Global Warming Projections

  In its 2007 report the IPCC projected temperature increases for several different scenarios, depending on the magnitude of future greenhouse gas emissions. For a “moderate” scenario—in which emissions grow slowly, peak around the year 2050, and then fall—the IPCC report projected further warming of 1.1 to 2.9 Celsius degrees (1.9 to 5.2 Fahrenheit degrees) by the year 2100. For a “high-emissions” scenario—in which emissions continue to increase significantly and finally level off at the end of the century—the IPCC report projected further warming of 2.4 to 6.4 Celsius degrees (4.3 to 11.5 Fahrenheit degrees) by the year 2100.

  The IPCC cautioned that even if greenhouse gas concentrations in the atmosphere ceased growing, the climate would continue to warm for an extended period as a result of past emissions, and with more dramatic effects than were observed during the 20th century. If greenhouse gas emissions continue to increase, scientists project severe climate changes.

In October 2007 a study published in the Proceedings of the National Academy of Sciences warned that climate models used to project future global warming may have been overly optimistic. The study found that atmospheric carbon dioxide levels had increased 35 percent from 1990 to 2006, a rate of increase far higher than most climate models had assumed. The researchers reported that the average rate of growth in carbon dioxide levels was 1.3 percent during the period from 1990 to 1999, but 3.3 percent from 2000 to 2006. In 2000 an estimated 7 billion metric tons of carbon were released into the atmosphere from burning fossil fuels; by 2006 that number had grown to 8.4 billion metric tons, according to the study. Scientists pointed to the unexpectedly rapid melting of sea ice in the Arctic Ocean during the summer of 2007 as evidence that climate models were failing to predict how quickly the climate was changing.