Large and Long-Lasting Human-Caused Climate Change

The carbon dioxide released into the air by human fossil-fuel burning will have large and long-lasting effects on climate and sea level, impacting humans and other living things in major, costly ways. The high scientific confidence in these conclusions is based on a “four-legged stool” of evidence: (i) understanding from fundamental sciences (physics, chemistry, biology); (ii) agreement among a range of models of different types implementing the scientific understanding and run by different groups; (iii) emergence of trends in recent observations; and (iv) confirmation from Earth’s climate history. Major changes expected include increasing heat stress and decreasing cold stress, increasing variability in the water system (more droughts and floods), increasing strength of the strongest storms, and sea level rise. Some of these changes are already occurring, but human decisions will control how large they become.