Fundamentals of Climate Change

Part 1 (20 minutes): What is meant by climate change? Clear explanations, analogies and examples

When we talk about "climate change," we mean the long-term change in the average weather conditions of the Earth or a region over decades to centuries. Unlike daily weather, which fluctuates hour to hour and day to day, climate refers to the statistical description of those fluctuations over longer periods. A simple analogy is the household thermometer: weather is the instantaneous reading of temperature, climate would be the averages and the typical seasonal variations that one observes over years.

From a natural science perspective, climate change includes changes in temperature, precipitation, wind patterns, ocean circulation, sea surface and other properties of the Earth system. The term covers both natural causes (e.g., volcanic eruptions, variations in solar radiation) and human-induced changes. The current increase in the global average temperature observed since about the mid-20th century is, according to the current state of research, predominantly attributable to human activities, in particular the increase of greenhouse gases in the atmosphere. This is summarized in the Sixth Assessment Report of the IPCC (Working Group I) (IPCC, 2021).

An illustrative example of the effect of greenhouse gases is the greenhouse principle: shortwave solar radiation passes through the atmosphere and warms the Earth's surface. The Earth emits this energy back as longwave infrared radiation; greenhouse gases such as carbon dioxide (CO2), methane (CH4) and others absorb some of this longwave radiation and re-emit part of it back toward the surface, causing further warming. This physical effect has been known for a long time and is demonstrated in laboratory and field measurements; the IPCC summarizes the findings on this (IPCC, 2021).

Everyday examples clarify why global changes appear relevant: a region can become warmer overall even though there are still cold winters; extreme heat days can become more frequent, precipitation can shift or become more intense in some regions and drier in others. Such shifts affect agriculture, water availability, health and infrastructure.

It is important to distinguish between the core fact—the physical process and measurable changes—and the societal interpretation of what this means for people. The scientific findings on warming and its causes are based on extensive measurements and analyses, which are summarized and assessed in several places (IPCC, 2021; IPCC, 2023).

Part 2 (20 minutes): Technical terms and how the climate is measured and documented

Basic technical terms

Greenhouse gases: Gases in the atmosphere that absorb and re-emit longwave thermal radiation. The most important include CO2, CH4, nitrous oxide (N2O) and fluorinated gases. The increase of these gases due to human activities is a central driver of recent warming (NOAA; Scripps).

Radiative forcing: A physical quantity that describes how strongly a factor (e.g., changed greenhouse gas concentration) alters the energy inflow or outflow in the Earth system. A positive radiative forcing leads to warming; the IPCC assesses total amounts and contributions of individual factors (IPCC, 2021).

Feedbacks: Processes that amplify or dampen the initial effect. A frequently mentioned example is the melting of sea ice: less ice means lower reflection of sunlight (albedo), so more energy is absorbed and it becomes even warmer. Feedbacks are part of the complex response of the Earth system and are included in climate models (IPCC, 2021).

How is the climate measured and documented?

The assessment of climate is based on several complementary measurement series and methods:

Instrumental observations: Thermometers at land stations, ocean buoys and ships, satellite measurements and radiosondes provide direct measurements for temperature, precipitation, sea level, ice cover and many other variables. Global temperature time series are produced by institutions such as NASA (GISS), NOAA and the Met Office/CRU; these datasets are based on large-scale compilations of measurement data and statistical adjustments (e.g., for station relocations) (NASA; NOAA).

Satellite observations allow consistent global coverage since the late 1970s and contribute important information on the development of land surface temperature, ice cover, sea surface temperature and sea level. Satellite data require calibration and validation with ground-based measurements; both data sources are evaluated in combined analyses (NASA).

Paleoclimate indicators (proxies): To reconstruct climate changes before the start of instrumental records, researchers use tree rings, ice cores, sediment layers, corals and pollen. These proxy series provide information on temperature, precipitation and atmospheric composition over hundreds of thousands of years, but with varying temporal resolution and uncertainty. Such reconstructions show, for example, that today's CO2 concentration and the current warming trend are unusual on many time scales (NOAA Paleoclimatology; IPCC, 2021).

Measurement of greenhouse gases: Direct measurement series like the Mauna Loa CO2 measurements (the "Keeling Curve") and global measurement networks of NOAA document the temporal increase of CO2 and other gases. These observations show a persistent, strong rise in concentrations of relevant gases since the Industrial Revolution, associated with combustion of fossil fuels and land-use changes (Scripps Institution of Oceanography; NOAA GML).

Documentation, assessment and uncertainties

Global syntheses such as the reports of the Intergovernmental Panel on Climate Change (IPCC) summarize observations, process understanding and model analyses and provide assessments of the confidence in conclusions. The IPCC, for example, makes clearly formulated statements in its summaries about attribution and the likelihood of certain changes; at the same time, uncertainties in regional projections, some feedback processes and the exact strength of certain climate events are identified (IPCC, 2021; IPCC, 2023).

A typical area of uncertainty concerns regional projections of precipitation and extreme events: globally, a warming trend can be determined with high confidence, while on a regional level the projections vary more and depend on local factors. Likewise, the attribution of individual extreme events to human influence requires methodological effort and yields graduated confidence: for some types of extremes (e.g., heatwaves) human influence can be detected with high likelihood; for others (e.g., single heavy precipitation events in complex local systems) statements are often less precise (IPCC, 2021).

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

Concrete applications of the knowledge learned can be found both in policy design and in technical and municipal measures. Climate models are used to investigate scenarios of future developments and to evaluate options for emission reductions and adaptation measures. Observational data are the basis for monitoring, early warning systems and the validation of models (IPCC, 2023; NASA).

At the same time, the limits of knowledge must be considered: models provide projections under certain assumptions about emission pathways and socio-economic development; they are tools for weighing risks, not predictions with one hundred percent certainty. Regional uncertainties, incomplete data in some parts of the world and difficult-to-quantify feedbacks remain research areas for which the IPCC reports explicitly recommend further work (IPCC, 2021).

Short thought exercises for consolidation

1) Imagine you are responsible for a water supply network in a region where precipitation patterns are becoming more uncertain. What direct consequences could changed precipitation distributions have, and which three measures would you consider to increase supply security? In your consideration, please separate measures to reduce the cause (avoidance/reduction of emissions) and adaptation measures (e.g., infrastructure changes).

2) Consider a simple measurement series: If a single weather station shows a warming of 0.5 °C over 30 years, what additional information do you need to put this local observation into a global context? Think of data homogenization, comparison with nearby stations, satellite data and longer time series.

3) Discuss in a short thought experiment which social consequences more frequent hot days can have (e.g., health, work capacity, energy demand) and which actor groups (urban planning, health sector, energy industry) are each directly affected.

For all tasks it is helpful to use the sources mentioned in this lecture as a starting point for deeper information. They show both the observed development and the uncertainties and the methodological foundations (IPCC; NOAA; NASA; WMO).