Existing climate change assessments could be underestimating the amount of future sea level rise, as well as the likelihood of other phenomenons like increased superstorms and glacier loss, warns a new high-profile paper in Atmospheric Chemistry and Physics. The study, by longtime climate scientist James Hansen and 18 co-authors, has gained attention recently for its radical projections of climate change impacts.
To conduct research for the paper, titled Ice melt, sea level rise and superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2 ◦C global warming is highly dangerous, Hansen and the other authors combined ancient climate data with new satellite readings and an updated climate model to show that ice melt is occurring more quickly than previously thought. Instead of incrementally melting, the ice sheets around the Earth’s poles actually melt at a non-linear rate, losing mass rapidly, according to Hansen and his team.
“We have uncovered information and a partial understanding of feedbacks in the climate system, specifically interactions between the ocean and the ice sheets. These feedbacks raise questions about how soon we will pass points of no return, in which we lock in consequences that cannot be reversed on any time scale that people care about. Consequences include sea level rise of several meters, which we estimate would occur this century or at latest next century, if fossil fuel emissions continue at a high level,” Hansen says in a video released about the paper. “That would mean loss of all coastal cities, most of the world’s large cities and all their history.”
Hansen notes that a positive feedback loop is created as ice melt influences the structure of the ocean’s layers. As cold freshwater runoff from exit glaciers flows into the ocean, it lowers the density of the surface water. This change of density shuts down the normal circulation in which cold salty water sinks and brings warm water to the surface, releasing the heat it carries into the atmosphere. But when heat stays in the ocean at a depth where ice shelves contact the trapped warm water, a feedback loop occurs. The warm water next to the deep ice makes the ice melt even faster.
Thus the ice melt in these regions causes further loss of ice sheets in direct contact with the ocean, which contributes to more rapid movement of exit glaciers and to faster sea level rise. In addition to quickening ice melt, the feedback loop also contributes to shutting down the ocean’s circulation, trapping warm water between layers of cold water in polar regions. The feedback loop creates a greater temperature gradient by increasing temperature differences between high and low latitudes, which increases the likelihood of superstorms.
Earth’s ice sheets are melting quickly, and the rate of melt is also expected to increase exponentially. As a result, we could see the sea level rise up to five meters, or about 16 feet, by the end of this century if no emissions reduction actions are taken. This puts many of the world’s coastal cities in danger of flooding, including cities like Miami, London, New York, Miami and Shanghai.
The paper forecasts a greater increase in sea level within a shorter period of time than other research has found. In its 2013 Fifth Assessment Report (AR5), the U.N’s Intergovernmental Panel on Climate Change (IPCC) predicts closer to three feet of sea level rise occurring at or after 2100.
“The models that were run for the IPCC report did not include ice melt,” Hansen said at a news conference.
But the paper has received criticism. Hansen and the other researchers first released their research as a discussion paper in the European Geosciences Union (EGU) Open Access journals. This made the paper visible to the public before the peer review process was finished, which is atypical of scientific research and generated some criticism.
There has been contention about Hansen findings within the scientific community, which can be seen not only in the papers reviews and comments but also playing out across Twitter and in the news. In an op-ed on the paper in the New York Times, the environmental journalist Andy Revkin quoted the climate journalist Eric Holthaus, who succinctly sums up the negative responses in the tweet, appended below.
Concern has been expressed that the predictions made in the paper are too extreme. For one, some critics found the assumptions, such as exponential rates of ice loss, to be improbable. Others raised objections to the particular way in which paleoclimate data was used to suggest future conditions. Kevin Trenberth of the National Center for Atmospheric Research strongly criticized the study, saying that “there are way too many assumptions and extrapolations for anything here to be taken seriously other than to promote further studies.” As the extensive comments on blog posts here and here show, the paper by Hansen and his team has attracted a great deal of attention, and sparked lengthy debates in the scientific community.
At a February 2012 TED talk titled Why I must speak out about climate change Hansen said: “Clearly I haven’t got this message across. The science is clear. I need your help to communicate the gravity and urgency of this situation and its solutions more effectively. We owe it to our children and grandchildren.”