Greenhouse gases (GHGs) accumulate in the Earth’s atmosphere, trapping the sun’s heat and warming the planet. This scientific fact was established more than 150 years ago. The higher the level of GHGs in the atmosphere, the greater the amount of heating at the Earth’s surface. The direct relationship between atmospheric carbon dioxide (CO2) concentration — the primary GHG — and global surface temperature change can be seen in Figure 1. If GHGs continue to accumulate in the atmosphere, climate scientists project the planet’s temperature will rise to levels not seen in millions of years.

Figure 1. Atmospheric CO2 Concentration and Global Surface Temperature Change

Source:  The Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report, “Technical Summary,” p. 44.

Temperature is a measure of average energy in a collection of molecules within some boundary. A 1°C rise in global average surface temperature is small in terms of the average molecular energy, but it is enormous in terms of total energy because the number of molecules comprising the Earth’s surface is vast. This large amount of extra energy in the land, oceans, and atmosphere increases the frequency and intensity of heat waves, droughts, hurricanes, heavy rain, and floods. There is no specific threshold at which these effects start to occur. It’s simply a direct relationship: the greater the total energy, the greater the probability of extreme weather events (as compared to the distributions seen in the last several hundred years).

Climate scientists have built models to predict how temperature and precipitation patterns may change as a result of increased global surface temperatures. Despite significant progress made in models in recent years and the use of state-of-the-art supercomputers, these models can only offer probabilities. One area of uncertainty is whether, and when, rising temperatures will trigger changes in the environment that will drive temperatures even higher. Often referred to as “tipping points,” examples include glacial retreat, permafrost thaw, and the slowing of the Gulf Stream.

Despite the uncertainty about how the climate may evolve, there is no doubt in the scientific community that there has been a rise in global average surface temperatures over the last century and that the increase is the result of GHG emissions from human activities. The thousands of scientists who contributed to the International Panel on Climate Change (IPCC) Sixth Assessment Report, issued in 2021, said, “Since systematic scientific assessments began in the 1970s, the influence of human activity on the warming of the climate system has evolved from theory to established fact” (p. 44).