very likely (>90%) increasing more quickly during the current era (1750–present) than at any other time in the last 10,000 years. Climate sensitivity Climate sensitivity is defined as the amount of global average surface warming following a doubling of carbon dioxide concentrations. It is likely to be in the range of 2 to 4.5 °C, with a best estimate of about 3 °C. This range of values is not a projection of the temperature rise we will see in the 21st century, since the future change in carbon dioxide concentrations is unknown, and factors besides carbon dioxide concentrations affect temperature. Model-based projections for the future Model projections are made based on an analysis of various computer climate models running within the different scenarios that were established in 2000 in the Special Report on Emissions Scenarios (the "SRES scenarios"). As a result, predictions for the 21st century are as shown below. Surface air warming in the 21st century: Best estimate for a "low scenario" is 1.8 °C with a likely range of 1.1 to 2.9 °C (3.2 °F with a likely range of 2.0 to 5.2 °F) Best estimate for a "high scenario" is 4.0 °C with a likely range of 2.4 to 6.4 °C (7.2 °F with a likely range of 4.3 to 11.5 °F) A temperature rise of about 0.1 °C per decade would be expected for the next two decades, even if greenhouse gas and aerosol concentrations were kept at year 2000 levels. A temperature rise of about 0.2 °C per decade is projected for the next two decades for all SRES scenarios. Confidence in these near-term projections is strengthened because of the agreement between past model projections and actual observed temperature increases. Based on multiple models that all exclude ice sheet flow due to a lack of basis in published literature, it is estimated that sea level rise will be: in a low scenario 18 to 38 cm (7 to 15 inches) in a high scenario 26 to 59 cm (10 to 23 inches) It is very likely that there will be an increase in frequency of warm spells, heat waves and events of heavy rainfall. It is likely that there will be an increase in areas affected by droughts, intensity of tropical cyclones (which include hurricanes and typhoons) and the occurrence of extreme high tides. "Sea ice is projected to shrink in both the Arctic and