Several things have contributed to the rising sea, but the two most important, scientists almost universally agree, have to do with climate change. Thanks to heat-trapping greenhouse gases, especially carbon dioxide (CO2) pumped into the atmosphere by the burning of fossil fuels, global temperatures are more than one degree F higher than they were 100 years ago. Since water expands as it warms, the oceans take up more space than they once did, and the only direction they can expand are up and out.
Warmer temperatures also make glaciers and land-based ice sheets melt, and make tidewater glaciers — glaciers that reach the ocean — slide more rapidly into the sea and calve more icebergs. In both cases, water that had been trapped on land enters the ocean, in either solid or liquid form, making sea level rise even more.
All of this has been thoroughly documented. Scientists understand the expansion of water very well; they have watched many hundreds of glaciers around the world retreat over the past century; and careful measurements from the ground, air and space show that Greenland and Antarctica have been losing ice at an accelerating rate over the past two decades, at least.
Scientists have also watched levels of greenhouse gases in the atmosphere rise steadily, thanks largely to the growing worldwide demand for energy. The increase in temperature is what you’d expect from the rise in greenhouse gases; the rise in sea level is what you’d expect from the rise in temperature; and the acceleration of sea level rise in recent decades is what you’d expect from the fact that there are more greenhouse gases like CO2 in the atmosphere now, trapping more heat, than there were at the beginning of the 20th century.
Next section: History of Sea Level