Beware the Doubling Time for Rising Seas
“Doubling” or exponential growth is deceptive. It can mean wealth with an investment, or disaster with something like rising sea level. As an illustration of doubling, perhaps you fell victim to this math puzzler in high school — or in a bar. Would you prefer to get a million dollars now or one penny the first day with the amount doubled each day for one month? That is, one cent today, two cents tomorrow, four cents the next day, eight cents the day after that, etc.
Most choose the one-time million dollars, but are stunned to learn the other offer would total more than ten times that in just 31 days, for a whopping grand total of $10,737,418. Such is the power of doubling, or exponential growth.
The time period for the doubling is the key. Sea level rise — SLR — is happening faster and faster, almost entirely due to the melting of glaciers on land in the Arctic and in Antarctica. A few decades ago, the doubling time for SLR was more than forty years. Now it is less than 20. Many experts believe it could get as short as ten years in the coming decades.
As the planet gets warmer and warmer due to higher levels of greenhouse gases (CO2, etc. ) the glaciers and ice sheets on Greenland and Antarctica will melt faster and faster, raising sea level – shortening the doubling time.
Over the last century the global average for SLR was 1.7 mm a year, or about a sixteenth of an inch, though there are huge variations by location, mostly due to vertical land movement. The rate has now doubled to 3.4 mm or about an eighth of an inch annually. Let’s follow the hypothetical case of it doubling every decade.
In ten years the current eighth of an inch would be a quarter inch each year. A decade later it would be a half inch per year. Then an inch, in another decade. Then 2, then four, then 8, etc. The stunning fact is that with a doubling time of ten years, in 50 years sea level would have risen more than 5 feet. Though frankly there is no way to know precisely the future acceleration rate of SLR, because of the uncertainty of how quickly the ice sheets will melt. Nonetheless, if it continues on it’s current path it will mean global catastrophe by the end of this century.
Looking at the geologic record, sea level rose dramatically in a relatively short period of time. Fourteen thousand years ago, sea level rose some 65 feet in just four centuries – that’s an average of more than a foot and half (half meter) every decade. In my recent blog post Sea Level Rise in the Rear View Mirror I pointed out that the last century no longer allows us predict the future, because of an abrupt change in rate, or inflection point.
It will be a huge challenge for the world to adapt to sea levels that are many feet or meters higher than present. The longer we pretend, or hope for the fantasy that it will not happen, the less time we have to adapt our communities, economies…our civilization. As I explained in Paris Climate Agreement: the Good, the Bad, the Ugly even if the world keeps the commitment to the 2015 Paris Climate Agreement, sea level is still going to rise — with only a modest slowdown of the acceleration.
Because of the doubling phenomenon, sea level is going to rise at a surprising rate. The next 50 years will not be anything like the last 50. It is time to adapt and prepare for the future.