What is Causing Global Warming?

What is Causing Global Warming?

While it is not our intent in this module to explore this question in detail, it is worth pointing out that many human activities clearly affect the climate system. Most notably, emissions of greenhouse gases, especially carbon dioxide and methane, are causing more heat to be trapped within Earth’s atmosphere. This effect, called the greenhouse effect, has been well understood since it was discovered by Svante Arrhenius in 1896. Figure 1 below, taken from the 2023 IPCC Working Group 1 Technical Summary shows the relative amount of heating or cooling of the climate system that can be attributed to the various factors that have changed between 1750 and 2019. The anthropogenic modifications to the climate system, enumerated in panel (a) of the figure, show the breakdown of radiative forcing. Anthropogenic forcing greatly outweighs the changes due to natural changes in solar irradiance. Panel (b) shows the effect each emitted component has on global surface temperature. The IPCC is quite careful to note the level of confidence associated with any given piece of knowledge, seen here with the black error bars of 5-95%. They are also transparent and are quick to point out when new understanding has significantly changed estimates or predictions, as has happened with our understanding of stratospheric water vapor, which was thought to be a significant contributor to warming in the Fourth IPCC Assessment Report (AR4, released in 2007), but has been found to be less significant.

Figure 1. (IPCC Figure TS.15) Effective radiative forcing (ERF) of climate change since the Industrial Era. Panel (a) shows the various radiative forcing emitters and their effect on global surface temperature, seen in panel (b). Black bars represent uncertainty in the data.

Source: IPCC, 2019