There is no doubt about it.
But why?
How much of the recent rise is due to greenhouse gas warming? How much of it is natural?
The situation is a bit more nuanced than described by some of the media and others (like Zillow in a recent prediction of large numbers of coastal homes flooding in the NW).
Well, let's get our feet wet in this topic by examining sea level records provided by NOAA for several cities. In each of them, NOAA has also put a "best fit" line for reference.
First Seattle, which has a sea level record going back over a century (1900). Over the entire period, there has been an average of 2.03 mm increase in sea level per year (.67 feet per century). The interesting thing is that the upward trend has been going on for a long time, well before the impacts of human emissions of greenhouse gases were significant. (The radiative impacts of increasingly CO2 became large in the 1970s and later). And rate of rise has been quite steady, with no hint of a recent acceleration. In fact, there has been minimal rise during the past 20 years.
San Diego? A very similar evolution, at a slightly greater rate of rise (2.15 versus 2.03 mm per year)
How about Key West? A little more: about 2.40 mm a year (.78 feet per century). Again, no hint of acceleration of sea level rise during the past decades as human-emitted greenhouse gases have increased rapidly.
From NOAA's National Climatic Data Center's website I secured this graphic of global sea level rise over the past 140 years. Pretty steady rise since roughly 1920 and even rising before that.
Pretty steady rise.
It is important to note that coastal sea level rise is not uniform around the world, with one reason being that the land is not staying at the same elevation! In some places, the ground is sinking, due to pulling removal of subsurface water or oil, or some other natural process. For example, the land is rising today in locations that were covered by ice-age glaciers that pushed the land surface down. After they melted roughly 14,000 years ago, the land rebounded. That is happening here in the Northwest, particularly for the Olympic Peninsula.
To illustrate all this, here is a a sea level trend map from NOAA. Some places like the Olympic Peninsula has sea level going down. Same in Alaska. But there are large rises where the land is subsiding, such as New Orleans.
So this sea level rise business is pretty nuanced.
Sea level rise is not accelerating appreciably, even thought greenhouse gas concentrations are rapidly rise. And the rise of sea level began more than a century ago, well before humans could have been a significant cause. In fact, there was something called the Little Ice Age that occurred during the 1500s to middle 1800s, with subsequent warming that was mainly natural. The current sea level rise period appears to have its origin in the demise of the Little Ice Age and the warming that followed.
So claims that all or most of the rise in sea level is due to human-emitted greenhouse gases appear problematic because it started before humans could be the main cause. The casual link is further weakened by the lack of acceleration of sea level rise during the past few decades.
On the other hand, our climate models suggest an accelerated rise of sea level rise due to greenhouse gas warming during this century. Will our models be correct or are they too sensitive to greenhouse gas impacts? Time will tell.
So, what sea level rise should we expect in Seattle during the remainder of the century?
Extrapolating the current, steady upward trend implies about a .6 ft rise. If we include the impacts of greenhouse gas warming, there would be more. A National Academy of Sciences report did such an analysis suggesting a 4-56 inch increase by 2100, with a mean change of 30 inches (2.5 ft). But whether such model-driven estimates are reliable is uncertain: I suspect it will be on the high side considering the slow rise of the past few decades.
from Cliff Mass Weather and Climate Blog http://ift.tt/2zCsO7r
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