When you ask yourself what the biggest unanswered scientific questions are, “how did sea levels change over the past 100 years?” is unlikely to appear at the top of your list.

After all, haven’t we already figured that out? It turns out that obtaining a complete picture of how our oceans have been changing is not a simple task, yet is vital for making future projections. In a paper published in Nature this week, a research group developed a more accurate yardstick for measuring rising sea levels, offering clues to a discrepancy that scientists have grappled with for years and potentially insight into future projections of the rise in sea levels.

Sea-level observations over the 20th century come from tide-gauge records, which, in their simplest form, are essentially yardsticks attached to coastlines around the world. The global coverage of these measurements is limited, particularly at the start of the 20th century and in the southern hemisphere. Additionally, even the most complete records can include significant gaps in time. The incompleteness of these records makes obtaining estimates of global mean sea level very difficult.

NASA Goddard Space and Flight Center, CC BY

The latest report from the Intergovernmental Panel on Climate Change (IPCC) included two different estimates of the global increase in sea level over the 20th century. The first estimate came from a suite of previously published studies that analyzed the tide-gauge observations directly. The 1901-1990 rate obtained using these approaches falls in the range of 1.5 to 2 millimeters per year.

The second estimate was computed by adding estimates from individual sources, such as melting water from ice on land and expansion of the ocean. This “bottom-up” approach produced a lower rate of 1 to 1.2 millimeters per year over the same time period.

Read more here http://theconversation.com/solving-the-puzzle-of-sea-level-rise-by-reexamining-the-past-36331