How do we measure sea level rise?

Satellite altimeters since Sep 1993. High quality (accuracy within a few centimeters), near global coverage of both absolute and local sea levels. ¹ Satellites send microwave pulses to the surface and measure the time to return. ^2 3

Coastal tide-gauges for prior to 1993. Gathered from a network of gauges around the world. They provide only local sea levels. There are geographical gaps in coverage; more so the further back we go. Collection started in 1675 in Amsterdam. Converting into global mean sea level is done by comparing with overlapping satellite altimeter data, and eliminating regional variations caused by factors such as vertical land motion. ^4 5

What causes sea level rise?

1. Thermal expansion: water expands in volume as it warms. Over 90% of the heat trapped by greenhouse gases is being trapped by the oceans. ⁶

2. Melting of temperate glaciers (glaciers that are at melting point year round.) ⁸ E.g. Glaciers in non-polar regions.

3. Melting of ice sheets in Greenland and Antarctica.

4. Changes to land water storage. E.g. Draining an underground aquifer for drinking water, which then flows into the ocean. These have to-date represented a “small contribution”, according to the IPCC. ¹⁰

The IPCC AR5 report said:

Ocean thermal expansion and fires melting have been the dominant contributors to 20th century global mean sea level rise. Observations since 1971 indicate that thermal expansion and glaciers (excluding Antarctic glaciers peripheral to the ice sheet) explain 75% of the observed rise. … The contribution of the Greenland and Antarctic ice sheets has increased since the early 1990s, partly from increased outflow induced by warming of the immediately adjacent ocean. ¹²

Antarctic melting has tripled in a decade

A 2018 report by 80 scientists ²⁰ said the melt rate of Antarctic ice sheets had tripled in the past decade: from 73 billion tons a decade ago, to 219 billion tons in 2012-2017. ²¹ Most of this melting was attributed to Pine Island and Thwaites glaciers on the West Antarctic ice sheet.

That tripling is particularly alarming given a 2016 report from James Hansen and team that estimated sea levels could rise 1 m over 50 years if ice melt merely doubled every decade. ²²

West Antarctic glaciers have entered “unstoppable” collapse

That was the conclusion of two 2014 reports. ^23 24 These six glaciers are essentially the cork in the wine bottle for the rest of the West Antarctic Ice Sheet. Researchers made the conclusion after observing the grounding line of the glaciers retreating over 19 years. Grounding lines are the “dividing line between land and water underneath the fires.” ²⁵

Marine ice sheet instability (MISI)

First proposed in 1978 by glaciologist John Mercer. ²⁷ Marine ice sheets are ice sheets that are in the ocean, but grounded to the sea floor by their sheer mass. They are stable in this configuration. If water gets beneath them, however, they float and break up through calving.

If the ground below the marine ice sheets slopes downwards the further inland it goes (reverse bed slope), the grounding line accelerates as it moves forward. This is because the ice above is gets progressively thicker, and the thicker the ice is, the more vulnerable it is to floatation and calving.

The trigger for this process is ice shelve breakup. Ice shelves float on the surface of the ocean, forming a buttress around the edge of the grounded ice mass. As the atmosphere and ocean warm, they accelerate the breakup of ice shelves, which eventually advances the grounding line beneath the ice mass to the point where it is traveling down the slope of the bowl of the marine ice sheet.

This process would hit West Antarctica hardest, as it has large marine ice sheets.

Marine ice cliff instability (MICI)

This is a newer hypothesis, and still being debated. The idea is intuitive: ice cliffs higher than 90 meters collapse due to the physics of ice. The collapse process will accelerate if the ice mass gets thicker the further back it goes (as most do), as the ice collapsing at the edge of the mass reveals ever-higher, faster-collapsing ice behind. ²⁹

Albedo effect

Albedo (al-bee-doh) is a measure of how much light that hits a surface is reflected without being absorbed. Something that appears white reflects most of the light that hits it (high albedo), while something that appears dark absorbs most of the light (low albedo). ³⁰

As white ice melts, it reveals darker ocean and land below. These surfaces absorb more of the sun’s light as warmth. It has been estimated this could mean a warming “…equivalent to 25 years of global carbon emissions.” ³¹ Forest fires could also contribute to the albedo effect, by depositing black soot on ice sheets.

Methane release from melting permafrost

The Arctic contains vast quantities of frozen permafrost. As the planet warms, this permafrost melts, and has the potential to release greenhouse gases, including methane, which has a greenhouse effect several dozen times more powerful than CO2. ³²

Antarctica is the primary cause for increasing projections

In the 2014 IPCC report, it was understood that Antarctica would be an increasingly important driver of sea level rise going forward, but it was felt that models of ice sheet breakup were too uncertain to incorporate into GMSL projections. Since then, we have 1) observed accelerating Antarctic melting, and 2) learned more about the processes by which ice sheets break up. And so newer reports are more confidently incorporating Antarctic melting into their projections. For example, compare the 2014 and 2019 IPCC reports:

2014 AR5:

There is low confidence in the available models’ ability to project solid ice discharge from the Antarctic ice sheet. Hence, these models likely underestimate the Antarctica ice sheet contribution. ⁵⁴

2019 Oceans and Cryosphere:

Mean sea level rise projections are higher by 0.1 m compared to AR5 under RCP8.5 in 2100, and the likely range extends beyond 1 m in 2100 due to a larger projected ice loss from the Antarctic Ice Sheet (medium confidence) ⁵⁵

In general, IPCC reports tend towards more conservative projections, in part because they reflect the consensus opinion of many authors.