques for measuring the tiny variations were pioneered by Keeling's son, Ralph Keeling).(59) Most of the "missing" carbon was finally located, with gradually increasing precision, in rapidly changing forests, soils, and other biological reservoirs.
Meanwhile the level of CO2 in the air kept rising, indeed faster than anyone had expected. Ever since the late 1950s, an increasing number of experts had been saying that effects on climate would become clearly visible around the year 2000. They were right. As the 21st century began, not only was the global temperature soaring in a way never seen before, but field evidence showed that the expected feedbacks were kicking in. The world's plants were taking up more CO2, but many ecosystems were under stress and their capacity to absorb was waning. Warmer oceans were absorbing less CO2, and gas was seen bubbling from melting Arctic tundra.(60) In sum, global warming was leading to more greenhouse emissions, which would lead to more warming... and so forth. Also as predicted, the world was beginning to suffer worse heat waves, droughts, floods, and severe storms, while the sea level rose and important ecosystems began to show signs of stress. (See the essay on impacts). For the scientists, as one of them remarked, "Seeing their own predictions come true has been a frightening experience."(61)
Still more sobering, people were just now coming to grips with the implications of a fact that scientists had known for decades — the climate system has built-in time lags. Even if human emissions of CO2 magically dropped to zero, the gas already in the air would linger for many centuries, trapping heat. Global temperatures would continue to creep upward until the ocean depths reached equilibrium with the heated air, until biological systems finished adapting to the new conditions, and until Arctic icecaps melted back to their own equilibrium. Whatever we did now, humanity was already committed to centuries of changing weather and rising seas.(62) Yet emissions of greenhouse gases, far from halting, were soaring at an accelerating rate.
The basic physics and chemistry of the problems raised by Tyndall were now well in hand. There were reliable calculations of the direct effects of CO2 on radiation, of how the gas was dissolved in sea water, and other physical phenomena. Further progress would center on understanding the complex interactions of the entire planetary system, and especially interactions with living creatures. The creatures who would count the most were humans. The climate a century hence would depend chiefly on what they chose to do about their emissions. |
