Sea levels are predicted to rise by about three quarters of a metre by 2100 due to human-induced climate change, according to the 2007 Intergovernmental Panel on Climate Change report—an increase that would threaten millions of people worldwide.
“That’s a conservative prediction,” says Richard Peltier, physics professor at U of T, an IPCC author, and the first Canadian to win the prestigious Bower Award and Prize for Achievement in Science. Considering how little scientists actually know about the stability of land ice when exposed to global warming, the IPCC’s current predictions for how much the sea level will rise come with very important caveats.
Speaking as a part of the Royal Canadian Institute for the Advancement of Science lecture series, Peltier addressed a large audience at U of T on January 24 about the scientific consensus on anthropogenic global warming and the findings from his research on rising sea levels.
According to Peltier’s research, global warming is currently causing the sea level to rise by one millmetre each year due to the melting of the ice sheets in Greenland, Alaska, and West Antarctica. An additional one millimetre rise per year is thought to be coming from smaller land glaciers melting, plus 0.5 millimetres annually from the thermal expansion of the oceans.
A steadily rising sea level isn’t the only concern. Scientists understand how the surface of a glacier will react to the increased temperatures of global warming, but they know much less about how this will impact structural ice dynamics. As a result, the current climate models cannot accurately predict how glaciers will melt with future warming.
Because of this uncertainty, we may unexpectedly reach some key threshold temperature, said Peltier. That threshold could trigger the collapse of major land ice sheets in Greenland, Alaska, and West Antarctica causing a rise up to 15 metres in sea level. The actual threshold temperature is unknown and ice dynamics are not well understood, so researchers are yet to reach consensus on a deadline.
Peltier’s research group has sought to address some of the gaps in knowledge about global warming and glaciers.
The group developed a theory of how sea level rise played out after the retreat of the glaciers 21,000 years ago. Land masses have been slowly rebounding ever since the weight of the ice retreated, creating a natural background level of falling sea levels as the land rises out of the sea. With the determination of these ongoing cycles, Peltier’s research group was then in a position to separate out the current impacts of global warming on sea level.
They applied their theory to new satellite data on global changes in sea level by subtracting ongoing hydrological cycles from the data, which allowed them to isolate the rate of sea level change caused solely by global warming.
The new satellite data came from a system called GRACE (Gravity Recovery And Climate Experiment). The two-satellite system is in orbit 480,000 kilometres above the earth and is passive, which means that it does not point any mechanisms at the earth’s surface. Instead, the satellites measure minute changes in the distance between the two satellites using microwaves. The satellites, 220 kilometres apart from each other, can detect a change in distance within the width of a human hair. The satellites move apart based on changes in the earth’s gravitational field, which indicates changes in the layer of water on the earth’s surface.
Peltier’s research found that land ice in Greenland, West Antarctica, and Alaska is melting and ending up in the oceans causing an increase of one millimetre per year. His team also found that melting ice will cause the sea level to rise in the opposite hemisphere from where the ice is located. This is because without the gravitational pull of the ice, water is no longer disproportionately drawn towards the large bodies of ice. If the West Antarctic ice sheet melted, for example, Halifax would experience about eight metres of sea level rise.
The GRACE satellites are producing data that our climate models cannot explain. Scientists continue to wrestle with the stability of land ice in a global warming era and refining measurements to more accurately capture the impact of global warming on sea level.
Until they figure out more details of this complex system, the predictions for future sea level rise will be fraught with uncertainty.
Peltier is starting work on the next Intergovernmental Panel on Climate Change report and expanding his research focus to address the still murky numbers surrounding the melting of the multitude of smaller land glaciers all over the world.
