As our globe heats up, the effects of those rising temperatures are complex and varied.

Overall, we call these effects “climate change,” but that’s an umbrella phrase that covers a vast number of changes. Melting polar ice, glacial melt, changing weather patterns, more extreme weather, rising sea levels, ocean acidification … these are the topics we hear about quite a bit. But there are other changes going on, and these are every bit as much a cause for concern.

For example, global warming is strangling our oceans.

By that I mean that oxygen levels in the oceans are affected by global warming, too. This occurs in two general ways. One is that warmer water has a harder time holding on to dissolved gases—that’s a basic law of chemistry. So oxygen levels drop as water warms. The other is that as surface waters warm, they expand, and mix less with deeper water. Surface water gets most of its oxygen from phytoplankton that breathe it out. That water sinks and mixes with deeper water. As the water becomes more stratified due to global warming, less of that oxygenated water gets to lower depths.

The question then becomes, when will we start to see the effects of global warming on the oceans’ oxygen content? Like so many other aspects of climate change, we’re seeing that deoxygenation due to human-generated global warming occurring now. Not sometime in the distant future. Now.

That’s the result of a new study done by a team of scientists led by Matthew Long, an oceanographer at the National Center for Atmospheric Research in Boulder, Colorado. He and his fellow scientists used a sophisticated computer model that calculates the amount of oxygen dissolved in the oceans over the globe and with ocean depth. The model accounts for a great number of factors, including air temperatures, water temperatures, layers in the water, sea ice, and much more.

Global warming changes the air temperature, which affects the model. In the past, global warming was small, and natural variations in the environment changed the oxygen levels far more than human-made warming. But, as the world warms more and more, the effect of warming increases, and at some point becomes noticeable over the natural variations.

Think of it this way: Imagine being in a loud room, with many people talking, and someone nearby whispers at you. You can’t hear them at first over the ambient noise, but if they keep speaking more and more loudly, at some point you can distinguish their voice from the crowd.

The scientists running the model wanted to know when they could hear the voice of warming over the babble of the natural variations. They ran the model many times, varying the air temperature a small amount with each run. This gave them a range of outcomes that they could compare to physical measurements; that gave them a check to make sure they had real-world testable outcomes.

What they found is sobering. Deoxygenation due to human-made global warming is already detectable in the southern Indian Ocean, and in some regions in the eastern tropical Pacific and Atlantic. By 2030 to 2040—two decades from now—they expect to see more and more widespread deoxygenation over the globe. By the year 2100 (which is how far into the future they ran the models) a significant fraction of the global oceans will see some deoxygenation due to human activity.

This is, obviously, bad. The amount of deoxygenation may not be very much, just a drop of a few percent. But as we learned with increasing carbon dioxide levels in the atmosphere, and rising temperatures, it doesn’t take much change to destabilize a system.

The worst effects will come from areas already low in oxygen, called hypoxic zones, where the levels can be as much as 70–90 percent lower than average, and in suboxic zones, where it’s even lower. In those regions, a few percent drop can mean the difference between life barely holding on, and death.

Even when the change isn’t so dramatic, it can be devastating. You might think of the ocean as one big fish tank, but it’s actually incredibly diverse, depending on water temperatures, currents, pressure, and more. Changes in oxygen levels in the water reduce marine life habitats, stressing the inhabitants there. Changes in regional oxygen levels have caused migrations of fish, and even massive die-offs. Besides the effect on the life there, this has an impact on human activity including fishing, on which many countries depend.

Mind you, about half the oxygen we breathe comes from ocean phytoplankton. Messing with their habitat is like setting fire to your own house. Which is pretty much what we’re doing.

So you can add this to the list of deleterious effects of global warming to our planet. And don’t forget that the past six months have all had record-breaking high temperatures, with many scientists already expecting 2016 to be the hottest year on record globally, and carbon dioxide levels still rising.

Global warming is our future, but it’s also our present. It’s now. Once again, Americans, I implore you to consider this, what is perhaps the greatest long-term threat facing humanity, when you go to the voting booth in November. And for the rest of the world, know that most of us here in the U.S. are aware of the problem and doing our best to urge our leaders to take action as well. We can minimize the damage of global warming, but first we have to make sure our government is facing reality.