It can be estimated using three main lines of evidence:. Despite its importance, equilibrium climate sensitivity is very uncertain and for many years the standard estimate has been 1. The lower end of the range has therefore risen substantially, meaning that scientists are now much more confident that global warming will not be small. This has important implications for climate change risk assessments.
In a risk assessment, it is normal to consider outcomes that are possible even if they are not the most likely. The latest climate models have a wide range of climate sensitivities, with our own Met Office models at the high end. This happens because climate sensitivity is not something that scientists input to the models, but rather it emerges from the same complex interactions the models simulate.
This diversity of models lets us understand the regional changes in climate and extreme weather associated with different climate sensitivities, and assess their potential impacts. This includes the high sensitivities that are less likely but still possible.
At the other end of the range, seeing the minimum changes we can expect will help inform climate change adaptation measures. The new study allows a key aspect of climate models, their climate sensitivity, to be seen in the context of other evidence. While there is still more to be done to assess more precisely how the global climate will respond to further increases in greenhouse gases, these advances provide a much more solid base of evidence on which climate change policy can be further developed.
This article is published in collaboration with The Conversation. The views expressed in this article are those of the author alone and not the World Economic Forum. Here's how the private sector can play a central role in co-piloting the European Green Deal, the EU's ambitious plan to become climate neutral by Where was the rest? Oceanographers calculated how much of the gas the oceans took up, while other scientists calculated how much the biosphere took up or emitted.
The numbers didn't add up — some of the carbon was "missing. Looking at large-scale climate changes, such as between ice ages and warm periods, they turned up a variety of possible interactions with climate involving plant life and ocean chemistry.
The papers addressing these topics became increasingly complex. Some scientists took up the old argument that fertilization of plant life by additional CO 2 , together with uptake by the oceans, would keep the level of gas from rising too sharply. Keeling, however, warned that by the middle of the next century, plants could well reach their limit in taking up carbon as every gardener knows, beyond some point more fertilizer is useless or even harmful.
Further, there would eventually be so much CO 2 in the ocean surface waters that the oceans would not be able to absorb additional gas as rapidly as at present. The curve did not climb smoothly, but stuttered through a seasonal cycle, plus mysterious spells of faster and slower growth.
But over a long term, say a decade, the rise was clearly as inexorable as the tides. And global temperatures began to rise again. It was getting increasingly difficult for scientists to believe that the greenhouse effect was no cause for worry.
Meanwhile global temperatures resumed their rise. The cooling from smoke particles had a limit, for the particles dropped from the atmosphere in weeks whereas the accumulating CO 2 would linger for centuries. It was getting increasingly difficult for scientists to claim that the greenhouse effect was no cause for worry. By the ever more powerful computers had confirmed that it was impossible to construct a model that could mimic the current climate and that did not warm up a few degrees if the level of the gas was doubled.
How would we know if we should take action to avert dangerous climate change? In a couple of experienced climate scientists reviewed the predictions of the best computer models, and compared them with the natural fluctuations of climate observed in the past. Concerns were sharpened by new evidence from holes arduously drilled into the Greenland and Antarctic ice caps. The long cylinders of ice extracted by the drills contained tiny bubbles with samples of ancient air — by good fortune there was this one thing on the planet that preserved CO 2 intact.
Group after group cut samples from cores of ice in hopes of measuring the level. For two decades, every attempt failed to give consistent and plausible results. Finally reliable methods were developed. The trick was to clean an ice sample scrupulously, crush it in a vacuum, and quickly measure what came out. In , a team published findings that were definite, unexpected, and momentous. These Greenland measurements were later called into question, but the dramatically lower ice-age level was quickly confirmed by other studies.
When scientists get the same numbers using utterly different methods, they begin to feel that they have touched reality.. The Vostok core, an ice driller declared, "turned the tide in the greenhouse gas controversy. All through these decades, a few geologists had continued to pursue the original puzzle raised by Tyndall and Chamberlin — had changes of CO 2 been responsible for the greatest of climate changes?
These were the vast slow swings, lasting tens of millions of years, between eras like the age of dinosaurs with summer-like climates almost from pole to pole, and eras like our own when continental ice caps waxed and waned. There was no consensus about the causes of these grand shifts, and nobody had found a way to reliably measure the atmosphere many millions of years back.
Nevertheless, by the s, scientists turned up evidence suggesting that CO 2 levels had been elevated during the great warm eras of the past. Lines of thinking converged to emphasize the importance of the greenhouse effect. Geologists had been puzzled for decades by a calculation that astrophysicists insisted was indisputable: billions of years ago the Sun had been dimmer, and had gradually grown brighter as it consumed its nuclear fuel. In the distant past the oceans should have been frozen entirely.
Yet somehow the Earth's temperature had remained neither too cold nor too hot to sustain life. The best guess was that CO 2 acted as a thermostat for the planet.
Volcanoes presumably put the gas into the atmosphere at a fairly constant rate. In the earliest times there could have been enough greenhouse gases to keep the planet unfrozen. But chemical processes run faster at higher temperatures, so as the Sun grew brighter the weathering of rocks would take up CO 2 faster. As the rocks eroded, rivers carried the soil into the seas, where the carbon eventually wound up in compounds deposited on the seabed.
Thus a rough self-sustaining balance would be maintained among the forces of solar radiation, volcanic emissions, greenhouse warming, weathering, and ocean uptake. To be sure, if there was some great disturbance the system might take millions of years to stabilize. Such great disturbances — even a totally glaciated "snowball Earth" — were not a fantasy of oversimplified models.
Geologists turned up evidence that more than half a billion years ago the oceans had actually frozen over, if not entirely then mostly. That seemed impossible, for how could the Earth have escaped the trap and warmed up again? There was at least one obvious way but it was only obvious once someone thought of it, which took some years.
Over many thousands of years, volcanoes would have continued to inject CO 2 into the atmosphere. There the gas would have accumulated, since it could not get into the frozen seas. Eventually a colossal greenhouse effect might have melted the ice. The planet Venus, on the other hand, seemed to have suffered a runaway greenhouse catastrophe: a surface that might once have been only a little warmer than the Earth's had heated up enough to evaporate the carbon in the rocks into the atmosphere while ever more CO 2 was created, making the planet a hellish furnace.
All this was speculative, and proved little about our future climate. But it added to the gathering conviction that CO 2 was the very keystone of the planet's climate system — a system by no means as cozily stable as it appeared.
Another unusual disturbance had begun. And the drill was still only partway down; by the time they stopped drilling a dozen years later, the team had recovered ice going back , years, through four complete glacial cycles. The CO 2 levels in their record got as low as parts per million in the cold periods and reached in the warm periods, never higher. But in the air above the ice, the level of the gas had reached — far above anything seen in this geological era and still climbing.
Level of CO 2 in the atmosphere, The curve has been climbing exponentially, much faster now than in the s. Despite some attempts to slow down emissions, the quantity of gas added to the atmosphere is doubling every years. During the s, further ice core measurements indicated that at the end of the last glacial period, the initial rise of temperature in Antarctica had preceded CO 2 changes by several centuries.
Scientists debated whether the dates could be measured so precisely, but certainly around Antarctica the temperature rise had not come much after the rise of CO 2. But in fact the discrepancy 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 s, painstaking studies had shown that subtle shifts in our planet's orbit around the Sun called "Milankovitch cycles" matched the timing of ice ages with startling precision.
The amount of sunlight that fell in a given latitude and season varied predictably over millenia. As some had pointed out ever since the 19th century, in times when sunlight fell more strongly on northern latitudes in the spring, snow and sea ice would not linger so long; the dark earth and seawater would absorb more sunlight, and get warmer. However, calculations showed that this subtle effect should cause no more than a small regional warming.
How could almost imperceptible changes in the angle of sunlight cause entire continental ice sheets to build up and melt away? The new ice cores suggested that a powerful feedback amplified the changes in sunlight. The crucial fact was that a slight warming would cause the level of greenhouse gases to rise slightly.
For one thing, warmer oceans would evaporate out more gas. For another, as the vast Arctic tundras warmed up, the bogs would emit more CO 2 and another greenhouse gas, methane, also measured in the ice with a lag behind temperature.
The greenhouse effect of these gases would raise the temperature a little more, which would cause more emission of gases, which would Many thousands of years later, the process would reverse when the sunlight falling in key latitudes weakened.
Bogs and oceans would absorb greenhouse gases, ice would build up, and the planet would slide back into an ice age. This finally explained how tiny shifts in the Earth's orbit could 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 CO 2 , or take it up Into the 21st century, scientists kept finding new ways that warming would push more of the gas into the atmosphere.
As one of them remarked, "it is difficult to explain the demise of the ice sheets without the added heating from CO 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, CO 2 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 CO 2 might bring in the future. What was now beyond doubt was that the greenhouse effect had to be taken very seriously indeed. 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.
No model that could simulate the Earth's climate — and some of the simulations had become very good indeed — failed to show warming if its greenhouse gas level was raised. Scarcely any expert with a record of contributing to climate science now doubted that CO 2 and other greenhouse gases were at least partly responsible for the unprecedented warming all around the world since the s.
A final nail in the coffin of scientific skepticism came in , when a team compiled accurate long-term measurements of temperatures in all the world's ocean basins. It was not in the air but the massive oceans, after all, that most of any heat added would soon wind up. Indeed natural fluctuations had kept air temperatures roughly the same since the late s; the significant question was whether the oceans were continuing to warm.
The team found that over many decades the planet's content of heat-energy had been rising, and was rising still this continued steadily after as well. There was only one remotely plausible source of the colossal addition of energy: the Earth must be taking in more energy from sunlight than it was radiating back into space.
Simple physics calculated that to heat all that seawater required nearly an extra watt per square meter, averaged over the planet's entire surface, year after year. The number was just what the elaborate greenhouse effect computations had been predicting for decades.
James Hansen, leader of one of the studies, called the visible increase of the planet's heat content a "smoking gun" proof of greenhouse effect warming see graph below. Moreover, in each separate ocean basin there was a close match between the pattern of rising temperatures measured at each location and depth and detailed model calculations of where the greenhouse effect warming should appear. Warming from other sources, for example a change in the Sun's output, could not produce these patterns.
Evidently the modelers were on the right track. Yet amid all the uncertainties about how carbon cycles operated, how much could we trust the computer models to work under circumstances different from the present? Scientists are more likely to believe something if they can confirm it with entirely independent lines of evidence, preferably from somewhere nobody had looked before. Just such new evidence came up in the s, 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 CO 2 , the structure of their leaves changed. Ancient fossil leaves showed just such changes. Several kinds of chemical studies of ancient rocks and soils helped pin down how the level of the gas had swung widely over geological ages, and the temperature too.
A sustained effort by many geochemists and their allies managed to get numbers for the "climate sensitivity" in past eras, that is, the response of temperature to a rise in the CO 2 level. Not only during the recent ice ages but back over hundreds of millions of years under radically different conditions, a doubled level of the gas had always gone along with a temperature rise of three degrees, give or take a degree, in full agreement with the computer calculations.
It was reassuring that there seemed scant possibility of a Venus-style runaway greenhouse apocalypse. It was less reassuring to see what the climate had looked like in the ancient eras when CO 2 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. To be sure, it would take many thousands of years to melt entire polar ice caps. But in the meantime even a modest sea-level rise would disrupt humanity's teeming coastal populations. If humanity's emissions continued they seemed bound to bring not only "a warming unprecedented in the past million years," as one of many worried scientists explained, but changes "much faster than previously experienced by natural ecosystems By , new studies had pinned down some truly disturbing numbers about eras in the distant past when CO 2 levels had been high — although no higher than we would reach by the late 21st century if emissions continued to rise without restriction.
In those eras global temperatures had been at least three degrees higher than at present, and perhaps as much as six degrees higher, that is, in the upper range of what computer models found plausible, if not higher still. For the real planet, a rise in temperature had evidently not been limited by increased cloud reflection or the like. The rise had instead apparently been amplified by positive feedbacks, as ice and oceans and vegetation responded over centuries to the changing conditions with darker surfaces and their own gas emissions.
The computer models did not take these slow feedback loops into account. Hansen and others argued that humanity risked setting off a chain reaction that would eventually bring an altogether catastrophic planetary change. In the first decade of the 21st century international panels of experts reviewed the evidence, and announced conclusions that were checked and endorsed by virtually all the major national science academies, scientific societies, government science agencies and other bodies representative of scientific expertise.
All of these bodies declared that the world faced a serious problem, andrecommended that governments adopt strict policies to restrict greenhouse gas emissions.
All, that is, except a few self-appointed panels composed primarily of people with limited expertise in climate science, representing ideological and business interests that opposed all forms of government regulation. Of course they continued to argue vehemently over details, as always in frontier research.
Critics pounced on every apparent discrepancy. They published long lists of scientists who denied there was any problem — although the lists included hardly any scientist who had made significant contributions to climate research. Through all these discoveries and controversies, Keeling and his colleagues had kept on quietly monitoring and analyzing the ongoing changes in atmospheric CO 2 levels.
Since the s, 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.
How much did changes in the level of CO 2 reflect changes in the growth or decay of plants, perhaps related to climate fluctuations? Most of the "missing" carbon was finally located, with gradually increasing precision, in rapidly changing forests and forest soils, along with other biological reservoirs. Another marker of biological activity was the rare isotope carbon Plants take less of it from the atmosphere than the lighter isotope carbon, so the latter is over-represented in coal and oil.
The fraction of the lighter isotope in the air was increasing, proving to a lingering band of skeptics that the rise in CO 2 came from humanity's use of fossil fuel, not from a mineral source such as volcanoes. The 21st century brought a grand expansion of studies of the way CO 2 was accumulating in the atmosphere, and where the carbon went in the land and oceans. Here as elsewhere in climate research, the global scope and complexity of the problem called forth massive international projects.
At thousands of locations instruments measured air, soils, trees, seawater, and more, providing "ground truth" for satellites that scanned the entire planet. Meanwhile the level of CO 2 in the air kept rising, indeed faster than anyone had expected.
Over the half-century up to , emissions of CO 2 had quadrupled. Efforts to reduce use of fossil fuels did have an effect, and by global emissions reported by national authorities were rising more slowly.
Of course even with a steady level of human emissions CO 2 would continue to accumulate rapidly in the atmosphere. For decades the rise had in fact been accelerating, perhaps because natural systems were not absorbing carbon efficiently. In this essay we have seen how, ever since the late s, an increasing number of experts predicted that effects on climate would become clearly visible around the year They were right.
As the 21st century began, the global temperature was soaring in a way never seen before. The rise in surface air temperature was irregular, but the oceans, where most of the heat energy was stored, warmed up steadily.
Worse, field evidence showed that the expected feedbacks were kicking in. The world's plants were taking up more CO 2 , but their capacity to absorb was waning. Warmer oceans were absorbing less CO 2 , and gas was seen bubbling from melting Arctic tundra.
In sum, global warming was leading to more greenhouse emissions, which would lead to more warming Starting around some scientists began to warn that these changes were coming on faster than the international panels had predicted.
Also as predicted only sooner, the world was beginning to suffer historically unprecedented heat waves, droughts, floods and storms. So while carbon dioxide contributes less to the overall greenhouse effect than water vapor, scientists have found that carbon dioxide is the gas that sets the temperature.
Carbon dioxide controls the amount of water vapor in the atmosphere and thus the size of the greenhouse effect. Rising carbon dioxide concentrations are already causing the planet to heat up. At the same time that greenhouse gases have been increasing, average global temperatures have risen 0. With the seasonal cycle removed, the atmospheric carbon dioxide concentration measured at Mauna Loa Volcano, Hawaii, shows a steady increase since At the same time global average temperatures are rising as a result of heat trapped by the additional CO 2 and increased water vapor concentration.
The degree to which temperatures go up beyond that depends in part on how much more carbon humans release into the atmosphere in the future. About 30 percent of the carbon dioxide that people have put into the atmosphere has diffused into the ocean through the direct chemical exchange. Dissolving carbon dioxide in the ocean creates carbonic acid, which increases the acidity of the water.
Or rather, a slightly alkaline ocean becomes a little less alkaline. Some of the excess CO 2 emitted by human activity dissolves in the ocean, becoming carbonic acid. Increases in carbon dioxide are not only leading to warmer oceans, but also to more acidic oceans. Ocean acidification affects marine organisms in two ways. First, carbonic acid reacts with carbonate ions in the water to form bicarbonate. However, those same carbonate ions are what shell-building animals like coral need to create calcium carbonate shells.
With less carbonate available, the animals need to expend more energy to build their shells. As a result, the shells end up being thinner and more fragile. Second, the more acidic water is, the better it dissolves calcium carbonate. In the meantime, though, more acidic water will dissolve the carbonate shells of marine organisms, making them pitted and weak. Warmer oceans—a product of the greenhouse effect—could also decrease the abundance of phytoplankton, which grow better in cool, nutrient-rich waters.
On the other hand, carbon dioxide is essential for plant and phytoplankton growth. An increase in carbon dioxide could increase growth by fertilizing those few species of phytoplankton and ocean plants like sea grasses that take carbon dioxide directly from the water.
However, most species are not helped by the increased availability of carbon dioxide. Plants on land have taken up approximately 25 percent of the carbon dioxide that humans have put into the atmosphere. Only some of this increase occurred as a direct result of fossil fuel emissions. With more atmospheric carbon dioxide available to convert to plant matter in photosynthesis, plants were able to grow more.
This increased growth is referred to as carbon fertilization. Models predict that plants might grow anywhere from 12 to 76 percent more if atmospheric carbon dioxide is doubled, as long as nothing else, like water shortages, limits their growth.
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