What does “sea level” mean? How do you go about measuring it? Those with any experience in large bodies of water know that “level” is rarely a realistic description. Even without the moon distorting the Earth and driving the tides as it orbits us, swells and waves mean that most ocean surfaces are constantly moving up and down. Beyond that, areas with a large amount of dense matter – like mountains and ice sheets – will pull water towards themselves, causing higher sea levels in their gravity wells, and lower sea levels in other areas.

Measuring sea level requires taking thousands of different sorts of measurements all over the world, and for all that complexity, sea level represents just a tiny fraction of what’s happening in the oceans, let alone global climate change as a whole.

So how can we measure the rate of climate change? What does that even mean? Calculating the rate at which heat is being trapped, based on greenhouse gas levels, is pretty straightforward. We’ve known the basics of that for over a century, and it’s how we have headlines like “Earth is heating at a rate equivalent to five atomic bombs per second“. The problem is that that heat doesn’t necessarily stay as heat. There are a myriad of ways in which thermal energy can be converted to kinetic or chemical energy, on top of things that are hard to measure like deep ocean temperature changes.

Most of the heat the planet has been absorbing has gone into the oceans, but even so, scientists have been detecting biological and physical changes all over the planet that are driven by the rise in temperature.

And that brings up another question – how much does a given change in temperature actually matter? For humanity’s purposes, there are two main lines of inquiry to look at. The one that tends to get the most focus, for obvious reasons, is the effect on day to day and year to year temperatures. Will heat waves get worse? Will rainfall change? These are important questions to answer, but they might be less important than questions about the non-human parts of the biosphere.

How will a given change in temperature affect the wildlife where you live? Some of that will be a matter of precipitation or heat tolerance – same as with humans – but some will be increased pressure from new species moving into areas that used to be too cold, or too wet for them to survive. The temperature change we’ve seen thus far has already been affecting ecosystems all over the planet. Figuring out what those changes are, and what, precisely, has been driving them, can help us understand what is likely to happen as the planet continues to warm.  These “proxy” measurements won’t tell us what temperature the planet is, but they will help us draw a connection between the heat we know has been trapped by rising greenhouse gas levels, and the changes we’re seeing on the ground. That’s how you begin to build a projection of “if CO₂ levels rise to Xppm, it will probably have Y result”. We can’t see or feel the change in atmospheric gas levels, but we can see and feel follow-on results of that change.

Every time a research team runs a model to try to calculate how all these lines of data will interact, they tend to run a pretty wide set, allowing for different scenarios. The “worst-case” and “best-case” models bracket the most likely outcome, based on the data currently available, and the current understanding of those data. The problem here is that the current global changes are unlike anything that has ever happened in recorded history. Every year we enter new territory, which means that historical data are always going to be less reliable.

That’s why proxy measurements are so important. “Bio-indicators” like migrating birds and flowering plants give us insight into what climate change is doing right now to those species whose lives are most closely attuned to climate conditions.

Ice melt is another such proxy – it lets us see how fast energy is being absorbed and “spent” on converting solid water into liquid. Even if our historical data continues to point to the planet being on a “middle of the road” trajectory, if the ice is melting in line with a worse trajectory, then we need to check our numbers, and think hard about what’s headed our way.

Melting on the ice sheets has accelerated so much over the past three decades that it’s now in line with the worst-case climate warming scenarios outlined by scientists.

A total of 28 trillion metric tons of ice was lost between 1994 and 2017, according to a research paper published in The Cryosphere on Monday. The research team led by the University of Leeds in the U.K. was the first to carry out a global survey of global ice loss using satellite data.

“The ice sheets are now following the worst-case climate warming scenarios set out by the Intergovernmental Panel on Climate Change,” lead author Thomas Slater said in a statement. “Although every region we studied lost ice, losses from the Antarctic and Greenland ice sheets have accelerated the most.”

Ice melt from sheets and glaciers contributes to global warming and indirectly influences sea level rise, which in turn increases the risk of flooding in coastal communities. Earth’s northern and southern poles are warming more than twice as fast as the rest of the planet. In 2020, a year of record heat, Arctic sea ice extent hovered around the lowest ever for most of the year.

As I’ve mentioned before, I think it’s reasonable to feel badly about news like this. The world on which most of us were born no longer exists, and beyond finding ways to take direct action, I think we also need to be thinking hard about what human life on Earth looks like, and how it will have to change. Food production is one obvious area of focus, but so is basic habitation. Science fiction as a field has spent decades imagining how humanity might survive on a variety of alien planets. Temperature extremes, toxic atmospheres, hostile wildlife – a lot of it involves putting ourselves in a situation where, despite all of our advanced technology, we’re required to once again struggle for survival against a lethal and indifferent world. Keeping homes cool is already shifting from a matter of comfort to one of survival, and that change is likely to accelerate. Higher temperatures are going to mean more dangerous air pollution, even without things like increasing wildfires or even crematorium smoke as new diseases cause mass death.

I’ve believed for about a decade now that the planet is almost certainly going to keep warming for the rest of my life, even if I manage to have a very long life.

That melting ice released CO₂ into the atmosphere. The thawing permafrost is doing the same. The tiny amount of warming we’ve already seen has been enough to cause measurable changes across the entire surface of this planet, and many of those changes are going to make the warming speed up, or at least continue even if humanity stops adding to the problem.

So, we need changes, not just to how we interact with our atmosphere, but also to how we conduct our lives day to day. The floating neighborhoods of The Netherlands are a good example of this – they know sea level rise is going to be an escalating problem, especially with so much of their population already living below sea level. They could have just responded by building up their dikes, or moving people to higher ground, and while those options are definitely still on the table, having residential areas designed to simply float up as the water rises is one way to literally stay on top of the problem.

This is one of the reasons I keep leaning on local organizing as a catch-all starting point for dealing with climate change and political problems (insofar as the two can be said to be separate). The lifestyle changes needed for the Netherlands will be useless in most of California. The changes needed for California won’t help people in Alaska. The changes needed in Alaska won’t help people in Vietnam. What changes are coming to where you live? Should you be thinking about how to deal with killer heat waves as a community, or is air pollution a more pressing issue? Has there been an increasing problem with flooding from the ocean? If so, should you be focusing on how to keep your homes dry, or on how to ensure that there’s safe food and water available when the flooding happens?

At best, we can be sure that the worst-case scenarios are still a very real possibility, and that means that regional differences – and regional organizing – are going to matter a whole lot more going forward.

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