Tropical Cyclones - Since 1878, about six to seven hurricanes have formed in the North Atlantic every year. Roughly two per year make landfall in the United States. The total number of hurricanes (particularly after being adjusted for improvements in observation methods) and the number reaching the United States do not indicate a clear overall trend since 1878.
Ocean Heat - In three different data analyses, the long-term trend shows that the oceans have become warmer since 1955.
Sea Surface Temperatures - Sea surface temperature increased over the 20th century and continues to rise. From 1901 through 2012, temperatures rose at an average rate of 0.13°F per decade.
Sea Level - After a period of approximately 2,000 years of little change, global average sea level rose throughout the 20th century, and the rate of change has accelerated in recent years.When averaged over all the world's oceans, absolute sea level increased at an average rate of 0.07 inches per year from 1880 to 2011. From 1993 to 2011, however, average sea level rose at a rate of 0.11 to 0.13 inches per year—roughly twice as fast as the long-term trend.
Ocean Acidity - Measurements made over the last few decades have demonstrated that ocean carbon dioxide levels have risen in response to increased carbon dioxide in the atmosphere, leading to an increase in acidity (that is, a decrease in pH)