The most important drivers of climate change

Part 1 (approx. 20 minutes): The topic explained clearly — analogies and examples

The term "greenhouse gas" can lead to the false impression that the atmosphere behaves like a glass roof. A more accurate picture is that of a warming blanket: gases in the atmosphere absorb part of the heat radiated from the Earth and re-emit it back toward the surface and adjacent air layers. This feedback increases the average temperature of the Earth's surface compared to the value without these gases. This basic principle is often compared in popular descriptions to a blanket or a greenhouse; the physical basis, however, is the absorption and re-emission of infrared radiation by certain gas molecules. This description and the underlying measurements are documented in the IPCC report and form the scientific consensus on the physical basis of the greenhouse effect [1].

A simple everyday example: imagine a heated room that retains heat better because of an additional layer of insulating material. Analogously, greenhouse gases increase the "insulating effect" of the Earth-atmosphere system. Different gases act with different strengths and over different time periods. CO2, for example, is relatively persistent in the atmosphere and acts over decades to centuries, whereas other substances such as soot particles or short-lived aerosols can cool or warm only temporarily. The relative importance and interplay of these effects are the subject of precise measurements and climate models; assessments of the contributions of individual gases and their lifetimes are summarized in relevant overviews [2][3].

Essential for understanding is the distinction between cause and amplification: Some processes (e.g., increased CO2 concentration from burning fossil fuels) act as direct causes of increased radiative forcing, while others (e.g., changes in water vapor or ice cover) act mainly as feedbacks that amplify or dampen an initial warming. The empirical attribution of the main cause of the observed warming in recent decades to human emissions is based on a multitude of independent measurements and model comparisons and is summarized in the IPCC report: The dominant role of the human-caused increase in greenhouse gases is a core result of the scientific consensus [1][2].

Part 2 (approx. 20 minutes): Technical terms and detailed explanation

Radiative Forcing (Strahlungsantrieb) is a central technical term: it describes the change in the Earth's energy balance due to a perturbation — positive when more energy remains in the system (warming tendency), negative when energy is removed (cooling tendency). Greenhouse gases produce predominantly positive radiative forcing; the IPCC reports summarize the known contributions of various components (CO2, methane, nitrous oxide, fluorinated gases, aerosols, land-use changes, etc.) and quantify them within ranges of uncertainty [1][2].

Another important concept is the atmospheric lifetime of a gas. This parameter describes how long a perturbation caused by an emission typically remains detectable in the atmosphere. CO2 does not have a single clear lifetime because it is removed by various processes (dissolution in oceans, uptake by vegetation, long-term storage in soils and sediments); therefore CO2 is often considered to have a long-term effect. Methane has a shorter average residence time but has a stronger effect per molecule on the radiative balance over its typical lifetime than CO2. These differences explain why policy measures must address both short-term effective reductions of methane and long-term reductions of CO2 [3][4].

The term Global Warming Potential (GWP) is used to make the climate effect of different gases comparable over a defined time interval. GWP values depend on the chosen time horizon (e.g., 20, 100 years) because gases have different atmospheric lifetimes. GWP is a useful tool for weighing strategies but has limitations: it does not directly account for regional effects, feedbacks, or chemical interactions in the atmosphere; therefore climate scientists complement GWP metrics with other indicators and model studies when making policy recommendations [2][4].

On the role of the carbon cycle: Anthropogenic emissions not only change the atmospheric composition but also alter fluxes between the atmosphere, oceans, and land ecosystems. A considerable fraction of the annual CO2 emissions is taken up by the ocean and by terrestrial sinks, which reduces the rate at which atmospheric concentration increases compared to what would happen without these sinks. At the same time, warming, changes in precipitation, and land-use changes introduce uncertainties in the future effectiveness of these sinks. Current estimates of these fluxes and their development are regularly summarized in global carbon budgets [5][1].

Natural factors: solar irradiance, volcanic aerosols, and changes in Earth's orbit (Milanković cycles) are natural forcings. Short-term fluctuations in solar activity lead to small changes in incoming radiation; however, large-scale warming in recent decades cannot be explained by solar variations. Major volcanic eruptions can temporarily cause cooling by injecting sulfur compounds into the stratosphere; such events are well observed and appear in climate models as short-term cooling impulses [6][7]. Orbital cycles drive ice ages over millennia but are not the cause of the rapid temperature increases of the last one to two centuries [7].

Uncertainties: there are uncertainties in the quantitative estimation of individual contributions. These include future emissions pathways, the response of clouds and aerosols to warming (these feedbacks are particularly difficult to quantify precisely), and the long-term behavior of land and ocean sinks. The IPCC explicitly names these uncertainties and provides justified ranges; the direction of the main conclusion — that human emissions dominate the observed warming trend — remains robust despite these uncertainties [1][2].

Part 3 (approx. 10 minutes): Applications, limits and short thought exercises

Concrete applications of the concepts presented can be found in the analysis of emissions pathways and their translation into expected temperature changes using climate models. In this way, policy scenarios regarding emissions reductions are translated by models into temperature projections. When evaluating measures, it is important to consider gases that act effectively in the short term (e.g., methane) separately from long-lived emission sources (primarily CO2) because their climate impacts play out differently across time horizons [3][5].

Limits: models are representations of reality with limited resolution and simplified processes; in particular cloud physics and regional feedbacks remain areas with higher uncertainties. Observational data are widely available, but in some regions (e.g., certain ocean areas and high latitudes) time series are shorter or spatially sparse, which complicates local attribution. Such data gaps are being actively addressed but remain relevant for the interpretation of regional projections [1][5].

Short exercise 1: Name three properties of greenhouse gases that determine their climate effect, and briefly explain why each property is relevant. Hints for the solution can be found in the sections on Radiative Forcing, lifetime, and GWP above as well as in the referenced sources [1][2][4].

Short exercise 2: Consider which time horizons should play a role when prioritizing climate protection measures if short-term warming (as a risk for extreme events) and long-term warming (as a result of cumulative CO2 emissions) are to be considered simultaneously. Use the distinction between short-lived climate pollutants and long-lived greenhouse gases from Part 2 as a starting point [3][5].

In conclusion, it should be emphasized again: the scientific consensus holds that the current warming trend is predominantly due to human activities, particularly the increase of CO2 and other greenhouse gases in the atmosphere. However, the precise quantitative breakdown of individual contributions and the regional impacts continue to involve research questions and uncertainties that are being addressed in current research programs [1][2].