Many climate scientists believe that the middle of the 21st century will witness a dramatic acceleration in global warming. In part, this acceleration will result from exhaustion of various natural sinks that have been absorbing greenhouse gases. At about the same time, various feedback loops are also expected to accelerate natural increases in atmospheric greenhouse gas levels and worldwide temperatures. These include several natural processes that add greenhouse gases to the atmosphere such as melting permafrost in the Arctic and, in the far future, possible gasification of solid methane deposits (clathrates) in the oceans, as well as an increasingly dark Arctic Ocean that will absorb more heat as its ice cap melts.

In each case, human-provoked warming caused by an overload of greenhouse gases in the atmosphere is expected to aggravate the natural feedback loops like a bank account drawing an environmentally dangerous form of compound interest. Evidence is accumulating that these processes have already begun. The danger, according to many people who are familiar with the paleoclimatic record, is that once this journey has begun in earnest, any return trip may become a matter of many centuries as well as copious human and natural pain and suffering.

In addition to possible increases in greenhouse gases from gasifying permafrost, other feedback mechanisms are expected to enhance the effects of greenhouse warming during the 21st century. Two of the most important are increasing amounts of water vapor (itself a greenhouse gas), as well as changes in the planet’s albedo, or reflectivity, as land and sea surfaces once covered by ice are replaced by darker-colored bare land and open water.

One of global warming’s most troubling aspects is its tendency to compound through feedback loops. Thus, its effects are not linear but accelerate as temperatures rise. Sea ice that melts, for example, exposes open water, which allows a darker surface to absorb more heat. Warming in the Arctic also melts permafrost and adds carbon dioxide and methane to the atmosphere. Extremes of weather such as heat waves, droughts, and floods can impede plant growth and decrease carbon absorption. The power of feedbacks is such that the worldwide rate of global warming may double, at least, during the half-century to come if greenhouse gases continue to increase in Earth’s atmosphere at current rates, according to climate simulation models analyzed by Steven Smith and his colleagues at the Pacific Northwest National Laboratory in Richland, Washington ( “Global Warming” 2015 ).

In the journal Nature (2013, 147), Quirin Schiermeier wrote:

Satellite observations have been combined with information from carbon dioxide measurement towers, providing a suggestion that extreme weather events reduce plant productivity in Europe as much 15 percent because of human emissions. Heat waves can turn forests that are usually carbon sinks into carbon sources. “In 2003 alone, a record-breaking heat wave in Europe led to the release of more CO₂ than is normally locked up over four years,” Schiermeier wrote (2013 , 1247). Drought also impedes plants’ ability to combat insect and pathogen damage.

By 2007, methane was bubbling up through melting permafrost, and Inuit hunters sometimes lighted impromptu fires to warm themselves with the gas ( Funk 2007 , 54). “We are taking risks with a system we don’t understand that is absolutely loaded with carbon,” said Steven Kallick, a Seattle-based expert on the boreal forests for the Pew Charitable Trusts. “The impact could be enormous” ( Struck 2007 ). “With permafrost, it may take longer for change to get moving. But it may keep moving, even if we get our emissions under control,” said Antoni Lewkowicz, a professor of geography at the University of Ottawa. “It’s like a big boulder. Once you get it moving, it won’t stop” ( Struck 2007 ).

Even after carbon dioxide and methane levels in the atmosphere stop rising, feedbacks will produce adverse conditions for decades to centuries. The amount of time that temperatures and sea levels may continue to rise after carbon dioxide levels stabilize is not known with any degree of certainty, however. According to one scientific observer, “The rise in mean global SAT [surface average temperature] and the world ocean level (due to thermal expansion) may … continue for several centuries after the stabilization of the carbon dioxide concentration [in the atmosphere], due to the gigantic thermal inertia of the oceans.” The same observer also believes that “The response of ice sheets to earlier climate changes may continue for several centuries after the climate stabilizes” ( Kondratyev et al. 2004 , 45).

Geophysical evidence suggests that Earth has suffered bouts of severe warming in the distant past from natural causes that were intensified by the release of stores of greenhouse gases. Considerable scientific inquiry is now aimed at determining just how much human-provoked warming might cause the process to reach a “runaway” status in which the feedbacks take control and force warming out of control.

Frozen bogs in Siberia contain an estimated 70 billion tons of methane. If the bogs become drier as they warm, according to one observer, “the methane will Page 130  |  Top of Articleoxidize and the emissions will be primarily CO₂. But if the bogs stay wet, as they have been recently, the methane will escape directly into the atmosphere.… with 20 times the heat-trapping power of carbon dioxide” ( Romm 2007 , 69). Some 600 million tons of methane are emitted each year from human and natural sources, so if even a small fraction of the 70 billion tons of methane in the Siberian bogs is released, warming will accelerate dramatically.

Another example of a feedback loop that will reinforce warming temperatures (and probably drought) occurs in the tropics. A rain forest or jungle creates its own weather. In our time, however, humans seeking food and shelter have competed with nature so that now roughly 20 percent of the Amazon rain forest has been destroyed, and another 20 percent has been damaged by logging to such an extent that sunlight can reach the forest floor and cause significant drying. According to some studies, “Models suggest that when 50 percent of the forest is destroyed (The year 2050 would be a reasonable estimate for that, at present rates) we will reach a tipping point at which drought and heat will combine to at the rain forest and its canopy, reducing local rainfall and further accelerating the drought and local temperature rise, ultimately causing the release into the atmosphere of huge amounts of carbon currently locked in Amazon soils and vegetation, another fearsome feedback loop at work” ( Romm 2007 , 72).

In cold water, methane clathrates form crystal structures that are somewhat similar to water ice. Warming temperatures could destabilize the clathrates and release some of their stored methane. Roughly 10 trillion tons of methane is trapped under pressure in crystal structures in permafrost or on the edges of the oceans’ continental shelves, “Earth’s largest fossil fuel reservoir,” according to Gerald Dickens, a geologist at James Cook University in Townsville, Australia ( Pearce 1998 ). The greenhouse potential of all the methane stored in clathrates on the continental shelves and in permafrost worldwide is roughly equal to all of the world’s coal reserves ( Cline 1992 , 34).

Atmospheric scientist Roger Revelle has estimated that, with a 3°C rise in global average temperature, methane emissions from clathrates would increase half a gigaton per year worldwide. Over a century, this rate could be enough to double the amount of methane in the atmosphere. Add to this another 12 gigatons of methane that could be released by clathrates liberated from ocean bottoms under the Arctic Ocean once the ice cap now covering them melts. “It is possible,” writes Jonathan Weiner, “that [this] … feedback effect is already underway and the rise Page 132  |  Top of Articlein Earth temperatures in the last hundred years has already sprung many gigatons of methane from their molecular prisons at the bottom of the sea” ( Weiner 1990 , 118).

Warming temperatures also change the behavior of Earth’s hydrological cycle. Warmer ocean water removes less carbon dioxide from the atmosphere than cooler water, so warming oceans may feed on themselves in coming years. Water vapor is also a potent absorber of heat in the atmosphere. A doubling of carbon dioxide in the atmosphere has been estimated as increasing its water content by 30 percent, raising temperatures an additional 1.4°C ( Hansen 1981 , 957). Many models project a rise in cloudiness and attendant atmospheric moisture in a warmer, more humid world. George M. Woodwell raises the possibility of a rapid surge in global warming beyond any possibility of human control:

Given Woodwell’s expectations, the peoples of Earth are approaching a point of no return with regard to climatic feedbacks. Deforestation is accelerating around the world because of growing populations and levels of material affluence. Use of fossil fuels, which has increased at an annual rate of roughly 5 percent during most of this century, shows no signs of stabilizing, much less falling by half in the next 30 years. China alone projects burning enough fossil fuel (mainly coal) by 2025 to account for around half the current consumption of fossil fuels by everyone on Earth ( Leggett 1990 , 27).

How much time remains before critical feedback loops lock into place? In January 2005, a world task force of senior politicians, business leaders, and academics warned that the point of no return—including widespread agricultural failure, water shortages and major droughts, increased disease, sea level rise and the death of forests—will occur as the average world temperature increases 2°C above the average prevailing in 1750, shortly before the Industrial Revolution began ( Byers and Snowe 2005 , 1). By 2015, temperatures already had risen an average of 1.0°C. The report also asserted that the tipping point will occur as the atmospheric concentration of carbon dioxide passes 400 parts per million (ppm). The level was 379 ppm in 2004, and the threshold of 400 ppm was crossed in 2015. Given half a century for thermal inertia to realize the effects of CO₂ at 400 ppm, serious damage is now factored into the system. The report was assembled by the Institute for Page 133  |  Top of ArticlePublic Policy Research in the United Kingdom, the Center for American Progress in the United States, and the Australia Institute. The group’s chief scientific adviser was Rakendra Pachauri, chairman of the U.N. Intergovernmental Panel on Climate Change ( McCarthy 2005 ).

The report concluded:

The science of warming feedbacks is not settled knowledge. In areas such as the role of organic decomposition in soils under warmer conditions, a robust debate continues. Knorr and colleagues have written in Nature that “the sensitivity of soil carbon to warming is a major uncertainty in projections of carbon dioxide concentration and climate” ( Knorr et al. 2005 , 298) because their findings indicate that “the long-term positive feedback of soil decomposition in a warming world may be even stronger than predicted by global models” ( Knorr et al. 2005 , 298). In the meantime, the very idea that organic soil decomposition is sensitive to temperature at all has been challenged by other scientists ( Giardina and Ryan 2000 ). For climate models, the solution of this debate is no small matter because soils contain twice as much carbon as the atmosphere ( Powlson 2005 , 204), so the rate at which warming may accelerate exchange from one to the other is and will continue to be an important factor as other feedbacks add more greenhouse gases to the air that sustains us.

“The biggest lag is in the political system,” said geoscientist Michael Oppenheimer of Princeton University. At least two decades have passed since scientists have pointed out the seriousness of the threat, he said, and another 20 years may pass before a worldwide program is established that is up to the task. In the meantime, the window of time before feedbacks take control narrows. “We can’t really afford to do a ‘wait and learn’ policy,” Oppenheimer said. “The most important question is: when do we commit to [contain global warming to] 2 [°C]. Really, there isn’t a lot of headroom left. We better get cracking.” The current pace, said Roger Pielke Jr. of the University of Colorado at Boulder, “isn’t going to do it” ( Kerr 2007 , 1231).

See also: Desertification ; Drought, United States ; Drought, Worldwide ; El Niño and La Niña ; Extreme Weather ; Heat Waves, Evidence of ; Heat Waves, Forecasts of ; Permafrost ; Pliocene Paleoclimate ; Sea Ice, Arctic ; Temperatures, Global ; Temperatures, Greenhouse Gas Levels and ; Thermal Inertia