Monday, December 17, 2007


Sea Level Rise

I had an argument about global warming this weekend and the subject of sea level rise came up during the discussion. (We were talking about the necessary third question, (after 1. Is it warming? (yes) and 2. Is it human caused? (partially)), Will it be a bad thing?). My opponent said that sea levels had risen alarmingly recently.

Here's a closer look at that assertion. During the depths of the last Ice Age 22,000 years ago, sea levels were 130 meters lower. That's a lot of warming, melting, and rising in 22,000 years, over 400 feet. Remember, however, 80% of the rise of the sea level after warming is thermal expansion of the sea water, not land ice melting and flowing in. But it hasn't been steady. Here's the s-curve graph.
From 22,000 to 8,000 years ago, most of the sea level rise took place, at least 120 meters of it or about .3368 in./year. In the last 8,000 years it's gone up at most 10 meters or .049 in./year.

Now we should take a look at sea level rise between 1880 and 2005, or for 125 years. Is it closer to the alarming rate 22,000 to 8,000 years ago or is it closer to the recent very stable rate?

Since 1880 the sea level has risen 20 cm (that's 7.8 inches in over a century, not alarming in my book) and the rate is .0629 in./year. Well, that's much closer to the recent, stable rate (only about 30% more) and much less than the alarming rate of the ancient past (about six times less, in fact). Nor is there a hint of acceleration in the rate of change; it's merely a straight line increase. So there's certainly no crisis in sea level rise. It's still getting warmer since the depth of the last Ice Age (as it always does during the first half of the interglacial and we've had at least 20 interglacials in the past few million years) and sea levels are gently and very slowly rising. Not exactly 'run for the hills, the sea's rising' sort of news.

Gee, I wonder why my opponent thought that sea levels had risen alarmigly recently?


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