IPCC Report AR 6: What does the Intergovernmental Panel on Climate Change say about our glaciers?

Anyone who spends time in the Alps on a regular basis doesn’t need a report to see what’s happening. The tongue of the Great Aletsch Glacier, which I’ve been photographing for years, is retreating. Moraines are now exposed where there was still ice just a decade ago. The meltwater roars louder in the summer than it used to. These changes are real—I see them with my own eyes.

But what exactly does science have to say about this? And above all: What constitutes a reliable measurement, what is physics, and where does interpretation begin?

These questions led me to read the Intergovernmental Panel on Climate Change’s Sixth Assessment Report—the IPCC AR6—from cover to cover. Not the summary for policymakers, but the entire report. At 184 pages, it took me a while, but I’ve finally finished it. And throughout the process, I made a point of consistently distinguishing between what we know, what we can calculate, and what we assume.

Why make this distinction?

In public discourse, measurement data, physical laws, and model predictions are often lumped together. This leads to two problems: Some people exaggerate and claim that everything has been proven. Others downplay the issue and say it’s all just speculation. Both views are wrong.

The truth lies in a clear distinction. That is why I have divided the IPCC’s statements into three categories:

Measurement data — what thermometers, satellites, and ice cores directly measure. Global warming of 1.09°C, glacier loss of 199 gigatons per year, sea level rise of 20 centimeters since 1901. These are facts, not opinions.

Physical laws—the mechanisms that explain why this happens. Planck’s law of radiation, the Schwarzschild equation, the latent heat of fusion of ice. These laws are universally applicable, have been confirmed millions of times, and are used in climate model calculations.

Interpretations — statements about the future that depend on assumptions. The extent of global warming depends on feedback mechanisms—particularly clouds and water vapor. This is where the greatest scientific uncertainty lies, and this is where the honest debate begins.

What surprised me

The direct warming caused by a doubling of CO₂ is physically about 1.15°C. This is not an estimate; it is radiative physics. The fact that the IPCC arrives at 3.0°C is due to amplification effects—and the biggest source of uncertainty here is clouds. It is not CO₂ itself, but how clouds react to warming, that determines whether the future will be closer to 2°C or 4°C.

I found that remarkable. And I thought it deserved to be presented transparently.

The article

In the following article, I have summarized the key findings from the IPCC report—strictly organized into the three categories. No exaggeration, no downplaying. Just what the data show, what physics explains, and where the unanswered questions lie.

“We can’t understand glaciers unless we understand what’s really happening”

Understanding the IPCC Report

Jürg Kaufmann | August 2025

The Intergovernmental Panel on Climate Change’s Sixth Assessment Report (IPCC AR6, 2023) contains hundreds of statements on temperature, sea level, glaciers, and future scenarios. However, not all of them have the same level of certainty. This article transparently categorizes the most important findings into three categories: observational data, physical laws, and interpretations.

The Three-Category Methodology

To properly contextualize the findings of the IPCC report, we use a strict three-part framework. This approach follows the basic scientific logic that distinguishes between observation, theory, and prediction.

Category 1 Measurement Data

Measurement data are direct observations of the physical world. They are collected using instruments—thermometers, satellites, ice cores, water level gauges—and are independent of models or theories. If a thermometer reads 15°C, that is a measurement. Measurement data may contain measurement errors, but they are, in principle, verifiable and reproducible.

Category 2: Laws of Physics

Physical laws describe the mechanisms underlying observations. They are formulated mathematically, confirmed experimentally, and apply universally: Stefan–Boltzmann’s law, Planck’s law of radiation, the Clausius–Clapeyron equation, and the Schwarzschild equation. These laws are not opinions or a consensus—they are mathematical relationships that have been confirmed millions of times.

Category 3: Interpretations and Assumptions

Interpretations refer to statements about the future or about complex system relationships that cannot be directly derived from a single physical law: climate models, emission scenarios (SSPs), equilibrium climate sensitivity (ECS), and projections regarding glacier melt or tipping points. These statements can be highly plausible, but they depend on assumptions. The IPCC labels them with confidence levels (high/medium/low confidence) that reflect the degree of expert consensus—not physical certainty.

Cat. 1 What the measurement data show

The instrumental records and satellite observations in the IPCC AR6 document clear changes in the climate system:

Measurement parameter	Value	Source / Method
Global Warming (2011–2020 vs. 1850–1900)	+1.09°C [0.95–1.20°C]	Thermometers, station networks
Warming over land	+1.59°C	Ground stations
Warming over the ocean	+0.88°C	Ship and buoy measurements
CO₂ concentration (2019)	410 ppm	Direct measurement (Mauna Loa, etc.)
CH₄ concentration (2019)	1,866 ppb	Direct measurement
Sea Level Rise (1901–2018)	+0.20 m [0.15–0.25 m]	Water level gauges, satellite altimetry
Rate of sea-level rise (2006–2018)	3.7 mm/year	Satellite altimetry
Glacier mass loss (global)	−199 Gt/year (2000–2019)	GRACE satellites, field measurements
Greenland Ice Sheet	−279 Gt/year (2010–2019)	GRACE satellites
Antarctic Ice Sheet	−150 Gt/year (2010–2019)	GRACE satellites
Arctic sea ice (September)	−40% since 1979	Satellite remote sensing
CO₂ vs. 800,000 Years of Ice Core Data	Today's figures are unprecedented	Ice cores (Vostok, EPICA)

These data are empirically substantiated. They show that the Earth is warming, glaciers and ice sheets are losing mass, sea levels are rising, and greenhouse gas concentrations are higher than they have been in the past 800,000 years. ^{[1] [2] [3]}

Cat. 2: What Physics Explains

Radiative transfer and CO₂

The Earth receives solar energy (~240 W/m² on a global average) and radiates the same amount back into space as infrared radiation. Greenhouse gases—primarily H₂O, CO₂, CH₄, and O₃—absorb a portion of this outgoing radiation and re-emit it in all directions. This process is precisely described by the Schwarzschild equation (1906).

The radiative effect of CO₂ is logarithmic: the central absorption band at 15 μm is already nearly saturated at current concentrations. Each subsequent doubling reduces outgoing longwave radiation (OLR) by about 3.7 W/m². The calculation by Wijngaarden & Happer (2020) using over 300,000 spectral lines confirms this: ^[7]

CO₂ concentration	OLR (W/m²)	Reduction
Transparent (no GHGs)	~390	—
0 ppm (H₂O and O₃ only)	~308	~82
200 ppm	~288	~20
400 ppm (current)	~277	~5
800 ppm (doubled)	~274	~3

The direct warming effect of a doubling of CO₂—without any feedback—is:

ΔT = F₂ × CO₂ /_λPlanck = 3.7 / 3.22 ≈ 1.15°C

This is pure physics: Planck's law, the Schwarzschild equation, and the Stefan-Boltzmann law. ^{[7] [9]}

Melting energy

Exactly 334 kJ is required to convert 1 kg of ice to water at 0°C (latent heat of fusion). According to the Stefan-Boltzmann law, a glacier at 0°C emits 316 W/m² of infrared radiation. The energy balance at the glacier’s surface determines whether a glacier grows or shrinks. ^[5]

Thermal inertia

The ocean stores enormous amounts of heat. Even if emissions were to stop immediately, the heat already stored would continue to have an effect for centuries—this is physically inevitable. ^[6]

Cat. 3 Where Interpretation Begins

The transition from physics to interpretation occurs where feedback mechanisms come into play. This is the critical point at which the direct CO₂ warming effect of ~1.15°C multiplies to the IPCC estimate of 3.0°C.

The IPCC estimates the equilibrium climate sensitivity (ECS) to be 2.5–4.0°C (best estimate 3.0°C). This implies an amplification factor of ~3 compared to pure radiative physics: ^{[1] [9]}

Feedback mechanism	IPCC value (W/m²/°C)	Physical Basis	Security level
Planck response (baseline)	−3.22	Stefan-Boltzmann's Law	Proven Physics
Water vapor + lapse rate	+1.30	Clausius-Clapeyron Equation	Subject to acceptance
Surface albedo	+0.35	Measured ice/snow loss	Empirical, small
Cloud Feedback	+0.42 (−0.1 to +0.9)	Model parameterizations	Greatest uncertainty
Biogeochemical	+0.15	Various processes	Depending on the model

Water vapor feedback

The Clausius-Clapeyron equation states that warmer air can hold approximately 7% more water vapor per degree. The standard assumption in climate models is that relative humidity remains constant. Wijngaarden & Happer calculate: With fixed absolute humidity, the result is 1.4°C; with fixed relative humidity, it is 2.2–2.3°C. The assumption is plausible, but not dictated by a law of nature. ^[7]

Cloud Feedback

This is the largest single source of uncertainty. Clouds both cool (through reflection) and warm (through IR absorption) at the same time. How clouds change in response to warming cannot be derived from any fundamental law. The IPCC range extends from −0.1 to +0.9 W/m²/°C—a range that alone accounts for several degrees of ECS difference. ^[9]

Future scenarios

All projections depend on emission scenarios (SSPs)—assumptions about human behavior, policy, and the economy. Sea-level projections through 2100 range from +0.28 m (SSP1-1.9) to +1.01 m (SSP5-8.5). ^{[1] [10]}

Why this distinction is important

First, it prevents overgeneralization. Anyone who says, “The IPCC has proven that the temperature will rise by 3°C,” is confusing a model-based estimate with a physical fact.

Second, it prevents the issue from being downplayed. Anyone who says, “CO₂ levels are saturated, so there’s no problem,” ignores the fact that the feedback loops—particularly water vapor—are based on sound physics.

Third, it allows for an honest discussion of uncertainties. The measurement data are clear: the Earth is warming. Physics explains why. The open question is not whether, but by how much —and this question hinges on feedbacks, particularly clouds.

Summary

Category	Examples	Security level
Measurement data	+1.09°C warming, +0.20 m sea level rise, −199 Gt/year glacier loss, 410 ppm CO₂	Empirically proven
Physics	Planck, Schwarzschild, Stefan-Boltzmann, Clausius-Clapeyron, 334 kJ/kg, ~3.7 W/m², ~1.15°C direct	Fundamental laws of nature
Interpretation	ECS = 3.0°C, SSP scenarios, glacier retreat at 1.5°C, tipping points, sea level projections	Depending on the model or scenario

The measurement data clearly show that the climate system is changing. Physics explains the mechanisms. Future projections depend on assumptions. A fact-based discussion of climate change requires that these three aspects be consistently kept separate.

Sources

1. IPCC, 2023: AR6 Synthesis Report. ipcc.ch/report/ar6/syr
2. WGMS, 2025: Annual mass change of the world’s glaciers from 1976 to 2024. wgms.ch
3. Wouters, B. et al., 2019: Global Glacier Mass Loss During the GRACE Satellite Mission. frontiersin.org
4. IPCC, 2023: AR6 SYR, Section 2.1.1 — Observed changes in GHG concentrations. ipcc.ch
5. Oerlemans, J. & Klok, E.J., 2002: Energy Balance of a Glacier Surface. tandfonline.com
6. IPCC, 2023: AR6 SYR, Section 3.1.3 — Long-term sea level rise and irreversibility. ipcc.ch
7. van Wijngaarden, W. & Happer, W., 2020: Dependence of Earth’s Thermal Radiation on the Five Most Abundant Greenhouse Gases. wvanwijngaarden.info.yorku.ca
8. IPCC, 2023: AR6 SYR, Section 3.1.2 — Climate change risks. ipcc.ch
9. Forster, P. et al., 2021: Chapter 7 — The Earth’s Energy Budget, Climate Feedbacks, and Climate Sensitivity. IPCC AR6 WGI. ipcc.ch
10. IPCC, 2023: AR6 SYR, Figure 3.4 — Sea level projections. ipcc.ch