evidence: The IPCC assessments also forecast the atmospheric GHG concentrations and global temperatures in 50 and 100 years if humanity continues on its current trajectory of burning fossil fuels, reducing forest cover (which means less carbon stored in plants and in the soil), and other activities. The latest estimates suggest that by 2100, global average surface temperatures will increase between 1.5 and 4.5 degrees Celsius (3 and 12 degrees Fahrenheit). And once this increase approaches 2 degrees Celsius, we may pass tipping points after which global warming may accelerate.10 For example, melting permafrost in the Arctic could release more methane, which, as a potent GHG, would raise arctic temperatures faster, thus melting permafrost faster and releasing even more methane in a self-propelling cycle. As the science progresses, the IPCC reports have become more confident in predicting a rising rate of extreme events like hurricanes. With powerful computer models that simulate hurricane development under different ocean temperatures and other factors, scientists now simulate the mechanisms which drove Katrina’s quick acceleration to a Category 5 hurricane. Thus, scientists can now confirm that many extreme weather events have been made worse by rising GHG emissions.