How Do We Know that Human Activities Have Influenced Global Climate?

Human activities have significantly altered not only the chemical composition of Earth's atmosphere, but also the climate system. Human influences have led to increases in well-mixed greenhouse gases, decreases in stratospheric ozone, and changes in the atmospheric burdens of sulfate and soot aerosols. All of these atmospheric constituents interact with incoming solar and outgoing terrestrial radiation. Human-induced changes in the concentrations of these constituents modify the natural radiative balance of Earth's atmosphere, and therefore perturb climate. Quantifying the size of the human effect on climate is a difficult statistical problem. 'Fingerprint' methods are typically used for this purpose. These methods involve rigorous statistical comparisons of modeled and observed climate change patterns. Fingerprinting assumes that each individual influence on climate has a unique signature in climate records. The climate fingerprints in response to different forcing factors are typically estimated with computer models, which can be used to perform the controlled experiments that we cannot conduct in the real world. One criticism of the findings of previous scientific assessments is that they have relied heavily on fingerprint studies involving changes in near-surface temperature. Recent fingerprint work, however, has considered a variety of other climate variables, such as ocean heat content, stratospheric temperatures, Northern Hemisphere sea ice extent, sea level pressure, atmospheric water vapor, and the height of the tropopause. These studies illustrate that a human-induced climate change signal is identifiable in many different variables and geographic regions, and that the climate system is telling us an internally- and physically-consistent story.