Positive Feedbacks and Climate Runaway – The Need to Act without Delay

LVW Climate Change Taskforce
Chad A. Tolman, lead writer

Whydo most of the world's governments—representedon the Intergovernmental Panel on Climate Change (IPCC)—as well as most climate scientists and the League of Women Voters of the United States—think that strong action needs to be taken to reduce greenhouse gas (GHG) emissions without further delay? Instrumental measurements for more than 50 years and ice core records going back over 650,000 years show that the concentrations ofgreenhouse gases are higher now that at any time in that long period.  We are now in uncharted waters.  Experts advise that GHG emissions mustpeak and begin declining before 2015.

AnIPCC report issued in November 2007 says that "Atmospheric concentrations of CO₂(379 ppm) and CH₄ (1774 ppb)¹ in 2005 exceed by far the natural range overthe last 650,000 years (emphasis added).  Global increases in CO₂ concentrations are due primarily to fossil fuel use—with land use change providing another significant but smaller contribution.  It is very likely that the observed increase in CH₄(methane) concentration is predominantly due to agriculture and fossil fueluse."²  Increasing concentrations of GHGs in theatmosphere are trapping heat near Earth's surface and causing the global average temperature to rise.

If we allow GHG emissions from fossil fuel burning anddeforestation to continue to grow at their present rates, the warming that will follow can be expected to decrease albedo³ (reflectivity) and increase the rate of absorption of solar energy near the poles, increase water vapor concentrations in the atmosphere, melt Arctic tundra, release stores of carbon from soils and sea floors, and accelerate the melting of ice.  Climate change is happening more rapidly than anyone expected.⁴  We could reach a tipping point where we could have a runaway climate change—one over which we no longer have any control—when releases of carbon (especially methane) from natural reservoirs greatly exceed emissions from burning fossil fuels.  The grave danger we face and the need to take vigorous global action to reduce GHG emissions without delay are clear.

Those whothink that the recent scientific assessment of climate sensitivity is preliminary, that we may be able to get away with higher levels of CO₂than 350 ppm, or that the rate of ice melting may slow down, should recall the moral and political Precautionary Principle which advises:

 "... a willingness to take action inadvance of scientific proof [or] evidence of the need for the proposed actionon the grounds that further delay will prove ultimately most costly to society and nature, and, in the longer term, selfish and unfair to future generations."⁵ 

To help you understand the seriousness of the current threat, several importantconcepts are discussed below:  (1) feedbacks in the climate system, (2)tipping points and abrupt climate change, (3) climate sensitivity—the dependence of temperature on CO₂, (4) natural climate change, (5) anthropogenic climate change, (6) global warming and sealevel rise, and (7) time lags and the lifetime of CO₂.

(1) Feedbacks in the Climate System

Climate feedbacks may be negative orpositive.  Negative feedbacks cause the climate to change more slowly the larger the change becomes.  An example is the more rapid uptake of CO₂by plants as its concentration increases.⁶  Positive feedbacks—which cause the climate to change more rapidly the larger the change becomes—predominate.  Positive feedbacks linked to rising temperatures⁷ include the following:     

• Decreasing albedo (reflectivity).  As snow and ice warm and melt, to be replaced by darker soil or deep blue sea, surface reflectivity decreases markedly, causing more heat to be absorbed and leading to more rapid warming.  A competing effect is that cloud cover may increase as Earth warms, tending to increase the reflection of sunlight out into space.⁸
• Increasing concentrations of water vapor.  Water vapor is a powerful, naturally occurring GHG,⁹ and the amount of it in the atmosphere at equilibrium increases as temperatures rise.  Rising concentrations of watervapor cause a greater greenhouse effect, leading to further warming.
• Increasing rates of release of carbon from natural reservoirs.  As temperatures rise, large natural reservoirs of carbon in soils and sea floors could be released as CO₂or methane—adding to the GHGs produced directly by human activity.  The releases could result from more rapid bacterial action on carbohydrates in decaying plants or from the melting of gas hydrates.  There is a huge amount of carbon in soils, Arctic tundra, and permafrost, and in methane hydrate on the sea floor. 
  
  Methane hydrate is a type ofice containing up to 13% methane by weight. If it melts, the methane is released as a gas.  Indeed, there are already reports that bubbles of methane can be seen rising out of the lakes that are forming in Siberia as the permafrost melts.¹⁰ 
  Methane hydrate is stable at sufficiently low temperature and high pressure. Because temperatures are lower in the Arctic, the pressure doesn't need to be so high, and methane hydrate can be found nearthe surface.  In much warmer areas, methane hydrate is found on the ocean floor only at depths of more than 500 meters (m) (over 1500 ft).
  
  The amount of carbon current lyin non-frozen soils and permafrost is estimated to be about 2000 GtC (gigatons[billions of metric tons]).¹¹  The amount of carbon in the form of methane present on the ocean floors is uncertain but is probably much larger; most experts put the amount at 5,000-10,000 GtC.¹²  Compare these huge numbers to about 300 GtC that humans have added to the atmosphere since 1750,¹³ and the800 GtC as CO₂ in our present atmosphere.  A very large methane release is a disaster waiting tohappen.
  
  Methane is a powerful greenhouse gas; it traps heat much more effectively near Earth's surface than CO₂ does.  Although its atmospheric concentration was only about 1.8 ppm (1,774 ppb) in 2005, it produced 30% as much warming as the significantly larger 379 ppm concentration of CO₂.¹⁴  To make matters worse, a warming of only 2 or3°C¹⁵ can melt additional methane hydrate, releasing the methane as a gas.  This is not just a hypothetical scenario.  It happened before, 55 million years ago, at a time called the Paleocene-Eocene Thermal Maximum.¹⁶  At that time, an estimated 2,000 GtC was released as methane, causing global average temperatures to rise several degrees.  Most of the methane was oxidized in the atmosphere to CO₂, part of which dissolved in the oceans, acidifying them and causing an "oceanic extinction event" (a loss ofnumerous marine organisms)¹⁷ at the end of the Paleocene.

(2) Tipping Points and Abrupt Climate Change

As a 2002 report from the National Academy of Sciences stated:
"Abrupt climate changes were especially common when the climate system was being forced to change most rapidly.  Thus, greenhouse warming and other human alterations of the earth system may increase the possibility of large, abrupt, and unwelcome regional or global climatic events. The abrupt changes of the past are not fully explained yet, and climate models typically underestimate the size, speed,and extent of those changes.  Hence, future abrupt changes cannot be predicted with confidence, and climate surprises are to be expected.¹⁸

(3) Climate Sensitivity—The Relationship Between CO₂ Concentration and Temperature

The build-up of atmospheric concentrations of CO₂is of concern because it is closely correlated with increases in global average temperatures.  This relationship can be explored using global climate models and by looking at Earth's past climate history.  Svante Arrhenius, a Swedishchemist, was the first to use a mathematical model to explore what would happen to Earth's surface temperature if the CO₂ concentration were cut inhalf or doubled.  He spent the year of 1895 doing tedious hand calculations to conclude that halving CO₂would reduce global average temperatures by 4-5°C while doubling it would increase the temperature by 5-6°C.¹⁹,²⁰  This effect on temperature of doubling came to be known as "climate sensitivity." 

The impacts expected from increases in global average temperature relative to its pre-industrial value are shown in Figure 1. 

Figure 1. Expected impacts ofglobal average temperature change.  Froma presentation by Sir David King.²¹

In 1750, before the Industrial Revolution, the global average temperature was about 14°C (57°F), and the atmospheric concentration of CO₂ was 280 ppm.  Many, including the LWVUS,²² have recommended that the average temperature not be allowed to increase more than2°C (3.6°F). The temperature since 1750 has increased 0.8°C (1.4°F), and CO₂is at about 385 ppm—and increasing at over 2 ppm/year.²³  Many climate models more sophisticated than the one used by Arrhenius and run on high speed computers have projected that holding the CO₂ at 560 ppm (abbreviated2xCO₂ [in 1750]) would increase the global average temperature about3°C above its pre-industrial value.²⁴  Based on that projection, Dr. James Hansen,the chief climate scientist at NASA, strongly urged in 2006 that CO₂concentrations not be allowed to increase above 450 ppm—limiting the total equilibrium temperature increase²⁵ to no more than about 1°C above what it is now.²⁶  Dr. Hansen wrote— 

"This does not mean that climate impacts will be negligible if global warming is kept under 1°C(relative to year 2000), but the planetary conditions will be within a range in which we know that the climate did not go seriously haywire in the past.  In contrast, if warming approaches the range2-3°C (a result that is extremely likely before 2100 with business-as-usual increasing emissions of CO₂),^* it is virtually certain that there will be large-scale disastrous climate impacts for humans as well as for other inhabitants of the planet…"²⁷ 

In more recent work in 2008,²⁸ basedon Earth's past response to changing concentrations of CO₂, Hansen and coworkers have determined that climate sensitivity is actually about 6°Cfor a doubling of CO₂—twice what was reported by the IPCC asrecently as 2007.²⁹  Hansen's 2008 paper concludes that the concentration is already too high, and that we should work hard to reduce global CO₂ emissions to zero and then negative (a net uptake [thatis, sequestration] of CO₂) as soon as possible to reduce CO₂concentrations to below 350 ppm. 

Table 1 summarizes atmosphericCO₂ concentrations and the equilibrium temperature increases that can be expected for climate sensitivities of 3°C and 6°C.  Concentrations and temperature changes after2008 (shown in italics) are projections based on a 2% per year compoundincrease in global CO₂ emission rates.³⁰ 

Table 1.  Calculated EquilibriumTemperature Increases as CO₂ Concentrations Increase

The temperature increase so far(0.8°C) is less than the equilibrium values shown because of the slow responseof the global average temperature to changes in CO₂ concentrations;the oceans take a long time to heat up. The projected future temperature changes shown may be underestimatedbecause they assume that the fraction of CO₂ taken up by oceans andplants remains constant and that sudden large emissions of CO₂ ormethane from natural carbon sinks— like Arctic tundra or methane hydrates onthe sea floor—do not occur.

(4) Natural Climate Change

Incomingsolar radiation, the Earth's orbital parameters, volcanic action and weatheringof rock, ocean circulation, and the concentrations of greenhouse gases can allaffect global climate.³¹ 

 The planet has experienceddramatic climate changes in the past; North America as far south as New York was coveredwith the Laurentide ice sheet a mile thick as recently as 20,000 yearsago.  Three million years ago Earth wasso warm that there was no Greenland ice sheet;40 million years ago it was even warmer, and there was no Antarctic icesheet.  Prior to the Industrial Revolution (that is, prior to about 1750),climate change was due to natural causes. Now humans are making large and rapid changes in Earth's atmosphere andland surface, causing climate change that could have enormousconsequences. 

 (5) Anthropogenic Climate Change

Burningfossil fuels (coal, oil, and natural gas), deforestation, population growth,and agricultural practices are changing the composition of the atmosphere(increasing the concentrations of CO₂ and other GHGs, soot, and sulfates)and changing the surface of the land, including melting large areas that havebeen covered by ice and snow.  The large climate changes that have been observed in thepast 30 years cannot be accounted for unless the effects of human activitiesare included.

(6) Global Warming and Sea-Level Rise

Of all the threats posed by global warming—more severedroughts and floods, crop yield losses, more intense hurricanes, the spread ofdiseases, increased forest fires, species extinction, and sea level rise—thelast poses perhaps the most obvious threat to modern industrial societies, withtheir major cities on coasts and their dependence on ports for internationaltrade.³²  As GHG concentrations and temperatures rise,sea levels also rise for two reasons:  (1) Seawater expands as it warms, and (2)water runs into the oceans from glaciers melting on land. 

Global mean temperature threemillion years ago was only 2-3°C higher than it is today while sea level was25±10 m (80±30 ft) higher.³³  When the atmosphere last had a concentrationof 560 ppm, twice what it was in 1750, about 7 million years ago, there was noGreenland ice sheet and considerably less ice in Antarctica.³⁴  If just the Greenland and the West Antarcticice sheets melt, this would raise sea levels by 15 m (50 ft), submerging largeparts of the Delmarva Peninsula,³⁵ Florida, much of Bangladesh, several small island states (e.g., the Maldivesand the Marshall Islands),³⁶ andother low lying areas.  A 50-ft risewould drown many large coastal cities and can aptly be called"catastrophic".  A recent paper using data on landelevation and population in coastal areas reports that a sea level rise of just6 m (20 ft) would inundate over 2 million km2 (720,000 square miles)and displace over 430 million people.³⁷  Even a 1 m rise would displace more than 100million. 

Figure 2. Relationship betweenglobal mean temperatures and changes in sea level relative to today's, from thepaleoclimate record.³⁸ 

The relationship between globalaverage temperature and sea level, based on earth's behavior for the past 40million years, is shown in Figure 2. Note that the solid points represent equilibrium conditions—with enoughtime for the oceans, ice, and vegetation to fully respond.  The open point, labeled Projection for 2100, which shows a projected temperature of about18°C and a sea level rise of 1 m in 2100, is based on the fact that the icewill not have had nearly enough time by then to fully respond to thetemperature change.  The best straightline drawn through the solid points has a slope of 20 m/°C (37 ft/°F).  This means that we can expect an equilibriumsea level rise of 20 meters (67 ft) for each 1°C rise in global averagetemperature.  A critical questionis: How rapidly will the ice melt?  If weare lucky and the melting is slow enough, we may be able to manage a stagedretreat from the coasts. 

While we do not have goodtheoretical models for the melting of the polar ice sheets and the rate of sealevel rise, an approach based on the observed sea levels and temperaturechanges during the 20thcentury suggests that the rate of sea level rise now is roughly proportional tothe increase in global average temperature above the 14°C that it was in 1750.³⁹  This means that the farther we drive up thetemperature by adding CO₂,the faster the sea will rise.  Attimes in the past it has risen as much as 5 m in a century⁴⁰—a ratethat would be very difficult to adapt to.

(7) Time Lags and the LongLifetime of CO₂ in theAtmosphere

The slow response of sea level to a change in global averagetemperature is a consequence of the huge thermal mass of the oceans, their slowmixing, and the time required to melt the polar caps.  There is also a lag between the time when thecomposition of the atmosphere is no longer changing (net emissions of CO₂and other GHGs are zero, i.e. their rates of addition no longer exceed theirrates of removal) and when the global average temperature becomes steady.  While part of the CO₂ released byburning fossil fuels is taken up by plants or dissolved in the ocean withindecades, nearly 20% is still in the atmosphere 1,000 years after its release.⁴¹  Because the solubility of CO₂ inwater decreases as the temperature increases, a larger fraction of the CO₂emitted will remain in the atmosphere as the oceans warm, adding to thegreenhouse effect. 

Added Note
While this paperwas in preparation, the Environment News Service reported that scientists fromsix national laboratories are launching a project to study abrupt climate change—inhopes of predicting dangerous tipping points before they occur.⁴²  The name of the project will be IMPACTS,which stands for Investigation of theMagnitudes and Probabilities of Abrupt Climate Transitions. The initialemphasis will be on four types of abrupt climate change: 

1. Instability among marine ice sheets, particularly the West Antarctic ice sheet
2. Positive feedback mechanisms in subarctic forests and arctic ecosystems, leading to rapid methane release or large-scale changes in the surface energy balance
3. Destabilization of methane hydrates—vast deposits of methane gas caged in water ice—particularly in the ArcticOcean
4. Feedback between biosphere and atmospherethat could lead to megadroughts in North America

Precautionary Principle

For more information, acontemporary discussion of the Precautionary Principle as applied to climatechange can be found on YouTube.⁴³