As humans emit heat-trapping gases into the atmosphere, it’s causing the Earth to warm. It’s also causing the ocean waters to rise. In fact, water rise is one of the clearest signatures of a warming world. The questions we want to answer are, how much will sea levels rise, and how fast?
The answers to this have large implications on what societies should do. It isn’t just coastal communities that will be affected. While there are approximately 150 million people worldwide that live within 3 feet of today’s water levels, because of the interconnected economies and societies, ocean rise will affect us all.
The prestigious Proceedings of the National Academy of Sciences recently published a series of sea level rise papers.
One paper covers the Antarctic ice sheet, and the authors look back in time at the world’ largest ice sheet. The authors use three tools to advance our knowledge of the ice. First, they use a very accurate calculation approach to quantify how the ice sheet interacts with the atmosphere. Second, they incorporate potential ice fractures into their analysis. Finally, they use information about changes to oxygen isotopes to improve their calculations. What they find is that the atmospheric concentration of carbon dioxide will soon be at levels not encountered since the Miocene period (23 million to 5 million years ago). They also find that newer computer calculations do a better job of quantifying changes to the ice sheet.
A second paper published by Roelof Rietbroek and colleagues looked at the sources of sea level rise. They wanted to know how much of the current rise is from water that is warming and expanding, how much is from melting of ice sheets in Greenland and Antarctica, how much is from melting glaciers, how much is from shifting continents, etc.
This study measured the mass of water in various regions of the Earth through special gravity-sensitive satellites that orbit the Earth. They conclude that the sea level rise from thermal expansion is higher than previously reported. The also find the water rise from melting of ice is consistent with measurements taken of ice melt around the world. Finally, they find that while the global oceans are rising steadily, there is tremendous regional variation so that some areas have very fast ocean water rise while others have slow ocean rise (or even ocean drop).
The third paper by Robert Kopp and his colleagues uses statistics to find the relationship between temperature and sea level for the Common Era. They find that the sea level rise accelerated in the early 19th century and the rate of water rise in the last century is likely higher than it has been in 2,700 years. Their findings were in good agreement with those of the most recent IPCC report.
The final part of the foursome was authored by Matthias Mengel and his colleagues. That study uses historical information on sea level and climate change projections to make predictions about where ocean levels will be in the next 100 years or so. This study tries to separate how much sea level will rise because of various contributors, such as thermal expansion, melting mountain glaciers, the Greenland ice sheet, and the Antarctic ice sheet.
When projecting into the future, the authors have to make decisions about how much more heat-trapping gases will be emitted into the atmosphere. To make the projections, the authors use the IPCC scenarios (RCP2.6, 4.5, and 8.5). A comparison between the new paper and the IPCC projections show excellent agreement.
It appears that, unless societies make significant changes, we will see approximately 3 feet of sea level rise by 2100. That may not sound like a lot, but it’s enough to cause enormous economic and societal problems. What’s great about this paper is they also include a discussion on the limitations of their work. For instance, they state that their method cannot deal with processes that are independent of the warming rate (such as a sudden collapse of an ice sheet).
I wrote to the lead author to get his summary of the study and learn more about its importance. He told me,
Human influence on climate pushed the world’s oceans, glaciers and ice sheets away from their equilibrium so that they now all contribute to rising seas. As they are inert, a lot of their contribution will only be realized in the future. There is knowledge on how much they will add to sea level before they reach a new equilibrium, a process that can take thousands of years. We now combined this knowledge on long-term sea level rise with past observations into a small and efficient model. This allows us to make probabilistic projections of sea level rise incorporating the main uncertainties.
While stopping sea level rise within this century is unlikely, our projections confirm that the world can still choose from a spectrum of sea level futures. It mainly depends on how much heat-trapping gases will be emitted. To avoid the risk of 1.3 meter higher seas in 2100, mankind needs to cut its carbon emissions.
As we saw in my last post , the actual costs to human society will depend not only on the rate and amount of seal level rise, but on how we humans adapt to this new reality.