IB Geography 2.1.2 Changes in the Global Energy Balance and Feedback Loops Notes

Forcing and Feedback: One Pushes the Balance, the Other Decides How Far It Moves

1. Radiative forcing is a change imposed on the energy balance from outside the temperature response, measured in watts per square metre at the top of the atmosphere.
   1. A volcanic eruption, a shift in solar output and a rise in carbon dioxide are all forcings, because none of them waits for the planet to warm first.
   2. A forcing can be positive, which adds energy to the system, or negative, which removes it.
2. The IPCC Sixth Assessment Report assesses the total human-caused forcing between 1750 and 2019 at 2.72 W/m², a net figure that already has the cooling from aerosols subtracted.
   1. By comparison, the Sun's contribution over the same period is 0.01 W/m².

Positive and Negative Feedback

1. A feedback is a change that the warming itself sets off, which then alters the amount of energy the system gains or loses.
   1. Melting sea ice is a feedback, because the ice only melts once the Arctic has already warmed.
2. Positive feedback
   1. Carries the system further in the direction it was already moving, so the original change grows.
   2. Warming melts ice, leaving a darker surface that absorbs more radiation and adds to the warming.
3. Negative feedback
   1. Pushes back against the direction the system was moving, so the original change shrinks.
   2. A warmer surface radiates more energy to space, which slows the warming that produced it.

Solar Variation: Measurable, Cyclical and Too Small to Explain Recent Warming

1. The 11-year sunspot cycle
   1. NOAA puts total solar irradiance at 1,361 watts per square metre on a surface held at right angles to the Sun, which spreads to the 340 W/m² planetary average given in 2.1.1.
   2. Across a strong cycle the Sun's total brightness varies by roughly 1 W/m² between solar minimum and solar maximum, under 0.1 percent of the 1,361 W/m² beam.

1. Divided by four for the disc-to-sphere geometry, then cut by the planetary albedo of 0.3, that swing amounts to roughly 0.17 W/m² of forcing.
2. Because the variation reverses every eleven years, it appears in the record as a small oscillation and not as a trend.

1. Milankovitch cycles
   1. Milutin Milankovitch described three slow changes in Earth's orbit and axis that alter where and when insolation arrives.
   2. NASA gives eccentricity, the change in the shape of the orbit, a period of about 100,000 years.
   3. Obliquity, the tilt of the axis, swings between 22.1 and 24.5 degrees over about 41,000 years and currently sits at 23.4 degrees.
   4. Axial precession, the wobble of the axis, completes a cycle in about 26,000 years.
   5. Climatic precession, the seasonal timing that matters for ice ages, shifts faster on a roughly 21,000 to 23,000 year cycle, because the orbit's own long axis rotates too.
   6. These cycles redistribute insolation between latitudes and seasons and barely change the annual global total.
   7. Acting over tens of thousands of years rules them out as the cause of a temperature rise measured over decades.
2. Sunspot minima
   1. The Maunder Minimum, from 1645 to 1715, is the stretch of almost no recorded sunspots that Gustav Spörer identified in the 1880s and that John Eddy named after Edward Maunder in a 1976 paper in Science.
   2. It falls inside the Little Ice Age, a cold interval conventionally dated from about 1300 to 1850.
   3. Two records moving together over one episode does not establish that one caused the other.
3. The size of the solar forcing
   1. The IPCC Sixth Assessment Report assesses solar effective radiative forcing since 1750 at plus 0.01 W/m², with a range of minus 0.06 to plus 0.08 W/m².
   2. NOAA puts the contribution of changing solar brightness since the pre-industrial period at no more than 0.01°C, against the 1.44°C by which the WMO placed 2025 above the 1850 to 1900 average.

Solar activity has declined since the middle of the twentieth century while global temperature has climbed, the opposite of what a solar cause would produce.

Global Dimming: Aerosols Reflect Sunlight and Mask Part of the Warming

1. Direct scattering, or aerosol-radiation interaction
   1. Sulphate, nitrate, organic carbon and mineral dust particles scatter part of the incoming shortwave beam back to space.
   2. Black carbon works the other way, absorbing radiation and warming the air that holds it.
   3. AR6 assesses the net of the two over 1750 to 2014 at minus 0.3 W/m².
2. Brighter clouds, or aerosol-cloud interaction
   1. Aerosol particles act as condensation nuclei, which spreads the same cloud water across a larger number of smaller droplets.
   2. A cloud made of many small droplets reflects more sunlight than a cloud made of fewer large ones.
   3. AR6 assesses this indirect pathway over the same 1750 to 2014 window at minus 1.0 W/m², larger than the direct one.
3. The size of the aerosol forcing
   1. AR6 assesses the total aerosol effective radiative forcing between 1750 and 2014 at minus 1.3 W/m², with a range of minus 2.0 to minus 0.6.
   2. AR6 puts greenhouse gas forcing over 1750 to 2019 at 3.84 W/m².
   3. The aerosol term therefore cancels roughly a third of that figure before any of the warming reaches the surface.
   4. The 2.72 W/m² total is what is left once the aerosols and the smaller land-use and contrail terms are set against it, not a figure to measure the aerosols against.
4. What the surface record shows
   1. Stanhill and Cohen (2001) found surface solar radiation falling by about 2.7 percent per decade between 1958 and 1992 across the global radiation network.
   2. Wild (2012) reviews estimates of the total dimming from the 1950s to the 1980s at 3 to 9 W/m² at the surface.
   3. The trend reversed in the late 1980s, a recovery that Wild and others (2005) named global brightening.
   4. Brightening has returned between 1 and 4 W/m² of the 3 to 9 W/m² that dimming took away.
   5. Since 2000 the pattern has been regional, with brightening in Europe and the United States, renewed dimming in China and unabated dimming in India.

Mount Pinatubo, 1991: Dimming You Can Date

1. Mount Pinatubo in the Philippines erupted climactically on 15 June 1991, injecting about 17 to 20 megatonnes of sulphur dioxide into the stratosphere.
2. The USGS reports Northern Hemisphere surface cooling of up to 0.5 to 0.6°C, plus cooling of perhaps 0.4°C over large parts of the Earth in 1992 and 1993.
3. NASA puts the lifetime of fine aerosol injected near the surface at several days, while fine volcanic sulphate in the stratosphere can stay aloft for years.
   1. That difference in residence time is why one eruption cools the planet for two or three years, whereas industrial dimming lasts only as long as the emissions continue.

Terrestrial Albedo Change: The Ice-Albedo Loop Is the Clearest Positive Feedback in the Course

How the Ice-Albedo Loop Turns

1. Warming raises air and ocean temperature in the Arctic until snow and sea ice melt.
   1. NSIDC gives open ocean an albedo of about 0.06, bare sea ice 0.5 to 0.7, and snow-covered ice as much as 0.9.
2. Albedo falls, because the dark water left behind reflects far less of the incoming beam than the ice it replaced.
3. The surface therefore absorbs a larger share of the incoming shortwave radiation.
4. That extra absorbed energy warms the surface further, melting more ice and closing the loop.
   1. Melt ponds carry the loop forward within a single summer, because ponded water on the ice has an albedo of only 0.4 to 0.5.

The Arctic Record: 12.1 Percent per Decade, and Not Steadily

1. NSIDC records the September Arctic sea ice minimum falling at 12.1 percent per decade between 1979 and 2025, measured against the 1981 to 2010 average.
   1. The 2025 minimum of 4.60 million square kilometres on 10 September tied for tenth lowest in the 47-year satellite record.
   2. NSIDC adds that there has been no significant downward trend in the September minimum over the past two decades, although every year from 2007 to 2025 still sits below every year from 1979 to 2006.
2. Pistone, Eisenman and Ramanathan (2014) measured Arctic planetary albedo falling from 0.52 to 0.48 between 1979 and 2011.
   1. That darkening added 6.4 plus or minus 0.9 W/m² of absorbed solar energy over the Arctic Ocean region.
   2. Spread over the whole planet it amounts to about a quarter of the rise in carbon dioxide forcing across the same period.
3. Rantanen and others (2022) put the warming of the region north of 66.5°N at 3.8 times the global rate between 1979 and 2021.

Clearing Forest North of 45°N Cools the Local Surface

1. Lee and others (2011) compared paired forested and open sites and found that clearing forest north of 45°N cooled the local surface by 0.85 plus or minus 0.44 K.
   1. Because these are paired local measurements at the surface and not a global mean, they change where warming shows up rather than how much of it there is.
   2. Open snow-covered ground reflects far more winter sunlight than a dark conifer canopy that sheds its snow.
   3. Lee and others name night-time temperature change, which albedo cannot explain, as an important contributor to the cooling they measured.
   4. Below about 35°N Lee and others find weak evidence that the sign reverses, because losing the canopy also loses the evaporative cooling that carries heat away as latent heat.
2. The same physical quantity, albedo, sits on both sides of the forcing and feedback distinction, because clearance is a human decision while ice melts in response to temperature.

Methane Release: Warming Opens a Carbon Store That Then Adds to the Warming

1. Why a tonne of methane matters
   1. NOAA's Global Monitoring Laboratory measured a global mean methane concentration of about 1,938 parts per billion in April 2026.
   2. That is about 2.7 times the 729 parts per billion AR6 gives for 1750.
   3. Methane has an atmospheric lifetime of around twelve years, much shorter than the time carbon dioxide persists.
   4. The diagram's about 80 times comparison means that, over 20 years, one tonne of methane produces about 81 times as much cumulative radiative forcing as one tonne of carbon dioxide; AR6 gives a lower 100-year value of 27.9 because methane lasts around twelve years.
2. The permafrost store
   1. Hugelius and others (2014) estimate about 1,300 petagrams of organic carbon in soils of the northern circumpolar permafrost region, with roughly 1,035 petagrams in the top three metres.
   2. NOAA's Arctic Report Card gives a larger figure of 1,460 to 1,600 billion tonnes.
   3. The gap between the two estimates measures how uncertain the deep and buried deposits remain.
   4. Frozen ground preserves plant and animal remains that microbes could not decompose while the soil stayed below 0°C.
3. What thaw releases, and in which form
   1. Thaw restarts decomposition, releasing carbon dioxide where the ground drains and oxygen reaches the microbes.
   2. Waterlogged ground such as a thermokarst lake bed excludes oxygen and shifts the output towards methane.
   3. The same thawed carbon therefore produces a weaker or a stronger feedback depending on local drainage.
4. Abrupt thaw at Batagay, Yakutia
   1. The Batagay megaslump in the Yana Uplands of Yakutia, Russia, is the largest known thaw slump.
   2. Kizyakov and others (2024) measured it at about 990 metres across, with a headwall 55 metres high, in 2023.
   3. The slump loses roughly one million cubic metres of frozen ground a year, releasing 4,000 to 5,000 tonnes of organic carbon annually.
   4. Abrupt thaw of this kind reaches deep carbon that gradual surface warming would take far longer to expose.
5. How large the feedback might be
   1. AR6 projects permafrost carbon dioxide release of 3 to 41 petagrams of carbon per degree Celsius of global warming by 2100.
   2. The report holds high confidence that thawing permafrost releases carbon, with low confidence in the timing, the magnitude and the split between carbon dioxide and methane.
   3. The range is wide because models represent abrupt thaw poorly and the observational network across the Arctic is thin.

Damping and Amplifying Loops: The Net Effect Slows the Change without Removing the Forcing

1. Increased longwave emission, the damping response
   1. A warmer surface radiates more strongly in the thermal infrared, the response that prevents any warming running away.
   2. This is why the planet settles at a new and higher equilibrium temperature instead of heating without limit.
2. Cloud, the amplifying response AR6 assesses
   1. Low cloud reflects incoming shortwave radiation and cools the surface beneath it.
   2. High thin cloud absorbs outgoing longwave radiation and warms the surface instead.
   3. AR6 assesses the net cloud feedback as positive at 0.42 W/m² per °C, with a range from minus 0.10 to plus 0.94.
   4. Cloud is therefore assessed as adding to the warming, not braking it, though its range is the widest of any feedback in the assessment.
3. The net effect across the whole system
   1. AR6 assesses the net feedback parameter at minus 1.16 W/m² per °C, a stabilizing value overall.
   2. That single figure already nets the amplifying loops against the damping ones.
   3. A stabilizing total therefore does not make the amplifying loops small.

Western Norway: A Local Advance That Was Not a Planetary Brake

1. Nesje (2005) records the maritime glaciers of western Norway gaining mass through the 1990s on increased winter accumulation, an explanation that invokes no planetary feedback.
   1. A regional shift in winter precipitation can push a glacier the other way for a decade while the global energy imbalance carries on unchanged.
2. Briksdalsbreen, an outlet of the Jostedalsbreen ice cap, then retreated 230 metres between 1996/97 and 2004.
   1. Its retreat of 130 metres in 2003/04 was the largest in a record that began in 1900.
3. The Arctic September minimum shows no such reversal, staying below the whole of the 1979 to 2006 range in every year since 2007.

A Feedback Explanation Must Return to Its Starting Condition

1. Use the same four-link structure for ice-albedo and permafrost methane feedbacks so the return to the starting condition is explicit.

1. A complete feedback answer names the trigger, the change in albedo or gas concentration, the change in absorbed or emitted radiation, and the return to the trigger.