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I'm discussing climate change with a friend who is a climate change denier, basically the argument is that looking at this graph, is not clear that something unusual is going on, so the sea level rise is just something that was there, all natural not caused by human activity. Could you please explain this ? enter image description here

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    $\begingroup$ I wonder where this graph comes from. $\endgroup$ – Trilarion Jan 28 at 8:31
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    $\begingroup$ @Trilarion NASA credits CSIRO $\endgroup$ – JollyJoker Jan 28 at 8:53
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    $\begingroup$ Unusual compared to what? How long have we been keeping accurate weather reports and how long has earth been around? Yes, something is changing... is it unusual, is it solely because of humans... none of that can be proven or disproved with current data sets. $\endgroup$ – BossRoss Jan 28 at 11:27
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    $\begingroup$ @BossRoss It's not like one does prove anything in natural sciences. You have a theoretical model like for example man made climate change and all observations should be congruent with that model at the very least. There might be different models also explaining all the data. Unusual in this context probably means human-made. $\endgroup$ – Trilarion Jan 28 at 11:52
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    $\begingroup$ Hard to believe nobody has posted either of the relevant XKCD links $\endgroup$ – Bill K Jan 28 at 18:20
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The problem is the increase in the rate of sea level rise. I pulled out some approximate numbers from the figure you presented:

enter image description here

Can you see now how the sea level is rising much faster today than a century ago?

Sea level rise, as well as climate change are normal things on Earth history. However, most times they happen at a very slow rate, allowing ecosystems and other processes to adapt to the change.

For example, if the climate changes over several hundreds of years, animals and even tree populations can "move" towards the side of their distribution where climate is still good for them. But if the climate changes in 50 years, all the trees can die before they had time to grow in the areas where they could thrive in the new climate.

In the case of sea level rise. The delta of a river for example, stays in equilibrium with the sea level because of the accumulation of sediments carried by the river. If the sea level rises slowly, the sediments can fill the delta and keep it roughly at sea level. But if the see level rises too quick for the sedimentation to keep up, it will be flooded by the sea, killing all the animals and people that live on such fertile environments.

Analogously, coastal infrastructure have a given lifetime. Let's say 50 years. If the sea level doesn't change much over that period (8.5 cm at the 1880-1940 rate), there is no problem. Once the infrastructure gets replaced, the new building will be set a bit higher. However, in the next 50 years the sea level could rise 50 cm, or even more (it would be 19 cm if we assume the last rate from the figure won't increase any further), and that is a big deal. That could mean that much coastal infrastructure will be flooded, and maybe destroyed during storms.

In places like Bangladesh there are hundred of millions of people that could be displaced due to sea level rise if the rate keeps increasing. People that will also need to find a new home.

Coastal infrastructure loss, coastal erosion and immigration could be some of the worst expressions of fast sea level rise.

ADDITION

Given the interest risen by this question I'm adding here some data beyond what is presented by the OP, and brought to my attention by @Bobson in the comments. It comes from the paper Recent global sea level acceleration started over 200 years ago? and shows a sea level reconstruction going back to 1700, and it shows also the changes in rate over that period. This is summarized in their figure 3:

enter image description here

Here you can clearly see how, with some ups and downs, the rate of sea level rise have been increasing over the last few centuries. And notably the current rate, about 20 years after the end of this plot is already out of the scale, and around 3.2 mm/year as pointed also by other answers.

I would highlight the following from their abstract:

Sea level rose by 6 cm during the 19th century and 19 cm in the 20th century.Superimposed on the long-term acceleration are quasi-periodic fluctuations with a period of about 60 years. If the conditions that established the acceleration continue,then sea level will rise 34 cm over the 21st century. Longtime constants in oceanic heat content and increased ice sheet melting imply that the latest Intergovernmental Panel on Climate Change (IPCC) estimates of sea level are probably too low.

Regarding to whether this is caused by humans or not, I rather stay out of that argument and point that our best science and models suggest that lowering $\text{CO}_2$ emissions can make a significant impact in slowing down sea level rise in the upcoming centuries, so we should ACT NOW, and stop arguing whether it was or not our fault in the first place.

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    $\begingroup$ Excellent answer which addresses the most frightening aspect of climate change: the rate. $\endgroup$ – Gimelist Jan 28 at 0:54
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    $\begingroup$ This goes a long way to pointing out the real risk of CC - not so much that the planet becomes uninhabitable, but that changes in its habitability profile wrecks our complex, globalised economic system. This will lead to refugees, famines, the breakdown of international co-operation and a return to a geo-politcal landscape that was last seen in the 1930s. And we all know what happened then. $\endgroup$ – Oscar Bravo Jan 28 at 14:08
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    $\begingroup$ To be clear the sea levels has risen before, 8000 years ago you could walk from France to England. The end of the last glacial advance caused the Early Holocene Sea Level Rise (60meters) and a lot of settlements were destroyed by that rise. But since then sea level has been very stable, until now. We know what caused that change and its not what is causing the current one. $\endgroup$ – John Jan 28 at 15:38
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    $\begingroup$ this graph of the last 140 years cannot be used to prove anything, considering the earth is 4.5 billion years old, and the fact that the sea level has risen in the past much, much faster than even 4mm per year $\endgroup$ – user3163495 Jan 28 at 16:22
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    $\begingroup$ @user3163495 No doubt about that. The sea level have risen much faster in the past. But you still need to explain why it was stable for thousands of years and now is suddenly rising at a fast pace again. $\endgroup$ – Camilo Rada Jan 28 at 16:27
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The problem is that sea level is increasing faster than ever in last couple thousand years. It is currently rising at 3.2 mm/year according to satellite data:

enter image description here

The curve you showed is not a straight line, it is rising at an increasing rate.

And the trend is expected to continue: The last IPCC report (2018) on the subject say:

Projections vary in the range 0.26–0.77 m and 0.35–0.93 m for 1.5°C and 2°C respectively for the 17–84% confidence interval (0.20–0.99 m and 0.24–1.17 m for the 5–95% confidence interval).

And projected sea level incrase for year 2100 are summarized in table 3.1:

enter image description here

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Sea level rise from thermal expansion is a very slow process: oceans are 3.7 km deep on average, and water has a very large specific heat capacity.

Here's a related diagram from the IPCC Third Assessment Report (page 17): sea level rise after emissions are reduced

Climate change didn't have much impact on the sea level, yet.

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    $\begingroup$ Quick question: in the chart, when $CO_2$ emissions drop to very low levels, why does $CO_2$ stabilization (presumably $CO_2$ levels) not decrease? $\endgroup$ – Underminer Jan 28 at 14:30
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    $\begingroup$ @Underminer For the same reason that if you stop putting water into the bath tub the water level in it doesn't go down. For the $\text{CO}_2$ level to go down you need a $\text{CO}_2$ sink. $\endgroup$ – Camilo Rada Jan 28 at 16:22
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    $\begingroup$ I'll not downvote, but cannot upvote because this answer is tangential to the question in this current form. Maybe you cam imporove it if you compare it with the most important factors in sea rising, like ice caps melting and explaining why natural processes takes ages, literally $\endgroup$ – jean Jan 29 at 13:41
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    $\begingroup$ @jean: There's a curve for "sea-level rise due to ice melting" on the diagram, and it appears to be really flat in the beginning. So it seems to fit well with the "not clear that something unusual is going on" argument in the original question. Also, the simple fact that oceans have a very large volume and can absorb a lot of heat is a good explanation why the process takes many centuries. If you want a more detailed explanation, the IPCC summary for policymakers is linked. $\endgroup$ – Eric Duminil Jan 29 at 14:28
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    $\begingroup$ @immibis Yes, that happen and is known as $\text{CO}_2$ fertilization. But the effect is rather small compared with the main $\text{CO}_2$ sinks and sources. $\endgroup$ – Camilo Rada Jan 29 at 18:47
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I think this XKCD says it all:

enter image description here

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    $\begingroup$ I really love this graph. I had seen it before and it really makes the point completely clear $\endgroup$ – arkaia Jan 30 at 14:35
  • $\begingroup$ @arkaia Yeah, it's so vivid. $\endgroup$ – Tim B Jan 30 at 15:23
  • $\begingroup$ That graph is very compelling! But then I looked at it a second and third time and noticed something. Is the solid part the part where we have recordings of temperatures, and the dashed part where we're making projections either forward or back in time? $\endgroup$ – bob Jan 30 at 20:37
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    $\begingroup$ @bob I believe so. Solid is where we have direct measurements. Dotted into the past is where we work out past temperatures based on other evidence (ice cores, tree rings, etc) and dotted into the future are estimates at the time this was made (2016) $\endgroup$ – Tim B Jan 31 at 9:33
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    $\begingroup$ @bob Extrapolated means taking a trend and extending it beyond the known points. i.e. if you have 1, 2, 3 then you can interpolate 1.5 as half way between 1 and 2 and extrapolate 4, 5 as coming after 3. Extrapolation can go wrong fast (interpolation can also go wrong, but generally not as badly). For historical data we neither interpolate nor extrapolate. We use indirect methods using temperature proxies and combine the results from those proxies. - theguardian.com/environment/2012/mar/07/… $\endgroup$ – Tim B Jan 31 at 19:18
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In addition to other answers...

This curve isn't a good comparison to the "natural" state of the Earth, because it starts in 1880. The Industrial Revolution had been in full swing for some time by then, and by that point it had been fuelled by coal for around a century. Every factory was powered by coal, every house was heated by coal or coal gas, and every light used coal gas.

Even then, the Earth's temperature and sea level graphs didn't change too dramatically. By the mid-20th century though, climate models fitted to past data were extrapolating to a mini Ice Age. No big deal - the climate cycles like that, as the historical climate record shows.

Except the climate didn't behave that way. Instead of following the natural cycles that have been observed over hundreds of years, the temperature escalated as the graph shows. So climate scientists had to improve their models to cover dramatic differences to "normal" climate behaviour, modelling climate events which normally would only be seen in a disaster such as massive volcanic eruptions.

The result is that with all this work, they can predict a lot of the causes and effects of rising global temperatures due to fossil fuel usage (amongst other things). Your friend needs to not just look at the graph of what the Earth is currently doing, but also consider what climate scientists even as far as the 1970s who looked at the historical record were expecting the Earth to do, which assumed not burning stuff.

Even then, the Earth will survive. The idea of "saving the Earth" is nonsense - the Earth will survive. Many species will go extinct, but that's happened a lot in the past too. Many major cities (and some entire countries) will be lost too, which is more of an issue for humans. The question is whether your friend is prepared to accept humans having control over whether this happens or not.

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  • $\begingroup$ Not a bad answer but it could do with some actual data... $\endgroup$ – Jan Doggen Jan 28 at 16:30
  • $\begingroup$ "natural cycles that have been observed over hundreds of years"? $\endgroup$ – BЈовић Jan 28 at 16:45
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    $\begingroup$ @JanDoggen Added a few links. I doubt they'd convince a hardcore climate-change denier, but then we already know that they hold that position through faith and not evidence. $\endgroup$ – Graham Jan 28 at 16:45
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    $\begingroup$ @BЈовић I meant "cycles over hundred of years have been observed". Although we've got directly-measured records starting in the 1850s, so that's 160 years of solid observations. We've got a number of older records as well, from ships' captains and from various gentleman scientists, which are not global records but give us documentation for specific locations (or journeys). The oldest records apparently go back to the 1650s. earthmagazine.org/article/… $\endgroup$ – Graham Jan 28 at 17:02
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    $\begingroup$ @MartinSchröder The Earth will still be there. Creatures on it, less so... :) Hansen's hypothesis isn't really fact-based anyway. Yes, if we burned an order of magnitude more fuel than exists on Earth, then things would go wrong - but that's got a "magic happens" step of where this fuel comes from. Assuming a Coal Fairy doesn't exist, the Earth isn't really going to turn into Venus. TBH though, I think it'd stop being a good place to live well before that anyway, and "civilisation" would already have either collapsed or left the planet; either way the human problem goes away. $\endgroup$ – Graham Jan 30 at 14:49
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I just took the sea level elevation rate (mm/yr) graph from Camilo Rada's answer and added:

  • A vertical axis at zero (when the curve is at the point, it means the sea level did not change that year).
  • Red and green colors showing the trend.

Hopefully it will speak more to your friend.

enter image description here

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