This video is sponsored by dashlane
As a youtuber who makes videos about the Earth's environment, and specifically climate,
I receive a lot of comments from people who dispute the influence humanity has on the natural world
One particular argument which gets brought up again and again is something along the lines of "why haven't sea levels risen?"
A frequent prediction of anthropogenic climate change is that mean sea levels will rise
threatening coastal communities, causing mass migration, and apparently the end of the world?
It certainly doesn't feel like that's happened
So in this video, let's review the data on global sea levels and work out if humans are actually changing anything
First of all, let's look at some data from coastal tide gauges, not satellite data,
nothing fancy, an actual physical set of gauges at multiple locations around the world.
Do these measurements show sea level ri-?
-Oh, wow, yeah, they do
The gauge has shown nearly 25 centimetres of mean sea level rise from 1880 to 2013
Sea levels have been rising a lot
But the term sea level rise isn't actually terribly useful when talking about climate change
Because if you look at historical data, from among other things coral cause,
It's clear that mean sea levels have been rising for around 20,000 years
As this figure suggests, this is a result of the earth coming out of the last glacial maximum,
what you might call the most recent ice age
About 20,000 years ago, the earth started warming,
largely due to cyclical variation in its orbit around the Sun, called Milankovitch cycle
Though there were likely other factors
As the earth warmed, which we can see very clearly in proxy data like ice cores,
sea levels rose rapidly due to ice sheets melting and pouring into the ocean basins
Water that previously been locked up as ice including a huge amount as ice on land enter the oceans and increase their volume
when these ice sheets retreated the landscape significantly changed
making for example, the British Isles separate from mainland Europe
Returning to the figure, we're very much at the tail end of this interglacial sea level rise
which appears to be either completely static or decreasing in pace,
but still changing the mean sea level
A smart question to ask then is,
"Given this background sea level rise the earth coming out of the last glacial maximum,
has humanity changed the rate of sea level rise?"
In other words, by influence in the climate through emitting greenhouse gases like co2 have we, as a species, accelerated sea-level rise?
Well, we have a time series of mean sea level stretching back to 1880.
The hypothesis is that humans have increased the rate of sea level rise since then
In terms of this graph, that the rate of sea-level rise (the slope) is greater at the end of the time series than at the start
Clearly the sea level hasn't risen by the same amount every year. There are plenty of wiggles
So let's take an average over a long period of time
Let's compare the average sea level rise in the first 30 years of data to the average sea level rise and the last 30 years of data
If I was being rigorous,
Then I'd test if my conclusions were sensitive to the length of time considered,
but I'd leave that as an exercise for the viewer.
In the first 30 years, sea level rose by an average of one point three one millimeters per year
In the last 30 years, the sea level rose by two point five millimeters per year
So what could have caused this increase? Factors which are known to have caused previous sea level change:
Milankovitch cycles and continental drift, operate on long timescales and haven't been observed to change significantly over the past century
Another mechanism that could force this increase is variation in solar outputs: the total solar irradiance
Over the past century, this has increased but by less than nought point one percent (0.1%)
not enough to have forced observed acceleration of sea level rise
that leaves one further mechanism known to alter sea levels: changes in radiative forcing
Over the past century, the concentration of radiatively active greenhouse gases in the atmosphere have changed significantly
due to human emissions, altering the Earth's energy budget
the resulting increase in global average temperature causes both increased melt of ice sheets like Antarctica and Greenland,
But also thermal expansion of the oceans. If you heat up water, it expands in volume ever so slightly
This is well known physics. This isn't noticeable on small scales, but when you expand an entire ocean ever-so-slightly, The results are absolutely measurable.
So we have observational evidence that sea level rise has accelerated and that natural forcings have not varied sufficiently on the timescales of observation
Therefore, we're left to conclude that our hypothesis that humans have accelerated sea level rise is correct
But this was just me messing around with one time series for 10 minutes
There is an entire scientific literature using lots of data sources including global satellite altimetry
Because sea level rise isn't the same everywhere and more powerful statistical techniques to establish that Yes, sea levels started rising more rapidly
Around the turn of the 20th century and that this rate of change has likely continually increased over the past 100 years
so we can be pretty sure that humans have accelerated global sea level rise
but only by a couple of millimeters per year
That hardly matches up with the predictions of the future looking like Kevin Costner's "Waterworld"
by which I mean, terrible
Well there are two parts as to why:
Firstly, no serious scientist has claimed that we would all be underwater or living on atolls or trimarans by the present day
Many of the scientific predictions that are mocked by climate change deniers have either been hyped out of all recognition from serious research
or outright fabricated
I had a mate who used to do exactly this. He would hype up stories about me to the point where he could start mocking me
for spreading such outlandish tales about myself to our friends.
For the record, for those who know me from uni
I never actually single-handedly saved a girl from drowning by abseiling bare-chested into the river. That-
That was Rob's version of the story.
Secondly, previous predictions of sea level rise actually match up fairly well with what we've observed
The very first IPCC report in 1990
made a series of predictions of sea-level rise by the present day
and satellite observations are right in the middle of their predictions for the first 26 years,
showing slightly less sea level rise than their best guess at the time
Current estimates still have uncertainties attached but much smaller ones as we
understand the systems much better and as our models for predicting sea level rise add new complexity
and more physical systems, our estimates as well as getting more accurate get higher and higher
It's considered very likely that sea levels in the year 2100 will be between 30 and
130 centimeters higher than in the year 2000
And a lot of scientists consider that to be too conservative an estimate. In other words,
the acceleration of sea level rise that we've observed humans calls to date, is only the thin end of the wedge
Sea levels are going to rise faster and faster every decade and by the end of the century,
the impacts are going to be very real. So to answer the question in the video's title
Sea levels HAVE been rising every decade. And in fact, the sea level has been rising faster every decade
Scientists understand the physical systems behind this increase in sea level
and have seen their prediction born out by observations thus far,
And the acceleration of sea level rise that we, humans, cause is going to continue for the foreseeable future
committing us to potentially more than a meter of sea level rise by the end of the century
This will put coastal populations at risk of inundation,
Species at risk of extinction, (something we think that's already happened actually)
And economies around the world
Especially those of island nations at risk of going under and that's just this century
We are committed to further warming and further sea-level rise for centuries to come
Every tonne of greenhouse gas that we emit into the atmosphere now,
only makes that long term change worse.
We can, of course, adapt to changes in our environment,
We are a hardy species. But why make the lives of our descendants, generations yet unborn,
more difficult, perhaps even impossibly difficult
when we can take action now
See the description for my video on what we can do today to enable
meaningful change in our climate outlook
Thanks to dashlane also linked in the description for supporting my climate change based content on this channel
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Thank you again and I'll see you in the next one