warming causes seawater to expand, which contributes to sea level rising. Sea level rose at an average rate of about 1.8 mm/year during the years 1961–2003. The rise in sea level during 1993–2003 was at an average rate of 3.1 mm/year. It is not clear whether this is a long-term trend or just variability. Antarctic sea ice shows no significant overall trend, consistent with a lack of warming in that region. Hurricanes There has been an increase in hurricane intensity in the North Atlantic since the 1970s, and that increase correlates with increases in sea surface temperature. The observed increase in hurricane intensity is larger than climate models predict for the sea surface temperature changes we have experienced. There is no clear trend in the number of hurricanes. Other regions appear to have experienced increased hurricane intensity as well, but there are concerns about the quality of data in these other regions. It is more likely than not (>50%) that there has been some human contribution to the increases in hurricane intensity. It is likely (>66%) that we will see increases in hurricane intensity during the 21st century. Table SPM-2 lists recent trends along with certainty levels for the trend having actually occurred, for a human contribution to the trend, and for the trend occurring in the future. In relation to changes (including increased hurricane intensity) where the certainty of a human contribution is stated as "more likely than not" footnote f to table SPM-2 notes "Magnitude of anthropogenic contributions not assessed. Attribution for these phenomena based on expert judgment rather than formal attribution studies." Factors that warm or cool the planet AR4 describes warming and cooling effects on the planet in terms of radiative forcing—the rate of change of energy in the system, measured as power per unit area (in SI units, W/m2). The report shows in detail the individual warming contributions (positive forcing) of carbon dioxide, methane, nitrous oxide, halocarbons, other human warming factors, and the warming effects of changes in solar activity. Also shown are the cooling effects (negative forcing) of aerosols, land-use changes, and other human activities. All values are shown as a change from pre-industrial conditions. Total radiative forcing from the sum of all human activities is about +1.6 watts/m2 Radiative forcing from an increase of solar intensity since 1750 is about +0.12 watts/m2 Radiative forcing from carbon dioxide, methane, and nitrous oxide combined is