Note: This page is intended to contain a complete list of all
significant known or hypothesized climate feedback mechanisms. If you notice any errors or
omissions, please tell me. -DAB
Feedbacks - Table of Contents:
- What are “feedbacks?”
- Negative (stabilizing/attenuating) climate feedbacks
- Positive (amplifying/destabilizing) climate feedbacks
- Unknown-sign climate feedbacks
What are “feedbacks?”
In Systems Science, a
or “feedback loop” is a mechanism through which the output of a system loops around or “feeds back,”
and affects an input to the same system (which, in turn, affects the output, which affects the
For example, when the thermostat in your house detects that the temperature is getting too cold,
it turns on the furnace to raise the temperature. That's a (manmade) feedback system: The
temperature causes a change in thermostat & furnace behavior, which, in turn, causes a change in
Feedback mechanisms (or simply “feedbacks,” for short) are grouped into two
categories: positive & negative. That doesn't mean good vs. bad. It means amplifying (positive)
vs. attenuating/reducing/stabilizing (negative).
A positive feedback is one which causes a same-direction response, so it tends to increase
(amplify) the effect of a change in input.
A common misconception is that positive feedbacks necessarily “run away,” and make a
system unstable. That is incorrect. Positive feedbacks of less than 100% don't make a system unstable.
For example, consider a linear system with a positive 10% (i.e. +1/10) feedback from the output to the input.
An input change of 1.0 will "feed back" +10% to become, effectively 1.1. The “.1” (additional
part) is also then amplified by 10%, becoming .11, etc. The +10% feedback ends up, in the long term,
asymptotically approaching 11.1111111...% (i.e., +1/9 = ×10⁄9) amplification.
Similarly, a +20% (i.e. 1/5) linear feedback causes a +25% (i.e., +1/4 = ×1.25) amplification,
a +33⅓% (i.e. 1/3) feedback causes a +50% (i.e. +1/2 = ×1.5) amplification, and
a +50% (i.e. 1/2) feedback causes a +100% (i.e. +1 = ×2) amplification. (Caveats:
in the real world, delays in the feedback path may mean that the full amplification effect of a
positive feedback isn't immediately seen; also, most systems are not perfectly linear, though many
are approximately linear over ranges of interest.)
A negative feedback is something which causes an opposite-direction response, and thereby
reduces the magnitude of the effect of the change. (Exception: if there are delays in the
feedback path, very strong negative feedback can cause oscillations in the system, but that's beyond
the scope of this little primer.)
The thermostat in your home is an example of a negative feedback mechanism (albeit a highly
nonlinear one). It reduces the effect on indoor temperature of input changes, like changes in the
weather, or someone leaving a window open.
Negative feedbacks abound in nature, including your own body. E.g., if your body overheats, you
will sweat in reaction to your elevated body temperature. Evaporation of perspiration cools
your body: a negative feedback.
“Course corrections” are another example: When you are driving your car, and it drifts toward the
edge of the road, in reaction to that drift you reflexively nudge the steering wheel toward the
center of the road: a negative feedback.
Feedbacks are at the center of the climate debate. The direct warming effects of anthropogenic
greenhouse gas emissions are known to be small, but climate alarmists believe that those slight
warming effects will be multiplied dramatically through positive feedbacks, with catastrophic
consequences. I find scant evidence of that.
The remainder of this page is a list of known and theoretical climate-related feedback
mechanisms, grouped into negative feedbacks, positive feedbacks, and feedbacks of unknown sign.
Climate feedback mechanisms
- Negative (stabilizing/attenuating) climate feedbacks:
- Planck Feedback. The most fundamental feedback effect is simply that when the Earth's
surface gets warmer, it loses heat faster, thereby reducing the increase in temperature.
“It is like pumping air into a tyre with a puncture: the harder you pump the faster the air
escapes.” –Clive Best
The simplest and easiest to quantify component of that effect is the radiative component,
called “Planck feedback.” Radiative emissions from a warm body are
the 4th power of the body's absolute temperature (temperature in in Kelvin).
It is calculated that a uniform global temperature increase of
1°C would increase radiant heat loss from the surface by about 1.4% (variously estimated to be
3.2 to 3.7 W/m²).
Coincidentally, 3.7 ±0.4 W/m² is also the approximate amount of additional energy calculated
to be retained (i.e., the “forcing”) due to a doubling of atmospheric CO2 levels
(though Prof. Wm Happer has found evidence that CO2's forcing is
commonly overestimated by about 40%, which, if correct, would reduce that to about 2.6 W/m²). ↑
- Convective Cooling Feedback. Convective heat loss increases with surface
temperature, as well. It is roughly
proportional to the difference in temperature between the surface and the
- Water Cycle / Evaporation Feedback. Evaporative cooling is expected to
increase with higher temperatures, because warmer water evaporates faster, accelerating the
The water cycle is a classic phase-change refrigeration cycle, just like the Freon
refrigeration cycle in your refrigerator: Water evaporates at the surface, absorbing
“Heat Of Evaporation” (evaporative heat loss). Because the molecular
weight of water vapor molecules is just 18 (compared to 28 for nitrogen), moist air is
lighter than dry air (contrary to intuition). So the moist air rises to the mid-troposphere,
where the water condenses into clouds, releasing the heat which it had absorbed at the
This process is the most important way in which heat is removed from surface of the Earth.
Warmer temperatures should increase the rate of evaporation, and thereby increase the rate
at which heat is transported away from the surface. ↑
- A number of researchers have investigated an apparent link between
tropical sea surface temperatures and clouds, which seems to regulate temperatures in the
These mechanisms are controverial. The misleadingly named “Skeptical Science”
(“SkS”) climate alarmist blog catalogs the contrary arguments
(Reader beware: I hesitated to link to those pages, because SkS is a notoriously
untrustworthy site, which routinely cherry-picks those studies and evidence that confirm
their bias, and ignores all other.)
- CLAW Feedback.
There is evidence that increased ocean temperatures and/or sunlight increase the abundance of
Pelagibacterales (“SAR11”) bacteria in the oceans, which produce dimethyl sulfide (DMS)
via an intermediate compound called DMSP. DMS escapes to the atmosphere and leads to increased
sulfate aerosols, which act as cloud condensation nuclei. That “seeds” clouds, increasing
cloud cover, which reduces the amount of sunlight reaching the ocean and thus reduces ocean temperature,
making it a negative (stabilizing) feedback
(But see also Ocean Acidification / Temperature Linkage.)
CLAW Feedback was
first hypothesized in 1987
by researchers named Charlson, Lovelock,
Andreae & Warren, hence the acronym: “CLAW,” from their initials. ↑
- Sea Ice / Evaporation Feedback. Decreased polar sea ice coverage (Arctic & Southern Ocean)
increases water evaporation, cooling the ocean by evaporative heat loss (but see also
“Ice / Albedo (positive) Feedback,” below).
The additional evaporation (due to more open water) also apparently causes additional
cloud cover, increasing albedo at altitude, and probably cooling the surface.
It also increases “lake/ocean-effect snowfall” (LEOS) downwind. Some of that snow falls
on the ice sheets and glaciers, increasing ice accumulation, and offsetting meltwater losses. Other
snow falls on land, increasing albedo and snowpack, decreasing land temperatures, and
Note that snow accumulation has a large effect on grounded ice mass, which in turn affects sea-level.
In both Greenland
and Antarctica, snowfall is the most important factor affecting ice sheet mass balance, greater in
magnitude than melting, sublimation, or iceberg calving. In fact, in Antarctica, snowfall accumulation
is approximately equal to the sum of those other three factors.
magnitude of ice accretion from snowfall on ice sheets was illustrated by the amazing story of Glacier Girl.
She's a Lockheed P-38 Lightning which was extracted in pieces from beneath 268 feet(!) of accumulated
ice and snow (mostly ice), fifty years after she made an emergency landing on the Greenland Ice
That's more than 5 feet of ice per year, which is equivalent to more than seventy (70) feet of annual
snowfall, which had piled up on top of the airplane!
That snow represents evaporated water, mostly removed from the Arctic and North Atlantic Oceans, which then
fell as ocean-effect snow on the Greenland Ice Sheet.
Diagram: Glacier Girl under the ice
Langen et al, Earth Sci., 12 January 2017. doi:10.3389/feart.2016.00110
A Tale of Two Winters [in Greenland], The Arctic Journal, May 9, 2017. ↑
- Sea Ice / Turbulence Feedback. Decreased polar ice cover increases water turbulence,
“stirring” the water, so that surface heat loss cools the water to greater
- CO2 Fertilization Feedback (“greening”). Higher CO2
levels increase plant growth rates, which reduces atmospheric CO2 levels.
AR5 estimates that this effect currently removes about 29% of anthropogenic CO2 emissions
from the atmosphere
but that's a very rough estimate. (Note: on p. 6-3 they give slightly numbers: 2.5 / 9.2 = 27% went
into the biosphere.) Other sources give
estimates, but there's general agreement that this is an important climate feedback mechanism,
The Earth is getting greener, and
that the primary cause is anthropogenic CO2, and the secondary cause is climate change.
However, since higher CO2 levels
make plants more water efficient,
the increase in green plants has
not been accompanied by much increase in water usage (transpiration).
(Note: the sum of the amounts of CO2 taken up by fertilization/greening and water
absorption is estimated by AR5 to be 55% of anthropogenic CO2 emissions, and that
estimate is not as rough as either of the two addends.) ↑
- CO2 Absorption By Water Feedback. Higher atmospheric CO2 levels increase CO2 absorption
by water bodies (mainly the oceans), removing CO2 from the atmosphere.
AR5 estimates that this effect currently removes about 26% of anthropogenic CO2 emissions
from the atmosphere
but that's a very rough estimate.
(Note: Since the oceans contain about 50 times as much CO2 as the atmosphere, the absorption
of atmospheric CO2 by the oceans affects the oceans much less than it affects the atmosphere.) ↑
- CO2 Coccolithophore Feedback. Increased CO2 levels dramatically increase growth of
calcifying coccolithophores, removing CO2 from the oceans. This effect seems to be much
greater than can presently be explained: ↑
- Greenland Ice Melt / Ocean Iron Fertilization Feedback. Increased Greenland ice melt fertilizes
the ocean via iron in the runoff water, increasing absorption of CO2 by photosynthesis in the oceans. ↑
- Lapse Rate Feedback. The “lapse rate” is the rate at which
air temperature decreases with altitude within the troposphere.
(It's why mountains have snowcaps.) At any given time and place, the lapse rate varies considerably,
but, on average, it is about 6.5°C per km of altitude (greater for dry air, less for very humid
air). Greenhouse warming is generally expected to slightly reduce the average temperature vs. altitude
lapse rate, disproportionately warming the atmosphere at higher altitudes, in the tropics. That should
increase radiative energy losses to space, thus reducing overall warming.
Physicist Clive Best discusses lapse rate feedback
- Rock weathering feedback. Atmospheric CO2 dissolves in raindrops, forming weak carbonic acid,
which causes chemical weathering of wollastonite and similar silicate rocks. That chemical process
removes CO2 from the rainwater, and hence from the atmosphere, and the process accelerates at
warmer temperatures and higher carbonic acid levels. Higher atmospheric CO2 accelerates this
process two ways: it increases carbonic acid content in rainwater, slightly lowering the water's
pH, and it also causes slightly warmer temperatures through global warming.
On less than multi-millennial time scales, this is a negligible feedback. Not only are the
feedback mechanisms weak, AR5 estimates that rock weathering
removes just 0.3 PgC/yr, which is only 3.4% of the estimated anthropogenic emissions of 8.9 PgC/yr.
AR5 section 184.108.40.206 (p.550 of WG1AR5_ALL_FINAL),
and Fig 6.1.
discussion here. ↑
- Thermohaline Circulation Feedback. Thermohaline Circulation or Meridional
Overturning Circulation (MOC) refers to the “Atlantic
Conveyor” and related currents which carry warm surface water from the tropics toward the poles,
and currents deep in the ocean which carry cold water back toward the tropics. (It's a slow process: the
Atlantic Conveyor is estimated to have a cycle time of around 1000 years.) Thermohaline Circulation is
driven, in part, by density differences between colder/saltier water and warmer/less-salty water.
Some scientists have speculated that global warming could reduce the density differences which drive
thermohaline circulation, by reducing the temperature difference between low and high latitudes, and by
adding fresh meltwater to the ocean near the poles. But if thermohaline circulation were to actually slow,
the slowed circulation should reduce warming at high latitudes, and thus increase the
temperature difference between lower and upper latitudes, making it a negative feedback, and mitigating
the effect. ↑
- Arctic Summer OLR Feedback. Swedish researchers Joseph Sedlar & Michael Tjernström report
that they have “identified a relationship between large-scale circulation variability and changing
cloud properties permitting LW [Long-Wave] radiation at both the surface and top of the atmosphere to
respond to variability in atmospheric thermodynamics. Driven by anomalous advection of warm air, the
corresponding enhanced OLR [Outgoing Longwave Radiation] cooling signal on monthly time scales represents
an important buffer to regional Arctic warming.”
Ref: Sedlar, J., and M. Tjernström (2017). Clouds, warm air, and a climate cooling signal over the
summer Arctic, Geophys. Res. Lett., 44, 10951103.
(Note: This is a new addition to the sealevel.info Feedbacks page, and I've not yet even read
the paper. -DB) ↑
- Organic Aerosol Temperature Feedback. Dr. Catherine Scott of The University of Leeds, and her
colleagues, conducted a model-based study of the hypothesis that, though a couple of natural mechanisms,
warmer temperatures cause higher concentrations of large (>100 nm) natural organic aerosols, which reflect
sunlight, cooling the surface: a negative feedback. The paper is
here and (as usual)
has a discussion of it.
(Note: This is a new addition to the sealevel.info Feedbacks page, and I've not yet even read
the paper. -DB) ↑
- CO2 / Evapotranspiration Linkage. Higher atmospheric CO2
levels cause plants to release less water. That improves drought resistence in crops, which is an
important benefit for agriculture.
But it also reduces humidity. Like CO2, water vapor is a greenhouse gas, so reduced
release of water vapor by evapotranspiration from plants should also reduce greenhouse warming, by
reducing water vapor feedback.
Here's an article
about it, with references to a couple of papers.
This is not a true feedback mechanism, but, like negative feedbacks, it might attenuate the warming
effect of additional atmospheric CO2, so I've included it in this list for the sake of
- Positive (amplifying/destabilizing) climate feedbacks:
- Water Vapor Feedback. It is generally expected that warmer temperatures should increase the
amount of water vapor in the atmosphere, because warmer air holds more moisture. This effect is
usually approximated in climate calculations by assuming stable relative humidity as temperatures
change. Under that assumption, warmer temperatures cause greater amounts of water vapor in the
atmosphere, and since water vapor is a greenhouse gas, increased water vapor in the atmosphere
should increase greenhouse warming: a positive feedback.
This is generally believed to be the most important positive climate feedback mechanism.
Quantifying it is difficult, though.
The latest version of the U. of Chicago's online MODTRAN interface
calculates that for
a Tropical Atmosphere water vapor feedback should increase the warming effect of CO2 in the tropics
by only about 8% to 9%. That's probably incorrect: most other sources give much higher
estimates, generally between 60% and 100% (i.e., up to doubling):
Here's a fairly in-depth discussion:
However, atmospheric water vapor levels do not appear to be increasing as expected:
Note that some scientists use the term “water vapor feedback” in a broader sense than I'm
using it, to encompass not only the direct greenhouse warming effect of atmospheric water vapor,
but also water cycle (evaporative) cooling, lapse rate
cooling, and/or perhaps clouds. For example: ↑
- Ice / Albedo Feedback. If warmer climate reduces ice and snow cover, reduced ice cover
(on water) and snow cover (on land) will decrease average albedo (reflectivity), and thus increase absorption
of sunlight during daytime, a positive feedback. (Ice has a low microwave albedo, but snow is a very effective insulator
which also reduces heat loss at night, making ice and snow cover a negative feedback mechanism at night.)
The net ice / albedo feedback effect is thought to be modestly positive during daytime / spring-summer (but see also
“Sea Ice / Evaporation (negative) Feedback,” above).
However, in the context of North American and
ice sheets, this is a form of
“positive integral feedback,”
which means the effect accumulates/increases as the ice sheets grow or shrink, and over extremely long
timespans this feedback mechanism becomes very important. It is believed to be a key factor in
glaciation/deglaciation cycles, which occur over (very roughly) 100,000 year periods.
A common misconception is that the dramatic glaciation/deglaciation cycles are evidence that the Earth's climate
is very unstable, suggesting that climate sensitivity is high. They aren't.
There are two reasons that Milankovitch cycles have a large effect on glaciation, and neither of them
implies that climate sensitivity to GHG forcings must be high.
The two reasons Milankovitch cycles have a large effect on glaciation are:
1. Milankovitch cycles change the breadth of seasonal temperature swings at the latitudes where it
matters for glaciation: mainly northern North America and northern Eurasia.
When those seasonal swings increase it increases ice melt in the summers and decreases snowfall in the winters.
(When temperatures are very low snowfall is greatly reduced, because the cold air cannot carry much moisture;
it is said to be “too cold to snow.”)
When the seasonal swings are reduced it decreases ice melt in the summers and increases snowfall in the winters.
2. Milankovitch cycles last for tens of thousands of years, which enables them to overcome
the extreme damping of the feedback mechanism.
When, due to Milankovitch cyles, the Earth's seasonal swings are reduced to the point that the great
northern ice sheets grow a little bit each year, by adding more snow in the winters than they lose in
the summers, after thousands of years of ice sheet growth it adds up to a very large change in planetary
albedo. Conversely, when the Earth's seasonal swings are increased to the point that the great northern
ice sheets shrink a little bit each year, as the ice sheets dwindle it eventually adds up to a large
reduction in planetary albedo.
That is an important positive integral feedback mechanism, but only on timescales of thousands of
years, and it is highly non-linear, because it only works when the extent of the great northern ice sheets can
significantly increase or decrease.
Those ice sheets are gone, now. The only remnant is Greenland, so ice sheet albedo feedback could only
become important in a warming climate if enough of the Greenland Ice Sheet were to melt to significantly
increase the exposed land area, which is not in prospect.
So Milankovitch cycle-driven glaciation cycles are evidence of very large natural long-period
climate variability, but they are not evidence for high climate sensitivity to anthropogenic GHG
- CO2 / Water Temperature Feedback. The solubility of gases like CO2
(and CH4) in water decreases as the water gets warmer, so as the oceans warm they outgas CO2
(or, if they're absorbing CO2, as is currently the case, they absorb it more slowly).
The CO2, in turn, works as a GHG to cause warming. This is a
positive feedback mechanism. ↑
- Permafrost & Clathrate / Methane Feedback. If the climate warms, it could melt
some of the Arctic permafrost,
and/or underwater methane clathrate
(hydrate) deposits, causing the release
of methane (and some CO2) into the atmosphere. Methane is a greenhouse gas,
so this should increase warming,
making it a positive feedback mechanism. However, according to
the latest research,
this effect is
and is likely to remain so; here's
a good article. ↑
- Methane / OH-Radical Feedback.
Prof. Lyatt Jaeglé of U. Washington
reports that as atmospheric methane (CH4) levels increase, OH radicals
are depleted, which reduces OH levels. That reduces the rate at which CH4 is removed from the atmosphere
by oxidation, which increases the atmospheric lifetime of CH4, making it a positive feedback mechanism.
Prof. Jaegle calculates that this
feedback effectively increases the atmospheric lifetime of additional CH4 by about 50%, from about 8 years
to about 12 years. ↑
- Sahara Dust Feedback. It has been hypothesized that
from the Sahara could play a significant role in the Earth's cycling between glacial maxima and
interglacials. Researchers have found evidence that as the Earth was warming after the last glacial
maximum, much less dust was being blown from the Sahara into the atmosphere. That could have
helped warm the oceans, which, in turn, may have increased monsoon rains in North Africa, helping
vegetation to grow, and further reducing windblown dust. ↑
- Tree Line Feedback. Boreal forests are dominated by evergreens. Since they remain green
even when there is snow on the ground, boreal forests have lower albedo (i.e., they are darker) than
unforested ground, in snowy conditions, hence they absorb more sunlight. If a warmer climate causes tree
lines to advance toward higher latitudes, this should be a positive feedback mechanism in those regions:
warmer temperature → increased forestation → lower albedo → warmer temperature
(This feedback is mentioned in the 7th paragraph of p.4 of Grantham's
“Briefing paper No 12,
June 2015, Biosphere feedbacks and climate change.”) ↑
- Ocean Acidification / Temperature Linkage.
Higher CO2 levels in the atmosphere cause increased absorption of CO2 in the
oceans, which slightly reduces seawater pH, which some researchers believe might impede the biogenic
production of DMS. If so, that would reduce the production of sulfate aerosols, which, in turn,
would presumably reduce cloud cover, and thereby increase ocean temperatures.
(See also CLAW Feedback.)
This is not a feedback mechanism. However, if real, it would be a mechanism which increases
(amplifies) the very long-term warming effect of anthropogenic CO2, so I've included it
in this list, for the sake of completeness. ↑
- Unknown-sign climate feedbacks:
- Cloud Feedback (Overall). Clouds are the elephant in the living room. They're obviously extremely
important, but they are very poorly understood.
High, wispy cirrus clouds have a warming effect, because they are made of ice crystals, which makes
them much more nearly opaque to outgoing longwave infrared than to incoming visible and near-IR solar
radiation. Lower clouds, which are made of liquid water droplets, have a strong cooling effect in
daytime, but a warming effect at night. How clouds are affected by warming or cooling climate is very complex.
- Ice Topography Feedback, a/k/a Ice-Elevation feedback. Temperatures decrease with elevation,
so if an ice sheet diminishes in height, the temperature at the surface will increase, which could accelerate
melting, a positive feedback. On the other hand, snowfall rates increase as temperatures approach 0°C,
and lower elevations reduce gravitationally-driven ice flows and iceberg calving, both negative feedbacks.
Additionally, melting at the edges of the Greenland ice sheet and ice accumulation at the center changes the
topography, which might change wind patterns and hence snowfall patterns, which might change the topography,
a feedback of uncertain sign:
On centennial and shorter time scales, ice topography feedback is probably of minor importance. ↑
Last modified: 30-Mar-2018 (version 54)
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