Sea level rose more rapidly during the 20th century than during any of the previous 27 centuries, and humans bear the lion’s share of the responsibility for that rise. That’s just one of the sobering takeaways from the U.S. Global Change Research Program’s Climate Science Special Report (CSSR), released today, but leaked to the New York Times in August. Billed as Volume 1 of the Fourth National Climate Assessment (NCA), the CSSR captures the state of sea level rise science and its implications for the coasts of our country.
Here are six noteworthy findings from the sea level rise section of the CSSR:
1. People are responsible for 80% of the sea level rise since 1970
The first key finding in the CSSR’s sea level rise chapter contains a bold statement that is backed up in the chapter’s main text: “Human-caused climate change has made a substantial contribution to [global mean sea level rise] since 1900…contributing to a rate of rise that is greater than during any preceding century in at least 2,800 years…”
This finding is based on eight independent studies published in the last three years that aim to quantify the human contribution to sea level rise since 1900. All of them conclude that the human contribution is “substantial,” and at least two find that, in the absence of human activity, sea level rise over the course of the 20th century would have been about 50 to 60% of what has actually been observed.
The human contribution to sea level rise is even more striking if we look at just the last 50 years: People are responsible for about 80% of the global mean sea level rise since 1970.
These findings broadly reflect the rapid evolution of attribution science–or assessing whether–or what proportion of–observed climate and weather events can be attributed to human activity. A recent study published by Brenda Ekwurzel and others takes this sea level rise attribution one step further by showing that about 30% of global sea level rise since the Industrial Revolution was caused by the burning of products from 90 major fossil fuel companies.
Given the tendency of climate-confused politicians such as Scott Pruitt to say things like “Science tells us that the climate is changing and human activity in some manner impacts that change…the human ability to measure with precision the extent of that impact is subject to continuing debate and dialogue, as well they should be,” this high-confidence finding would ideally help to lift some of their “confusion.”
2. Sea level rise is accelerating, and a growing proportion of that rise is due to loss of ice on land
Estimates of how much sea level rose over the course of the 20th century have been changing, which has implications for our understanding of how the pace of sea level rise has been changing. The average rate that has long been quoted for the 20th century, from a 2011 study by Church and White, is 0.06 inches/year. But a few more recent studies, including one by Hay et al. cited in the CSSR, have found that rate to be slightly lower–0.05 inches per year. That amounts to 4-5 inches of sea level rise in the 89 years between 1901 and 1990.
Since 1990, less than 30 years ago, global sea level has risen by about 3 inches. The rate of sea level rise is now 0.13 inches per year–more than double the 20th century average–with both tide gauges and satellite data confirming the changing pace.Over the course of the 20th century, the pace of sea level rise varied. This recent acceleration is different for at least a few reasons. First, it’s coming on the heels of a century of already above-average sea level rise that we know is attributable to human activity. Second, projections show that this acceleration has only just begun. And third, loss of land-based ice is contributing more to sea level rise than it did during the 20th century.
The six scenarios used by the CSSR to project future sea level rise show that the rapid pace of sea level rise we are experiencing today could pale in comparison to what lies ahead. With an intermediate scenario, the pace of sea level rise would increase to 0.2 inches per year in 2020 and to 0.6 inches per year in 2090. With a high sea level rise scenario, those rates increase to 0.4 inches per year in 2020 and 1.7 inches per year in 2090.
Changes in sea level arise largely from two sources: loss of land-based ice and warming of the ocean, which causes seawater to expand and take up more space. Over the course of the 20th century, warming oceans contributed the bulk of the sea level rise signal. But since 2005, about two-thirds of observed sea level rise has come from loss of ice. When we look into the future, there is still a considerable degree of uncertainty about how much ice loss will contribute to sea level rise.
3. Antarctic ice loss is still a wildcard, but its game-changing potential contribution is becoming clearer
Quantifying the response of the Greenland and Antarctic ice sheets to future warming has been a consistently large source of uncertainty in global sea level rise projections for over 15 years (here’s the third IPCC report from 2001, for example). In the past couple of years, however, major developments in the ability to model the response of the Antarctic ice sheets to warming have begun to hone our understanding of Antarctica’s potential contribution to sea level rise this century.
And it’s scary.
The CSSR is unequivocal that Antarctica (and Greenland) are losing ice, and there is a growing body of evidence suggesting that the pace of that loss is accelerating. The rate of ice loss is about 100 gigatons per year–an amount that this Washington Post article can help us to wrap our heads around.
Models suggest that ice loss from Antarctica could contribute more than three feet to global sea level rise this century on top of rise from other sources. This growing body of knowledge is reflected in what the CSSR calls an “extreme” scenario in which sea level rises by an average of 8.5 feet by 2100. The previous NCA report’s highest sea level rise scenario projected about 6.6 ft by 2100.
There’s a concerted effort in the CSSR to incorporate the latest science about Antarctic ice loss into sea level rise projections. But there’s still enough uncertainty that Antarctica’s potential contribution couldn’t be fully accounted for when assigning probabilities to potential sea level rise futures.
4. Sea level rise scenarios tied to emissions scenarios and assigned likelihoods
NOAA has developed a new set of sea level rise projections that are designed for this round of the National Climate Assessment. New SLR scenarios designed for understanding risk given a range of different carbon emissions scenarios. Each sea level rise projection is assigned a probability based on emissions pathways, as in “with a high emissions scenario (RCP8.5), the ‘very likely range’ of SLR is about 1.7-4.3 ft by 2100”.
Here’s where the big Antarctic wildcard plays in, though: The probabilities do not factor in the possibility of major ice loss from Antarctica.
5. Communities will be affected by more frequent, more severe flooding before they are permanently underwater
Since 2014, there’s been an increased focus on what happens between now and the point at which coastal regions are permanently underwater due to sea level rise. This in-between time will be characterized by an increase in the number and extent of high tide flooding events, and a number of studies in the past three years have painted a picture of what that looks like quantitatively and qualitatively.
The CSSR puts this issue of tidal flooding up front in key message #4: “As sea levels have risen, the number of tidal floods each year that cause minor impacts…have increased 5- to 10-fold since the 1960s…Tidal flooding will continue increasing in depth, frequency, and extent this century.”
While the tidal flooding findings described here won’t be news to regular readers of this blog or to residents of flood-prone places like Charleston and Annapolis, their elevation to key finding status will hopefully highlight the insidious threat of frequent flooding that hundreds of communities in the U.S. could face in the coming decades.
6. Buckle up for centuries of sea level rise
When we look at projections for how much sea level will rise through the end of this century, it’s tempting to assume that 2100 is so far off that of course we will have cracked the climate change nut by then and be back to a climate that feels right. A climate in which people will know what a month of below average temperatures feels like. A climate in which coastal towns see a couple of high tide floods per year instead of dozens.
But sea level takes time to respond to the emissions we are pumping into the atmosphere, and even if temperatures stabilize, sea level is projected to continue rising for centuries if not millennia. Emissions through 2100 could lock us into a sea level rise of 12 feet by 2200 and up to 33 ft in the next 2,000 years.
Again, major ice loss from the Antarctic ice sheet is the big wildcard because once that ice is lost, it cannot easily be regained. The CSSR states: “Once changes are realized, they will be effectively irreversible for many millennia, even if humans artificially accelerate the removal of CO2 from the atmosphere (DeConto and Pollard 2016).”
This new report shows us that we are on the comfortable end of the steep sea level rise curve. As this report gains attention in the coming days there will be those who will wave their hands and insist “nothing to see here.” Clearly, there is far more to see here than we want. Thank you to the dozens of authors and researchers who are enabling us to see it coming.