Winterschule 2025

Man-made emissions of greenhouse gases (CO2, CH4, N2O, ….) and other anthropogenic disturbances (surface coverage, aerosols...) forces the Earth energy budget out of balance. This anthropogenic forcing of the climate system by greenhouse active substances reached 2.72 W/m2 (range 1.96 to 3.48 W/m2) in 2019 relative to 1750 and at the present rate of greenhouse gas emissions is increasing by 0.061 W/m2 (2.7%) per year. Accordingly, the unbalance between received solar radiation and reemitted long wave radiation to space has been recognized in observations of increased outgoing long wave emission to space (~1.82 W/m2), which primarily reflects the greenhouse gas induced change in the radiative balance of the atmosphere. The difference between the radiative forcing and increased outgoing radiation was also noticed in the overall heating of the climate system by 0.90 ± 0.15 W/m2 for the period 2005 – 2019. The latter primarily serve to heat the ocean, which constitutes the largest heat capacity (91%) of the climate system, and the rest (9%) warms the land (5%), causes ice loss (3%) and warms the atmosphere (1%). 

 

In the context of climate change, the most important quantity is the equilibrium climate sensitivity (ECS), which is a measure by how much the globe will warm for a given amount of radiative forcing. For a doubling of atmospheric CO2, for which the best estimate is 3.7 W/m2 in radiative forcing, ECS is assessed to 3.0 C with the most likely range being between 2.5 to 4.0 C. However, the ECS is an intrinsically uncertain quantity since in the complex climate system non-linear and difficult to be quantified positive and negative feed-back mechanism are at work, not to mention that certain elements of the climate system may reach tipping points at certain thresholds. The latter refers to rapid and irreversible changes in some climate subsystems (sea ice coverage, glaciers, meridional oceanic circulation (MOC), vegetation cover, permafrost thawing, … ) with increasing surface temperatures.

 

In this lecture series, I will provide further details on the essential features of the climate energy budget, including observations and constraints from theoretical studies and modelling, as well as on the equilibrium climate sensitivity estimated from past and present observations of the climate system, modelling and from other constraints. The lecture course is accessible for experimental and theoretical PhD students, and only requires a basic understanding of thermodynamics and environmental physics.