While the Earth has been warming every year, this year’s temperature records have been broken month after month, and this last month, September 2020, has been the warmest September on record. The question arises: Is the Earth’s heating accelerating? Is something shifting in our climate? And what will be its impact on one aspect of climate change related to this warming: the sea level rise. Regarding this component, the physics is clear and undeniable – there is some unavoidable sea level rise to come. We can opt to bury our head in the sand and wait for it to happen, or we can act now to prepare and adapt to it.
Water expands: As the air temperature increases due to the greenhouse effect resulting from the accumulation of greenhouse gases in the atmosphere, the water in the oceans warms as well. We must realize that we humans are experiencing only a small portion of the total warming. Much of the heating is occurring in the ocean where the heat is stored. In fact the heat resulting from the greenhouse effect is accumulating in the ocean at a rate of 90%, compared to only a few percent in the atmosphere. As a result, physics, and in particular thermodynamics, dictates that thermal water expansion will occur as a result of this oceanic heat storage and therefore sea level will increase globally. Other processes (such as precipitation variations over the oceans, and water run off from the land) play a minor role in local variations to sea level changes, with some regions experiencing more increase than others, but overall the sea level continuously rises everywhere.
Arctic Ice melts: Besides water expansion, another process is at play regarding sea level increase. It is the loss of ice in the Arctic (and the Antarctic but here we focus on the Arctic). The warming of the air and water globally is inducing an accelerating warming in the Arctic region due to a process called positive feedback. As the ice melts, the darker ocean and land surfaces below the ice get exposed to the radiation from the sun (which they were previously shielded from by the ice) and then this radiation is reflected back to the atmosphere. When exposed, the darker land and ocean surfaces absorb the sun radiation and warm up. The warming induces accelerated ice melting which leads to ice cover opening up and thus further warming the surface, particularly over the ocean. Overall this positive feedback process is like a cascading effect that leads to much larger local temperature increases. In the Arctic the increase is 2 to 4 times larger than the global temperature rise. Now, when the melting occurs over the ocean it accelerates the water temperature change in that region, but it doesn’t affect the sea because the ice is already in the water. On the other hand, when the melting occurs over land covered by glaciers (like the Greenland icesheet), the glaciers melt, lose their anchor, slide down, and break down when they reach the coasts, giving rise to spectacular views of huge chunks of ice breaking down into smaller pieces and falling into the water, where they melt. The glaciers’ melt water is added to the ocean salt water, increasing the sea level height. These two processes, ocean water expansion and land glacial melt, join to increase the global sea level in roughly similar proportions.
What is different this year that makes this slow but inevitably evolving sea level increase different is, in part, a heat wave in Siberia last Spring. As a result, the frozen sea ice in the Arctic Ocean started thinning and shrinking, and therefore melting earlier than in previous years, inducing an Arctic Sea Ice extent reduction. As a result, this year has the second lowest sea ice extent value ever recorded. Now like every year, the question arises whether the usual thickening and expansion that occurs during the colder months of Fall and Winter following the melt will be sufficient to allow for the recovery of all, or most, of the summer ice loss. Scientists are doubtful these days because of the accelerating atmospheric and oceanic warming tendency. If that is the case, this will induce further shrinking of the Sea Ice in the future and an unavoidable disappearance of Artic ice.
Why is it that we are concerned by what happens so far from us , in the Arctic? Naturally the sea level increase that comes from the ice melting is very concerning to us now and even more in the future since it is global and nothing can stop the irremediable reduction in glaciers in the foreseeable future (centuries), even when (if?) the Earth surface were to stop warming. That is if we were to reduce drastically our emissions due to burning of fossil fuels. That is why it is urgent to both slow down the warming by reducing burning of fossil fuels while at the same time preparing for the sea level change to come in the future. Not only beaches around the world will be shrunk like around Santa Barbara, but coastal areas and cities worldwide will be inundated, low lying islands will be submerged, deltas will disappear and all the natural barriers that have protected humans will be reduced if not destroyed.
There are other concerns related to the Arctic Ice melting which are the impacts on the atmospheric and oceanic circulation and all the northern ecosystems. They will be discussed in future blog postings.
Catherine Gautier – UCSB Professor Emerita