Recently I saw a NOAA vessel in Seattle. I have heard that tides have been getting larger recently. In fact, the largest tide in over 113 years happened last winter. Of course this brings up concerns related to the oft dismissed topic of climate change (previously known as global warming).
Are these fears legitimate? Is there a way we can distil data to help answer this question? The government and various agencies such as NOAA focus efforts upon answering questions such as these. Unfortunately, they don't have much to say about tidal changes. Nonetheless, they do gather these data. While NOAA may be moving, I welcome their presence. I have even noticed a recent increase in their presence in Seattle. Don't believe me, here is a picture from a few days ago:
How have tides changed over the last five years? Examine this by asking: how have predictions accounted for the expected changes in tidal volumes? Fortunately for concerned citizens and data enthusiasts everywhere, NOAA makes important tidal prediction data available for public use.
I have taken the time to summarize these data into a single visualization that illustrates the daily changes in tidal depth. I utilized the powerful d3 library and the calendar plug-in to create this graph. The advantage of this calendar graph is the ability to show a lot of information in compact space. There are five rows within this graph, each row describes an entire year. From left to right within a single row we progress from January to December. From top to bottom within a column of any single year we move from Monday to Sunday. In such a fashion, all 365 days are recorded for each year. This simple visualization is rich since it records data for more than 1300 days into a single picture.
The graph above represents daily changes in high tide depth. The graph below represents these changes for low tides.
This view of the data helps to clearly show several patterns. First, there are several diagonal bands present within each year. These bands line up across all five years. Next, the pattern of red and green changes with each column. This means that the pattern of rising or falling for tides is consistent within each week. It also appears that tides alternate between rising and falling every other week.
Back to the main question: Have tides increased or decreased?
These graphs seems to suggest that the rise and fall of tides is more pronounced in 2015 than in 2011. Notice how the diagonal bands are larger in 2015 than in 2011. Notice also that the magnitude of changes appear larger in 2015 than in 2011. This may indicate that tides are getting more extreme. But, it could also mean that low tides are getting lower since similar changes will cause larger relative changes as the low tide gets lower. I'll dig into this issue more in the future with further analysis that explores this question in more detail.