Feedback Mechanisms in Climate
you're the windshield,
M.C. Carpenter, "The Bug"
Any change in the environment leading to additional and enhanced changes in that system is the result of a positive feedback mechanism. Alternatively, if a change in the environment leads to a compensating process that mitigates the change it is a negative feedback mechanism. In climate change discussions the focus is on the atmospheric radiation field as a forcing of the climate system (radiative forcing). Currently the discussion concentrate on the radiative forcing associated with the steadily increasing concentrations of different gases in the atmosphere - the so-called greenhouse gases : CO2, CH4, N2O, CFC-gases etc. Other changes in the environment can also lead to changes in the radiative budget, like deforestation, changes in land use and air pollution (ozone, SO4-aerosols, CONTRAILS, É). Also non-anthropogenic changes are important in disturbing the radiative balance: fluctuations in the Solar output and volcanic activity.
Important positive feedback mechanisms include:
Negative feedback mechanisms include
Clouds are important factors in the climate system. They are known to have a negative impact on the surface temperatures in the present climate system. However, a changing climate may involve changes in the clouds with both positive and negative effects on the radiative balance. Currently it is not known whether the total effect of these changes will be a negative or positive feedback.
Ocean warming provides a good example of a potential positive feedback mechanism. The oceans are an important sink for CO2 through absorption of the gas into the water surface. As CO2 increases it increases the warming potential of the atmosphere. If air temperatures warm it should warm the oceans. The ability of the ocean to remove CO2 from the atmosphere decreases with increasing temperature. Hence increasing CO2 in the atmosphere could have effects that exacerbate the increase in CO2 in the atmosphere.
Similar examples can be drawn for warmer air temperatures increasing the rate of glacier and sea ice melting. As the ice melts it changes the surface characteristics of the surface as the underlying ocean or land will have a lower albedo than the ice and hence an enhanced ability to absorb solar radiation.
Likewise, the increase in temperature would permit more water vapor to be stored in the atmosphere. The amount of water vapor the atmosphere can hold increases exponentially with temperature so increases in temperature can yield increases in atmospheric water vapor. The increased water vapor, as a greenhouse gas, enhances the greenhouse effect and could lead to further warming as long as this positive feedback isn't modified by an increase in cloudcover that could lead to a negative feedback (bee below).
A good example of a negative feedback mechanism will be if the increase in temperature increases the amount of cloud cover. The increased cloud thickness or extent could reduce incoming solar radiation and limit warming.
On the other hand it is not obvious that if additional cloud cover happens at what latitudes and at what times might it occur. Also it is not obvious what types of clouds might be generated. Thick low clouds would have a stronger ability to block sunlight than extensive high (cirrus) type clouds.
Low clouds tend to cool, high clouds tend to warm. High clouds tend to have lower albedo and reflect less sunlight back to space than low clouds. Clouds are generally good absorbers of infrared, but high clouds have colder tops than low clouds, so they emit less infrared spaceward. To further complicate matters, cloud properties may change with a changing climate, and human-made aerosols may confound the effect of greenhouse gas forcing on clouds. With fixed clouds and sea ice, models would all report climate sensitivities between 2°C and 3°C for a CO2 doubling. Depending on whether and how cloud cover changes, the cloud feedback could almost halve or almost double the warming.
2. Effects of Aerosols
Aerosols are tiny (0.001 to 10 µm) airborne particles. In the troposphere, the lower about 10 to 15 km of our atmosphere, human-made aerosols have greatly increased since about 1850. They present a large source of uncertainty in assessing human influences on climate. `Fine' aerosol particles with sizes between about 0.1 and 1 µm can influence climate in two ways. Under clear skies they scatter and absorb solar radiation; some of the scattered sunlight goes back to space (the direct effect). They also can act as cloud condensation nuclei, they may enhance reflectivity and lifetime of clouds (indirect effect).
Sulfur dioxide from fossil fuel burning, yielding sulfate particles after oxidation, is presently the largest source of fine human-made aerosols. Another large source is organic and elemental carbon from burning of tropical forests and savannas. Globally averaged, fine human-made tropospheric aerosols may currently cancel about 50 % of the warming effect of human-made greenhouse gases. So far, though, the uncertainty range is large, stretching from roughly 10 to 100%. Moreover, global averages are misleading. Even if the global averages of aerosol and greenhouse gas forcing cancel, their different distributions may cause climatic changes. With life spans of up to over 100 years, human-made greenhouse gases are fairly evenly distributed. Most tropospheric aerosols are washed out after about a week, they are unevenly distributed. Human-made sulfate aerosols occur mainly down- wind of northern industrialized areas. Most biomass smoke rises from tropical land areas during the dry season.
Cutting back sulfur dioxide emissions or biomass burning reduces the aerosol load quickly, leaving over the more long-lived greenhouse gases. By the way, roughly one third of the tropospheric sulfate load has natural precursors, mainly oceanic dimethyl sulfide (DMS) and volcanic sulfur dioxide. Violent volcanic eruptions, like Pinatubo 1991, give rise to stratospheric sulfate aerosols which, being more long-lived than their tropospheric cousins, tend to warm the stratosphere and to cool the troposphere and surface for a few years.
'Coarse' aerosols with particle sizes
between 1 and 10 micrometers include mineral dust raised by wind blowing over
dry soils. Human influences like over-cultivation and soil erosion may have
up to doubled the flux of mineral dust. Mineral dust is most abundant over
Except for atmospheric CO2, carbon reservoirs and natural fluxes are hard to measure. Their estimates vary somewhat across the literature. Carbon enters and leaves the atmosphere largely as CO2. Other fluxes involve various carbon compounds. The above irreverently lumps land animals with soils and detritus, and it omits many other details as well. For instance, both volcanic CO2 and CO2 removal via silicate weathering are in the order of 0.1 GtC/year and play a role on geologic time scales only. Here's what is estimated:
Carbon Reservoirs (GtC)
CO2 uptake by land plants through photosynthesis is roughly balanced by plant and soil respiration. Depending on whether photosynthesis exceeds or falls below respiration, the net result is CO2 drawdown or CO2 release. Today, photosynthesis is probably slightly ahead. In future, climatic changes or rising CO2 level may trigger feedbacks that curb or speed up the rise of atmospheric CO2. To name a few:
The overall effect of these and other feedbacks is hard to tell. Ecosystem models tentatively suggest that carbon storage in vegetation and soils may eventually win out. Temporarily, however, carbon may be released, especially if large and rapid changes should cause forests to die back.
Turning to the ocean, a sea surface warming of 1°C may increase atmospheric CO2 by up to 10 ppmv through degassing. More importantly, marine life, in spite of its low biomass, takes up and releases about 50 Gt of carbon annually. Marine biological production occurs largely in the sunlit surface and is thought to be limited mostly by nitrogen. Surface nutrient supplies are replenished primarily through transport from deeper ocean layers. (In the open ocean, iron can be limiting; it enters the ocean mainly in airborne dust and via rivers.) The export of organic carbon from the surface to deeper ocean layers, known as the biological pump, is not or little affected by CO2 availability, but it may be affected by changes in temperature, cloud cover, ocean currents, nutrients availability, or ultraviolet radiation. These and other marine biological processes are complex. Researchers cannot yet say how they will respond to disturbances. It has been estimated that, with no biological pump, pre-industrial atmospheric CO2 would have been 450 instead of 280 ppmv, whereas a marine life seizing all available surface nutrients could have lowered this to 160 ppmv. On the other hand, preliminary results suggest that changes in the biological pump may affect atmospheric CO2 only by 10s rather than 100s of ppmv.
Back to the land, spreading of boreal forest into tundra may lead to warmer winters. Trees protrude above the snow-covered ground, they reflect less sunlight back to space than snow-covered tundra. During and after deglaciation, the expansion of boreal forests amplified the warming of northern land areas. The reverse process, displacement of boreal forest by tundra, probably played a role in the onset of the last glaciation. For another example, rising CO2 tends to improve the water-use efficiency of vegetation. Plants may then release less water vapor to the ambient air. Regionally, this may warm the surface and affect precipitation and soil moisture.
Living in the
The breadth of feedback mechanisms in the climate system is enormous. Figure 4 tries to articulate some of the feedback mechanisms without quantifying their importance. The bottom line is our attempts to forecast future climates holds considerable uncertainty. Nonetheless it should be understood that the uncertainty could go either way and our estimates of temperature rise could be overestimated ...or underestimated.
Figure 4. A concept map of potential feedback mechanisms in climate change. Click on the map for a larger view.
1. Feedback mechanisms exist that can potentially exacerbate or mollify temperature increases.
2. There are a number of reasons to believe that temperature increases will enhance processes that either increase CO2 concentrations or the absorption of incoming solar radiation.
3. Many of the feedback mechanisms are poorly understood, so there is ample uncertainty in our estimates of future climates, nonetheless it is just as possible we are underestimating future temperature rises as overestimating them.
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