Oceans and Ice – Inertia and Momentum in the Climate System
Oceans have absorbed carbon dioxide and heat – Massive ice caps delay melting
A true revelation for me while compiling these essays has been recognizing the role that the oceans and ice play in delaying the impacts of greenhouse gas emissions. Oceans have absorbed a significant fraction of emitted carbon dioxide and most of the excess heat generated by the warming climate system. Global ice mass is far out of balance with current temperatures, as the sheer mass of the major ice caps causes major delays in the rate of melting. These two crucial facts don’t seem to be part of our climate conversation. Together, oceans and ice have given us time to respond to the climate challenge, but also now represent irresistible momentum for continuing change into the future.
Oceans Buffer the Impact of a Major Greenhouse Gas
Increasing concentrations of greenhouse gases in the atmosphere drive increasing global temperatures. If oceans were not absorbing both carbon dioxide and heat, increases in both would be far greater than they have been.
Carbon dioxide dissolves readily into sea water and the rate of absorption depends on the concentration in the atmosphere. As concentrations have gone up, increased net uptake of carbon dioxide by the oceans has followed. This is a two-way process that we have tipped in favor of increased storage of carbon in the oceans.
How much of human emissions of carbon dioxide are now in the oceans? That is an interesting question. You can find estimates that range from 25% to nearly 50% of the total. We know that about 50% of carbon dioxide emissions have remained in the atmosphere, driving concentrations up to levels higher than any in the last 2 million years. The uncertainty surrounds how much of the other 50% has been taken up by the oceans and how much by land ecosystems.
I lean towards more in the ocean and less on land, but we can leave that argument for another essay, and just thank the oceans for causing a major delay in what would be a much faster increase in atmospheric carbon dioxide and temperature.
As greenhouse gases warm the atmosphere, as much as 90% of the excess heat generated has been transferred to the oceans. This is reflected in global ocean water temperatures which have been increasing steadily for several decades. In the figure on the right below, note the peak increases in 1998 and 2016 due to El Niño events and the moderation in 2020-2022 due to La Niña.
Again, oceans have delayed the negative impacts of a warming climate system.
And yes, this does relate to sea level rise – more on that in a minute.
Ice Caps Are Melting More Slowly Than Temperature Increases Would Predict
And what of ice? The steady reduction in late summer ice cover in the Arctic Ocean is one of the most visible indicators of a warming climate system. While the melting of this floating ice does not contribute to sea level rise, open water absorbs about 8 times as much of the radiant energy delivered by the sun as an ice-covered surface. This drives a strong feedback between melting arctic ice, increasing water temperatures, and additional melting.
Melting of land-based ice (glaciers and ice caps) does drive rising sea levels. More than 99% of the ice and 68% of all the fresh water on the surface of the Earth are stored in the immense ice caps covering Greenland and Antarctica.
Measurable net ice loss is occurring from both Greenland and Antarctica, but losses would be much higher if those ice caps were in equilibrium with current temperatures. The next two essays will explore the existence of “Zombie Ice” (that already committed to melting) and describe how the Earth looked the last time temperatures like those predicted for the end of the century occurred. Essentially, the very massive nature of these ice caps exposes only a small fraction of that ice to the warming atmosphere. Net losses should continue for centuries but future loss rates are uncertain.
Both Heat and Melting Ice Contribute to Rising Sea Levels
Through ice melt and ocean warming, sea level is rising at a steady pace. So far, one estimate ascribes about one-third of that rise to thermal expansion due to increasing ocean temperatures, and two-thirds to melting of land-based ice.
And Warmer Oceans Increase the Likelihood of Tropical Storms
Sea level rise is not the only impact of warming oceans and melting ice. The intensity of tropical storms is determined in part by the surface temperature of the oceans over which they pass. Heat storage in the ocean is reflected in an increase in average sea surface temperature. The fourth and fifth essays in this set will look at trends in the frequency and intensity of tropical storms in the North Atlantic in relation to ocean temperature and a more energetic climate system.
Inertia Becomes Momentum
Oceans and ice are responsible for significant inertia in the climate system, delaying the impacts of greenhouse gas emissions. The absorption of carbon dioxide and heat, and the delay in melting from massive ice caps have reduced both atmospheric warming and sea level rise. The flip side of this inertia is irresistible momentum going forward. Ice would continue to melt, and the oceans would continue to absorb heat, even if we reached net zero emissions today, and both would continue to drive sea level rise.
Recognition of this momentum is reflected in the longer timeline that now appears in many images projecting future sea levels. While 2300 might seem a long way off, this figure also captures how sea level rise will continue up to and beyond 2100 even if we reach zero net greenhouse gas emissions by that year. Increases measured in meters are sobering given the disruptions already occurring with an increase to date of only 10 inches (about one-third of a meter).
The Bottom Line
We are far from equilibrium in the climate system. Change has been slower than it might have been thanks to oceans and ice, but momentum now in the system will continue to melt ice and raise sea level even as we take actions to reduce greenhouse gas emissions.
Sources
Two sources for data on the fate of carbon dioxide emitted to the atmosphere
This one says nearly 50% is in the atmosphere and land uptake is negligible
https://scied.ucar.edu/image/carbon-cycle-diagram-ipcc
This one says ocean uptake and land uptake are about 25% each of emissions
https://www.csiro.au/en/news/All/Articles/2023/June/oceans-absorb-emissions
The image of change in heat storage in different parts of the climate system is from:
https://essd.copernicus.org/articles/15/1675/2023/
The graph on changes in average ocean temperature is derived from:
https://www.ncei.noaa.gov/news/noaa-updates-its-global-surface-temperature-dataset
Data on Arctic Sea ice extent can be found here (figure redrawn by author):
https://nsidc.org/arcticseaicenews/charctic-interactive-sea-ice-graph/
The numbers on fraction of ice and fresh water stored in Greenland and Antarctic ice caps is from:
https://nsidc.org/learn/parts-cryosphere/ice-sheets/ice-sheet-quick-facts
Figure on cumulative ice loss from Greenland is from:
https://climate.nasa.gov/vital-signs/ice-sheets/
The image of sea level rise is from:
https://www.climate.gov/news-features/understanding-climate/climate-change-global-sea-level
The figure on contributions by ice and thermal expansion to sea level rise is from:
The image of sea surface temperature anomalies for August 25, 2023, is from:
https://earthobservatory.nasa.gov/images/151743/the-ocean-has-a-fever
The image of projected sea level rise through 2300 is from:
https://earthobservatory.nasa.gov/images/148494/anticipating-future-sea-levels