(Image source: AVISO)
Since 1992, the average pace of sea level rise has remained constant at about 3 mm per year. This ongoing rise comes from a combination of thermal expansion of water as it warms due to human-caused global warming and contributions of melting ice in the form of glaciers.
Over the past decade, however, the pace of melt from glaciers in Greenland and West Antarctica have been on the rise. Their contribution to sea level and the overall pace of sea level rise is expected to grow as the glaciers continue to soften up and melt under the stress of human heating.
During the last two years, sea level rise tripled — jumping from an average rate of about 3mm per year to nearly one centimeter per year. So was this tripling due to increased glacial melt? The likelihood is that a moderate portion of this enhanced sea level rise came from jumps in glacial melt. But total volumes of melt are still not enough to account for the 2 cm of sea level rise over the past two years.
For the rest, we must look toward climate variability. In this case, volatility is probably a better word. During 2010-2011, more than half a centimeter of ocean water evaporated and ended up in the atmosphere. But the atmosphere couldn’t hold all of this extra moisture for long, so it ended up coming down in a series of unprecedented storms. This major event spawned epic floods and major rain events across the world. Over time, all this dumped water returned to the world’s oceans, re-contributing the more than half centimeter that was lost.
But even taking into account this half centimeter of water cycling in and out of the ocean, we still get a 5 mm annual rise in sea level since 2010. This, almost doubling, of sea level rise over the past three years is too early to count as an ongoing trend. But it may well be the result of enhanced glacial melt and steadily rising ocean heat content (and related thermal expansion) over this period.
Long Term Trend is Not Linear, Contains Risk of Outlier Events
Expectations are that sea level rise will drastically increase along with rising ocean temperatures and increasing glacial melt rates over the next century. The 3 mm per year rate of sea level rise would lead to a one foot increase were it to continue on until 2100. But this rate of rise is something we can hope for only if worldwide CO2 emissions stop now or very soon. Since this event is unlikely to happen, we can expect an amplification of sea level rise as ocean temperatures and glacial melt continue to increase.
At the 2010 to 2012 rate of sea level rise, oceans will have risen by more than 1 and a half feet come the end of this century. But, more likely, final sea level rise will be much closer to a meter by 2100 as glacial melt and thermal ocean expansion accelerate. This one meter rise would correspond closely to the one centimeter per year rise we’ve seen in 2011 and 2012, although greater than 1 centimeter per year averages will be more likely after 2040.
There is also, an outside potential for a major melt or ice sheet destabilization that will push sea levels much higher than 1 meter. Outside events of this nature are not taken into account in current climate models but are increasingly likely in worst-case scenarios produced by the IPCC (note, that IPCC does not model or predict for extreme responses like catastrophic ice sheet collapse). If such events were to occur, sea level rise could jump by 3 meters or more. The long-term likelihood of such events are difficult to predict. But they are worth noting, especially in the context of global temperature rises in the range of 2-6 degrees C over the next century.
Ocean Taking on More Heat Than Expected
With observations showing that more of the current GHG temperature forcing is going toward warming the oceans than previously anticipated, it is worth noting that, should this trend continue, sea level rise is at risk of being further amplified. Both thermal expansion and contact of warmer oceans with glaciers that lock in large ice shelves would be greater in such an event and would lead to higher rates of sea level rise.
Getting a better measure of how ocean and atmosphere heat balance changes in response to human caused forcings will be necessary if we are to have a more clear understanding of likely ocean changes and sea level rise over the next century.