Climate signals are long-term trends or changes in the climate system due to human-caused greenhouse gas emissions. The ‘signals’ of human-caused climate change are distinct from the ‘noise’ of natural climate variability as well as other ways, such as aerosol pollution, that human activities can affect the climate.

Climate signals embody changes that are well-established through climate change attribution as well as changes that are widely accepted based on available scientific evidence (e.g. observations, modeling, or basic physics).

Examples of climate signals that are well-established through attribution science include temperature and precipitation extremes, sea level rise, and intensifying storms. Some climate signals are more elusive, however, such as decreased precipitation in the southwestern US or intensifying El Niño events. These and other signals are supported by observations and/or modeling but it can be difficult, due to data limitations or the complexity of the systems involved, for scientists to attribute – with the usual standard for statistical significance – the changes to human-caused greenhouse gas emissions. This does not mean the human fingerprint is absent from a signal under question, or even small. It simply means more evidence is needed to determine the nature and extent of human influence.