The Science Background

Abstract

The pioneering work of three nineteenth-century scientists is briefly described. From the middle of the twentieth century, researchers recorded data that demonstrated the rise in carbon dioxide (CO₂) emissions at a rapidly increasing rate. Historical data from 400,000 years before present indicate that the levels of CO₂ emissions achieved in the late twentieth century were unprecedented in recent geologic time. Recent global annual average emissions of CO₂ exceed 40 gigatons, an amount that is inconsistent with limiting global warming to between 1.5 and 2 degrees Celsius. Radiative forcing calculated in watts per square meter is the preferred method for expressing global warming. Globally mixed greenhouse gases (GHGs) in the atmosphere are responsible for atmospheric warming. Representative concentration pathways describe the amount of warming that can be expected at different levels of GHG emissions between the present time and 2100 given assumptions about future levels of GHG emissions. Hotspots are defined as areas where greater warming has occurred beyond the global mean surface temperature data that are regularly reported. Climate change affects day-to-day weather and increases risks of disasters resulting from heavy winds, floods, hurricanes, and extreme cold. Human life inhabits a limited area named by scientists as the “critical zone” between the sky and soil.