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.
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 1.4% (variously estimated to be
3.2 to 3.7 W/m²).
Coincidentally, 3.6 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
overestimated by about 40%). ↑
- Convective Cooling Feedback. Convective heat loss presumably increases with
temperature, as well. ↑
- 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
- 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 ice cover (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-effect/ocean-effect” snowfall 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.
The magnitude of ice accretion
from snowfall on ice sheets was illustrated by the team which salvaged Glacier Girl
from under 268 feet(!) of accumulated ice, 50 years after she landed on the Greenland ice sheet.
- 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 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 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.
(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 / 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, by 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 18.104.22.168 (p.550 of WG1AR5_ALL_FINAL),
and Fig 6.1. ↑
- Thermohaline Circulation Feedback. Thermohaline Circulation 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 thermohaline 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. ↑
- 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). ↑
- 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 modest positive feedback
- 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 temperatures → increased forestation → lower albedo → warmer temperatures.
(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, whispy 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. Melting at the edges of the Greenland ice sheet and ice accumulation
at the center changes the topography, which might change snowfall patterns, which might change
This is almost certainly very minor. ↑