Models of the effects of climate change on sea levels have proven accurate for the early 21st century, new research shows. The projections have been confirmed not only globally, but for more challenging regional changes. Although this is encouraging for the scientists responsible, particularly in the face of the skepticism they experience from fossil fuel industry lobbyists and others, it's also worrying given their grim projections for the future.
A warmer world increases the volume of water in the world's oceans in ways both simple and complex. Consequently, there is always some uncertainty about efforts to predict how sea levels will change in response to a hotter climate. Moreover, such rises are not spread evenly across the globe. Ocean currents and temperature variations cause substantial regional differences that can change with time. All of which is further modified by local subsidence or geological uplift. That makes the job of predicting sea levels hard enough, even without those keen to pounce on any mistakes or present temporary fluctuations as representing much more than they do. (That's when they're not just straight out making things up).
In Nature Communications, University of New South Wales oceanographer Professor John Church conducted an assessment of how he and his peers performed in their predictions. Church and co-authors compared the projections made in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) from when projections started in 2007 to 2018. They found that trends from the report agree with satellite and tide-gauge observaions for that time period with a confidence level of 90 percent.
Like other IPCC reports, AR5 didn't provide a single prediction, instead making multiple projections of what would happen depending on how much action the world took to reduce emissions of greenhouse gasses. The most optimistic, a scenario where the world gets serious about reducing greenhouse gasses was known as RCP2.6, the worst case RCP8.5 with a median situation RCP4.5.
After allowing for known geologic changes, an average of 177 tide-gauges around the world gives an average rise of 3.6 millimeters a year (0.14 inches per year) accelerating through the period.
“Our analysis implies that the models are close to observations and builds confidence in the current projections for the next several decades,” Prof. Church said in a statement. Although he noted an 11-year period is insufficient to really be certain of which track sea levels are on, given fluctuations caused by major weather events. Nevertheless, “The analysis of the recent sea level data indicate the world is tracking between RCP4.5 and the worst-case scenario of RCP8.5,” he added.
Given the utterly catastrophic consequences of RCP8.5 on quite short timescales that's dire news. “If we continue with large ongoing emissions as we are at present, we will commit the world to meters of sea-level rise over coming centuries,” Chruch noted.